1998 4.0, manual

I was cruising at around 60 and lost most of my power for 10 seconds. Felt like the throttle completely shut for a moment. Kept driving and made it the rest of my trip, about 20 minutes without any more problems. It’s started happening more frequently though, pretty much every time I drive, and always after the engine’s warmed up. Similar thing happens when idling.

Not sure if related, but I’ve been having a really bad time starting it cold, as well. It cranks forever and only fires after several tries. Occasionally won’t start at all. Thought it was the crankshaft positioning sensor, but I replaced it and the problem stayed.

 

You didn't mention it but have to ask. Is the check engine light on or have you ever seen it on? If answer is yes, you must hook up a code reader or a scan tool and check for codes.

The losing power symptom you are having. Important question to ask. Did this symptom start AFTER you replace the crankshaft position sensor or did you also have this symptom BEFORE replacing that sensor?

Reason I'm asking is this. The cold start extended crank time (more on what might be happening there below) could be related to a failing check valve on the fuel pump so you could very likely have two separate things going on here. If you replaced the crank sensor for the extended crank time symptom and that is when the other symptom of losing power started, it could be because of the new crank sensor you installed. What brand crank sensor did you install? There is a lot of difference in how aftermarket crank sensors work, quite hit and miss with a lot of miss on aftermarket crank sensors.

Good luck and keep us updated!
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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 but lately, they can be near impossible to find. Carter is my second choice. Airtex is my least favorite choice and I will not recommend or install them. A Google search on this and other forums will show you why.

 

CE light is on for O2 sensor and evap leak. Rough idle/power loss started after CPS replacement. CPS was stock, as far as I know, but I’ll check.

 

The O2 sensor is a major component of the fuel delivery map. Get an NTK sensor.

The problems arising after the CKP sensor was replaced leads me to believe it's a low quality sensor. It's well documented that the Jeep crank sensors have a relatively low success rate if they're an aftermarket brand.