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Gone thru almost everything..still wont run right...


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Having serious problems w/ my '88 4.0L 5-speed... :help:

Traveling along @ 60mph and suddenly lost power...started running extremely lean, bogging out, couldn't go faster than about 10mph in 1st gear as I limped home. Realized the catalytic converter was glowing red, presumably running extremely lean due to symptoms...managed to get home and been going through it ever since with no definitive answer as to what is going on...

 

Spent the last week searching and replacing & testing (almost) everything except trying another ECU... First thought was the catalytic converter was clogged...it is brand new (as was front pipe and muffler) and first thing I did was pull it (totally clean inside). Ran the truck with the cat disconnected and no change. Only thing it did (close to) right was idle. Replaced the TPS, new O2 sensor, new Coolant temp sensor, MAT (ambient air) sensor tested & good, MAP sensor tested/vacuum tested & good, new fuel filter (fuel pressure also tested @ rail & found to be at spec at idle & during attempted acceleration).

 

Scan tool and timing light both showed timing was off (jumping around). Picked up a rebuilt distributor (and new cap & rotor) and a new crank position sensor. Installed new (rebuilt) distributor and then while replacing the CPS found the wire was laying on the exhaust manifold & melted up a bit, swapped it out and the truck ran 95% better BUT now the idle was off...truck sounded like it was "loping"... checked and cleaned the IAC, and while adjusting the TPS and letting it run, I looked back under to find the new cat glowing red, again. :headpop:

 

Did find some wiring issues while searching and testing and would like to swap in another wiring harness to double-check that is not the issue. Have located a relatively inexpensive parts truck and am looking for interchange info on the wiring harness and ECU.

 

So 2 questions... any ideas? anything, please any suggestions, I'm sure I've left out things out that we checked... :mad:

Second, will the engine wiring harness & ECU from an '89 Limited Cherokee interchange?

 

Thanks in advance ~

Catherine :(

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Second, will the engine wiring harness & ECU from an '89 Limited Cherokee interchange? Thanks in advance ~

Catherine :(

 

The 89 ECU from the donor will work IF everything matches your 88; i.e. engine, tranny, and the 89 donor does not have the factory security system. This should be the next thing you should do before anything else. BTW, is your timing still jumping around using the timing light?

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The cat glowing red indicates, if I'm not mistaken, a RICH mix, not lean. If you're running too lean, there's nothing in the exhaust to burn up in the cat. If you're running way rich, you're dumping unburned fuel down the pipe and it burns up in the cat.

 

Are the manifolds tight? If there are any leaks, air can get in and fool the O2 sensor into "seeing" a lean condition where there really isn't one at the engine. That tells the ECU to richen up the mix, and much hilarity ensues.

 

On one of our NAXJA NAC trail rides a couple or three years ago one of the guys busted a manifold. His O2 sensor was sending so much gas to the engine that there were flames shooting out the tailpipe when he let off the gas. He limped off the trail and put it on a trailer to get it home.

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Second, will the engine wiring harness & ECU from an '89 Limited Cherokee interchange? Thanks in advance ~

Catherine :(

 

The 89 ECU from the donor will work IF everything matches your 88; i.e. engine, tranny, and the 89 donor does not have the factory security system. This should be the next thing you should do before anything else. BTW, is your timing still jumping around using the timing light?

 

Hiya,

Only thing that doesn't match is the trans...the Limited donor has an automatic trans and I have a 5-speed BA-10 (haven't ever seen a Limited standard...assumed they were all automatics?) *shrug* Don't know if the donor has the factory security system but can find out. So no good on the ECU but is the wiring harness to the sensors & injectors the same?

 

I didn't get to recheck the timing as the rain finally started coming down in buckets and I'm working outside, so won't be able to recheck until tomm (first on the list to do). We had chalked the timing mark, so I pulled the cap off and manually rotated the crank around to confirm the rotor was at the first cylinder (again), and it was slightly off. I actually put the old distributor back in (since I hadn't turned the core in yet) to see if there was any difference, and altho it lined up more accurately, it ran exactly the same as with the new one. I did pull the timing light out but was soaked by then. Will do first thing.

