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No injector pulse, 1989 4.0 MJ

xjtrailrider

NAXJA Forum User
NAXJA Member
Location
Roanoke VA
I have spark, I have pressure on the rail. It will start if i prime it with carb cleaner, runs for a few seconds then dies.

I checked a couple of injector connectors to see if I had a pulse and nothing.

I had already replaced the CPS due to no spark. I turned to the synch sensor when I found out there was no pulse.

I dropped in a "known good" distributor from my XJ and still the same issue!

My next step is another ECU(I have a "known good" spare.

Any other ideas, need help quick so I can get my new project home!
 
did you try to check for a pulse with a test light or a noid light. the test light wont show injector pulse only a noid light will. i believe there should be a constant 12 volt signal to one side of the injectors.the ecm then grounds the return side and that what makes the pulse. if there is no pulse check all your fuses. also check your relays.
 
did you try to check for a pulse with a test light or a noid light. the test light wont show injector pulse only a noid light will. i believe there should be a constant 12 volt signal to one side of the injectors.the ecm then grounds the return side and that what makes the pulse. if there is no pulse check all your fuses. also check your relays.

I'm with Jeeptec I think your injector test was flawed.

I've never known the sync sensor in a Renix to cause many issues, I forgot and ran around with mine unplugged for months and never noticed. If it were to short it would likely cause grief.

The *Renix* pulses on the plus side and all the injectors have a common ground. Maybe ohm test the ground at injector 1, that ground circuit goes through a bunch of splices. It is my understanding all the injectors have separate drivers, all failing is unlikely. They all do share a common ground.

You may have some pressure at the fuel rail, but IMO you really have to know how much pressure. It can prime and have pressure and as soon it cranks the pressure falls.

I surely don't know what would shut down the injectors and still allow spark. There may be a necessary sensor input, that when wrong or missing may shut down just the injectors, but I sure don't know what it is.
The ECU only has two power sources as far as I know, one is a fusible link and pretty much supplies power to everything and the other is from the ignition switch and is likely just a signal and not a power supply.
 
Go over all your grounds as everyone here agrees how important they are. Check battery voltage and connectors for wiring harnesses. Check the TPS as "if the ECU detects a full throttle position it will assume flooding and shut down the injector pulse". (According to the FSM) Good Luck!
 
I'm with Jeeptec I think your injector test was flawed.

I've never known the sync sensor in a Renix to cause many issues, I forgot and ran around with mine unplugged for months and never noticed. If it were to short it would likely cause grief.

The *Renix* pulses on the plus side and all the injectors have a common ground. Maybe ohm test the ground at injector 1, that ground circuit goes through a bunch of splices. It is my understanding all the injectors have separate drivers, all failing is unlikely. They all do share a common ground.

You may have some pressure at the fuel rail, but IMO you really have to know how much pressure. It can prime and have pressure and as soon it cranks the pressure falls.

I surely don't know what would shut down the injectors and still allow spark. There may be a necessary sensor input, that when wrong or missing may shut down just the injectors, but I sure don't know what it is.
The ECU only has two power sources as far as I know, one is a fusible link and pretty much supplies power to everything and the other is from the ignition switch and is likely just a signal and not a power supply.

Your dead on 8mud! I went back to work on it with my son tagging along to help out. I have injector pulse(easier to test with someone else cranking the Jeep!). I have good pulse at all 6 injectors. I have a nice, snappy spark.

What I do not have is fuel pressure while the jeep is cranking! With the key in the run position I have pressure on the rail but as soon as I crank it I lose pressure.

I went through and cleaned all of the grounds, put in new and known good relays. I almost think the issue is at the starter relay?

Give me some input, i need to get this beast home so I can begin the restoration.
 
The Orange and back wire coming off of the starter relay (usually at the pin labeled "BAL") is a straight shot back to the fuel pump. The circuit does go through a splice and 3-4 connectors, which are possible trouble spots.
I'd pull off the orange and black wire at the starter relay and do a quick ohm test, just to check you don't have an open circuit, a dead short, really excessive resistance or bad ground.
Then jump the orange and black wire to the large lug with all the red wires on the starter relay, Maybe flash it a couple of times before holding it on there, just in case there is a short someplace, you don't want to burn out a fusible link.
If the pump has been sitting for awhile and the fuel is old the pump may be a bit gummed up. Jumping and running the pump for awhile may help to clean it out, I just did the same thing on my 88 that had been sitting for months. A cup full of Acetone (occasionally) in the fuel tank does a pretty good job of cleaning gum out of fuel system.

