• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

rough idle at startup

blysccr

NAXJA Forum User
Location
C-burg, pa
every once in a while when i start my 00 xj it idles low and rough. usually it goes away with the bleep of the throttle. but today it did it and i drove around for, say 6 seconds, and it had absolutely no power. any ideas?

thanks
 
When was the last time it had a basic tune-up? (air filter, plugs, wires, cap, rotor, cleaned the Idle Air Control, and perhaps cleaned the throttle body). When the XJ starts low and rough does it take the same amount of time to fire up as during a normal start? My first thought was Crank or Cam position sensor but there are a number of experts on here who might have more insight. Are you getting a check engine light when its failing? It sounds like a sensor......perhaps a dirty IAC or maybe the beginnings of an 02 sensor failing? Just a couple of ideas.
 
Tune-up for distributor-less 2000 or 2001 is fresh plugs, a fresh air filter, and a Throttle Body cleaning.



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 (1997-2001) 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 PSI.


POSSIBLE CAUSES

· Failed check valve on the fuel pump in the gas tank (1995 ½ to 2001 models ONLY)
· 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 gage at the fuel rail and start the engine. The gage should read 49 psi plus or minus 2 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.
 
Tune-up for distributor-less 2000 or 2001 is fresh plugs, a fresh air filter, and a Throttle Body cleaning.



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 (1997-2001) 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 PSI.


POSSIBLE CAUSES

· Failed check valve on the fuel pump in the gas tank (1995 ½ to 2001 models ONLY)
· 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 gage at the fuel rail and start the engine. The gage should read 49 psi plus or minus 2 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.

I am having the same problem on my 91 model and i have 31 psi on rail but with engine off it takes 3 or 4 key on's to get fuel pressure up to 31 but it will always start 1st try. could it be the check valve still? where is the check valve on these models? and is there any way to test it?
 
Back
Top