2001 XJ dies on cold start

I am at a complet and total loss and my mechanic has even given up asking I take the jeep somewhere else. I am the original owner, with 145,xxx miles and every morning and evening when I start it, she fires right up then stumbles and dies. It takes anywhere from 3-6 tries to get her to stay running with me keeping on the throttle. After 10 min or more of driving she always fires right up and stays running no problem. I've replaced the crank position sensor, cam position sensor, idle air controller, intake temp sensor all to no avail. The oil pressure gauge shorts out on cold starts and reads 80psi but after driving for an hour and restarting the gauge works and oil pressure is at 50psi driving and 25-30psi at idle.

What do you mean the oil pressure gauge "shorts out" on cold starts?

You have two problems.

1. One of the issues is your oil pressure reading. Replace the sending unit with one from a Jeep dealer or from moparpartsonline for a reduced price. Oil pressure sending units are a very common failure on the Jeep 4.0 engine.

2. The other is your cold start issue.

For the cold start, I would highly recommend some fuel pressure testing. Your vintage is notorious for having a problem with failing check valves on the fuel pump assembly which can lead to a loss of pressure and issues with cold starts. Because it is such a common problem, I have a copy and paste on this. Here you go. See if it sounds familiar.
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The check valves (there are two of them; one on the fuel pressure regulator and another on the fuel pump) are part of the fuel pump assembly on 95.5-2001 vintage 4.0 XJs. When a check valve starts to fail, it can reduce fuel pressure to a point where an extended cranking time is needed to start the engine. It is a very common failure. I experienced it on my 99 at around 75K.

A good (and simple) first step in helping you troubleshoot this problem is called “the poor mans prime”

1. Turn key to ON position (do not crank the engine!)
2. The fuel pump will energize and run for a couple of seconds
3. Turn key to OFF position
4. Repeat above steps 1-3 two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have a check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the fuel pressure leaking down and the subsequent stumble upon startup is the engine clearing the excess fuel that has leaked into the cylinder because of the faulty injector.

Also be aware that a bad battery can give you starting symptoms as well. Have your battery load tested (any parts store will do this for free) if you have any doubt; your XJ does NOT like low available battery voltage. Be sure battery connections and posts are CLEAN.

Some troubleshooting tips for extended crank times. You will need a fuel pressure gauge. Many parts stores will rent you this tool inexpensively.

*Hook up fuel pressure gauge on the schrader valve on the fuel rail under the hood

*Start engine and bring to normal operating temperature.

*Observe fuel pressure gauge. Normal operating pressure should be 49.2 psi (plus or minus 5 psi)

*Shut engine off.

*Observe pressure on gauge. Pressure should not fall below 30 psi for five minutes.

If pressure falls below 30 psi, it must be determined if a fuel injector, a check valve within the
fuel pump assembly, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult the manual that comes with the fuel pressure gauge and the Factory Service Manual for your year XJ for more information on this exact procedure, but here is basically how it works:

*Turn the engine off and immediately clamp the fuel line at the adaptor hose. Watch the pressure gauge and see how long it takes to lose pressure.

*If the pressure remains at 49 psi for an extended period of time then the problem is in the tank – most likely a check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky fuel injector.

**Note. Where check valve is suspect, a quick loss of fuel pressure is often the fuel pressure regulator check valve and a slow loss of pressure is often the fuel pump check valve. But keep in mind that either or both check valves can be to blame.

You can limp a check valve problem along indefinitely if you like, as it doesn't mean the fuel pump itself is going to fail. Perform the “poor mans prime” before cranking to speed up your starts. However, if you have a check valve issue and want to resolve it, it is recommended that you replace the entire fuel pump assembly for two reasons. First, you have to drop the gas tank to access the assembly. Secondly, with two check valves, replacing just the fuel pressure regulator where one of the check valve resides may or may not resolve your problem; remember there are two check valves; the other one is on the fuel pump.

The fuel pump assembly consists of fuel pump, fuel pressure regulator, fuel gauge sending unit, fuel gauge float, pickup filter and pigtail wiring harness. If you do replace the fuel pump assembly, purchase a high quality unit; you do get what you pay for here. Some cheap aftermarket assemblies have been known to fail prematurely. I personally prefer Bosch fuel pumps as they manufacture the best fuel pump assemblies for the XJ. Carter is my second choice. Airtex is my least favorite choice and I will not recommend or install them.

I suggest testing the battery.

For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator in not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging.

Your symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. The ECU will relearn the idle settings after a short period of driving and the engine will start and idle normally.

Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem. Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables and replace them as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body.

If the symptoms reoccur, you need to have your battery and/or the alternator load tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

Cleaning the Idle Air Controller (IAC), regular tune-ups, and using fuel injector cleaner should be part of your routine preventative maintenance.

Thanks for the input, the jeep did get a new battery when troubleshooting the problem, it was 5 years old and suspect anyways, but no change. The jeep has been maintained within in an inch of it's life. New spark plugs every 30K miles, all fluids flushed every 30K with oil changes every 5K using mobil 1 10w-30 and a bottle of BG 44k every year. I've tried the poor man's prime as my buggy has a fuel pressure regulator that bleeds down and requires a prime to start. No matter how many times i turn the key on and off it still starts and dies. What has me puzzled is:

1. Why does the oil pressure gauge NEVER read correctly on cold start up, but after 30 min or more of driving, i can shut it off and start up and the gauge works perfectly and the jeep holds great oil pressure.

2. Once up to operating temp, even if just 10 min of driving, the rig fires right up and idle's just fine.:twak:

You describe a low voltage ECU re-boot with difficulty starting, needing to nurse the gas pedal, and normal starting after the 5-10 minute ECU re-learn. I think you need to do some additional diagnostics and preventative maintenance focusing on battery wires and ground connections.

While some of us, (as RENIX readers), read this post, is there any real big difference as to the two diagostic trouble shooting proceedures.., the fuel side, and the wiring side to the RENIX? Thanks

Have you changed the oil pressure sensor?..in a 2001 xj... it uses a 5volt power that other sensors also use...the 5 volt power comes from the PCM... if the sensor is pulling down the 5 volts it will cause all kinds of starting problems.

You might want to try disconnecting the oil pressure sensor and see how it starts that way.

McQue said:

Have you changed the oil pressure sensor?..in a 2001 xj... it uses a 5volt power that other sensors also use...the 5 volt power comes from the PCM... if the sensor is pulling down the 5 volts it will cause all kinds of starting problems.

You might want to try disconnecting the oil pressure sensor and see how it starts that way.

Click to expand...

I agree. Gotta start with a new oil pressure sending unit before moving onto more complex possibilities. They are a VERY common failure.

Do yourself a favor and buy it directly from Jeep. Aftermarket oil pressure sending units are "hit and miss" with a lot of miss for reliability and longevity.

Thanks for the input, i'll be back home next week and will swap out the sending unit. But i still don't understand why it works when warm but not cold. A while back (at roughly 90K miles) it starting doing this. I changed the rear main seal at 120K miles and also put all new orings in the oil filter adapter, and had to pull the oil sending unit at that time. I cleaned everything up, put it all back together and it worked just fine. I was under the impression that oil was getting in the electrical connection from the leaking orings on the adapter, causing an electrical short in the sending unit. My theory is that when hot the oil heats up and burns off after 30-45 min of driving causing it to work when restarting at that time, but when cold it has some oil in the connection shorting it out....any thoughts?