rogh start/idling when warm

[Prom3theus]

Hey folks,

I have a starting issue when engine is warm. Before details about the issue: 99 XJ, 4L, fresh tuneup (few months ago), new O2 sensors, about 125K miles.

I have no CEL light and when I try reading the codes from ECU I get none (not even a pending). I have noticed the issue first time couple weeks ago. When cold, engine starts and idles normally, however, if I attempt to accelerate half pedal, the car stalls (does not die, just stalls, then eafter I release the pedal, it picks back up). Sometimes it sounds like it has a backfire (I hear that "bang"). After letting it idle for 10-20 seconds, it drives almost perfectly, but I can feel it doesn't ride at it's full potential (it feels like it's accelerating at 70%-ish from how it would normally). When engine is hot, and I stop it for couple minutes (turn off), it barely starts, and it idles very rough and almost dies. I can also smell it runs very reach. After again 10-20 secs, it starts to improve and in few more other seconds, it starts idling normally, but I have the same lack of power as when cold. If I just stop it and immediately start, it starts and ides fine, like it hasn't been stopped at all. Every now and then it runs normally (I fell it accelerates way better), then again at about 70%ish from it's potential. I have new IAT, TPS, MAP, CPS, idle control valve (from few months ago). When I first noticed this, I had a CEL for vacuum pump (sorry, do not remember the code), but it suddenly disappear. I had changed the vacuum pump also just in case, but with no results.

I would like and appreciate some suggestions in what could be the cause, I know this engines can be finicky and fortunately I haven't had such issues till now. I am surprised I have no CEL at all. In my opinion it could be from the coil, I will replace it tomorrow since I have a spare one. I also think it may be related to the evap system, but I have absolutely no extensive experience with this engine and again, I'd appreciate any suggestions. Thanks a lot!

 

[Prom3theus]

Bump. Anybody has any clue? I also think it may be the fuel pressure regulator, I will have it tested this week. Pump in itself seems good, since the jeep starts fairly quick, it just doesn't accelerate w/o hesitation and /or misfire. Meantime, anybody has any other suggestion?

 

[SBpunk]

Check your spark too. Fuel pressure test wont hurt. TPS if it has gotten wet at all. Test the fuel injectors too. I know you said you just replaced a lot of these but sometimes they come off the shelf defective. Wiggle the connectors to the fuel injectors see if it gets worse or better.

 

[Prom3theus]

Check your spark too. Fuel pressure test wont hurt. TPS if it has gotten wet at all. Test the fuel injectors too. I know you said you just replaced a lot of these but sometimes they come off the shelf defective. Wiggle the connectors to the fuel injectors see if it gets worse or better.

Thanks for answering. I will do a pressure test for fuel for sure, and then I will start chasing those suggested. I am out of idea anyways, so I might as well start somewhere, lol.

 

[joe_peters]

You may be dealing with multiple faults.

1. Test your TPS.

2. Test your fuel pressure and volume.

3. You may have "hot" or "heat" soak, normally found in 00-01, but what is your production date and what emissions package?

 

[Prom3theus]

You may be dealing with multiple faults.

1. Test your TPS.

2. Test your fuel pressure and volume.

3. You may have "hot" or "heat" soak, normally found in 00-01, but what is your production date and what emissions package?

I'll do that, took off few days so I have time to deal with this hassle. It's a 99 XJ with evap canister.

I think it may be fuel pressure regulator, since few years ago I had a mechanic replaced the pump, but I doubt he changed the regulator. Just ordered a fuel pressure gauge from summit so I can test the fuel pressure.

Thanks for answering Joe!

 

[SBpunk]

I'll do that, took off few days so I have time to deal with this hassle. It's a 99 XJ with evap canister.

I think it may be fuel pressure regulator, since few years ago I had a mechanic replaced the pump, but I doubt he changed the regulator. Just ordered a fuel pressure gauge from summit so I can test the fuel pressure.

Thanks for answering Joe!

Look inside your door just to make sure. If its late 99 it might have the 2000 emissions

 

[Prom3theus]

Look inside your door just to make sure. If its late 99 it might have the 2000 emissions

Its evap canister for sure, been so many times under this sucker, I'm 100% sure. There is also a sticker under the hood, driver side under the window with the vacuum system schematics (leak detection pump, solenoid, map, evap, gas tank).

 

[Prom3theus]

Update: Checked the fuel pressure, was little under 20. Ordered a new pump assembly, replaced, pressure at 32 psi. Jeep runs like new. I have a question though, I thought the normal pressure for a good fuel pump is above 50 psi. Is the 32 psi a good pressure for XJ? Thanks for all your help guys!

