RENIX Hell...

'88 MJ RENIX 4.0 AW4 NP231

I'm just gonna start of by saying I've cleaned the grounds and the C101 connector on the firewall, took care of that a few months ago when I first started driving the truck. The other day it just died on me all at once, no warning, no stumbling or studdering, just shut down and as best I could tell at the time was out of gas, no fuel at all in the rail. Simple enough, I pour 10 gallons in and it fires right up... for about 15 seconds all is well, then it dies. Crank it again and the same thing, springs to life with no hesitation and runs perfect for about 10-15 seconds. Crank it and touch the throttle at all and it dies immediately. Since then I've triple checked everything I can think of, can't figure it out. No fuel pressure gauge available, but judging by how it squirts out when i push the valve in the test port, I'm convinced that's not an issue. Now if i crank it and jump out quick enough with a can of starting fluid, I can keep it running as long as i want, so it's definitely a fuel problem.

I should probably mention that about a week prior, I sprung a power steering leak near the exhaust and set myself a nice little engine fire. Picked up an engine harness and 6 injectors off a neighbor that was parting one out, crossed my fingers and prayed the map sensor was still ok, and got it back on the road for a few hundred miles before it abruptly decided to die on me. Didn't notice anything strange at all during this time after the fire other than awful oil leaks, so I'm not sure if my current issue is related or not, but definitely worth mentioning.

Now when I unplug the map sensor, truck won't start at all, and key on engine off the sensor tests good. Engine won't idle long enough to test it running. That was the only sensor I had a question about, due to it's proximity to the fire. TPS is brand new and tests fine. I wasn't able to test the knock, CTS or MAT sensors before it got dark, but I get no change when i unplug the CTS and knock sensor, with the MAT unplugged it won't start at all. The wiring harness leading to the knock sensor connector has a third non-insulated wire with nothing at the end. Wasn't going to tear apart the harness to find out where it went, but i did try grounding it and saw no difference. Not really sure what else to look for, but in the morning I plan to crawl under and test the CTS for good measure. Other than that, or a bad ECU, I'm stumped. Any ideas? I know I have to be missing something, especially since I'm still new to these jeeps. This is my first electronic vehicle that's not OBD2, and it's giving me fits! :helpme:

Probably the fuel pump relay. When starting, the power to the fuel pump is provided differently than when in run. It can start fine and die shortly after letting go of the ignition switch if the fuel pump relay is bad. It's one of the relays near the diagnostic connector.

This feature came and went on some years and even partial years, but if your Jeep has a ballast resistor, failure of this or bad contact to it will cut the fuel pump off after starting just as you describe. Look for a big white power resistor on the driver's side inner fender. If it's there, simply bypass it. It's supposed to make the fuel pump quieter but it will run fine without it.

It's not anything related to fuel supply, I just re-wired all that because the guy that owned it before me hacked up some of the wiring. I've currently got power to the fuel pump anytime the engine is turning, and I've verified that I have fuel at the rail during the very instant that it dies. The injectors aren't putting in enough fuel for some reason

It sure sounds like fuel supply. Just because you have pressure at the rail doesn't mean you have enough pressure or enough volume.

Numerous times I've had good pressure, then the pressure falls off to half when the motor is running. Anything from a partially clogged fuel filter, to a hole in the fuel pump plumbing, to an electrical problem.

A fuel pressure gauge is a part of my Renix tool kit. Not expensive and helps eliminate a whole list of possibilities when I'm troubleshooting. A little trick I learned, you can use a freon (R 12) low pressure gauge and the same hose hookup to test your fue pressure.

The first thing to do is jump the ballast resistor like mentioned. The ballast resistor cuts back the voltage after the motor starts up, it sometimes cuts back the voltage too much or is fried and cuts off the voltage altogether.

The starting relay supplies voltage to the pump during cranking after the motor starts the power to the pump goes through the ballast resistor.

Could also be the starter solenoid. My 90 XJ did the same thing. Replace the starter solenoid behind the sits behind the battery and all problems were fixed.

In order to verify it is a lack of fuel, when it doesn't start, spray some carb cleaner down the throttle body. If it fires up, you have a fuel problem. If it doesn't, you have a different problem.

If it starts, runs for a second, then dies, you are not getting fuel. If there is pressure at the rail, you are not getting drive to the injectors. That can be caused by a bad ECU. If the CPS were bad, you wouldn't be getting the spark you needed to have it fire, so the CPS is most likely fine. In this case, you need to get a NOID light and verify a lack of drive to the injectors, and trace the wiring back to the ECU. If you are getting no drive from the ECU, the ECU is most likely hosed.

