Running problem with 99 WJ 4.0

[JACKED88]

My sons 99 4.0 4x4 WJ has a bogging problem. When he starts it it'll idle and run fine for about a block or so and then start bogging out and running rough at idle. If he turns it off and waits a few minutes sometimes it will and sometimes it won't idle and run right for another block or so. We've replaced tha fuel filter/pressure regulator and checked tha fuel pressure at tha rail. His pressure is higher than what they say its supposed to be at about 65 psi. We've checked his coil packs to be sure they didnt have any corrosion and were on properly. Also checked the air filter and inlet for any blockage. No service engine lights come on when its running bad so no, we havent run it for codes.

I don't really think its a hot or cold issue since it starts bogging within a block or two of cold starting the engine. Hot or cold once you get it started again it'll run about the same distance each time before it bogs and won't idle again.

The problem seems to have a mind of its own. At no time will it run for any more tha a few blocks but when it come to restarting it, its 50/50 on how its runs.

 

Here tha routine;

 

Cold start --- runs OK for a couple of blocks and then starts bogging and won't idle. Dies.

Try to restart --- runs rough and will not excelerate. Dies.

Let sit for 5-10 minutes and restart --- about 50% of tha time this works however it'll start bogging again within a couple of blocks. The other 50% of tha time you have to let it sit longer.

Just to make things more confusing --- Sometimes when you restart it and its running rough, if you turn it off and restart it again it'll run right. (but only for a couple of more blocks)

 

We'd appreciate any suggestions on what may be causing his problem.

[Automan2164]

Have you checked the exhaust for restriction, namely the catalytic converter?

 

Oh, and just because the light isn't on, don't mean that the computer isn't trying to store codes that could help.

Rob L.

[JACKED88]

Have you checked the exhaust for restriction, namely the catalytic converter?

 

Oh, and just because the light isn't on, don't mean that the computer isn't trying to store codes that could help.

Rob L.

Thanks for tha help.

 

Can't say I've checked the exhaust but I will. One of my other sons has a scanner and he's supposed to bring it home from work within tha next couple of days. Hopefully it'll have some codes stored that'll tell us what its doing. (or not doing) :dunno:

[Automan2164]

Sounds like you could be building exhaust pressure, and letting it sit equals it out. Maybe try taking the exhaust loose at the downpipe before the cat and see what happens.

 

Rob L.

[kro10000]

:agree:

[Eagle]

Oxygen sensor? Runs okay cold but goes nuts once it gets warm?

[knuckledragger]

If the exhaust is not the issue ,look at the cam sensor housing/oil pump drive to see if it shows signs of having moved.they have a tendency to lock up and cause interesting driveability issues.

Dan

[aemsee]

I think that fuel pressure is supposed to be 48 psi? If you have 65 at the rail, that is way to high. Are the plugs gas/ carbon fouled when you pull them?

[JACKED88]

Sounds like you could be building exhaust pressure, and letting it sit equals it out. Maybe try taking the exhaust loose at the downpipe before the cat and see what happens.

 

Rob L.

I'll definitely check it. THANKS!!!

 

Oxygen sensor? Runs okay cold but goes nuts once it gets warm?

I'll be sure to check tha sensor too. I think we had one left over from doing his engine swap several months ago.

 

If the exhaust is not the issue ,look at the cam sensor housing/oil pump drive to see if it shows signs of having moved.they have a tendency to lock up and cause interesting driveability issues.

Dan

Sorry but I'm not sure what your talking about. Please explain.

 

I think that fuel pressure is supposed to be 48 psi? If you have 65 at the rail, that is way to high. Are the plugs gas/ carbon fouled when you pull them?

To be honest we havent pulled tha plugs. They were replaced about 3 months ago so we didnt think they'd have anything to do with this particular problem.