Thanks for the info =)

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Hiya, Only thing that doesn't match is the trans...the Limited donor has an automatic trans and I have a 5-speed BA-10 (haven't ever seen a Limited standard...assumed they were all automatics?) *shrug* Don't know if the donor has the factory security system but can find out. So no good on the ECU but is the wiring harness to the sensors & injectors the same?

 

I didn't get to recheck the timing as the rain finally started coming down in buckets and I'm working outside, so won't be able to recheck until tomm (first on the list to do). We had chalked the timing mark, so I pulled the cap off and manually rotated the crank around to confirm the rotor was at the first cylinder (again), and it was slightly off. I actually put the old distributor back in (since I hadn't turned the core in yet) to see if there was any difference, and altho it lined up more accurately, it ran exactly the same as with the new one. I did pull the timing light out but was soaked by then. Will do first thing.

Thanks for the info =)

 

The donor auto ECU can be swapped to troubleshoot your lean condition, but you will need the standard tranny matching ECU if in fact the ECU is bad and is causing your lean condition. The donor auto tranny harness will not be the same as it has provisions for the auto tranny TCU and Neutral Safety Switch (NSS). Let us know how the timing check comes out.

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The cat glowing red indicates, if I'm not mistaken, a RICH mix, not lean. If you're running too lean, there's nothing in the exhaust to burn up in the cat. If you're running way rich, you're dumping unburned fuel down the pipe and it burns up in the cat.

 

Are the manifolds tight? If there are any leaks, air can get in and fool the O2 sensor into "seeing" a lean condition where there really isn't one at the engine. That tells the ECU to richen up the mix, and much hilarity ensues.

 

On one of our NAXJA NAC trail rides a couple or three years ago one of the guys busted a manifold. His O2 sensor was sending so much gas to the engine that there were flames shooting out the tailpipe when he let off the gas. He limped off the trail and put it on a trailer to get it home.

 

Hey Eagle =)

 

I will 'fess to be quite dyslexic, but if I'm running lean then the ECM is sending instructions to add fuel to the mix, no?

 

Either way, hilarity has *absolutely* ensued and it happened very suddenly, was a split-second change and whatever it was it was catastrophic, so I'm inclined to think if it had been a loose manifold or a bad gasket that it would have happened a bit more slowly over time. (I do recall a blown exhaust manifold on my old '89 Cherokee and it was very obvious sound-wise but did not effect the way the vehicle ran, and I drove it like that for a few months.)

 

I didn't specifically check the manifold or gaskets, but will do. If all is tight and in place, any other thoughts other than a wiring issue or possibly a bad ECM (beyond what has already been replaced/checked) that would cause enough fuel to be getting dumped that it would light up the cat?

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A glowing cat can be caused by either condition. In any event, your A/F mixture isn't right. Most logical next step, since you have been thru most of the Renix sensor dance, is to swap in another ECU, preferrably a "borrowed" one. After you make sure the timing is stabilized of course w. the light.

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Scan tool and timing light both showed timing was off (jumping around).

 

First off.........The '88 Renix system can not really be "scanned" and a timing light will only get you "close" to the correct timing. The actual timing is controlled by the ECU.

 

But.......with your "check" of the timing, and "Jumping around" sure make it sound like the timing chain is shot, jumped a tooth or stretched out of specs. If the timing is off, that would cause your lost of power, no advance on the timing, and cause the unburnt fuel to end up in your cat, causing 'that' problem.

 

A bad timing chain will let the engine fire up, after alot of cranking, idle, but total lack of power under a load.

 

With all the parts you tossed in there so far, you might as well change out the timing chain and gears.

 

Just wondering..........How many miles you have on this '88????