Have you checked the cam sensor in the distributor? I don't recall exactly how the Renix system works related to the later Chrysler, but I think in both of these, the cam sensor regulates injector timing. It was my understanding that a Renix can run with that sensor not working (a Chryco can run but cannot start unless you unplug and replug it before cranking). But that seems like a good place to look if your injectors aren't injecting.

I would also mention, though it's less likely, to check for continuity to the unswitched side of the injectors. I believe this is also the reverse of the Chrysler, in that the continuous side is a ground, but you need to make sure it's really there and not interrupted by a harness fault. In the later versions, the continuous side is hot, and sent to the injectors by splices which sometimes fail, though not usually all six.

If the starter relay isn't working it can also cause the same problem. I ran into the same problem as you are having. Checked everything. When I replaced the starter relay, everything was fixed.

8Mud said:

It sure sounds like fuel supply. Just because you have pressure at the rail doesn't mean you have enough pressure or enough volume.

Numerous times I've had good pressure, then the pressure falls off to half when the motor is running. Anything from a partially clogged fuel filter, to a hole in the fuel pump plumbing, to an electrical problem.

A fuel pressure gauge is a part of my Renix tool kit. Not expensive and helps eliminate a whole list of possibilities when I'm troubleshooting. A little trick I learned, you can use a freon (R 12) low pressure gauge and the same hose hookup to test your fue pressure.

The first thing to do is jump the ballast resistor like mentioned. The ballast resistor cuts back the voltage after the motor starts up, it sometimes cuts back the voltage too much or is fried and cuts off the voltage altogether.

The starting relay supplies voltage to the pump during cranking after the motor starts the power to the pump goes through the ballast resistor.

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As I mentioned, I don't have access to any kind of gauge to test fuel pressure, but for 15 seconds it starts and runs perfectly so I know the pump isn't failing. I also re-wired all that and have it running off a single relay, so the latch relay doesn't even provide power to the pump anymore. The pump will always run anytime the ecu has power and CPS signal (key on, engine turning). I've double checked my wiring and all is good, and judging by the authority with which fuel escapes the rail when i push on the schrader valve, I feel confident I don't have a low pressure issue. It's definitely an issue with the injectors not opening, or not opening enough.

Gecko321 said:

Could also be the starter solenoid. My 90 XJ did the same thing. Replace the starter solenoid behind the sits behind the battery and all problems were fixed.

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Gecko321 said:

If the starter relay isn't working it can also cause the same problem. I ran into the same problem as you are having. Checked everything. When I replaced the starter relay, everything was fixed.

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Hmmm, how would this affect injector pulsewidths? It does look awful nasty, and the truck came with an even nastier looking one in the floorboard when i got it, so it's possible it could be on it's way out, but how could it be related to my problem?

old_man said:

In order to verify it is a lack of fuel, when it doesn't start, spray some carb cleaner down the throttle body. If it fires up, you have a fuel problem. If it doesn't, you have a different problem.

If it starts, runs for a second, then dies, you are not getting fuel. If there is pressure at the rail, you are not getting drive to the injectors. That can be caused by a bad ECU. If the CPS were bad, you wouldn't be getting the spark you needed to have it fire, so the CPS is most likely fine. In this case, you need to get a NOID light and verify a lack of drive to the injectors, and trace the wiring back to the ECU. If you are getting no drive from the ECU, the ECU is most likely hosed.

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As I said, it starts and runs perfectly for the first 15 seconds, then dies. I also said that i could keep it running with starting fluid, so you're correct in that it's a fuel issue. More specifically a fuel control issue, as opposed to fuel supply. I didn't bother to mention the CPS in my original post because it runs fine when it has fuel. If I had a noid light, or a suitable LED to rig one up, I might try it. I already pretty much know my issue is injector control, but it might help to know whether the signal is going weak or if it's cutting out completely when it stalls. With all the electrical issues these renix systems are notorious for, why do you assume it's the ecu? Generally that's the very last component to suspect both because in general automotive ecu's rarely fail and because they are expensive and nearly impossible to test. I'm certainly not ruling it out as a possibility (leaning towards a probability at this point, unfortunately) but I'd like to test every other possible component first, since there's no good way I can test it and I'd like to avoid dumping money I don't have replacing parts I don't need.

Matthew Currie said:

Have you checked the cam sensor in the distributor? I don't recall exactly how the Renix system works related to the later Chrysler, but I think in both of these, the cam sensor regulates injector timing. It was my understanding that a Renix can run with that sensor not working (a Chryco can run but cannot start unless you unplug and replug it before cranking). But that seems like a good place to look if your injectors aren't injecting.

