Starts fine, idles good, then stumbles bad when you drive away

99 XJ, 4.0, auto, 170k miles...

runs good, starts fine, and it does this more often when the engine is warm or its not really cold out.

start it up, pull out of the driveway or parking space, then seems like soon as you pull out onto the road it wants to die really bad. It sputters and barely runs. But cleans up and runs fine after maybe 10 seconds. Also it seems like you can power through it.

Check engine light is also on for both o2 sensors. I replaced both sensors with zero change to the problem. Also the check engine light keeps coming back on for o2 sensor heaters.

I swapped the air intake sensor and the idle air control valve with my 98 TJ, the tj runs perfect and the xj has no change.

this has been going on now for 2 months, just out of the blue. Also just within the last couple days i've been experiencing a long crank time, at least double what it used to be.

Have a friend with an 01 that had a light on for the 02 sensor heaters, took some research but found that there is a small (in his case yellow colored fuse I think 15 or 20 amp) in the engine fuse box on the passenger side that controls that and was blown.

Did you install Bosch O2 sensors ? The Jeep 4.0L and Bosch O2 sensors do not play well together, you should be using NTK O2 sensors.

The symptoms are caused by whatever is causing the Check Engine Light to illuminate, research the trouble codes. As suggested, check the O2 sensor heater fuses.

Longer cranking times often are caused by a failed check valve in the fuel pump assembly.

When a 1997 to 2001 Cherokee won’t start on the first try day after day, the first thing to suspect is a failed check valve on the fuel pump. The fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is located on the fuel pump and located in the gas tank. A failed check valve allows the fuel to drain back to the tank. This results in one or more of the following symptoms:

SYMPTOMS

• Longer than normal cranking times
• Never starts on first try, almost always starts on the second try
• Rough idle for a few moments of idling
• Rough running for first ½ block of driving
• Little or no fuel pressure at the fuel rail test valve similar to this -

1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45-50 PSI.


POSSIBLE CAUSES

• Failed check valve on the fuel pump in the gas tank.
• Leaky fuel injector(s) may be the true cause of your problem, but more likely they are simply contributing to the check valve issue.

Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail, and you may be dealing with a bit of vapor lock as well, especially in warmer weather.

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.

2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.

If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably one or more leaky injectors.

There are a few solutions to the problem –

1. Turn the key to ON for 5 seconds and let the fuel pump prime. Turn the key to OFF, turn the key back to ON for 5 seconds, start the engine.

2. Crank the engine for 3-5 seconds, pause, and crank the engine again. It will almost always start on the second try.

3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.

You can use solutions 1 or 2 for as long as you want to, they do not harm anything and the fuel pump will continue to function for many more years.


If you do replace the fuel pump assembly, purchase a high quality OEM style pump assembly such as Bosch or Carter. Some cheap aftermarket fuel pump assemblies, like Airtex, and Precision don’t hold up and fail completely within 12-18 months.

Thanks for the tips... I can't remember what o2 sensors I picked up. I didn't know about the fuse.

I figured it was the check valve in the fuel tank, it always starts but sometimes after sitting it takes a good 5 seconds of cranking. Its not a big deal to me

Did you inspect the wiring harness for frayed or melted wires?

Fuel pressure regulator or fuel pump.

I have a similar problem in my 99. Starts and idles great. Doesn't matter how long I let it idle, but when I start to drive it stumbles bad. It recovers immediately and it won't do it again until the next time I shut it off. Also doesn't seem to matter how long it sits. No codes. I pretty much just live with it.

I have a 98 4.0 manual with the exact same problem. In the last year I've replaced the plugs, wires, cap, rotor, coil, fuel pump assembly (Bosch), exhaust from manifold back, both O2 sensors and the TPS. Not all of that was specifically to cure this problem, but I was hoping that it would. I just did the coil and it seems to be better, but there is still a noticeable loss of power, usually around when I grab for second gear, which makes people think I don't know how to drive a stick.

I thought maybe it was vapor lock, but it is still happening in the fall, so I don't think that's it. I don't have any codes now, although I did have some before for the O2 sensors which had frayed wiring.

At this point I'm just planning on living with it. I'm going to try cleaning the throttle body with some spray, maybe that will help.

my 99 would stumble everytime i started it or restarted it, sometimes not immediatly, but usually when i would first start to take off, no matter how much i let it sit idle

the second time i replaced the front O2 sensor it cured the problem. and yes i used the nkt both times, but other issues caused my front drive shaft to rub on the O2 sensor. there was no code, eventually the o2 sensor finally crapped the bed and threw a code 50 miles from home, and it was almost undrivable.

after i replaced it it has never done it again

RD

alwayzAMC said:

my 99 would stumble everytime i started it or restarted it, sometimes not immediatly, but usually when i would first start to take off, no matter how much i let it sit idle

the second time i replaced the front O2 sensor it cured the problem. and yes i used the nkt both times, but other issues caused my front drive shaft to rub on the O2 sensor. there was no code, eventually the o2 sensor finally crapped the bed and threw a code 50 miles from home, and it was almost undrivable.

after i replaced it it has never done it again

RD

Click to expand...

Well, I have 218K and as far as I know it is still the stock sensor (previous owner was meticulous about logging repairs and O2 was not on the list). I guess I'll throw one in and see what happens.

so NTK sensors only huh... its been too cold and snowy for me to crawl under and check it out. 4 day weekend coming up though, so should be able to get it in the garage.

I keep looking online and lots of posts about this exact same problem. I don't think its related to the 02 sensors though. It started doing this way before i replaced the sensors, and there was zero change with the new ones.

So now with the weather getting cold it has not been stumbling much at all. But i also just replaced the blown fuse for the 02 sensor heaters, check engine light is off now and it has been running fantastic. Not sure if all that was related or not. I guess we'll drive it a bit and see.