starionesir

I recently bought a 98 grand cherokee 5.9 limited and I have been fixing all its issues and it just started doing this a couple of days ago. When you first start it up it will idle for about 2 minutes and then start to pop and miss. You can hear it backfiring in the intake manifold. I hooked my matco scanner to it and told it to log the live data and when I played it back the intake air temp sensor was all over the place. So I replaced the sensor and it still does the same thing. The weird thing is it will not stall out and if you let it go it will stop on its own. Once it gets to about operating temp then it stops and runs fine. I have checked all the sensors on the logger and the oxygen sensors also act up at the same time as the intake air temp and also the map sensor. I don't have any dtc's stored in the computer. I'm a import mechanic so I'm not really used to working on american vehicles. If anyone has any info or ideas I will greatly appreciate it. Thanks in advance.

Bustedback

Hows the coolant temp sensor reading?

starionesir

Coolant temp is good. It has a steady rise on the graph to operating temp. I really hope Its just a sensor and not a bad computer. Its just so weird that when its missing and backfiring I can rev the engine and it stops what its doing and if the engine is warm enough then it stops missing all together. Its only on cold starts not once the engine is warm? Maybe the heated O2 sensor not working properly? I just hate to keep replacing sensors and that not fix the problem. Thanks for the input though.

starionesir

Also how many O2 sensors are on the 5.9 engine. My scanner only comes up with a bank 1 upstream sensor and a bank 1 downstream. So does this engine only have 2 sensors? I'm used to mitsubishi and anything bigger then a 4 cylinder has 4 sensors 2 before the cat and 2 after.

Bustedback

I think it has two if it is fedral emissions.

E-Rok

if you are a mechanic, replacing sensors without knowing what the problem is shouldnt even cross your mind

starionesir

Thats why I posted on here to see if someone else has had the same or similar problem. If this was a mitsubishi or toyota it would have already been fixed. The problem is it is not throwing any codes and I'm not sure how touchy chrysler products are before they throw a code. Or course it would be much more helpful to offer help then it is to just post and make a worthless statement. And also as far as the temp sensor goes according to my matco reader the temp was all over the place so that is why I replaced it.

starionesir

From reading the chrysler service manual I found the O2 sensor could not be the problem because on open loop the pcm does not look for O2 sensor input. I'm going to have to look into the map sensor and run the test thats in the manual. Too bad my scan tool can't perform the same functions as the chrysler drb because it would make my life much easier. Anyone else have a map sensor act up without setting the check engine light?