I would also mention, though it's less likely, to check for continuity to the unswitched side of the injectors. I believe this is also the reverse of the Chrysler, in that the continuous side is a ground, but you need to make sure it's really there and not interrupted by a harness fault. In the later versions, the continuous side is hot, and sent to the injectors by splices which sometimes fail, though not usually all six.

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These seem like two good points, and I'll definitely check them out. I'll have to get my hands on another analog meter so I can properly test the stator. Damn digital $&@#% :smsoap:
Any idea which side of the injector is switched by ecu logic? Better yet, does anyone know where I can find a copy of the FSM or at least some good complete wiring diagrams?

The injectors have separate drivers in the ECU, they switch on the plus side and all the injectors share a common ground through a series pf splices. The injector drivers don't screw up often. The wiring to a single injector does occasionally fatigue break near the bend of the harness by the firewall. The shared ground is more often problematic, the eventual end of the ground circuit near the dipstick holder always needs to be checked. Ohm the ground at the number one injector to battery negative and see what you get.

If you just want to tear into it and see what makes it tick, have fun been there done that.

I've only been working on the Renix for around twenty five years now and I wouldn't assume the fuel was OK unless I did a pressure and/or a volume test. And even then I'd pull the vacuum line off of the pressure regulator to check for leakage.


It starts and runs for 15 seconds then the ballast resistor overheats, any one of six major connectors between the power source and ground overheats, the pump bearings heat up and partially seize, there is a restriction someplace in the fuel delivery and the injectors need 15 seconds to suck down enough fuel to lower the pressure enough to starve the motor. That is just the short list. I wouldn't assume any of the above are OK unless I did a pressure test.

The fuel line to the fuel pump (in the tank) may be partially rotted or loose. This will allow the engine to start and run (most of the time) but as soon as ANY throttle is applied the engine will either stumble, backfire, or die. I chased the same issue for a week last year until I finally broke down and bought a fuel pressure gauge and used it to verify the loss of pressure when applying the throttle. Went to install a new pump, found the rotten hose (most likely caused by ethanol gas and age), replaced the line and NOT the pump, and have had zero issues since.

Buy a fuel pressure gauge. There are several available under the $50 mark, and are worth every penny. My .02¢

Stator will not cause this issue. Prove it to yourself. Unplug it and see if anything is different. It won't be. MT Mike is spot on here. TEST the fuel pressure.

I just had this one myself. turns out the rubber hose on the fuel pump was loose and was not sending enuff fuel to run. put it back on and everything ran fine. so my key diagnostic moment was when I thought it is acting like it is out of fuel. I removed the rubber boot on the intake and pours a teaspoon of gas down it and it ran. so I then went right to the fuel pump.i could hear and feel the pump run, so I had to remove it and see what was going on.

MT Mike said:

The fuel line to the fuel pump (in the tank) may be partially rotted or loose. This will allow the engine to start and run (most of the time) but as soon as ANY throttle is applied the engine will either stumble, backfire, or die. I chased the same issue for a week last year until I finally broke down and bought a fuel pressure gauge and used it to verify the loss of pressure when applying the throttle. Went to install a new pump, found the rotten hose (most likely caused by ethanol gas and age), replaced the line and NOT the pump, and have had zero issues since.

Buy a fuel pressure gauge. There are several available under the $50 mark, and are worth every penny. My .02¢

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Gunter said:

I just had this one myself. turns out the rubber hose on the fuel pump was loose and was not sending enuff fuel to run. put it back on and everything ran fine. so my key diagnostic moment was when I thought it is acting like it is out of fuel. I removed the rubber boot on the intake and pours a teaspoon of gas down it and it ran. so I then went right to the fuel pump.i could hear and feel the pump run, so I had to remove it and see what was going on.

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Excellent advice from you and MT Mike. The new "gasoline" we use is very hard on our 25 plus year old fuel lines.

The path out of Renix H*ll is paved with diagnostic stones.

8Mud said:

The injectors have separate drivers in the ECU, they switch on the plus side and all the injectors share a common ground through a series pf splices. The injector drivers don't screw up often. The wiring to a single injector does occasionally fatigue break near the bend of the harness by the firewall. The shared ground is more often problematic, the eventual end of the ground circuit near the dipstick holder always needs to be checked. Ohm the ground at the number one injector to battery negative and see what you get.

If you just want to tear into it and see what makes it tick, have fun been there done that.

I've only been working on the Renix for around twenty five years now and I wouldn't assume the fuel was OK unless I did a pressure and/or a volume test. And even then I'd pull the vacuum line off of the pressure regulator to check for leakage.


