backfire then dead now wont start

88 cherokee 4.0 renix motor rebuilt with 35-40k on it 5 speed.

driving to school today the jeep decided to break, it backfired one big loud pop probably as loud as a small gunshot and shut off and i had to pull over.

it is getting fuel, its getting air, there is exhaust pressure out the tail pipe, had plenty of oil and the temp was fine. it cranks and cranks and cranks but wont start it gives an occasional light backfire pop out the tailpipe.

i put a new cap and rotor on it because they were destroyed and it seemed like a timing issue/not firing on all cylinders but had no luck. checked plugs and its getting spark.

then someone suggested maybe it blew up a piston but i figured i would hear that like the other motors i have seen blow up pistons.

im aiming for broken timing chain/skipped a tooth. and im not excited to fix this if this is what the problem is. i have not dealt with timing chains before.

any ideas on what would be causing this would be appreciated.

Thanks,
Mitch

Check if you are getting a spark. If not, it is most likely the CPS. Beyond that given the circumstances, I would most likely do a compression test.

Also check inside the Distributor to make sure all the parts are in the right spot. Sounds like the same thing that happened to a friend with his rig....loud backfire then no start...it was a broken part in the distributor. Just a thought.

old_man said:

Check if you are getting a spark. If not, it is most likely the CPS. Beyond that given the circumstances, I would most likely do a compression test.

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Crank position sensor? how do i test that? and i was planning on doing a compression test just have to get the tools

also as stated in original post i AM getting spark.

FiveNdime said:

Also check inside the Distributor to make sure all the parts are in the right spot. Sounds like the same thing that happened to a friend with his rig....loud backfire then no start...it was a broken part in the distributor. Just a thought.

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do you mean under the cap where the rotor is? or take the whole distributor off?

i know the cap/rotor are on correctly and brand new as of today, wires are good and the firing order is correct.

Before I did anything else I'd set up the motor to TDC number 1. Before removing anything, just as a check.
If the number one piston is all the way up, with the number one cylinder on the compression stroke and the timing mark lined to zero, your timing chain is good (good meaning at least in the right gear tooth). If your rotor is pointed to about the five o'clock position your distributor drive shaft pin is likely OK.
Then you can go searching for a crossfire or whatever else is causing you grief.

This might be helpful, but try it the easy way first.
http://www.naxja.org/forum/showthread.php?t=964617


I've had my motor crossfire, pop and blow the MAP vacuum tube out of the TB.

8Mud said:

Before I did anything else I'd set up the motor to TDC number 1. Before removing anything, just as a check.
If the number one piston is all the way up, with the number one cylinder on the compression stroke and the timing mark lined to zero, your timing chain is good (good meaning at least in the right gear tooth). If your rotor is pointed to about the five o'clock position your distributor drive shaft pin is likely OK.
Then you can go searching for a crossfire or whatever else is causing you grief.

This might be helpful, but try it the easy way first.
http://www.naxja.org/forum/showthread.php?t=964617


I've had my motor crossfire, pop and blow the MAP vacuum tube out of the TB.

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okay i just have two questions, how can i tell if its on the compression stroke? and in that article what is a dizzy?

I tried to test my CPS. I ended up buying a new one and it worked. When mine went out it was random. Sometimes it would start somtimes would not. I would die at random times and it left me stranded on the side of the road 3 diferent times before i replaced it.

Mitchen said:

okay i just have two questions, how can i tell if its on the compression stroke? and in that article what is a dizzy?

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LOL I'm guessing a dizzy is a distributor.
It may build a little pressure on the exhaust stoke but it builds a lot of pressure on the compression stroke. I usually put a compression gauge in the number one cylinder. They say you can put your finger over the number one spark plug hole and tell that way, in my experience it is kind of hard to get your hand or finger in there.
When your going for number one compression, I try to only turn the motor to the right (facing the motor) for two reasons. One is takes up the slack in the timing chain and the other it reduces the chance of loosening the harmonic balancer bolt. It may take a few tries to get it right. If you have to turn it back to the left, back more than you need and make the last turn to the right and hopefully the balancer bolt doesn't back out when you turn it to the left.

mikeam7750 said:

I tried to test my CPS. I ended up buying a new one and it worked. When mine went out it was random. Sometimes it would start somtimes would not. I would die at random times and it left me stranded on the side of the road 3 diferent times before i replaced it.

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Very likely, sometimes it starts sometimes it doesn't, dies at random times and if it does start it sometimes runs like crap. Been there done that.

mikeam7750 said:

I tried to test my CPS. I ended up buying a new one and it worked. When mine went out it was random. Sometimes it would start somtimes would not. I would die at random times and it left me stranded on the side of the road 3 diferent times before i replaced it.

Click to expand...

did it backfire at all?

alright i did a compression test today.

starting from cylinder 1 (radiator) going back to the fire wall.

