Rough running only after start up

I have a '98 4.0L w/ AW4 & 231 that has started running rough on start up only. This started a few hundred miles ago, and I originally thought it was bad gas, but a second and third tank has not cured it.

Symptoms:

-after start up cold or hot
-seems to take a little longer than normal to start, ~5-8 seconds of cranking.
-usually idles just fine after start up
-initial acceleration, if more than slight, will induces missing and bucking
-WOT will run normally, and is sometimes the only way to accelerate
-idle will sometimes be rough for a minute
-symptoms disappear after a minute or so, usually faster with WOT

I have not had a chance to put the car on an OBDII scanner because the auto parts stores here are not legally allowed to provide that service.

Anyone had this problem before?

TIA

All your tune-up parts fresh ? Cleaned the Throttle Body and Idle Air Controller ? Used and fuel injector cleaner. Cleaned the wire connections ?


When your 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 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 a leaky injector.

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 unit. Some cheap aftermarket fuel pump assemblies, like Airtex, don’t hold up and fail completely within 12-18 months.

I haven't replaced anything on the engine since I purchased the vehicle a year ago. I will run some injector cleaner through it on the next tank and clean the TB and IAC. I never have to try twice to start it, it just takes a few cranks, especially if I just start cranking as soon as I put the key in the ignition.

Get a tuneup done. Basics first; always. Failure to do so when you have a driveability symptom often results in the chasing of your tail. Plugs, wires, cap, rotor. Money well spent.

Check the MAP and TPS sensor.

Ordinarily I would start with plugs, wires, cap, rotor, etc, but the symptoms are ONLY apparent immediately after start up and are gone within the first 1/2 mile. Also, they came on suddenly rather than progressively, seemingly with a tank of gas that I got a couple weeks ago.

This issue went away with a little injector cleaner and a few tanks of gas, then returned again a week ago. I can here gurgling sounds coming from the gas tank after shut down. Relatively certain that it is the check valve. My question is if I should replace the pressure regulator while I'm in there.

98cherokeedriver said:

This issue went away with a little injector cleaner and a few tanks of gas, then returned again a week ago. I can here gurgling sounds coming from the gas tank after shut down. Relatively certain that it is the check valve. My question is if I should replace the pressure regulator while I'm in there.

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The check valve is in the pump. You should change the entire fuel pump module, which will have a new filter/regulator. If you have the bucks, install a Bosch module, if not, go with a Carter module. The OEM module uses the Bosch pump (the entire module may be Bosch but I have no proof of that, only a visual on the pump).