 

[joe_peters]

Update: Checked the fuel pressure, was little under 20. Ordered a new pump assembly, replaced, pressure at 32 psi. Jeep runs like new. I have a question though, I thought the normal pressure for a good fuel pump is above 50 psi. Is the 32 psi a good pressure for XJ? Thanks for all your help guys!

I believe you should have 49 psi.

 

[Prom3theus]

Re: Re: rogh start/idling when warm

I believe you should have 49 psi.

So I wasn't too far away... thanks Joe. Whats funny is that the pump I got is oem, from dealer, not a cheap China knockout. I wonder if it doesn't need a break-in, which I doubt. Ill check it again, but with each ignition on, pressure raises by 10-ish psi, up to 32-ish, when it stops. It is also properly installed, I'm 100%sure.

 

[Ecomike]

Did a prior owner swap out stuff with and OBD-I rig?

Or did they give you the wrong year pump assy?

Did the new pump include the pressure regulator?

Is there a damaged fuel line, bad rubber or crushed metal?

 

[Prom3theus]

Did a prior owner swap out stuff with and OBD-I rig?

Or did they give you the wrong year pump assy?

Did the new pump include the pressure regulator?

Is there a damaged fuel line, bad rubber or crushed metal?

Thanks for answering Mike!

I ordered the pump with the part number from 99 parts fiche (04874173AB), this is the full assembly (with regulator on top, filter and fuel level). It looked exactly the same as the one I had, maybe less few cosmetic unimportant things (the clamp holding the pressure regulator, color of plastic, stuff like that). And as an update, the pressure of the new pump is 34 PSI, not 32 as stated previously (which is still low anyways), but apparently the jeep seems to run just fine... Pressure doesn't change when jeep is started, and it holds 30+ PSI for more than 5 minutes after engine is stopped.

I will need to go under the jeep and thoroughly inspect the fuel pipe for bends/choking points. Quick release hose is ok, I checked it when I replaced the pump. Worse case, I could get the pressure right at the exit from tank, to see if the pump outputs the correct pressure.

Do you think a different year fuel pump would still fit in, but have a different output pressure?

I have no knowledge about what previous owner did, Jeep seemed to be in pretty good shape and had a lot of receipts from when the car had been serviced, prev owner didn't seem to be the kind of do it yourself person. I know when I got the Jeep in 07 fuel pump was bad (took 3+ cranks to start) and had it replaced (took it to a shop then), after less than 30k it went bad again and needed to be replaced (this thread issue).

 

[joe_peters]

Did you replace just the fuel pump or the entire fuel module/assembly?

If just the pump, then maybe you got a mis-boxed pump.

 

[Prom3theus]

Did you replace just the fuel pump or the entire fuel module/assembly?

If just the pump, then maybe you got a mis-boxed pump.

I replaced the whole pump assy (regulator, pump, lever), as it came from the dealer. Didn't want to go with anything else (autozone, NAPA etc), just to be sure I get the right part...

If I drain the rail (using the pressure relief valve from the pressure gauge), so it has no pressure and no gasoline, with each ignition turn on pressure raises by about 10 psi, and stops at 34, as if the regulator would not let more pressure. I know that a good pump and bad regulator could cause pressure above 49PSI, towards 60-65. I wonder if it is a badly calibrated regulator... Do you guys know if there are Jeeps that use 34-ish PSI pressure?

 

[Ecomike]

Somewhere around 96-97 they raised the pressure from about 30-39, up to 49 psi.

 

[Prom3theus]

Somewhere around 96-97 they raised the pressure from about 30-39, up to 49 psi.

That makes sense, I think I will return it and have them send me the right one. Jeep runs fine, but i think the injector mist is not the optimum one.

Thanks Mike!

 

[Prom3theus]

I just researched this fuel pump ordeal a little more. If I didn't misunderstand, the 96 and below had the pressure regulator on the rail (calibrated for 39+/- psi), and 97+ in the gas tank (calibrated for 49+/- psi), as part of the fuel pump assembly. Also confirmed this with fsm for both prior to 96 and after 97. Fuel pressure regulators are different in between and cannot be swapped. So, there is no way the pressure regulator to be a mismatch with a 39 psi since 97+ has a different regulator compared to 96 and below, unless the regulator is badly calibrated from the manufacturer. In other words I have no clue what happened and why my pump outputs only 34 psi when its supposed to have 49+/- ... ill try another one and see, or maybe just screw everything and go with a custom pump with adjustable regulator, and mount it outside of the tank.

Thanks for all advices guys!

 

[joe_peters]

Have you tried a different fuel pressure gauge?

Could be a bad reading.

 

[Prom3theus]

Re: Re: rogh start/idling when warm

Have you tried a different fuel pressure gauge?

Could be a bad reading.

Good idea, thought about that but not too seriously since the gauge I got is brand new... but very possible to be bad, especially since it hasn't been tested and confirmed as accurate. Ill try a different one and post the results.