1- came out to 120
2-120
3- 120
4- 120
6- 100
7-100

i triple checked each cylinder for accuracy and they were the same every time
do these numbers look good bad?

Based on my experience, I nominate the coil, or connections to it. When I first installed the lt1 injectors in my stroker, the pcm sent too much fuel and melted the cat. I then ran catless for awhile and then had a huge backfire that blew up my precious borla muffler(2.25"in to 2"out was too small for the 4.6 anyhow). The coil didn't fire a sequence or two and when it did, the chamber in the muffler was where the explosion took place.

gradon said:

Based on my experience, I nominate the coil, or connections to it. When I first installed the lt1 injectors in my stroker, the pcm sent too much fuel and melted the cat. I then ran catless for awhile and then had a huge backfire that blew up my precious borla muffler(2.25"in to 2"out was too small for the 4.6 anyhow). The coil didn't fire a sequence or two and when it did, the chamber in the muffler was where the explosion took place.

Click to expand...

but its getting spark so how could it be the coil? and i havent got a cat :laugh3:

bump?

well today i turned the motor over with a wrench until i could feel the piston i checked several times and when piston #1 is at its highest point the distributor rotor is pointing at the plug wire for cylinder # 1 which makes me think the timing is fine. if it was off a tooth i wouldnt think the distributor would be pointing at the correct cylinder plug.

any input would be appreciated need to get this thing running.

You need fuel, spark and air to get the motor running. Even with pretty radical deviations from the proper air fuel ratio the motor will still try to pop.
This is the what, now comes the where, then you move on to the why.
If you have spark and it isn't seriously weak, then you have a fuel problem or an air problem.
Even firing randomly on the wrong cylinders it is still going to pop and maybe try to start, unless it is completely flooded or has no fuel or has no air.
It will likely run and maybe drive somewhat on 3 cylinders, it will definitely drive on four cylinders, it will run and drive pretty darned good on five cylinders.
Air problem means air going in and air (exhaust) is going out. I've seen guys fiddle with their motors for hours and eventually find a rodent nest in the TB or a wet wadded up air filter in the induction tube. Not common but it does happen.
What did the plug you pulled out of number one look like, did it smell like gas? Did it spark a nice sharp blue white spark or a weak fat yellow spark.
Do you hear the fuel pump prime when you turn the key to the run position?
Do you have any pressure in the fuel rail when the fuel rail has been primed and you push in the test valve on the fuel rail? A weak little dribble of fuel isn't going to get it running.
Sometimes turning the key to off and then run 2-4 times will get the fuel rail primed and makes for an easier start.
A backfire is a bang out the exhaust and usually comes from too much unburnt fuel in the exhaust that ignites. Weak spark, intermittent spark, may allow unburnt fuel and air to ignite at odd times. Cross fire can also do it, crossfire is when the spark jumps from one spark plug cable to another and the wrong cylinder fires. This most often results in the motor firing back through the induction system and can sound like a pop.
Hope I've helped, now go forth and find your problem, then replace your CAT and sin no more.

im getting air if i hold my hand on the intake tube it has suction im getting spark it made a blue arc.

i checked the valve on the fuel rail when i pushed in the needle it shot out with pressure and got gas on the air box then no more fuel came out.

the plugs smelt like fuel and when i crank it for a couple seconds i can smell gas.

there is exhaust pressure when it is cranking.

i cant hear the fuel pump but if i prime it drain gas/pressure from fuel rail then prime it again more gas gets to the fuel rail.

and if the fuel pump went bad it wouldnt get any gas and would not backfire i would think?

If it's getting fuel, air, and spark, generally something happens. Try cranking it over with the throttle full open for 15 seconds, push the gas pedal slightly (maybe an inch) and try to start. If the IAC is stuck closed you have to give a little pedal to get it started or it will flood the motor. The IAC being open is the only way you get air to the motor, without pushing the pedal, to start.

It can buck significantly when the motor is running way to lean. Which might be mistaken for a backfire.

It's possible to have a head gasket leak and get coolant into the cylinder, this can drown the spark plug. But normally this is only one or two cylinders, unlikely to have a coolant leak into all of the cylinders.
Watered gas is also a possibility, though this is unlikely to kill a motor, it can sure make them run really bad for the same reason as a head gasket coolant leak, the moisture and steam drowns/shorts the spark plug. I've actually filled a glass quart jar with fuel and let it sit overnight, you can see the water in the bottom the next morning.
CPS can be an intermittent type thing, I've had them idle OK but refuse to run up the RPM's enough to make them drivable.
A bad MAP sensor can make it run really rich to the point of stalling from too much fuel and spewing clouds of soot and even flooding.

well im not sure where to go from here, maybe i will pull the throttle body off and see how it looks. i pulled of the CPS and it looked fine other than some dirt, cleaned it off and threw it back on. all my vacum lines look good cant find any loose broken or cracked lines. nor can i find any bad wiring.

is there a way to check fuel pressure to see if im getting enough fuel?