Apparently the fuel filter is also a pressure regulator. We didnt check the pressure on the old filter/reg before replacing it. At tha time I though his problem sounded like an all too common clogged filter so we replaced it. Wrong!!! Still runs tha same.

Although I agree the pressure is too high I don't think thats tha problem. We'll exchange whats apparently a defective filter/reg for another one and see if that solves tha high pressure issue.

Talked to my son with tha scanner and he's gonna bring it home tomorrow. We'll hook it up and go for a drive around tha block and see if it reads any failed sensors or anything else thats malfunctioning.

 

Thanks for all yalls help. Hopefully by this time tomorrow I'll have some good news. Meanwhile, let me know if you think of anything else.

 

Robert

[ratrapp]

you need to check fuel volume,not just pressure.if you fuel gauge has a relief valve then turn the hose into a bucket and hold the valve open while running and see if the jeep bogs down or completely cuts off.you can have pressure but no volume.

[aemsee]

you need to check fuel volume,not just pressure.if you fuel gauge has a relief valve then turn the hose into a bucket and hold the valve open while running and see if the jeep bogs down or completely cuts off.you can have pressure but no volume.

 

:no: . do a volume test WITHOUT the engine running. much less chance of it going BOOM :D . if you can't find an exact spec, approx 250 mls in 7 secs is a rough spec for that Jeep.

[ratrapp]

it's hard to check volume without the vehicle running unless you jump the relay over.i've been a mechanic at a jeep dealer for 5 years and worked in independent shops for the last 10.i regularly check fuel volume with the car running.my snap-on fuel pressure gauge has a hose with a relief valve that i stick in a jug to check the fuel flow.if in doubt have a helper hold the hose ina jug for you.

[aemsee]

it's hard to check volume without the vehicle running unless you jump the relay over.i've been a mechanic at a jeep dealer for 5 years and worked in independent shops for the last 10.i regularly check fuel volume with the car running.my snap-on fuel pressure gauge has a hose with a relief valve that i stick in a jug to check the fuel flow.if in doubt have a helper hold the hose ina jug for you.

 

Well, you can certainly do what you want, but that is not the correct way to check fuel volume. You are not even supposed to check it through the schrader valve. It is an open line test. If a pump passes using your method then it is certainly OK, but I wouldn't condemn one using that test without verifying failure with an open line. Try using Miller 8978. It is a great tool.

 

Think about it, if you have enough pressure in the rail to keep the Jeep running, then you obviously have a resistance to flow.

 

 

I have the same Snappy tester. That hose is more for relieving pressure and fuel from the guage than for testing volume.

 

AND, since you brought it up. I have over 25 years as a professional tech, most with Jeep, and 3 1/2 years as a Chrysler Instructor. :D

[87Warrior]

Pull the throttle body and scrub it inside and out with any toothbrush, other than yours, and some TB cleaner. Clean the IAC and passage twice. A dirty TB can cause the IAC to stick.

 

Since it has coil packs it has a cam position sensor. That thing may need to be re-synchronized at the dealer as well.

[JACKED88]

We hooked up tha scanner today but it didnt read any codes. Checked to make sure all the injectors were firing and they are. Checked to see if there was exhaust coming out of the tail pipe. It seems to be flowing normally. (compared it to both my MJ's 4.0 and my other sons XJ's 4.0)

 

How can we do an open line test on an erratic problem? Won't we need to do the test while its not running right? I mean what good would it do to test it while it is running right?

 

Take today for example. Although it took some cranking we started the engine and drove around tha block for about 10 minutes before it started bogging, idling rough and finally dying. Limped it back to tha house and checked for codes again. Notta. After we scratched our heads for a few minutes we decided to start the engine again. Cranked up and ran fine for several minutes before it started idling rough and bogging again.

 

We could do an open line test for volume right out of the tank while we're exchanging that defective filter/regulator but again since the prob is erratic how will we know which "mode" tha pump is in at tha time? (running right mode or bogging mode)

 

Hope that makes since.

