skoob

I have a 99 with 175,000 miles.

Frequently, after I've been driving about 40 miles on the highway, it will start jerking/surging and the rpms jump back and forth every second or so between 2250 and 3250.

Then it might run fine, but it will be running at 3250. Then it might sort itself out for a few mins, then it starts all over again.

It's happened when it's 50 degrees out, it's happened when it's 100 degrees.

It's only at 65 mph+ and only after about 40 miles. It will last for a few mins, then be ok for a few mins.

It happened earlier this spring once or twice, then once a month, now it's happening more frequently.

Any ideas?

Thanks

tjwalker

How many miles on the oxygen sensors? Surging is a common symptom of a failing upstream O2 sensor.

Also would be a good idea to verify fuel pressure. You should have a steady 49 psi, plus or minus 5 psi.

Any check engine light? If so, you must pull codes. Even if you have no light, it might be a good idea to
check to see if there are any "stored codes" that might yield a clue. Need a code reader or a scan tool
to do this on a 99.

skoob

Hi,

any O2 sensors are original to the car, so 175k miles.

The engine light did come on for the first time yesterday. I'll check it.

Thanks

tjwalker

You are lucky that the check engine light came on. Absolutely read those codes and post them here for comment. Those codes are invaluable in figuring out your issue. Anything less is guesswork.

With 175k, those oxygen sensors are lazy at best. Even if they are not the cause, replacing both of them would not be a mistake if you plan on keeping the vehicle. The fuel savings will pay for them in pretty short order. I use NTK oxygen sensors where available; they are the oem supplier of oxygen sensors to Jeep.