Strange Problem

[BlueWeasel]

Having a strange problem with my '97 XJ.

After its been running at normal operating temperature for awhile (regular driving around town) I'll shut it off and be away for maybe 20 or 30 minutes, come back and start it up to take off and when pulling away it will hesitate like its starving for fuel. It lasts about 5 seconds then runs fine for the rest of the trip.
Truck always starts on the first crank and usually runs great. It has about 176,000 easy miles on it from a previous nerdy owner, and 3,000 of mine. I've encountered this strange behavior 3 times now, all on different days over the past couple weeks.
Toady when it happened I had already driven about 100 miles and started it multiple times with no problems.

I'm guessing its the fuel pump or filter. I don't think the pump unit has ever been replaced, and the fuel guage works intermittently. Could it be a vapor lock?

...Oh and its never been off-road before I bought it and lifted it.

 

[eflores]

Troy? Care to chime in now?

 

[Beezle]

Sounds like a "heat soak" problem (I think that's the term) when you shut off the engine the heat from the manifold heats up the intake and when you start it again it gets a blast of hot air into the engine and stumbles for a minute. My 99' would do this when I first got it then I put the electric fan on a manual switch and would run it for a few minutes while the engine was off to blow out some of the hot air and it would run fine with no stumble.

I've since replaced the radiator with a 3 core and lifted the rear of the hood for better air flow and haven't had to run the electric fan after I shut it down.

Someday I plan to put a timer on the e-fan so when I shut the engine off it will run for about 3mins then shut itself off to knock down some of the under hood temps. Someday......

 

[BlueWeasel]

Well that don't sound too bad. Not anything serious anyway.
I have hood louvers and removed my AC so I use the fan AC selecotr to run the e-fan when its hot. Might have to run it before I shut 'er down more often.
The timer idea sounds great.

 

[Hypoid]

Fuel filter should be cheap and easy to replace. Do it anyway.

 

[Beezle]

On a 97' the only fuel filter is in the tank isn't it?

 

[YELLAHEEP]

Having a strange problem with my '97 XJ.

Problem identified. I have a passionate hatred for the '97's............

Strip the good stuff off yours and go get a '98. Problem solved. That's my next plan of action with my lemon of a '97. Then I'll kill it with fire.

But I digress.


Fuel pumps are cooled by the fuel they pump. If you're having that stumbling issue during the heat of the day, particularly when the fuel is low in the tank, then I suppose the pump could be getting old and over heated....... Give a listen to the sound of the pump when you first start it up "cold" then listen to it running when it's been stumbling or when it's hotter out. See if it sounds different - as in more noise or sounding like its working harder.

The non-working/intermittently working fuel gauge is a common problem with the '97's. Mine does it. Reads fine from full to just above 1/2 tank, then goes to empty until it uses another 1/4 tank of fuel. Then it'll read again from 1/4 tank to empty. Generally it's the sending unit in the tank and not generally related to the pump.

However, if you're noticing that the gauge goes funky when the jeep is stumbling then I'd say you have a power supply problem - be it positive or ground.

If you haven't scanned the ECU for codes, do it. Codes can be stored without setting the check engine light. Mine constantly stores a low volt at the fuel sending unit code. (P0462 Fuel Level Sensor Circuit Low Input).

 

[Fred]

So Troy, have you ever checked the resistance to ground on pins 31 and 32 of the A-plug on the ECU? The grounds are supposed to be located on the firewall, but they weren't on Zac's 97, nor on Jeff's. When I checked Zac's, I think one was open, and the other showed some ohm's. I just piggy backed wires to the harness, and ran them to the firewall for grounds. Seems to run like a champ now, it's just the front end that's screwed up again, I'd suspect from fast driving on a rough road.

 

[YELLAHEEP]

Yeah, you PM'd me that ground thing before but I just can't picture what/where you're talking about.

"The grounds are supposed to be located on the firewall but they weren't......"

So, where were they located? Or were they non-existent?

I have a ground to the firewall (block to firewall) but I've never seen another XJ with any more firewall grounds than the one braided copper strap. :dunno:

You'd probably have to just show me the next time we meet up for some function/event.

 

[BlueWeasel]

I checked the codes and got none, fuel pump is not "noisy", and will have the problem even on 3/4 tank. I think it is the heat soak problem, but I'm saving dough to replace the fuel pump anyway for the long-term.

 

[Beezle]

Check your exhaust manifold for cracks too, that could also be raising your underhood temps.

 

[BlueWeasel]

WILCO