brbasketball

Hi everyone!

I am brand new to this forum but am desperately seeking help with my issue with my Cherokee. 1997, 4.0, 4x4, I-6, Laredo. I have been searching for a solution on this forum and others, but still have not found one for my Cherokee.

I have owned my Cherokee for 3-years now. The last 2-years, it has been having a bad hesitation, stumble, bucking (call it what you will) problem after it gets driven for 20-minutes or longer. It does not ever do this when it is cold (short trips). When the weather outside is hot too, this happens way more frequently (summer months, etc.) and it can happen a lot quicker. In the winter, it happens less or takes longer for this to happen. So, I want to say, that it happens when the Cherokee gets fully warmed up (full operating temp-HOT). I have seen many posts indicating that their hesitation happens when the car is cold, not many only when it is hot. Also, many talk about it stalling. Mine has not been stalling, just the bad hesitation. It does idle rough though when this is happening. The AC on brings it on faster too (under load I am guessing).

When the hesitation starts to occur after it is warmed up (20-minutes or so of driving), then the Cherokee will start to buck and stumble when you give it gas. Laying off on the pedal will get it to stop of course, but pressing it again gets it to hesitate again. It won't stop this hesitation phase until the car becomes cold again. So it will do this even after stopping, turning it off, then starting again. That solution has seemed to work for a few with hesitation problems, but not mine. I even tried stopping at a store, etc. to let the car cool down. If it is not almost cooled down completely, it will happen as soon as you get back on the road. Getting it up to a certain RPM does not change it either. When it is doing this, it will hesitate no matter the RPM, speed, as long as it is getting gas (pedal is being pressed).

Thinking heat and what it can do to electrical components, I have replaced the coil, TPS, O2 (before the cat), distributor cap, rotor, plugs, wires, to no avail. We only use the Cherokee for short trips but on the occasional long trip, this will happen all over again.

It will not turn on the check engine light all of the time, but does so once in a while. It will then turn itself off after a lot of short trips. When the light is on, I get a P0300 code. I don't have a scanner but got this code from the free O'Reilley's scan. I was thinking of getting a scanner so I could get a "live" scan when this is happening. I am not a mechanic and don't know if this would help or not to maybe get a better idea of what is going on. Thoughts?

Could this be a fuel pressure problem? Is it hard to check the fuel pressure? Would I have to check the pressure when it is hesitating (having these symptoms) or would I be able to get a good idea if this is the problem when it is not hesitating? Would I have to check it when I am driving? Is so, how? If any of you could let me know how to do this, I would surely appreciate it.

Anyway, if any of you have any ideas I would surely appreciate the help! I don't want to keep throwing parts at it, but the parts that I did change were from suggestions from other posts with symptoms similar to mine.

Thank you all for your help and time!

dave1123

Okay, your P0300 is a multi-cylinder misfire code and will reset itself if the problem is intermittent. This, to me, would indicate a drop in fuel pressure probably caused by fuel boiling in the rail due to heat. Your "AZ" clued me in. You may also have injectors that stick when they get hot.

Try a set of junkyard injectors and see if they make a difference. If they do, buy a set on new ones, or not. In the yard, you can grab a complete set with the rail for only a few bucks.

Autozone rents a fuel pressure gauge that screws into the fuel rail test port.

brbasketball

Thanks for the reply!

I will try to see if I can find some cheap injectors. I do have a couple of questions though. Why would the fuel "boil" in the fuel rail? Is it because the injectors might be sticking causing the fuel to get stuck in it? If I replace the assembly, will this still happen? Also, I am in AZ but this still happens at night, in the winter, etc. Do you think this could still be the cause?

If I can find the rail with injectors cheap, I will surely give it a try. I just thought I would ask these questions just to make sure.

I will keep you updated on the results.

Thanks!

dave1123

The only reason I suggested buying the rail was I saw one guy ahead of me in the check out line with the complete assembly and he was only charged $10 at Pick&Pull. I also know of a guy who walked out with a complete set of injectors in his cargo pants pockets! I'm not advocating theft, but it worked!

Also, I just had a brain fart! Try changing your IAT or intake air temp sensor. It's screwed into the manifold by the throttle body just ahead of it. This senses the temp and the PCM controls the mixture with this information, in addition with other sensors. Try this first before you go changing the injectors. Again, a JY part.

Two of the things that guys do for increased performance on jeep 4.0s are change the injectors to better ones from some other vehicle like Ford or Dodge Neon and moving the IAT from the manifold to the air intake system ahead of the throttle body. I've looked at my WJ and I think I could drill a hole in the bottom of my throttle body adapter directly above where the IAT is mounted in the manifold and all I would have to do is invert it and stick it in, no rewiring involved. The only thing I may have to do is run a ground wire but IDK.

The one reason I suspected the injectors is because they are right next to the exhaust ports and get a lot of heat. Another poster had this same problem with his 4.0 and insulated his fuel rail which helped for a while. Let me see if I can find his post and get back to you.

dave1123

Nope, can't find it. Anyway, his problem was only when the air temp got above 90*. He was going to cut vents in his hood to help cool the engine, but I told him to space up the rear edge of the hood at the hinges to do the same thing. He drilled 1 1/4" holes in the support members of the hood and in the back support. Not your problem at all.

I'm still thinking it's the IAT. The Haynes repair manual has a complete list of resistance values for different temperatures. Also, the supply voltage should be 5.0 volts from the PCM.

brbasketball

I have not had a chance to hit the junk yard yet but have another question/concern.

While searching for answers to my hesitation problem, I came across one suggestion that I thought I would look into. One suggested that a bad battery may cause some hesitation problems since it would send a weak signal to run the car. Well, I decided to have my battery checked by the guys at O'Reilly's. It turns out that the battery was okay, but the voltage regulator failed the charging system test. Therefore, they suggested that I get a new alternator since the regulator is inside. My Cherokee for a long while, has been cranking slow at start up so I was thinking it was a battery going bad.

Anyway, would a bad voltage regulator be a possible cause for the hesitation as the car gets driven for a long while (gets hot)?

Thanks!

dave1123

I believe your voltage regulator is built inside the PCM, not inside the alternator. There is, however, a temperature sensor underneath the battery that tells the PCM to cut back the charge rate when the battery gets hot. That might be the problem. Most people don't even know it's there.

There are companies that make external regulator systems, but I haven't had any experience with them to say how well they work.