1990 xj runs like poo after warmup

Ok, I am at my end with this jeep. 1990 xj 4.0/aw4/231. The jeep runs great once first started up, runs like a champ. Once it hits its warmup temperature it starts to fall on its face and only gets worse with the rising temperature until it won't run anymore or start. If it sits for a half our or so it will start again but after a few seconds it runs like crap again.

Here is a long list of what I have done already:
I have replaced the exhaust
02 sensor
Map sensor
TPS tested good
IAC replaced
EGR functioning
Ignition coil replaced
No vacuum leaks
Plugs, wires, cap, rotor replaced
Temp sensors tested good
Knock sensor not tested
CPS is new
Fuel pressure is good


What the hell am I missing that this jeep still runs like poo? Any suggestions besides off to the scrap yard with the damn thing?

Kim

My 89 4.0 did this until I disconnected Coolant Temp. Sensor. I later replaced it and have had no further issues. Drivers side, side of block.

I have tried running the jeep without it plugged in and plugged in with no difference. This temp sensor also tested good.

Is it throwing any codes?

kevswens20 said:

Is it throwing any codes?

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It's a RENIX... so no it probably isn't

only thing I know about RENIX's are they don't like bad grounds, so I'm probably no help to you.

Is the replacement crack sensor a factory part? I've read numerous sources that said they have had mixed results with aftermarket ones and the best luck with factory parts.

These jeeps also like nice o2 sensors. I've had good luck with my Bosch replacement.

bdahlg said:

It's a RENIX... so no it probably isn't

only thing I know about RENIX's are they don't like bad grounds, so I'm probably no help to you.

Is the replacement crack sensor a factory part? I've read numerous sources that said they have had mixed results with aftermarket ones and the best luck with factory parts.

These jeeps also like nice o2 sensors. I've had good luck with my Bosch replacement.

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I replaced the 02 sensor with a brand new Bosch. What do you mean by crack sensor? That is a new one to me. :dunno: I will go through all the grounds again, but why would this cause my problem? With a bad ground wouldn't it run bad from startup, not just after warmup?

Thanks for the suggestions everyone.

A bad ground can take a while to heat up and cause problems. Sometimes.

catalytic convertor

I would try and figure a way to test the sensors live, when it heats up and misbehaves.

Have you tried disconnecting the O2 sensor to see if the problem goes away? If it does, I would suspect the O2 sensor heater (internal) is not getting power!!! Check for an exhaust leak, overheating the CPS or a fuel injector.

I would suspect the IAT, intake manifold temperature sensor but you say you tested it? Did you test it at several temperatures and measure the temperature, or just test it cold?

Did you change the HV coil only, or the entire unit, with the circuit board that fires the coil?

Overheating ECU is a 1 in a million shot for the Renix.

I recently had a bad, brand new CPS from Autozone, like to drove me nuts confirming the problem source, but it was random no start issue that time. Try using a hair dryer to heat up parts like the CPS, before you start it, see if that causes the problem.

Is it starting OK, no starting problems? How long is this warm up before the problem?

85xjwoody said:

I replaced the 02 sensor with a brand new Bosch. What do you mean by crack sensor? That is a new one to me. :dunno: I will go through all the grounds again, but why would this cause my problem? With a bad ground wouldn't it run bad from startup, not just after warmup?

Thanks for the suggestions everyone.

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A loose dirty ground on a sensor to the ECU, or other places can raise hell anytime it wiggles, moves or expands from heat.

Also try a vacuum gauge to see if the vacuum readings change hot to cold, like a bad valve that only acts up once it is hot enough?

The grounds are super important. The dipstick tube stud is the grounding point for the O2 sensor, ECU, and injectors. Most other important sensors ground through the ECU and end up grounding at the dipstick tube stud. The connection needs to be refreshed by removing the nut, scraping paint from the block. recrimping the connectors on the wires, polishing them til shiny, and then reattaching them securely. accept no substitutes for this procedure except for maybe soldering the wires to the connectors.

There is also a chance that the sensor grounds in the harness are no good. A 20 second check with an ohmeter. PM me your email address and I'll send you the instructions on checking that circuit along with a simple way to verify your TPS is working properly.
If the sensor grounds are not within spec, I can provide instructions on repairing it easily.

