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2001 Cherokee misfire when cold

Lanser1999

NAXJA Forum User
NAXJA Member
Location
Wisconsin
I have a 2001 Jeep Cherokee 4.0 I6. I live in Wisconsin and right now it is in the 30’s. When starting my Jeep cold (in morning or after work) the Jeep will run rough and misfire when cold. When I start driving it will be misfiring but if I pull over and turn it off and back on it will run fine no misfire. It has new plugs and I don’t think the coil pack is bad but if anyone has any way of diagnosing the coil or any information that could help.
 
A faulty check valve will cause the starting symptoms described. Test the fuel pressure after 8 hours of not running.


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 inside 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 will fail completely within 12-18 months.
 
I would start by checking the fuel pressure, that will help guide you as to what to do next. The injectors are definitely easier to change that a fuel pump!
 
Had an injector with the oring that goes into the fuel rail leaking. Replaced the oring and misfire has gone away for now. Of course my check valve is still bad but I’ll wait for the fuel pump to go.
 
Misfire when cold is still present. The Jeep will run fine after it’s warmed up but when it’s cold, especially after it sits all night, when it is started will have a bad misfire on cylinder 1 and 5. After it warms up drives fine. I have noticed when it misfires if I turn it off after i start it and restart the engine it runs better. Has new spark plugs, cleaned the throttle body, new throttle body gasket.
 
How long does the misfire last ? What are the fuel pressure test results ?

Are you are using the recommended Champion copper, or AutoLite APP-985 spark plugs, and the coil rail tests good ? Have you recently used any fuel injector cleaner in the gas tank ?
 
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Used Seafoam a few tanks ago, misfire lasts less than 5 minutes. Using champion copper. Havent tested the coil, not sure on the procedure to do it. Need to check fuel pressure last time my gauge was broken, what is proper fuel pressure readings?
 
1997-2001 are 49 +/- 5 psi.

Have you tested the O2 sensors and inspected the O2 sensor fuses ?
 
Yes they were Bosch, all I could get that day. I should add, in the morning/when it’s cold it has a smell of fuel coming from the exhaust/running rich.
 
Numerous threads on 4x4 forums report that the Jeep 4.0L does not like Bosch or generic O2 sensors. Watch this >> https://youtu.be/-lUYRqaYvYI . Install some NTK brand O2 sensors, reboot the PCM, and the symptoms will likely clear up.

This procedure will erase the 1996+ OBD-II “Adaptive Memory” stored inside the PCM and allow a new “Adaptive Memory” to be developed. After performing this procedure the PCM will re-learn and store into Adaptive Memory your engines performance characteristics. Perform these steps exactly as they are written, in the order they are written. This will cause the adaptive memory in the PCM to be erased and cause the PCM to go into Fast Learn adaptive mode.

Disconnect the POSITIVE battery Terminal clamp and touch it to ground ( not the battery negative terminal ) for 30 seconds. The engine block or the A/C compressor pump will work as a ground. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory).

Reconnect the Battery Cable
Turn Ignition Switch to the “On” position but DO NOT start the engine
Turn Headlight “On”
Turn Headlights “Off”
Turn Ignition Key “Off”

The PCM Adaptive memory has now been flashed, or erased from the PCM. When you start the engine it will be running off a set of pre-programmed tables that come with the PCM from the factory. When you get the engine up to operating temperature the PCM will start to collect data for the “Adaptive Memory”. The PCM will collect data for Adaptive Memory for the first 50 Warm-up Cycles.
 
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The rear O2 sensors monitor the results of the cat converter function. The front O2 sensors are a primary input to the computer for fuel trims.
 
Sorry, I had that backwards^ I was told the after cat o2 sensors are just to monitor cat and not mixture. I think I will replace b1 s1 and b2 s1 because those are the two with boschs in.
 
Replaced o2 sensors. Fuel mileage has gone up but still has a misfire when cold.

Have you check the temperature sender for indicating proper temp?
If the computer thinks the engine is warmer than it is, it will not provide the needed richer mixture, instead the mix will be lean and can cause backfire.
Once the engine warms up, the problem should cease.

If you have a scanner that can read the OBDII information, you can see the sensor's indicated temp.
When my 01's temp sensor went bad, I tried several that did not read correctly, until trying a Mopar sensor that did read correctly.
I check the engine's actual operating temperature with an infrared thermometer, aimed at the thermostat housing.
 
Have you check the temperature sender for indicating proper temp?
If the computer thinks the engine is warmer than it is, it will not provide the needed richer mixture, instead the mix will be lean and can cause backfire.
Once the engine warms up, the problem should cease.

If you have a scanner that can read the OBDII information, you can see the sensor's indicated temp.
When my 01's temp sensor went bad, I tried several that did not read correctly, until trying a Mopar sensor that did read correctly.
I check the engine's actual operating temperature with an infrared thermometer, aimed at the thermostat housing.
I will have to check this. Did yours throw a code when yours went or no code?
 
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