I have an engine miss and would like to have some assistance in diagnosing. The "patient" is my 1999 Jeep Wrangler Sport. It has 85k miles on it and a manual transmission.
The miss exists at all rpm's, though it is most noticeable in the low RPM range (under 2k rpm). If I am in 5th gear and driving up a hill or into the wind, there is a noticeable backfire when the motor is turning 1500-2000 rpm. The backfire seems to go away when over 2000 rpm and only seems to occur in 5th gear. I have an exaust leak, though I am not sure where it is at. Originally, I thought it was coming from where the header and the pipe connect, but I have not yet confirmed this.
Here is what I have done:
1. I cleaned the throttle body
2. I replaced the IAC
3. I disconnected the upstream O2 sensor
4. Checked spark plugs - they appear to be tan in color (all six of them)
5. replaced distributor cap with a working one off of a friend's Wrangler
6. replaced rotor with a working one off of the same friends Wrangler
One of my buddies has expressed that he believes it likely is a "burned valve" - he diagnosed this by using a $1 bill, holding it at the exhaust? (i have not yet checked the compression, which should confirm if it is valve related).
The compression check is on my list to be done, as is replacement of the upstream 02 sensor (just for sake of knowing that was done).
What other steps/ideas do you have? What else could it be?