 

 

87Warrior .... Thanks for tha TB tip. Even if that turns out not to be tha problem it couldnt hurt to clean it. I won't be tha one doin tha cleaning anyway. :chillin:

[dasbulliwagen]

With no codes and a running problem, I might look at fuel too. Sounds like it might be getting pressure on startup, but maybe the fuel pump slowly slows down till it can't push enough fuel to keep it going. When it starts sputtering after running for a while, jump out and hammer on the bottom of the fuel tank, see if it comes back to running OK. If it does, then you know you need a fuel pump, either that or theres a bunch of crud in the tank building up on the pump filter sock, restricting flow enough to cause this problem. Then after it stalls, the stuff falls off, and starts over. Good luck!

[JACKED88]

With no codes and a running problem, I might look at fuel too. Sounds like it might be getting pressure on startup, but maybe the fuel pump slowly slows down till it can't push enough fuel to keep it going. When it starts sputtering after running for a while, jump out and hammer on the bottom of the fuel tank, see if it comes back to running OK. If it does, then you know you need a fuel pump, either that or theres a bunch of crud in the tank building up on the pump filter sock, restricting flow enough to cause this problem. Then after it stalls, the stuff falls off, and starts over. Good luck!

We didnt exactly hammer on tha tank but my son did lay on tha ground and kick it a few times. I'll try it again using a rubber mallet.

 

I'd hate to just start throwin money at it but at this point I'm thinkin tha hell with it and go ahead and order a pump off ebay. We're only talkin $33 bucks.

[Automan2164]

With no codes and a running problem, I might look at fuel too. Sounds like it might be getting pressure on startup, but maybe the fuel pump slowly slows down till it can't push enough fuel to keep it going. When it starts sputtering after running for a while, jump out and hammer on the bottom of the fuel tank, see if it comes back to running OK. If it does, then you know you need a fuel pump, either that or theres a bunch of crud in the tank building up on the pump filter sock, restricting flow enough to cause this problem. Then after it stalls, the stuff falls off, and starts over. Good luck!

We didnt exactly hammer on tha tank but my son did lay on tha ground and kick it a few times. I'll try it again using a rubber mallet.

 

I'd hate to just start throwin money at it but at this point I'm thinkin tha hell with it and go ahead and order a pump off ebay. We're only talkin $33 bucks.

 

But do you really want to cheap out on an item in which you might have to drop the tank again if it fails?

 

Rob L.

[JACKED88]

With no codes and a running problem, I might look at fuel too. Sounds like it might be getting pressure on startup, but maybe the fuel pump slowly slows down till it can't push enough fuel to keep it going. When it starts sputtering after running for a while, jump out and hammer on the bottom of the fuel tank, see if it comes back to running OK. If it does, then you know you need a fuel pump, either that or theres a bunch of crud in the tank building up on the pump filter sock, restricting flow enough to cause this problem. Then after it stalls, the stuff falls off, and starts over. Good luck!

We didnt exactly hammer on tha tank but my son did lay on tha ground and kick it a few times. I'll try it again using a rubber mallet.

 

I'd hate to just start throwin money at it but at this point I'm thinkin tha hell with it and go ahead and order a pump off ebay. We're only talkin $33 bucks.

 

But do you really want to cheap out on an item in which you might have to drop the tank again if it fails?

 

Rob L.

:hmm: No!!! But then who's to say one from tha local auto parts store that cost 4 times as much ($121) will be any better? :dunno: Now days their all comin from across an ocean with who ever's name on um.

I know what your saying and thanks for pointing that out. But when your as broke as my son is ya gotta do what ya gotta do.

I did find an AC Delco for about $65 shipped. Maybe we'll go with that one instead of some no name. $65 is stll better than $120+. Oh, and that $120+ doesnt include a new screen. The AC Delco for $65 does.

 

http://cgi.ebay.com/ebaymotors/ACDelco- ... ccessories