Thanks to everyone for the help so far. I just picked up a multimeter yesterday and now need to figure out how to use it! LOL I have to wait for it to warm up so I can wash the jeep before I get into trying to figure out where things are going wrong. It got to 30 degrees here this morning.
I will be checking the grounds first, then on to testing everything again. The TPS tested good and was within range. The temp sensor was tested cold and hot and showed to be working well. These things that were tested, I tested them and told the results to a friend on the phone and he said the numbers were where they were supposed to be. I don't remember what the numbers were, sorry.
The IAC I took out and cleaned the best I could. When unplugged while the motor was running it gave the motor a very high idle, which I am assuming it is functioning.
Could the EGR cause a serious problem like this? I have no cat and a brand new exhaust with a minor crack in the exhaust manifold. The knock sensor was unplugged when I went to check everything. I plugged it in and no change, I was told the knock sensor can stay unplugged without causing the problem I am having.

When the motor is cold, it starts up great and runs great!! But once it hits warm up temperature it slowly but surely starts to fall on its face and backfire out the intake. If I take it for a ride and stay at a very high RPM, it will run good but not great. As soon as I slow down or come to an idle, it gets really bad and will shut off. Then startup is a PITA. Once I had the exhaust downpipe cherry red. Never had that happen before.

Hopefully some of this info helps.

Kim

Do not let water, when washing, get near the electricals, especially the TPS!!!!

"Once I had the exhaust downpipe cherry red." How about more details on this? When, under what conditions? Before or after the exhaust work was done? Has the dizzy been touched, replaced,....

The exhaust crack needs to be fixed. It can cause erratic behavior, confuse the O2 sensor....

Run the vacuum tests, report the data, and then you may need to run a compression test, HOT!

The only thing I wash in the engine compartment is the radiator. I took the jeep for a ride to see if some unplugging was going to help. Well, it didn't and I proceeded to get stuck in a mud hole. So, full pedal back and forth for a bit I had someone come up and tell me my exhaust downpipe was glowing red hot. I got out to see it cherry red. The entire exhaust is brand new from the downpipe back with no cat and a cherry bomb glass pack.
This is the same style exhaust I was running before but was replaced because it was beat to hell. The exhaust crack has been there for two years and has never given me a problem. I just can't see that causing such a severe problem.
The dizzy? Are you referring to the distributor? If so it has never been touched except for cap and rotor replacement. Plugs look good and are less than a year old. No burnt or gassy smell to them. Wires are new also.
Thanks again for all the help.

Have you checked the plugs recently?

I would disconnect the knock sensor (since you say it is bad?) until you find the real problem. The cherry red Exh is an indication of very late ignition (late spark) or burned or leaking exh valves, or a jumped timing chain, or problem with the dizzy (distributor), or a bad ECU, but I have never heard of a Renix ECU having that problem.

Check the dizzy to see if is pointing to number #1 plug at zero degrees, TDC on the compression stroke! But do the vacuum tests first! Google the test procedure. Vac gauge is cheap, and tells a lot of info on the engine!!!!

Your catalytic converter is plugged.

DetroitStyle said:

Your catalytic converter is plugged.

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That does not fit the data in his first post.

Ecomike said:

That does not fit the data in his first post.

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How so?

DetroitStyle said:

How so?

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A clogged Cat converter would give problems all the time.

He said his runs fine at a cold start up.

A clogged cat converter would always, at all temperatures restrict exhaust flow and cause the engine to run rough as you increase engine speed with more gas, more throttle. And depending on how bad the restriction is from a clogged Cat converter it might or might not run rough at idle.

A broken, internal honey comb, loose guts in a Cat converter would usually rattle as you give it gas, and would randomly bog down on acceleration as the guts rotated around into different positions, blocking and unblocking exhaust flow.

85xjwoody said:

I have no cat and a brand new exhaust with a minor crack in the exhaust manifold.

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DetroitStyle said:

Your catalytic converter is plugged.

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Ecomike said:

That does not fit the data in his first post.

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Mike, I'd say that's an understatement:cheers:

a lean mixture will give you a cherry red exhaust as well-- I'd recheck fuel pressure at temperature and verify pressure at 1500rpm to help rule out fuel starvation.