jeeptrick

ok I have a 1998 Jeep cherokee classic 6cyl 4.0L 4wd. When you first warm up the jeep, after a minute or so you will hear it idle rough to the point it sounds like it wants to cut off. If you tap on the gas a a few times while its doing this it will be fine. If you dont tap on the gas to "clear it" (per say) It will just idle like crap for a few mins and then idle fine. Another example is if the jeep is cold and you just start it up and drive it will not want to accelerate and misfire. I don't have ANY check engine lights on what so ever. I have checked the spark plugs, changed the TPS, changed the IAC, changed the map sensor, had a new fuel pump installed (thinking it was maybe the fuel filter.) got a multi meter and checked the injectors and they all read at 11-12 so they seem fine. but as soon as you let the jeep warm up it runs PERFECT. The rpms arent jumping or anything. I did notice the IAC was pushed in when I installed it today well the pintle or what not. But i think its fine seeing when you start up the car the rpms go high for a second and then go normal (one notch below the 1) when i disconnected the IAC the rpms kept going up. But I dont know what else it could possibly be causing the jeep to idle rough. And the temperature in ct is pretty warm so its not like its Antarctica out here. If you have ANY INSIGHT PLEASE respond.

tjwalker

Finish your tuneup. Change your plug wires, distributor cap and rotor. Tuneup hardware has to be ruled in or out, or you can often chase your tail.....

If still symptomatic, the engine management sensor that I would verify would be the "coolant temp sensor". It is located on the thermostat housing. It provides the computer with input data on the temperature of the coolant so that proper fuel/air ratio can be achieved. This sensor can be tested and the procedure involves a calibrated thermometer and a beaker of water that you heat, etc. but what I generally do is shortcut that test and simply test the sensor cold, then test it hot and compare those resistance numbers to the factory resistance chart for that sensor to see if they are close. You should have BIG resistance swings between cold and hot. Refer to any manual for specifics.

Good luck and keep us updated!

jeeptrick

TJ,

The wires and cap and rotor are about 28 months old. Should I still change them out?

salad

At least inspect cap and rotors. Always go for the premium-type with brass contacts.

The warming up is a clue to me. Since you already changed the MAP I'd look at the Coolant Temperature Sensor and O2s. Once the ECU/PCM decides the engine is warm enough it starts considering the O2 sensors when determining fuel mix. They can either create or fix a problem, depending on the condition of everything else. Generally they only last 80k miles though, so if they're original, definitely consider installing new NTK units. 1998 is OBD-II so both sensors should be replaced.

I'd also recommend ruling out real fuel issues. You can rent a testing gauge and hook it up to the fuel rail. Should give you approx 49 PSI

jeeptrick

TJ,

Im gonna switch out the wires, distrubitor cap &rotor. And coolant temp sensor. The 02 sensors I got a feeling havent been changed. I got 160 on the jeep. Wouldnt codes come up? And would the coolant temp sensor make the car run rough?

salad

Codes don't come up for a lot of things. It doesn't need to be a perfect storm - just the sensors still sorta kinda work. There are codes for shorts and complete failures, that sorta stuff.

The involvement of the CTS is that the engine dumps a ton of fuel in when it's cold, then less when it's hot. If the CTS is reporting the wrong values then the fuel map will be off.

tjwalker

There are many conditions where you can have symptoms but no codes. These are called "escapes" as they escape from detection by the OBD system.

Good idea to get new wires, cap, rotor and coolant sensor in there. The interval (28 months) doesn't matter as much as miles do when it comes to tuneup hardware. As long as you're at it, I'd get 2 new oxygen sensors in there too. Even if they aren't your problem, they are likely not working efficiently. You'll make up the cost in fuel mileage efficiency. As mentioned, I would only use NTK oxygen sensors. Search rockauto.com, etc. to find the best price.

jeeptrick

So I ordered the ngk 02 sensors new plugs, wires, distributor cap amd rotor and coolant temp sensor. That hopefully solves the problem or im doomed. And with my iac it doesnt snap in but is on there prerty tight in. U start the car and it idles high for a sec and goes normal. I disconnected the iac and it reved high so im thinking its ok seeing it idles good. It just doesnt snap in. Well the connector to it.

jeeptrick

So any more insight please feel free to respond

jeeptrick

So I ordered the ngk 02 sensors new plugs, wires, distributor cap amd rotor and coolant temp sensor. That hopefully solves the problem or im doomed. And with my iac it doesnt snap in but is on there prerty tight in. U start the car and it idles high for a sec and goes normal. I disconnected the iac and it reved high so im thinking its ok seeing it idles good. It just doesnt snap in. Well the connector to it.

salad

Holy double post batman.

What do you mean by your IAC "snapping in"? The stepper motor inside is fairly fragile, if you force the pintle it can break.