backfire then dead now wont start

[8Mud]

I can't really think of anything in the TB to keep it from starting except a stuck closed IAC or a birds nest. Maybe a bad TPS messing with the supply voltage to other sensors.
Let me get this straight, the plugs smell like fuel, you are getting spark and the motor doesn't even make a pop when you try to start it?
Did you try what I suggested, holding the throttle all the way open and cranking it over for 15 seconds. Full throttle at start shuts off the injectors and all that happens is you purge (blow) the excess fuel out of the cylinders. Then hold the pedal open 1-2 inches and try to start it, which will give you enough air for a start even if the IAC is stuck shut.
Did you fill it up with diesel or water the last time you tanked up?
With fuel, air and spark something is going to happen.
I've fixed my junk with less trouble in a snow storm, at midnight, with no light or tools. First thing is to figure out if it's spark, fuel or air.
Maybe it's the fuel and you are mistaken about the plugs being wet. Or maybe the plugs are fuel wet and you are mistaken about the spark or air in or exhaust out isn't flowing as nicely as you think.
You have compression on two cylinders around 5% below the lower limit, not good, but not a motor killer.
We are missing something here. Pour a little fuel down the TB and see what happens, 1/3 shot glass full, don't over do it. And don't look down the TB when you do it, unless you are already bald and have no eyebrows.

 

[8Mud]

Are we having fun yet.

 

[djgrayxj]

Editted by Administrator to remove links to copyrighted material.

Read the sticky at the top of the forum about links to FSMs.

 

[Mitchen]

I can't really think of anything in the TB to keep it from starting except a stuck closed IAC or a birds nest. Maybe a bad TPS messing with the supply voltage to other sensors.
Let me get this straight, the plugs smell like fuel, you are getting spark and the motor doesn't even make a pop when you try to start it?
Did you try what I suggested, holding the throttle all the way open and cranking it over for 15 seconds. Full throttle at start shuts off the injectors and all that happens is you purge (blow) the excess fuel out of the cylinders. Then hold the pedal open 1-2 inches and try to start it, which will give you enough air for a start even if the IAC is stuck shut.
Did you fill it up with diesel or water the last time you tanked up?
With fuel, air and spark something is going to happen.
I've fixed my junk with less trouble in a snow storm, at midnight, with no light or tools. First thing is to figure out if it's spark, fuel or air.
Maybe it's the fuel and you are mistaken about the plugs being wet. Or maybe the plugs are fuel wet and you are mistaken about the spark or air in or exhaust out isn't flowing as nicely as you think.
You have compression on two cylinders around 5% below the lower limit, not good, but not a motor killer.
We are missing something here. Pour a little fuel down the TB and see what happens, 1/3 shot glass full, don't over do it. And don't look down the TB when you do it, unless you are already bald and have no eyebrows.

it will pop every once and a while when cranking but not very loud and not alot just a very light pop nothing loud. and no i didnt put diesel or water in it :moon: i havent tried spraying anything in there i will try gas today. im not at home right now but will be soon. i will double check everything when i get home, is there a way to tell if the fuel pressure regulator went bad? it sprayed out of the rail with alot of pressure but i was expecting more volume to come out.

 

[8Mud]

Two different fuel pump electrical systems, one comes from the starter relay and bypasses the ballast resistor, splices into the harness, then back to the fuel pump. And the other from the fuel pump relay, through the ballast resistor then back to the pump. It is possible to get electricity to the fuel pump in run *or* start.
Like mentioned a fuel pressure tester might tell a story. But first I usually go to a handy spot, like one end of the ballast resistor and check voltage there during start and run ( or cranking *with spark* even if it isn't actually running). The fuel pump relay will close for a second or two when the key is turned to run for a prime, it won't close again unless the ECU gets a pulse from the CPS (safety factor).
If I have electrical power to the fuel system and still suspect fuel, I hook up a fuel pressure gauge and crank it over.
In my experience the fuel pressure has to be way low for it not run somewhat. I really can't remember the lowest pressure I've seen and had the motor still run, at a guess, somewhere around thirty percent low and it will still run at idle (though likely not well).
Just for info, I've put the distributor in one tooth off it still ran, completely screwed up the firing order and it tried to run, had the injectors firing out of sync and it ran, had one bad intake valve and it ran, had so much mud and water on the engine it had multiple shorts and it still tried to run. What I'm saying is, if it doesn't even try to run, chances are you are missing a basic component of combustion, spark, fuel or air.
I usually troubleshot form the basics, to the complicated, just because the basics are usually easier. I've found the wiring or electrical system to screw up more often than the sensors or the ECU in the Renix.
It could be as simple as a partially plugged fuel filter. It might be as complicated as a voltage or amperage loss at an electrical connector (which can be really hard to find).
If you have low fuel pressure or volume and your getting spark, it is likely something in the fuel delivery system. If you have spark and good fuel pressure, your problem is likely near the fuel rail, most likely to be in the harness someplace, from my experience.
I have an inductive timing light, I tape the trigger closed and lay it up someplace where I can see it blink, then I know I have spark. Intermittent problems can drive you nuts, sometimes it works and sometimes it doesn't, can send you down the wrong path.

