'97 4.0 I am fixing up for a DD for my daughter.

After sitting for an hour or two, or overnight, it cranks for an abnormally long time. Sometimes as long as 10 seconds. When it catches, it runs rough for 3-7 seconds or so before clearing up. Then it idles fine.

If the time between restarts is not long, it fires right up and idles smooth. Runs good on the highway.

Sounds to me like a weak fuel pump, or fuel system is losing pressure and draining back.

Edit: I should also add that sometimes the engine will shut off completely, like you turned off the key, if you try to move the Jeep after it does what is described above. Then you can restart it.

Any ideas?

 

Sounds exactly like a fuel pump to me. I had a Chevy pickup that did the exact same thing and a new fuel pump took care of the problem.

 

The check valves (there are two of them; the primary one is on the fuel pressure regulator and a secondary one can be found on the fuel pump itself) 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 about 2 seconds. Be sure to wait until it stops running.
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 normally 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 check valve is part of 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 assembly. Know this….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. A search will show you why.

 

TJ, this was exactly what I was trying to find in a recent thread where we dealed with this (remember, me thinking you were the inventor of the term : poor man's prime" ?). Should copy-paste it this time to my notes

Fire, I have exactly the same, already for many many months. The PMP is something I do when i get tired of the long cranktimes, and always works. One day I will address it and fix it, bu you can keep it going for ever, it will not cause any harm.

 

Theres a small check valve that can be installed in that cures this. From a Volvo or some thing of that nature that bandaides this if its the check valve is the issue.

 

This is interesting. More info available on where to install what?

 

An inline check valve is not a good idea for this vintage XJ (97). Check out this link. There are others that have had issues as well, including one fire.

https://www.cherokeeforum.com/f2/fue...k-valve-21921/

 

Update: Had the fuel pump and sender replaced yesterday.

Cold crank time to start is now 1.5-2 seconds. No stuttering so far. Seems fixed.

 

Sounds good but IF it turns out it's not fixed your symptoms sound exactly like a bad fuel injector. If a fuel injector is stuck partly open or is "leaking down" fuel will continue to drip from it until the pressure goes down enough. If it sits a few hours or over night there is enough fuel in the cylinder that you need to crank a good while until it fires and then burns off the excess fuel.

 

A good theory. But the affected cylinder will clear itself in one to two cycles. It wouldn't take 10 seconds.

And it wouldn't stutter badly and/or die if it was just one leaky injector.