Jeep hesitates soon after starting

August 26, 2007 8:51 PM

I just bought a '98 Jeep Cherokee. It runs great except commonly within 5 minutes of driving it hesitates during acceleration, sometimes accompanied by a popping sound under the hood. After it recovers it drives fine.

It never dies, but it just doesn't accelerate however I press on the gas. The popping sound made me think of spark-plug related stuff, but it all looks pretty new and I reset all the wires.

I googled the best I could, and some forums mentioned TPS (throttle position sensor), or cleaning the throttle body.

It always starts right up, and runs fine until I accelerate from a stop.

Any ideas?

posted by chadparker to travel & transportation (8 comments total)

If it has the inline 6, check for a leaking or cracked exhaust manifold. It's a common problem on the jeep 4 liter engines in the Cherokees and Wranglers, might make the popping noise you describe if it is bad enough.
posted by Forktine at 9:34 PM on August 26, 2007

Also, I've had very good luck with this Jeep forum; there are many others, as well.
posted by Forktine at 9:37 PM on August 26, 2007

Might be completely useless to you, but this:

http://www.cartalk.com/menus/info.html

These guys have a radio show on NPR where people call in with car problems. It can be a funny show but these guys really know what they are talking about. A little outlandish of a suggestion but hey why not?
posted by deacon_blues at 9:39 PM on August 26, 2007

A splutter when cold, accompanied by a pop (a muffled backfire in all likelihood), is usually an indicator of a mixture that is incorrect for conditions, probably too lean.

On an older carburetter engine, this would be a choke problem, but I imagine the Cherokee is fuel injected, which might mean a sensor is feeding the wrong info to your engine management system, leaning the mixture before the engine has warmed up adequately.

It would be worth plugging a code reader into your OBD II port and seeing it if comes up with anything.
posted by maxwelton at 4:12 AM on August 27, 2007

Hesitation under load followed by a bang sounds like a backfire, generally caused by a spark plug firing when the inlet port is open and causing all the fuel in the inlet manifold to explode. My guess is that you have leakage between the spark lines, which usually occurs at the distributor.

Take the distributor cap off, clean it very thoroughly in soapy water, dry it perfectly, check for hairline cracks (carbon will build up in the cracks preferentially) and reinstall it without getting it greasy. Obviously you must put the plug leads back on the distributor cap in the same order... and it can't hurt if those get a good wipe-down with a clean damp rag then a dry rag to get rid of any dirt buildup.

If your distributor is arcing internally, it will leave a thick carbon deposit all over the inside which conducts and causes the wrong spark plugs to fire occasionally. Less voltage is required to fire a plug at atmospheric pressure than at full cylinder pressure, so if there is a resistive connection between the distributor terminals, the WRONG cylinders will fire by preference and it will backfire.
posted by polyglot at 4:18 AM on August 27, 2007

Could be a sticky Idle Air Controller (or whatever is equivalent on the Cherokee). It's basically used to control the same function as the old choke used to. As it warms up, it doesn't actuate smoothly and you get the hesitation.

If you're going to check forums though, go with the Jeep-specific one and not so much the CarTalk one. CarTalk is okay, but their forums are hard to search and have a lousy interface. Also, the people answering the questions are not Tom and Ray, the guys you hear on radio. It's whoever gets a logon ID and posts answers to questions. If you're going to do that anyway, stick with a group of people already familiar with your car; go with the Jeep forum.
posted by Doohickie at 11:21 AM on August 27, 2007

I had a similiar type symptom in my jeep when there was bad mix in the gas tank. I must have gotten the bottom of some station's tank (the consensus advice I got) and in the fill-up took in some bottom tank churned particulate matter and water in the gas. The matter clogged the gas filter and caused some hesitation . I guess the water messed with the air mixture and didn't combust properly, not driving the pistons properly.

If you dont want to spend on a mechanic diagnostic visit try changing the gas filter and getting a fuel additive.
posted by Kensational at 1:33 PM on August 27, 2007

it was the oxygen sensor.
posted by chadparker at 12:42 AM on September 27, 2007