You mentioned the cap and rotor were "destroyed", there is a rolled pin that holds the distributor shaft to the gear drive, this pin can shear. If it does, the rotor can rotate pretty much interdependently from the motor. Kind if a random timing and firing order, most likely to manifest as periodic popping out of the throttle body.

 

[Mitchen]

Two different fuel pump electrical systems, one comes from the starter relay and bypasses the ballast resistor, splices into the harness, then back to the fuel pump. And the other from the fuel pump relay, through the ballast resistor then back to the pump. It is possible to get electricity to the fuel pump in run *or* start.
Like mentioned a fuel pressure tester might tell a story. But first I usually go to a handy spot, like one end of the ballast resistor and check voltage there during start and run ( or cranking *with spark* even if it isn't actually running). The fuel pump relay will close for a second or two when the key is turned to run for a prime, it won't close again unless the ECU gets a pulse from the CPS (safety factor).
If I have electrical power to the fuel system and still suspect fuel, I hook up a fuel pressure gauge and crank it over.
In my experience the fuel pressure has to be way low for it not run somewhat. I really can't remember the lowest pressure I've seen and had the motor still run, at a guess, somewhere around thirty percent low and it will still run at idle (though likely not well).
Just for info, I've put the distributor in one tooth off it still ran, completely screwed up the firing order and it tried to run, had the injectors firing out of sync and it ran, had one bad intake valve and it ran, had so much mud and water on the engine it had multiple shorts and it still tried to run. What I'm saying is, if it doesn't even try to run, chances are you are missing a basic component of combustion, spark, fuel or air.
I usually troubleshot form the basics, to the complicated, just because the basics are usually easier. I've found the wiring or electrical system to screw up more often than the sensors or the ECU in the Renix.
It could be as simple as a partially plugged fuel filter. It might be as complicated as a voltage or amperage loss at an electrical connector (which can be really hard to find).
If you have low fuel pressure or volume and your getting spark, it is likely something in the fuel delivery system. If you have spark and good fuel pressure, your problem is likely near the fuel rail, most likely to be in the harness someplace, from my experience.
I have an inductive timing light, I tape the trigger closed and lay it up someplace where I can see it blink, then I know I have spark. Intermittent problems can drive you nuts, sometimes it works and sometimes it doesn't, can send you down the wrong path.

You mentioned the cap and rotor were "destroyed", there is a rolled pin that holds the distributor shaft to the gear drive, this pin can shear. If it does, the rotor can rotate pretty much interdependently from the motor. Kind if a random timing and firing order, most likely to manifest as periodic popping out of the throttle body.

well i took you advice and started checking some basic things and basic components for an engine to run. like you said fuel air and spark. its getting spark i took the plugs out and held them while someone turned the ignition. (that was a bad idea shocked the shit out of me) but there was a bright redish white spark they are the four prong plugs and they look fairly clean. a little carbon i cleaned off but the gap is good and they sparked.

fuel was next, i had my buddy turn the key to "run" and had him crank it also. both times i was under the fuel tank and could here the pump clicking *click click click* pretty fast i might add it was not like the normal hum/buzz i have heard XJ fuel pumps make. could my fuel pump be bad? it got a new one in 2008. my buddy also said he has never heard a fuel pump that made that sound. and he thinks there is not even close to enough fuel pressure on the fuel rail.

