4.0 L Humidity Issues
 

I have an 1989 Cherokee with the 4.0 L engine.

It has a problem running and accelerating when it is raining or humid out.

It runs great in the winter and on cool mornings though. As soon as the weather warms up it's performance goes downhill.

It has to speed up gradually and occasionally "hiccups". Step on the gas to try and pass or speed up to fast and it acts like

you took your foot off the gas instead of put the pedal down.

Plugs, ignition coil, battery, etc., have been replaced and it still does this. It has also had the fuel and air filters

replaced. It gets regular fuel treatment.

Any ideas on what the problem is and how to fix it?

Re: 4.0 L Humidity Issues
 

It was raining today when it went into a Jeep/Dodge dealership's shop and put on the computer to check the codes.

Nothing wrong showed up but the mechanics said they could see that it was running rough. It also had some other inspections done but nothing came up either.

It's supposed to go back on Thursday for further inspections. They want to do a compression test as a start to see if this engine is worth putting more money into.

Re: 4.0 L Humidity Issues
 

My 1990 comanche would quit when it warmed up. The wiring to the distributor shorted on the block near the distributor. I used a zip tie to keep it from touching the block and covered the wire with liquid electrical tape to fix it.

Re: 4.0 L Humidity Issues
 

The dealership did the compression test and said it was good. All cylinders were in the the 160 - 180 range. They didn't find anything else though. They thought the air filter, spark plugs, plug wires, distributor cap, and rotor should be replaced in a complete tune-up that would cost about $300.

I did that and more myself over the last couple of days along with a buddy who is a licensed mechanic who used to work at the dealership. It still didn't fix the problem.

He suspected the fuel pump because I have had problems with it requiring the Jeep to be shaken or tapping the fuel tank in order for it to start again. He said fuel pumps usually seem to go in warmer weather, rarely in winter.

I have the Chilton manual and we were using that as well as his Ohm meter and fuel pressure gauge. We tested the fuel pressure and most of the sensors that he thought might have an effect on the problem.

When the fuel pump, strainer, and fuel filter were replaced it ran a little smoother but it still sputters when accelerating too fast or just by putting the pedal to the floor. It no longer just "powered out" or acted like your foot came of the pedal instead of putting down.

We then began checking sensors again and think it's been narrowed down to one or all three of these:


Throttle Position Sensor - We adjusted it a couple of times and drove the Jeep. We also revved it by hand using just the sensor. It consistently gets to about the half throttle position when it starts to sputter. Could be worn out inside?


MAP sensor - He said he knows these are a fairly common problem sensor


Ballast Resistor - It is right along the edge of the hood and is cracked. Seems that temperature and humidity would have a big effect on this piece given it's job and location.


Going to get these priced out and take it from there. Kinda leaning towards replacing the TPS though.

Any one have experience or advice on dealing with these components? Would temp and humidity have an effect on them?

Is there any connection with these or any of the other engine sensors and the transmission?

Re: 4.0 L Humidity Issues
 

This is what happened after putting in the new distributor cap and rotor and putting the pedal down while on the highway to see if it would sputter or hesitate:

http://i34.tinypic.com/2emndd0.jpg


It had never backfired before until that morning. The first rev test made a little pop before the next one that made the big bang and did this to the muffler.

I have seen some pretty big fireballs come shooting out of that hole during rev testing over the weekend.

A new ballast resistor went on today without any change. It was cracked and needed to be replaced anyways. Oh well, it's been one of the cheapest parts in the last couple of days.

Re: 4.0 L Humidity Issues
 

Since my last post we have replaced the TPS and the CPS (crank not camshaft). The throttle body has also been taken apart and cleaned. Extensive wiring and ground checking has been done. Sensor and component testing has been done with an Ohm meter and manual.

The sputtering problem still remains. It sounds like a machine gun.

After putting on the new TPS, I noticed that it consistently started to sputter at 2500 RPM's. At 3000 the engine is still revving and sputtering but the needle drops to zero. It doesn't matter if it is parked or moving in any gear.

The needle will jump back up when letting off the gas to 3000 and then drops down as the engine's RPM's do. It will not go above 3000 on the tach.

We hooked the tach directly to the computer or ECU to under the dash and it still does this, so it doesn't seem to be a tach problem but the information it is receiving.

When I Googled for the keywords "Jeep 2500 RPM" quite a few results came up about the CPS, so I had researched info for them.

One of the results (http://www.lunghd.com/Tech_Articles/...iagnostics.htm) showed there was even a recall for the CPS on 87-90 Jeep's with the 4.0L:

http://www.lunghd.com/Downloads_and_...-90-update.pdf

I checked at the dealership to see if one was on the records for mine. They said there wasn't but there had been a recall on an oxygen sensor that had already been done on it.

A new crank sensor went on yesterday and it still machine guns when it hits 2500 RPM.

One of the sites I had checked about CPS's had mentioned C-101 Connectors:

We had jiggled it as suggested but it didn't seem to help. After I had researched it a bit and found weatherproofing was one of the issues with it (which would explain the raining and humidity problem), I even tried soaking it with WD40 this morning. No change.

The manual didn't seem to have any info on it. We would like some more info before doing anything else to it.

Anyone have experience or more detailed info on them like wiring diagrams, schematics, etc.

Our other suspect is the camshaft sensor.

Anybody have other ideas on what the problem could be now?

Re: 4.0 L Humidity Issues
 

Have you checked the fuel pressure? If the regulator is no good, a new fuel pump will do nothing for you. Try that, and keep us posted.

Re: 4.0 L Humidity Issues
 

I would look into the wiring for the CKP sensor because of the way the tach keeps dropping off. The next thing may be your PCM. Also be careful using an ohmmeter, I had a vehicle with a bad wire which created a noticable voltage drop but must have had one or two strands still connected which read no resistance on my ohmeter. ' Lost money that day!
Good Luck.