Rough Idle/Bogging At Startup

**danw77** · 08-18-2008, 10:46 AM

99 XJ, 4.0, AW4, 135,000 miles. For last 2 weeks or so, when I start the Jeep after it has sat over night, I have a rough idle for about 10 seconds. The engine will also bog and almost die with no power if I step on the accelerator. After 10 seconds or so, it clears up and runs fine, although I do notice a slight hesitation accelerating on the highway arund 2200-2300 rpms. I ran a can of fuel injector cleaner through it, and the fuel filter is relatively new. This only occurs if I let it sit over night. Even the 8 hours or so it sits while I'm at work don't cause this. But it has happened every single morning for the past two weeks.

**danw77** · 08-18-2008, 01:36 PM

bump for the afternoon crowd...

BDJCL3 · 08-18-2008, 03:39 PM

Go through all of the typical tune up things. Spark plugs and wires, make sure your throttle body is clean, air filter. Injectors might be getting clogged.

**danw77** · 08-18-2008, 03:49 PM

That has all been done within the past six months.

**CJ7-Tim** · 08-18-2008, 09:08 PM

Classic symptoms of a failing fuel pressure check valve. The check valve, located on the fuel pump in the fuel tank, holds pressure in the line while the engine is off. It will begin to slowly fail, allowing the fuel pressure to drop, slowly at first and then faster and faster. The symptoms are most noticeable on hot days, you also will have a little bit of vapor lock due to the loss of fuel pressure.

To test the fuel pressure, find the pressure relief valve on the fuel rail. After the motor has run for a bit, shut the motor off and press in the center of the valve, just like a tire valve. Of course, flammable GASOLINE will spray out, so be careful. Now, let the Jeep sit overnight and try the valve again. If you get little or no pressure the check valve is bad.

The simplest solution to the situation is to crank the engine for 3-4 seconds, let off the key, and then start it.

To fully fix this you will need to drop the gas tank and install a brand new fuel pump assembly.

**danw77** · 08-19-2008, 07:45 AM

Thanks, the check valve it is. No pressure this morning (only a dribble out of the relief valve). If only this had happened a couple of months ago before I put my hitch on. Is this an indication that the fuel pump itslef is going? I'd like to put off dropping the tank as long as possible.

**CJ7-Tim** · 08-19-2008, 10:08 AM

Quote:

Originally Posted by danw77

Is this an indication that the fuel pump itslef is going?

No, the fuel pump will run for years like that. My check valve has been bad for 3+ years. I am too cheap/lazy to replace the fuel pump.

**danw77** · 08-19-2008, 10:30 AM

Quote:

Originally Posted by CJ7-Tim

I am too cheap/lazy to replace the fuel pump.

x2 I just did my wife's Suburban (45 gallon tank) and I'm not pulling another tank anytime soon.

NUBOPOLY · 08-19-2008, 03:46 PM

I have a 98 cherokee sport with 135k with a bad check valve, but I just recently started having problems with it running odd. It only does this once in my drive but it almost does it EVERY drive. Usually the first time I accelerate and below 2000rpms it will just start to misfire it seems and will sit at one rpm and eventually seem like it is stalling if I keep the throttle in one position, but if I let off then get back on the throttle it will clear up and accelerate normal. Sometimes it takes a bit longer (pulling out of neighborhood) sometimes it comes sooner (driving through neighborhood almost at the exit). I just replaced plugs, wires, cap, rotor, and both o2 sensors(I still get codes P0135 and P0138). I had a pretty bad exhaust leak (my front hanger ripped out of the piping making a nice hole, but that got fixed with everything else. But anyways could this be my check valve causing this? Maybe its not getting sufficient fuel pressure for the first slow acceleration? or something

Didnt want to post a new thread about it, but does anyone also know an effective hanger design for the front, seeing what happened to my hanger(The hanger peice fell off too so I couldnt just weld it back on but the rubber mount part for the hanger is there)

bryris · 08-19-2008, 07:05 PM

I am willing to bet it is your TPS. Throttle position sensor. Mine did the same thing exactly! And it fixed it.

Assuming you can't fix the problem with the check valve thing.

Muddeprived · 07-18-2009, 02:55 PM

Quote:

Originally Posted by CJ7-Tim

Classic symptoms of a failing fuel pressure check valve. The check valve, located on the fuel pump in the fuel tank, holds pressure in the line while the engine is off. It will begin to slowly fail, allowing the fuel pressure to drop, slowly at first and then faster and faster. The symptoms are most noticeable on hot days, you also will have a little bit of vapor lock due to the loss of fuel pressure.

To test the fuel pressure, find the pressure relief valve on the fuel rail. After the motor has run for a bit, shut the motor off and press in the center of the valve, just like a tire valve. Of course, flammable GASOLINE will spray out, so be careful. Now, let the Jeep sit overnight and try the valve again. If you get little or no pressure the check valve is bad.

The simplest solution to the situation is to crank the engine for 3-4 seconds, let off the key, and then start it.

To fully fix this you will need to drop the gas tank and install a brand new fuel pump assembly.

I have a question about testing this. I'm supposed to ride around till the jeep is at operating temps then shut it off and test the valve. gas will shoot out. Then i leave it overnight and test it again in the morning with a cold engine? or do i drive it around again and warm it up?