98 XJ idle problems

[timmyxj]

ok, have major idle issue.

98 xj 5 Speed maanual. 4.0L 179,000 miles

idles poorly without any kind of regularity. i can go days with problem and days without. it will all of a sudden start to lose RPMs, down to 500 or so, then rev itself up to 1500, down to 500 up to 1500, etc. it does this several times, decreasing intervals until it stays at 1200 or so, and then process starts all over again. will do it driving down the road, and when the accelerator is pushed to the floor, it will run fine, no matter what gear I am in. (a side note, i do have a small fuel leak, but idle problem has been persisting for 3+ months and fuel leak issue only recently.

fuel pressure stays constant through good and bad idle, battery is 4 mos. old, (interstate), alternator is 1 yr old. (No starting issues at all though, will fire everytime) replaced the IAC, cleaned out TPS, replaced the crank sensor, replaced computer with friend's XJ, who has no issues with his, fixed all broken/damaged vacuum lines. up stream, and downstream O2 sensors are new along with catalytic converter and muffler. fuel sending unit (pump, and filter together) came out of salvage yard from jeep with 81,000 miles replaced, plugs/wires/cap/rotor/distributor are all new.

I want to post a video of it idleing to help the diagnosis...anyone know how to do so? is it with simple HTML? ahref..etc?

anyone experience similar issues? been doing some looking through the forum and have a few other ideas im going to test, but would entertain some direction...thanks!

[Mudderoy]

ok, have major idle issue.

98 xj 5 Speed maanual. 4.0L 179,000 miles

idles poorly without any kind of regularity. i can go days with problem and days without. it will all of a sudden start to lose RPMs, down to 500 or so, then rev itself up to 1500, down to 500 up to 1500, etc. it does this several times, decreasing intervals until it stays at 1200 or so, and then process starts all over again. will do it driving down the road, and when the accelerator is pushed to the floor, it will run fine, no matter what gear I am in. (a side note, i do have a small fuel leak, but idle problem has been persisting for 3+ months and fuel leak issue only recently.

fuel pressure stays constant through good and bad idle, battery is 4 mos. old, (interstate), alternator is 1 yr old. (No starting issues at all though, will fire everytime) replaced the IAC, cleaned out TPS, replaced the crank sensor, replaced computer with friend's XJ, who has no issues with his, fixed all broken/damaged vacuum lines. up stream, and downstream O2 sensors are new along with catalytic converter and muffler. fuel sending unit (pump, and filter together) came out of salvage yard from jeep with 81,000 miles replaced, plugs/wires/cap/rotor/distributor are all new.

I want to post a video of it idleing to help the diagnosis...anyone know how to do so? is it with simple HTML? ahref..etc?

anyone experience similar issues? been doing some looking through the forum and have a few other ideas im going to test, but would entertain some direction...thanks!

It really sounds like the TPS (throttle position sensor). If you had an automatic and the shift points were all screwy I'd be sure. The IAC (idle air controller) can cause problems as well, but on mine it just keeps the idle up about 1200 rpm, and I tap the gas petal and it drops to the 700 rpm norm.

BTW there is a funky situation with the wiring harness rubbing on the fuel rail near the firewall. It made my 98 act just like a bad TPS. After replacing the TPS for the 2nd time and the problem persisted I searched and searched finally reading about the very obscure problem. To fix it I just used a big tie wrap and pulled the harness up off the fuel rail!

Get some other opinions on here but I bet it is the TPS.

[xj4life2]

First disconnect he battery and let her sit for a 1/2 then reconnect and drive it. This will reset the comp. the driving will let it re-learn. If it continues remove and clean the IAC (idle air control) after reinstall it should rev to 3k or so then smoth out and idle normal. I would certianly test the TP as tony said but to me it seems more of an IAC issue. Oh do you have a check engine light on ?

[4.3LXJ]

I agree with this initial step. This will rule out the computer. Also I would suggest removing and cleaning the IAC. If it straightens out then that was the problem. If not, then the TPS needs attention.

[timmyxj]

The IAC is brand new. Ill try the battery disconnect and get back. There is a check engine light is on, ill have to write down the codes.

[prcjeep]

I say TPS... Ive probably replaced mine about 4 times in2 years of ownership... I had all the same symptoms.

[Mudderoy]

I say TPS... Ive probably replaced mine about 4 times in2 years of ownership... I had all the same symptoms.

I'm getting all new Chrysler sensors! I'm tired of rolling the dice on these after market sensors.

[BlueXJ]

No one has mentioned the one thing I always check on an XJ. Vacuum lines either broken, flattened or disconnected. Vacuum plays an important role in how the several sensors get their data to send to the ECU. So they bear checking no matter the year. They are more relevant to the Renix years but in the later H.O. years they are still quite important.

[Mudderoy]

No one has mentioned the one thing I always check on an XJ. Vacuum lines either broken, flattened or disconnected. Vacuum plays an important role in how the several sensors get their data to send to the ECU. So they bear checking no matter the year. They are more relevant to the Renix years but in the later H.O. years they are still quite important.

This is very true. I didn't mention it since I haven't had a vacuum issue (yet) :love0007:

[timmyxj]

Replaced the TPS, worked great idleing for 1/2 hour and 2 trips around the neighborhood, then shut off and went to the store an hour later and 5 miles down the road, same things happening. Haven't tried the battery disconnect yet, started to rain.

On the vacuum line issue, I forgot to mention, I did replaced 4 broken vacuum lines. Luckily, a good friends of mine owns a shop and a leak detection device and made it super easy. going to try the battery disconnect and let computer relearn. And then after that...rag in the gas tank and a zippo are the next cheapest parts to buy!

[prcjeep]

I just realized that I stopped having these issues after changing my spark plugs... Even after several TPS sensors and just assuming that I had an undiscovered vacuum leak once i changed out my plugs(It had bosch platinums[no bueno for our 4.0's]) and put in Champions I havent had any issues in 6 weeks

[Mudderoy]

Replaced the TPS, worked great idleing for 1/2 hour and 2 trips around the neighborhood, then shut off and went to the store an hour later and 5 miles down the road, same things happening. Haven't tried the battery disconnect yet, started to rain.

On the vacuum line issue, I forgot to mention, I did replaced 4 broken vacuum lines. Luckily, a good friends of mine owns a shop and a leak detection device and made it super easy. going to try the battery disconnect and let computer relearn. And then after that...rag in the gas tank and a zippo are the next cheapest parts to buy!

Did you raise the harness off the fuel rail?

[timmyxj]

I did not raise the harness, forgot about that, zip tie?

Spark plus are AC delco (I think...) 6 months or so old. I will try new ones, they're in expensive enough..

[LizardRunner]

Timmy are you still having the problem?