My engine is backfiring once it has warmed up.I've recently replaced all gaskets and did an oil change. Since then, the engine will start sputtering small backfires after running for about ten mins. I replaced the plugs, wires, rotor and cap. Still did it. I checked the timing but that did not solve problem. Running out of ideas. Has anyone got any advice? 1990 bronco xlt 5.0 .

 

Pull the codes.
Check fuel pressure.

 

The last time I worked on a problem like this (running badly after warm up), it was caused by a problem with the mass air sensor. In that case, a mechanic who recently worked on it did not fully push the electrical connector onto it, so it wasn't working at all. When the engine is cold, it does not use the sensor for measuring air flow; it's only when the engine has warmed up that it comes into play.

So check for that, or anything else (oxygen sensor) that gets used once the engine warms up.

 

I pulled the codes using paperclip and got 25 (knock not sensed). While screwing around with the starter and oil pan, it's possible I may have damaged the oxygen sensor. Both can be the probable cause of my problem. Thanks, conanski and xlt4wd90, for the advice. I'll let you know how it turns out.

 

put in a new oxygen sensor, and checked the codes again after hearing some backfires again. With the KOEO I got the code for MAP sensor. Would that be probable cause for the backfiring?

 

I'm guessing your engine does not have a MAF sensor if it's using the MAP sensor. That part does not usually fail on its own. But it's possible that a severe backfire can damage it, since it's connected to the intake manifold.

Is the engine backfiring under acceleration? Other causes can be vacuum leaks or a faulty throttle position sensor. The TP sensor is easy to check with an analog ohmmeter. Connect one lead to the wiper terminal and the other lead to one of the other terminals and slowly turn it and watch the resistance change. It should vary smoothly as you turn the center piece, with no jumps. That is a part that commonly fails on its own.

 

The backfires are small and only when driving slow after driving at normal speeds, like when parking or pulling up to a light.

 

With the engine running check that there is solid vacuum on the line going to the MAP sensor, if not remove the line and verify that the port on the vacuum tree at the intake is clear, if it is replace the vacuum line.. if it's not clear the port with a screwdriver or by removing the tree and cleaning it with solvent.
If you do have good vacuum on this line without having to clean or replace anything then swap in another MAP sensor, a junkyard part will do for a test, if this makes a difference then you found your problem, if not you have more work to do.
Other possible causes for backfiring through the intake are broken or bent intake valves or incorrect cam timing, to test the valves do a compression check, each cylinder should make and hold at least 110-120psi.

 

Well , I fixed it and I can kick myself for being such an easy fix. After seeing all the vacuum lines were cool, I decided to check the exhaust to manifold,and to muffler. There were no donut rings! $ 16 and fixed. Funny how something so easy can be overlooked. I appreciate all the advice/help. I'm in no way a mechanic, but enjoy the frustrations of working on my own vehicles. I couldn't have narrowed it down without the help. Thanks!