I have a 1998 XJ that has about 285 thousand miles on the original motor whenever I start the car after it's already been warmed up ie going into the store or something and I come out ready to leave the Jeep will already be warmed up and I will start it and back it out and when it hits 1900RPMs the first time it will hesitate and make 2 to 4 clunking noises and then move on normally and won't do it again the entire time the vehicle is on. It will not do it when it's cold and it will not do it if it is shut down there immediately restarted. I've checked plugs and wires to no avail. No check engine codes. 1998 XJ 4.0 aw4

 

cammy,

Due to the high-mileage and model-year, I suggest getting a bright light and inspecting the fuel injectors very closely. They may be cracked. You want to inspect all of them closely but you want to pay close attention to the #3 injector.

Let us know what you find and we'll go from there.

 

Same thing the nozzle to test fuel pressure is on correct?

 

They look ok. From all sides.

 

OK, good shots! Did this running condition start occurring since you moved to Colorado or was it occurring prior to that?

 

Nope. Only been going on sbout a week.

 

I figured as much but I didn't want to put words in your mouth so I can help.

Sounds like a classic case of vapor lock/ heat soak. This is not surprising now that you live in a higher elevation. Higher elevation = lower boiling point. That's why you may have noticed a different set of cooking instructions on many food labels.

At any rate, you're going to need to install a "heat soak kit" of some type, like this:

https://www.quadratec.com/products/17604_1031.htm

Or, you can make one by buying the material and using thin wire to secure it. It would be cheaper doing it this way.

You want it to look like this when you're done:


https://www.cherokeeforum.com/f2/eng...issues-128947/

And this: http://jpfreek.com/2015/09/24/dei-4-...tor-cover-kit/

Notice the larger flat piece on the intake plenum under the injectors and fuel rail. Need to cover the area around the #3 injector mostly but doesn't hurt to make it large enough to cover more.

 

But but it's not even close to overheating and usually about 200 to 210 degrees. And without explaining only doing it once during a startup and never does it again I would see how that would explain it never doing it cold though

 

And would that explain it always doing it right around the same spot on the RPM band at about 1900 RPM?

 

Yes, doesn't have to be overheating as we would normally describe it but it is basic physics really.

Everything you're describing screams vapor-lock/ heat-soak, including no codes. That's a big clue right there.

That's my story and I'm sticking to it. Lol!

 

Yes.

 

Oh no. I know you know your stuff. I'm just trying to be thorough. Because there's a lot of mention on that heat soak page about it having rough idols and such and this thing doesn't idle rough at all it only does it when I'm starting to get going when I get going at that RPM usually in 2nd gear

 

Just so I know there's nothing actually wrong with the Jeep it's just that it wasn't built properly to work at this altitude so I need to retrofit something onto it that'll help it correct?

 

I don't believe there's anything "wrong" with it; it just can't overcome certain laws of physics.

I will allow that I could be totally incorrect here. Maybe someone else will chime in but I believe installing thermal barriers will solve the issue. The 1900 rpm thing is probably just the point at which the f.i. system is able to purge itself and overcome the issue. I'm over-simplifying but you get the point.

 

No. The PCM will relearn everything for the new "conditions". You're fine there. Have you tried resetting the PCM? Might be best to start from a clean slate. Basically you just reset, and it learns the new conditions over 50 or so key cycles.