 

[Mitchen]

well i took you advice and started checking some basic things and basic components for an engine to run. like you said fuel air and spark. its getting spark i took the plugs out and held them while someone turned the ignition. (that was a bad idea shocked the shit out of me) but there was a bright redish white spark they are the four prong plugs and they look fairly clean. a little carbon i cleaned off but the gap is good and they sparked.

fuel was next, i had my buddy turn the key to "run" and had him crank it also. both times i was under the fuel tank and could here the pump clicking *click click click* pretty fast i might add it was not like the normal hum/buzz i have heard XJ fuel pumps make. could my fuel pump be bad? it got a new one in 2008. my buddy also said he has never heard a fuel pump that made that sound. and he thinks there is not even close to enough fuel pressure on the fuel rail.

alright never mind we cranked it whille holding that valve open and it was dumping alot of fluid with alot of pressure. he works in a shop and he said that should be plenty of fuel to start. he said exhaust pressure is good and if you hold your hand on the intake tube it sucks your palm in and gets smaller.
the rockers are moving and the timing is good. also i bypassed the ballast resistor for the fuel pump to get more fuel flowing and it made no difference. i got more volume but it still did not want to start. and the injectors are ticking

also there is a sensor on the side of the block drivers side. between block and oil pan. it broke off what is it? it had two wires and was ripped off (probably from wheeling trip)

and he checked the spark and said its definantly sparking.

so im back to CPS he thinks i should replace that as its the only thing he can think of that would not allow it to start when its getting fuel air and spark. (i might add i did a transmission swap about 600 miles ago my ba 10/5 went bad so i put in a new to me tranny same one with 80k on it not sure if this would affect the CPS) but i used the same cps on the new to me transmission.


also what is a MAP sensor? and what is an IAC sensor? what do they control

 

[8Mud]

If your getting good spark it is doubtful to the be a bad CPS, though I have seen them miss a beat or two and give erratic spark. Missing a beat or two can also load your exhaust up with unburnt fuel and may cause a backfire.

Figure out some way to tell of the injectors are cycling, I usually use a plastic tube and listen right on an injector when the motor is being cranked over, I can hear the injector going clackity clack. They say you can buy a noid light injector circuit tester, I've never used one. I've also used a analog volt meter and can see when the needle jumps that injector is getting a pulse. I'd also ohm test the injector ground wire (black wire), they all share the same ground wire that goes through a bunch of splices and eventually ends up at the engine dipstick holder.
MAP sensor is manifold atmospheric pressure, in effect it measures vacuum. Most that I have seen fail, have failed full rich and can flood the motor. I had one, that some way or the other, was getting 12 volts in the 5 volt supply circuit, which fried it. I replaced the MAP and the ECU (I had a spare). Though as far as I can tell this was a one off problem, I've never heard of it before or since. Most times it is the vacuum line for the MAP that is cracked, broken or plugged into the wrong hole on the TB or the electrical connector is partially plugged in or a pin has been bent over. I use a mirror to look in the plug socket.
The IAC is the idle air controller, when the throttle is closed , the throttle plate in the TB is shut and the air for a start comes through the IAC valve. If the IAC valve is stuck closed, the only way to get enough air for a start is to give it a little pedal. If the IAC valve sticks closed, the motor can also flood and is unlikely to start again easily. The IAC is also called a stepper motor.
I don't really know if it is possible to loose power to ECU and the injector drivers and still have other ECU functions working (like spark). I do know the ECU has two power sources, one is from the ignition which may just be a on/off switch. The other power source is from a fusible link, near the starter relay and is the main power.

The best test for a CPS is an ohm test 200 ohms plus or minus 75 ohms. Most good ones I've seen test out at right around 200, to maybe 225 or so. Though an ohm test if it is way high is an indicator the CPS is toast, one that ohm tests good may not be.

If you aren't getting any injector function you should still be able to get the motor to kind of start using a *little* fuel or ether down the TB, while holding the throttle open a little. Be careful this is dangerous, have a rag and extinguisher handy, wear gloves and keep your face away from the TB. The over use of ether can also cause damage to the motor, but may help you get a start if the cylinders are flooded with fuel or not getting any fuel at all.

That sensor on the side of the block may be the engine temperature sensor. I've never had one go bad, you'll have to ask someone else what happens when they do go bad or break off. The other is the knock sensor also something that I've never had break or screw up, other people have said they never really noticed any issues when it was broken. I have seen the harness where it runs from the knock sensor, the engine temp sensor and the O2 sensor cook on the exhaust manifold in the front of the motor and cause all sorts of assorted mischief, when it melt through and shorted out.

 

[Mitchen]

If your getting good spark it is doubtful to the be a bad CPS, though I have seen them miss a beat or two and give erratic spark. Missing a beat or two can also load your exhaust up with unburnt fuel and may cause a backfire.

