'98 Jeep Wrangler runs rough when started.

[Jim Spahr]

My dau.s' '98 Jeep Wrangler with 4 cyl. and manual trans sometimes runs rough when first started, then smooths out quickly as it warms up. This 'rough running' doesn't happen when it is a cold start but when it is warm and she restarts it, as when you go into a store and come out and restart the car. It runs rough as though it was firing on 3 cyl. but then smooths out as it warms up. What could cause this? New plugs about one year ago. There is an outstanding Emmisions Recall on the PCM and cat. converter. This hasnt been done yet. I am thinking a bad coolant temp sensor.
A second unrelated issue. The passenger side front seat won't lock in place. It will slide forward and back instead of locking in place. What could cause this problem?

 

[artificialist]

First, get that recall stuff done.

The one thing I can think of is the Idle Air Control motor. Sometimes it doesn't open as it should, causing the engine to feel like it will die out. Sometimes I'd have to put one foot on the gas and one on the brake to keep it from stalling out.

Also, havw you run a can of Seafoam through the vacuum line, and a can of Techron in the gas tank?

 

[Jim Spahr]

I would think the IAC being defective would cause an irratic idle the entire time the engine has warmed upand is idling. It doesn't idle irratically once the engine has started and run for a while.?? What would explain that?

 

[Quest]

While it never seized to amaze me when people jumping into conclusions, My rather uneducated guess would be something to do with engine running rich during startup.

If that is the case, I would go through the usual series of tests such as fuel rail pressure, cold start valve/sensor/injector, spark plugs and wires, ignition, etc. before jumping right into IAC valve.

Afterall: cold start calls for better control over extra rich fuel being delivered into the cylinders so it's still the same old story line of proper provision of fuel (anything that has to do with fuel), air and ignition.

 

[gomes512]

Hmmm my 4.0 did this in my ZJ and as you first guessed, yes it was the coolant temp sensor. However, like stated above it may be fuel pressure related though and as stated checking the rail pressure would be a good idea.

BTW the IAC isn't a wild conclusion at all, those things are notorious for causing all sorts of idle problems on Jeeps. Mine has failed twice (geniune Mopar replacements by the way) in a span of 60K. With the air cleaner off check to make sure it extends and retracts correctly when starting.

I'm not sure if the 4cyl has one or not, but if it is similar to the 6cyl it should have a throttle position sensor next to the IAC in the throttle body. I have seen those act funny when cold and then behave normally after a bit of running. The way to test is when you have it idling step on the gas a little bit and then let off it. If you see it hang for a bit then the tps is on its way out. Also if you depress the gas very very slowly and the rpm's do not change let off the gas quickly and see if it stumbles.

 

[99TA]

Two problems both fuel related you will need to check to fix this trouble.No 1 is the fuel pressure dropping when the jeep is shut off?If it is two things will happen if the engine is hot.1st the fuel will boil in the fuel rail causing a very rough nasty running engine until it clears up.It might also start harder than it should.Another problem could be dirty injectors leaking down fuel when its shut off hot.Starting cold leaking injectors are not much of a problem because the engine needs the extra fuel the injectors leaked out to start.Hot the engine won't tolerate this and it will run terrible until it burns all the extra fuel that was leaked into the engine.I've owned many jeeps,these are common problems with them.Fuel pump check valves leaking back and leaking injectors.You may want to try a GOOD fuel additive such as Lucas or fuel power etc.Then after a few tank fills see if it helps,if not then its time to either test the fuel system or let a GOOD pro do it for you THAT ACTUALLY KNOWS WHAT TO DO.(most don't IMO).