It's also possible to have a starter that uses excessive amps and/or a weak battery that just doesn't leave enough juice to successively run the pump while the motor is cranking. If the headlights dim *way* down when you are cranking, this can be a sign.
My battery took a dump this morning, the dome lights dimmed to almost nothing when I was trying to crank it up.
If the circuit is good and you can hear the pump running, you almost have to do a fuel pressure test. Personally I'm not going to replace something as expensive as a fuel pump on a hunch, I'm going to do some tests and troubleshooting first.
 
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The MJ has been sitting for 3 months. I'm buying it from a good friend and a very honest person and he tells me it ran great but was "hard starting". It could be that something was on the way out.

Ive been told that if you have spark then the ECU is good, I'm leaving it to the last thing to try. I have a "known good" RENIX ECU that was given to me by our local Jeep dealer, it was their test ECU.

The battery is brand new, I cleaned the grounds, rechecked the index on the new distributor.

What I'm confused by is that I have fuel pressure with the key in the "run" position but nothing in the "crank" position. If I prime the TB it will fire and run for a few seconds but it cuts out before the fuel pressure comes up on the rail. The fuel pressure is 0 until it cuts out no matter how long it runs(I tried keeping it running for 30 seconds or so one time and no fuel pressure until the engine cut out)

I'm still leaning towards the starter relay but I will test the wiring/connectors as well.
 
sounds like the fuel pump relay is stuck in the run position.....or the ignition switch is not putting 12v to the relay in the start position.
 
If I was in your situation, I would put a jumper wire from the battery to the fuel pump, and drive her home. I have done it once by jumping at the ballast resistor. On second thought did you check that resistor, and see if it still functions.
 
The MJ has been sitting for 3 months. I'm buying it from a good friend and a very honest person and he tells me it ran great but was "hard starting". It could be that something was on the way out.

Ive been told that if you have spark then the ECU is good, I'm leaving it to the last thing to try. I have a "known good" RENIX ECU that was given to me by our local Jeep dealer, it was their test ECU.

The battery is brand new, I cleaned the grounds, rechecked the index on the new distributor.

What I'm confused by is that I have fuel pressure with the key in the "run" position but nothing in the "crank" position. If I prime the TB it will fire and run for a few seconds but it cuts out before the fuel pressure comes up on the rail. The fuel pressure is 0 until it cuts out no matter how long it runs(I tried keeping it running for 30 seconds or so one time and no fuel pressure until the engine cut out)

I'm still leaning towards the starter relay but I will test the wiring/connectors as well.

It has been a long time since I had a starter relay apart, but if I remember correctly the solenoid and the fuel pump connectors are joined by a copper strip inside the relay, basically the same connection. If the starter solenoid is working, the power should be getting to the fuel pump (BAL) outlet (spade connector) on the relay. Easy enough to test for voltage at the "BAL" connector on the starter relay while your cranking and see if you are getting any power *into* the fuel pump circuit. Whether the power is making it all the way to the pump is another question.

The ECU closes the fuel pump relay for a few seconds when the key is turned to run on the way to start, this primes the fuel rail. Then when the starter relay closes, this supplies power to the fuel pump, once the CPS sends a pulse signal the ECU starts spark and closes the fuel pump relay. I'm not exactly sure when the fuel pump relay closes, likely an RPM thing.

Could be as simple as a partially plugged fuel filter. Be careful of the fuel filter, the fuel may siphon on when it is disconnected. The filter also often comes off of the hoses hard and is a good spot to get a face full of fuel. Empty the fuel filter out the inlet end and see what comes out. If chunks of rubber come out it is a sign the hoses are deteriorating on the inside.

You may be getting some pressure, but very little volume.

I had a 87 with a fried connector, it tested 12 +/- volts (or available voltage) while I cranked, but didn't have enough amperes to run the pump very well. Only seen it once, it drove me nuts.

Just for the heck of it, turn the key to run then off, then run then off, about three or four times. This gets you a good prime to the fuel rail. Some Renix loose prime in the fuel rail pretty fast because of a leaky check valve at the pump. Though extended cranking will usually allow enough fuel to get pumped up to the fuel rail for a start, even if the check valve is leaking, it just takes longer cranking to start.
 
Also, a previous owner has "hard wired" the CPS. I hooked the new CPS up the same as the old one but could it have been wired backwards and if so, would it matter?
 
Plugged fuel filter or weak pump. That would allow pressure to rise as long as the injectors were not operating, but soon as they start using fuel the pressure would drop.
 
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