!990 Jeep Wrangler Sahara Mystery Problem!

What can be the cause of my Jeep starting and idling fine one minute and then the next time I start the car the idle stays around 100rpms and if you touch the gas the car shakes, then after a few minutes all of a sudden jumps to 700 rpms and runs fine? This problem will also occur randomly while driving,rpms will suddenly drop and then after a few minutes jump back up…

In a 1990 I think the possibilities include a failing fuel pump or fuel pressure regulator, a failing igniter or distributor, an erratic (failing) coil, or even a flaky injector (if it’s a throttle body injection) or carburator (if it’s carbed).

If it’s a carb, the problem will probably be in the float bowl, either a sunken or hung up float or a flaky needle valve. But other possibilities include a bad accelerator pump (the pump provides a spray of gas to start without which an engine will stumble at startup) or even sticky linkage for the high idle cam.

You could even have a vacuum leak.

Can you provide more detail as to under exactly what conditions it stumbles? Or is it random?

It will stumble most frequently after the car has been driven and then let to sit for a 1/2 hour or more,however,again randomly. The stumbling while driving is completely random.

Does this Wrangler have the L4 or the L6 engine?

It has the L6 and has a carburator which was last replaced 3 years ago. Some more background info: car has 87,000 original miles, problem began after 85,000 mile service (6 months ago) at which time the oxygen sensor and fuel filter were replaced. Car has been looked at numerous times, however, the problem has yet to occur when in the shop.

Those new details add the possibility of a coil becoming heat soaked. When you shut your engine down, the heat in the interior of the engine and in the exhaust manifiold slowly dissipates out the sides and heats up the space under the hood and all the peripheral components. Underhood temperatures rise. It probably takes about 1/2 hour for temps to peak and begin to cool.

I’m going to change my mind and suggest getting the ignition system on a scope. Perhaps even with a heat gun to assist (used judiciously).

And when the car has sat for 5 to 6 hours and starts fine, but then stumbles within a mile or 2 of driving?

Coils that have become heat sensitive won’t typically begin to makfunction until the engine bay heats up. Yet in the original post you describe symptoms typical of fuel delivery problems.

You may have both carburation and ignition system problems. The Jeep is 20 years old. My grown daughter was a toddler when the Jeep was made. Perhaps it’s time for a good going-over of both the fuel and ignition systems.

That work that was done 3 years ago, was that a rebuilt carb that was installed? Why was the work done? What were the symptoms back then?

For the carb, start by checking first thing in the morning to see if you have fuel in the float bowl. This is a simple process wherein you remove the air filter, prop the choke open, look down the barrel with a worklight, operate the throttle linkage by hand, and see if gas sparays into the carburator. This checks to see if the float bowl is full, checkes the accelerator pump, and affords an opportunity to check the high idle cam for operation.

I’d be inclined to change that filter they put in too. Just in case.

For the ignition system, the best way I know to check it is on a scope. You’ll need a shop to do this. With a heat gun to check the coil for heat sensitivity. Bring it in hot to give them a good starting point.

The crank angle sensor was flakey on these models I think. I would also check for possible fuel delivery problems.

It was a rebuilt carb and it was no longer able to be tweeked to pass emissions testing.

On a different note, thanks for all your input to date!!

With just dying out, without what one would associate with running out of gas, this should be ignition/electrical in nature.

First check the wiring. Wiggle the harness while it’s running good. See if it reacts.

Problem solved…it was the ignition. Thanks!!!