 

One other thing, your asking about the '88 vs. the '89 wiring harness, the '89 engine harness will not have the C101 connector on it, thats the large black connector mounted right next to the MAP on the firewall. Other wise, its a direct swap. The auto tranny has its own wiring harness that is plugged into the tranny controller, which in turns connects to the ECU.

 

If you need to "borrow" a '88, 4.0, manual, ECU, just ask, I've got a couple to loan out :D

 

You can check the intake manifold for leaks with a "un-lit" propane torch, just fire up the engine, turn on the gas, and run it around the intake manifold, it the engine revs up, you found the leak :brows:

 

Like you wrote, an exhaust manifold leak/crack is very easy to "hear" or even feel the exhaust blowing out.

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Scan tool and timing light both showed timing was off (jumping around).

 

First off.........The '88 Renix system can not really be "scanned" and a timing light will only get you "close" to the correct timing. The actual timing is controlled by the ECU. But.......with your "check" of the timing, and "Jumping around" sure make it sound like the timing chain is shot, jumped a tooth or stretched out of specs. If the timing is off, that would cause your lost of power, no advance on the timing, and cause the unburnt fuel to end up in your cat, causing 'that' problem.

 

A bad timing chain will let the engine fire up, after alot of cranking, idle, but total lack of power under a load. With all the parts you tossed in there so far, you might as well change out the timing chain and gears. Just wondering..........How many miles you have on this '88???? If you need to "borrow" a '88, 4.0, manual, ECU, just ask, I've got a couple to loan out :D

 

Hey stranger!

 

Engine was rebuilt prior to my ownership, approximate mileage is about 70k. I have actually had excellent luck in the past scanning my previous plow truck ('88 Pioneer) - it pulled the codes right away...but with my Comanche the system specs are much more vague though and I'm not getting codes, just the data while it's running (I am waiting to hear back from my friend that I borrowed the scan tool from since he worked for Chrysler for a decade back in the 90's...hoping he can give some insight as well).

 

One of the things I was watching on the scan tool was the timing advance, as I'm aware it cannot be adjusted...is why I'm thinking it's the ECU. Before I replaced the CPS, I was concerned the timing chain was shot/slipping over teeth, but it started right up *every time* and the "loping" issue didn't start until afterward when the new distributor & CPS were in...so if the timing was off before and after, then why 2 different sets symptoms?

 

With the new CPS in, it accelerates sooo smoothly, but the idle lopes...before the idle was ok but nothing close to smooth acceleration, just bogging. But either way the fuel ending up in the cat is still the main problem :dunno:

 

So now I have power, but it's loping and still dumping fuel. I'm heading over shortly to recheck the timing with a light, and would like to try a donor ECU and different wiring harness before I tear it down and replace the timing chain and gears. I should have emailed you! I knew you'd probably have one (or more) spare ECU units :shake: ~ got that wiring harness as well? I can pull the one down here but if you have one laying around that was from a standard... will post the timing results...thanks again :bowdown:

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Use the ECU you have -- the one from the '89. The fact that it's for an automatic won't make any significant difference. As noted, the tranny runs off a separate, free-standing harness.

 

Double check the bolts on the exhaust manifold. Yes, you are correct that a lean RUN would call for more fuel, but assuming the system is functioning somewhat, the mixture only gets rich enough to bring the A/F ratio back to the optimum at the O2 sensor. The reason an air leak at the exhaust maifold creates such problems is that it adds air (oxygen) into the exhaust stream after combustion, so it alters the ratio at the O2 sensor so the sensor isn't actually reading what the mix was in the combustion chamber.

 

One other thought: Is it getting up to temperature? The Renix ECU doesn't respond to sensor inputs while it's in open loop (warm-up) mode. It uses a pre-mapped fuel curve, which is necessarily on the rich side because it's intended to make the engine run when the air coming in is cold and dense. If your ECU isn't getting into closed loop mode, it's not even seeing what the O2 sensor is trying to tell it.

 

On the Renix, the temp sensor for the ECU is the one on the driver's side of the block (near the knock sensor). You might check that to ensure that it's operating in-spec.

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