Figure out some way to tell of the injectors are cycling, I usually use a plastic tube and listen right on an injector when the motor is being cranked over, I can hear the injector going clackity clack. They say you can buy a noid light injector circuit tester, I've never used one. I've also used a analog volt meter and can see when the needle jumps that injector is getting a pulse. I'd also ohm test the injector ground wire (black wire), they all share the same ground wire that goes through a bunch of splices and eventually ends up at the engine dipstick holder.
MAP sensor is manifold atmospheric pressure, in effect it measures vacuum. Most that I have seen fail, have failed full rich and can flood the motor. I had one, that some way or the other, was getting 12 volts in the 5 volt supply circuit, which fried it. I replaced the MAP and the ECU (I had a spare). Though as far as I can tell this was a one off problem, I've never heard of it before or since. Most times it is the vacuum line for the MAP that is cracked, broken or plugged into the wrong hole on the TB or the electrical connector is partially plugged in or a pin has been bent over. I use a mirror to look in the plug socket.
The IAC is the idle air controller, when the throttle is closed , the throttle plate in the TB is shut and the air for a start comes through the IAC valve. If the IAC valve is stuck closed, the only way to get enough air for a start is to give it a little pedal. If the IAC valve sticks closed, the motor can also flood and is unlikely to start again easily. The IAC is also called a stepper motor.
I don't really know if it is possible to loose power to ECU and the injector drivers and still have other ECU functions working (like spark). I do know the ECU has two power sources, one is from the ignition which may just be a on/off switch. The other power source is from a fusible link, near the starter relay and is the main power.

The best test for a CPS is an ohm test 200 ohms plus or minus 75 ohms. Most good ones I've seen test out at right around 200, to maybe 225 or so. Though an ohm test if it is way high is an indicator the CPS is toast, one that ohm tests good may not be.

If you aren't getting any injector function you should still be able to get the motor to kind of start using a *little* fuel or ether down the TB, while holding the throttle open a little. Be careful this is dangerous, have a rag and extinguisher handy, wear gloves and keep your face away from the TB. The over use of ether can also cause damage to the motor, but may help you get a start if the cylinders are flooded with fuel or not getting any fuel at all.

That sensor on the side of the block may be the engine temperature sensor. I've never had one go bad, you'll have to ask someone else what happens when they do go bad or break off. The other is the knock sensor also something that I've never had break or screw up, other people have said they never really noticed any issues when it was broken. I have seen the harness where it runs from the knock sensor, the engine temp sensor and the O2 sensor cook on the exhaust manifold in the front of the motor and cause all sorts of assorted mischief, when it melt through and shorted out.

well i poored gas down there and it didnt even try to start it ccoughed out the exhaust a little did with no pedal and with pedal floored. i was also thinking CPS sensor because if it went bad or gave an abnormal pulse some cylinders would ignite others wouldnt and fuel would build up in exhaust and when it finally sparked cause one big backfire. like what happened the day it died.

 

[8Mud]

Sometimes you have to go with your gut, but I'd still do an ohm test on the CPS. Check a couple of plugs and see if they are wet after you try a start. Too much fuel can be as bad as too little fuel, the plugs can get so wet they won't fire right. I've had the best results with regular old Champion plugs, without the trick electrodes or exotic platting.
A good spark is blue white and you can hear it crack. Weak spark is usually yellow thick looking and even splashes sometimes on the electrode.

Don't rest your crotch against the fender when checking the plugs :wow:, rubber gloves are your friend :clap:.

 

[Mitchen]

Sometimes you have to go with your gut, but I'd still do an ohm test on the CPS. Check a couple of plugs and see if they are wet after you try a start. Too much fuel can be as bad as too little fuel, the plugs can get so wet they won't fire right. I've had the best results with regular old Champion plugs, without the trick electrodes or exotic platting.
A good spark is blue white and you can hear it crack. Weak spark is usually yellow thick looking and even splashes sometimes on the electrode.

Don't rest your crotch against the fender when checking the plugs :wow:, rubber gloves are your friend :clap:.

i will check it, it just seems like its the only sensor thats been fiddled with at all sense i have had the truck so it would make sense it would be the messed up one, but i will check it and i will re check/ possibly replace the plugs and wires. if the is reads good i may still replace it.

and i will keep my boys away from the fender:lecture:

 

[Swamprat]

oddly enough, i had the same thing happen to my 88 MJ just yesterday. it was sucking down gas, the stalled out, started then stalled, and then one big pop and it wont start again. today i'm tearing into it to see if i can get it going long enough to rebuild on of the HO's i have laying around.

 

[8Mud]

oddly enough, i had the same thing happen to my 88 MJ just yesterday. it was sucking down gas, the stalled out, started then stalled, and then one big pop and it wont start again. today i'm tearing into it to see if i can get it going long enough to rebuild on of the HO's i have laying around.

