'98 XJ HELP!!!!

I just bought a '98 XJ Classic 4x4 Automatic with 3.55 Chrysler 8.25 open diff and a NP231 Transfer case. It is completely stock for the next month til my goods come in.

I have a few questions or should I say issues.

OK for starters, when the vehicle sits for a while for instance over night and I start it up and go to leave for work in the morning, It is very hesitant for the first couple blocks and no matter how much pedal I give it there is still no response. Once I make it a few blocks like that it clears up. I tested the fuel pressure and it has a constant 50 psi and I just did a full tune up on it (plugs, wires, cap, rotor, sea foam). I have no check engine light on either. Any ideas?

Now problem two...

My friend has a stock '98 XJ Sport with the same differentials and gears and t case. His accelerates a lot faster than mine as if mine is being bogged down. Any Ideas?

Last Problem... I just replaced all three u-joints in the front drive shaft, the two in the rear drive shaft and I am now doing the front axle joints. I put all new bushing in the front end too. I have a nasty vibration in the front end which feels like it is in the drive train that starts right around 55+ mph. I checked the balance of the tires already and checked the hub bearings and the tie rod ends are all good.
Any ideas?


I am new to the jeep world and plan on having fun with this XJ hopefully one my stuff comes in and once I figure out this craziness.

Thanks for all your help.

Welcome to the forum! 98 is a good year for the XJ!

1. For that hesitation when cold, I would look for sensors that might be a bit off in open loop. Coolant temp sensor, map sensor would be the first ones to test. And the oxygen sensor input comes into play pretty quickly after the engine is run a bit as well. If there is between 100-150k on the sensors, replacing them makes some sense, at least the pre-cat sensor which provides the signal that helps determine fuel/air ratio. NTK or Bosch are good sensors here.

2. Test your "throttle position sensor". Directly involved with how the engine accepts tip in of the throttle. More on the throttle position below.....

3. Good luck with your vibe. Vibes in that 55 zone are not uncommon with the XJ and finding the cause can be a challenge. Question for you. Does this vibe continue at speeds faster than 55+ or is there a point where the vibe goes away? Try removing your front driveshaft to rule that in or out.
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture and vibration leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following: -

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics! It should be verified early in the troubleshooting process, when a transmission issue is suspected!

• Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".
• High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.
• Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
• Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted acordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
• Engine Misfire: A fault TPS can report values outside the deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

Thanks for the info as I will check the tps today. As far as the vibes go they start around 55 mph and get worse as the speed increases. I have not tried driving it without the front driveshaft yet but I will give it a shot and see if it eliminates the problem. Is the driveshaft in the front a common issue with the XJ's?

1_Sick_XJ said:

Is the driveshaft in the front a common issue with the XJ's?

Click to expand...

Not necessarily. Just something to rule in or out.

Ok I found out the vibration. I put all new u-joints in the front driveshaft and the front on at the differential is not quite wide enough to touch both tabs on the differential side so is has a slight slide/slop up and down so I gotta see if I can find a u-ioint that fits properly. It fits fine on the clip side but not on the clamp side... hmmm wierd.

As far as the hesitation goes, I checked the TPS and it read fine and swept through the analog just fine. I did however put my fuel pressure gauge back on and is took about 15 seconds for the pressure to get to 50 psi but that was after sitting all night and after I had relieved the fuel pressure last night with the gauge so maybe it may still be good. I'll check it again in the morning. What do you think about Crank Sensor?

Crank sensor not usually involved unless there is a miss/skip or engine dies. Throttle response is almost assuredly not related to the crank sensor. I'm still thinking engine management sensor for this, especially because you mention that this problem clears up as the engine warms up.

Also: you might be experiencing a loss of fuel pressure because of a failing check valve. Here is more. It is VERY common on your vintage.
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The check valve is located on top of the gas tank and is part of the fuel pump assembly on 95.5-2001 vintage 4.0 XJs. When the check valve starts to fail, it reduces fuel pressure to a point where an extended crank 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) way to 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 a two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have the check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the bleed down and the 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 this symptom. Sounds strange but it definitely does happen. Have battery load tested if you have any questions; your XJ does NOT like low available battery voltage. Be sure battery connections and posts are CLEAN.

Some troubleshooting tips for extended crank times:

Hook up fuel pressure gauge on the schrader valve on the fuel rail

Start engine and bring to normal operating temperature.

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

Shut engine off.

Pressure should not fall below 30 psi for five minutes.

If pressure falls below 30 psi, it must be determined if a fuel injector, the check valve within the
fuel pump module, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult gauge manual for more information on this 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 - possibly the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky fuel injector.

You can limp a check valve problem along indefinitely if you wish as it doesn't mean the fuel pump itself is going bad. Normally, the check valve is replaced as an entire assembly as you have to drop the gas tank to access the assembly. Fuel pump assembly consists of fuel pump, regulator, check valve, filter. If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket assemblies don’t hold up and fail prematurely. I specifically seek out Bosch fuel pumps as they are the best in the business. Stay away from Airtex, as IMHO, they have a bigtime reliability problem.

birchlakeXJ,

Can you confirm for me EXACTLY where the check valve is? I have a '98 that has an intermittent fuel delivery problem.

Last December I got some bad gas that had rust in it and it really screwed things up.

I had to replaced the the fuel pressure regulator (FPR) about two years ago due to a leak. And I remember reading on somewhere that the check valve is in the FPR.

After the bad/rusty gas issue I replaced the FPR again. But when I removed the old one, I blew through the input tube and it flowed freely. Then I blew through the new one and it was free as well.

I went ahead and put it all together and it works just fine for a month or so then just decides to get weird and lose pressure randomly for about 3 days then goes back to normal again.

I also replaced all the injectors with rebuilt Bosch units at the same time.

Thanks.

Original_MudButt said:

birchlakeXJ,

Can you confirm for me EXACTLY where the check valve is? I have a '98 that has an intermittent fuel delivery problem.

Click to expand...

I believe the primary check valve is part of the fuel pressure regulator. There may be a secondary check valve at the output of the fuel pump itself, but this is not confirmed.

So I decided to go ahead and change the upstream o2 sensor and the TPS and it seems like the hesitation and dead pedal has gone away. Now I just gotta figure out the front driveshaft vibration from hell! I did manage to replace that u-joint that was not machined properly and that helped a little bit but it is still bad around 65 so I am gonna drop the front shaft off today to have the balance checked.

1_Sick_XJ said:

So I decided to go ahead and change the upstream o2 sensor and the TPS and it seems like the hesitation and dead pedal has gone away. Now I just gotta figure out the front driveshaft vibration from hell! I did manage to replace that u-joint that was not machined properly and that helped a little bit but it is still bad around 65 so I am gonna drop the front shaft off today to have the balance checked.

Click to expand...

Glad you are making some progress....

Be sure to let us know if that driveshaft is in balance, and if not, if rebalancing cures the vibe.

will do! Thanks for all the help!