Timing Question

For a couple of months now I've had an erratic idle. it will drop or jump a couple 100 RPM sitting at a light. Once in a while (1-2 x a week max) when I put in park after driving for a while, it will jump to 1200-1300 rpm and stay there until I put it in drive or shut it off then it starts and runs normal (except for the idle fluctuation).

Today, just for giggles, I put the OBDII scan tool on there and it said the timing was anywhere from 11* to 18*...

Thoughts?

1998, 4.0, 110,000 miles, new plugs cap &rotor about a year ago. New air filter today. No codes....

TPS is the first, primary suspect, but a vaccum line or intake manifold or throttle body gasket leak, or stuck or sticking IAC can cause a high idle, or a bad CTS or IAT sensor,or leaking fuel injectors on your year.

Clean the Throttle body and the Idle Air Controller. Use a bottle of fuel injector cleaner. Makes sure all the tune-up parts are fresh. OBD-II, the base idle is set by the distributor orientation and adjusted by the ECM. Any changes you make in the base idle is overruled by the ECM.

Ecomike said:

TPS is the first, primary suspect, but a vaccum line or intake manifold or throttle body gasket leak, or stuck or sticking IAC can cause a high idle, or a bad CTS or IAT sensor,or leaking fuel injectors on your year.

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Thanks for your help. I forgot to mention that I replaced all the injectors about 9 months ago with rebuilt Bosch multi-ports. They worked perfectly when I installed them.

I put a vacuum gauge on the manifold port and I can see no indication of anything wrong.....

Tim_MN said:

Clean the Throttle body and the Idle Air Controller. Use a bottle of fuel injector cleaner. Makes sure all the tune-up parts are fresh. OBD-II, the base idle is set by the distributor orientation and adjusted by the ECM. Any changes you make in the base idle is overruled by the ECM.

Click to expand...

Thanks Tim. I know the timing can't be changed by me, I just thought it was odd that it would jump around like that at idle. It's all over the place from 11*-18*. Seems odd to me. I was hoping somebody could tell me if this is normal or not.

I'll take the advice given and clean the throttle body and check into the IAC. IAC seems to make the most sense at this point.

I'm just wondering if the timing chain isn't stretched....

Thanks all....

I'd like to add for you to freshen your grounds till they're nice and shiny. Every sensor on your jeep relies on 2 ground points. It they're corroded very strange things start happening.

Same thing with the battery terminals.

dukie564 said:

I'd like to add for you to freshen your grounds till they're nice and shiny. Every sensor on your jeep relies on 2 ground points. It they're corroded very strange things start happening.

Same thing with the battery terminals.

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Thanks.... New battery as of three months ago, but I'll add the ground points to my list of things to check.....

Especially the sensor/ECU ground near the oil dip stick on the engine block. It grounds the battery, the engine starter and the ECU and sensors. It and the battery ground are essential, or starter current/voltage will feed back and confuse the ECU. Also do this one:

http://www.naxja.org/forum/showthread.php?t=1072534

Ecomike said:

...... Also do this one: http://www.naxja.org/forum/showthread.php?t=1072534

Click to expand...

If you mean TPS adjustment, 1996-2001 OBD-II TPS is non-adjustable.

Tim_MN said:

If you mean TPS adjustment, 1996-2001 OBD-II TPS is non-adjustable.

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This is what I have been lead to believe....

Check the MAP sensor on the firewall too. These have been known to cause erratic idle and stalling. Start with the vacuum lines. The hard plastic lines can split causing a leak. Same with the rubber connector ends.

While your Jeep is running, lightly spray some carb cleaner on all your vacuum lines. The key word here is lightly. If you have a leak the carb cleaner will be pulled into the intake system and you rpms will increase. If a leak is discovered, simply replace what ever hose is leaking.

The thin walled hosed on the charcoal canisters are prone to cracking. I would not be surprised if you find a vacuum leak there.