Just for the heck of it, grab the rotor and turn it counter clockwise hard. Maybe put an old rotor on there, because it may take some significant force to turn the shaft even if the pin is sheared and you don't want to shatter your good rotor. I used to see that a lot on old Chevy's, it would pop once and shear the shaft pin (or maybe shear the shaft pin then pop, who knows). Never seen it on an XJ, but seeing as most of the old ones have been around for twenty years, things are bound to show up.

 

[chiw00]

just have the same issue on my 1998 xj classic, backfired and then it didnt started.

had a good cps, iac, fuel pump, good spark, but it wouldnt started.

tried with a new coil and in half a second it started, even check engine light went off, maybe it will be good to borrow a coil and just test see what happend..

 

[8Mud]

just have the same issue on my 1998 xj classic, backfired and then it didnt started.

had a good cps, iac, fuel pump, good spark, but it wouldnt started.

tried with a new coil and in half a second it started, even check engine light went off, maybe it will be good to borrow a coil and just test see what happend..

Not a bad idea, spark in the open air and spark under compression are two different things. Ten times compression means ten times air density. Coils can get pretty hot quick.
The Renix coil and the ignition module are one unit, two pieces. The contacts where the two pieces plug together get hot and relax making for iffy contact. Trying to bend the contacts together to get them tighter can be dangerous, the metal contact clips get really soft.

Periodically I take an inductive timing light and hook it to each spark plug cable one at a time and shine it on the relay cover (on the Renix). You can see the missed beats and weak spark pulses pretty easily, the next best thing to an oscilloscope.

 

[Mitchen]

Not a bad idea, spark in the open air and spark under compression are two different things. Ten times compression means ten times air density. Coils can get pretty hot quick.
The Renix coil and the ignition module are one unit, two pieces. The contacts where the two pieces plug together get hot and relax making for iffy contact. Trying to bend the contacts together to get them tighter can be dangerous, the metal contact clips get really soft.

Periodically I take an inductive timing light and hook it to each spark plug cable one at a time and shine it on the relay cover (on the Renix). You can see the missed beats and weak spark pulses pretty easily, the next best thing to an oscilloscope.

CPS didnt solve anything. my budy wrecked his jeep and its just sitting now, so i will ask him to barrow his coil. which whole on the trhottle body does the vacumm line go to? the top on or bottom one? i dont think it got changed but figure it was worth asking.

 

[8Mud]

The bottom hole goes all the way through on my 87 TB. If your not sure use a flashlight and a mirror and look.
I periodically blow through there, the same with the MAP vacuum line. More of a why not type thing than a why thing, I've never found one plugged but if I blow through there I'm sure it is open.
The only time I've seen fail to start, if it had spark and fuel (and a close initial setup) is when it was seriously flooded, water in the cylinders or the plugs were oil coated. Really hard starts have usually come from bad fuel, weak spark, air in the fuel rail.
I'm sure there are other reasons, these are the ones that come to mind.
How fast is your motor turning over? If it is turning over really slowly it can cause issues.

 

[old_man]

Like 8Mud said, fouled plugs cause lots of problems. At this point, go get yourself a set of cheap Champion plugs and install them. Also, there is a problem with the computers where you can still be cranking quite well but the voltage getting to the computer is not enough to let it work, so put the battery on a charger for a few hours.

 

[djgrayxj]

Information for Mitchen
From DJ with a 88XJ
Regarding you’re wires hanging down.

Engine temperature sensor wires are probable Tan & Black, located tucked up next to exhaust manifold down pipe.

Knock sensor wires are probable Violet & Black, they are probable wrapped in aluminum foil inside of the plastic covering. It is located tucked just behind the motor mount.

Quote from 8Mud
“That sensor on the side of the block may be the engine temperature sensor. I've never had one go bad, you'll have to ask someone else what happens when they do go bad or break off. The other is the knock sensor also something that I've never had break or screw up, other people have said they never really noticed any issues when it was broken. I have seen the harness where it runs from the knock sensor, the engine temp sensor and the O2 sensor cook on the exhaust manifold in the front of the motor and cause all sorts of assorted mischief, when it melt through and shorted out.”

Make sure these dangling wires are not touching any metal or each other. If either conduction is present you are sending an unrecognizable signal to the ECU.
The Black wire is spliced together with several other sensors. The poor ECU is all confused.

Keep us posted

 

[1990JEEPXJ]

im still thinking its the coil. when mine went south it would do the same thing. pop and backfire when warm then eventually it just wouldnt start. had spark, just not strong enough to run the motor. you can have it tested at most auto parts stores. i would check that before getting too deep into this.

KISS. keep it simple stupid.

good luck