It starts and runs for 15 seconds then the ballast resistor overheats, any one of six major connectors between the power source and ground overheats, the pump bearings heat up and partially seize, there is a restriction someplace in the fuel delivery and the injectors need 15 seconds to suck down enough fuel to lower the pressure enough to starve the motor. That is just the short list. I wouldn't assume any of the above are OK unless I did a pressure test.

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I fully agree with you guys on testing the fuel pressure. However I do not have access to a suitable gauge to test it, even if I did have the money to buy one. The ballast resistor is no longer a part of the system, nor is any of the original wiring or connectors. I was unable to trace the existing wiring faults a while back, and decided to just run all fresh wiring since the existing wire going to the pump was very small and corroding inside. It might be possible the contacts are slightly corroded in the relay I'm using, but I doubt it. Again if I had any chance of getting my hands on a gauge I'd just test the pressure as a matter of course. However as someone who's used to working on cars on a regular basis I'm very certain it's not fuel pressure related. If I had a video camera I'd post a clip on youtube to prove it to you guys, it's quite obvious by the sound and the way it dies that something electronic is abruptly getting switched off. It doesn't bog/stutter as it dies, it just shuts down from a smooth 1500rpm high idle as if someone turned the key. If I were in ya'lls shoes I'd be advising the same thing, but I simply have no way of testing fuel pressure and I'm 99.99% certain that's not the issue. If fuel pressure were dropping it wouldn't die abruptly the way it does from a nice smooth high idle. I'm almost tempted to rip an old LED out of something to use as a noid light and prove it to you guys so we can move on :laugh3:

As far as the injector drivers, if they were bad I'd expect to see a single cylinder misfire, not all switch off at once, no? Common ground, switched on the hot side eh? That's a first, most newer ecu's switch the ground side of the injectors... interesting. Any idea where I can find a copy of an fsm or at least some good wiring diagrams for this thing? Seems to me the ecu is possibly switching strategies at a certain point a few seconds after startup, and for some reason calculating an injector pulsewidth of near zero.

NOTE: I should add that today I discovered if I aggressively pump the throttle during the first few seconds while it runs I can sometimes get it to rev, if it goes above 3k rpm it runs fine as long as I keep it there. Allow it to drop below 3k rpm and it shuts down immediately. More reason to believe that I have sufficient fuel pressure.

cruiser54 said:

Stator will not cause this issue. Prove it to yourself. Unplug it and see if anything is different. It won't be. MT Mike is spot on here. TEST the fuel pressure.

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Wish I had a way to test the fuel pressure, but I have no money and it's about a 40 mile walk to somewhere I could get one. I know the stator has a function in controlling injector timing, but I didn't think it would cause a fuel issue other than possibly a drop in fuel economy. Good to know.

joe_peters said:

The path out of Renix H*ll is paved with diagnostic stones.

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The path out of renix hell is paved with a weber 2bbl...

The ECU will shut down the injectors if it gets no good reference signal from the MAP. But in this case you wouldn't even get 15 seconds.

Kind if reaching but I've seen fuses with a hairline crack work like a bimetal, open and close as they heat and cool. One of those things that will drive you nuts. The fuel pump is relatively high amperage and a connector with a corroded or flaky connection may be heating up, when the pins in the connector heat up they flex. Pretty much the same with the ignition witch, you can often see scorch marks on the rear of the ignition switch connector. Just a couple of long shots to think about.

8Mud said:

The ECU will shut down the injectors if it gets no good reference signal from the MAP. But in this case you wouldn't even get 15 seconds.

Kind if reaching but I've seen fuses with a hairline crack work like a bimetal, open and close as they heat and cool. One of those things that will drive you nuts. The fuel pump is relatively high amperage and a connector with a corroded or flaky connection may be heating up, when the pins in the connector heat up they flex. Pretty much the same with the ignition witch, you can often see scorch marks on the rear of the ignition switch connector. Just a couple of long shots to think about.

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I'll keep that in mind. Going to try pulling the ecu out of the dash tomorrow and load testing the full length of each wire in the harness, which I probably should have checked already considering the reputation these things have for awesomely well designed wiring. Too bad there's not a good way to test the ecu, without another ecu.

I think the member here posting as "5-90" has wiring diagrams and info on Renix systems digitized. I'm a bit surprised he has not chimed in here, being a Renix guy. You might try a PM to him, because I think he may have something emailable that would not violate the strict FSM posting rules here. I have, if needed, the schematic from a Chilton's fuel injection manual, and may have some other stuff, but I would try 5-90 first.