misfire diagnosis??

[stiltner1972]

ok guys so my ohv 4.0 has had a dead miss at idle for a year or so, now ive noticed it getting worse. i did some testing had about 165psi give or take 7 in all cylinders but the second one back on passenger side it was sitting at about 90 , so i had the bright idea to pull my cai off and run open throttle body, this revealed that im getting serious "pop-back" at WOT so im wondering if i dont need to pull it down put a bigger cam in it and a new set of heads? because also my valves ratttle like a slot machine, i always run 93 octane and on occasion ill run a 75/25 120 octane mixture oil is always changed at 3000 with royal purple only mods are straight piped back in to one 4 inch tip and a cai

 

[malohnes]

What condition are the plugs? That should help you locate the dead cylinder. Severely carboned plugs can cause misses. I have a continual issue with my LIM bolts getting loose and leaking coolant and oil and fouling the plugs on #5 and 6 cylinders. CLeaning the fouled plugs brings the performance right back up to normal. My new heads don't seem to make a difference when it comes to the LIM bolts issue.

 

[arco777]

Try to isolate which cylinder is misfiring. There are several ways to do this. One way is mentioned above, checking for a spark plug that is not like the others. Here are two other methods which may be useful:

#1. Run a cylinder balance test. This follows a KOER test and begins a test mode to determine which cylinder is contributing the least. The PCM will spit out a code indicating which cylinder is weak, or all cylinders are even. The test can be run three consecutive times, with different threshold values used to determine how weak a cylinder is.

#2. Use an infrared thermometer pointed at the exhaust manifold at each cylinder's exhaust exit to determine which is coolest. All should be about equal temp but if one is cooler than the rest, that cylinder is not firing. If you do not have an infrared thermometer, you can squirt water onto the manifolds to check it the same way, but it is less precise by far.

Once a weak cylinder is identified, it is as simple as checking the basics. Fuel, compression, and spark.

 

[stiltner1972]

Im to tired to think about what number is weak i know its the passengerside middle plug ... compression is magical.. yesterday i could only get 90psi but long behold today im seeing a steady 146 psi... wth is goin on with this thing