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Q: The truck intermittently, has a very rough idle, and runs rough,usually after warming up.

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The truck was running fine until a fill up a few weeks ago. Shortly after I filled the tank, it started running rough and idling rough. I ran some sea foam, and then fuel injector cleaner through it. Then changed the fuel filter, and now have changed the spark plugs. I had changed the plugs previously and wires at 100k. The truck seems to run fine when cold, but once at operating temperature, the idle becomes very rough, and sluggish on acceleration. I have also cleaned the M.A.F., the EGR valve, and checked the pcv valve. Motor appears to change and run better if I disconnect the MAP, but not convinced that's the issue. The fuel economy has been about 15mpg for some time, down from 19 mpg when new. Had a mechanic put on a test, and the reading was first a bad O2 reading on cylinder 4. Then, cylinders 1,3, and 5. Sometimes, the Check Engine light blinks. It did cut off (the light) at first when I changed the plugs. O2 sensors have never been changed.

My car has 172000 miles.
My car has an automatic transmission.

A: Hello. This can be caused by a number of di...

Hello. This can be caused by a number of different things. If this occurred right after you had gotten fuel then I would start by testing the fuel pressure. I would do this when the vehicle starts running poorly. It is possible that the bad fuel contaminated the fuel pump and it is not putting out the right pressure. If the fuel pressure is fine then I would check the other sensors readings with a scan tool. One other common thing that I have seen on this engine is a leaking intake manifold gasket. It starts leaking on the bottom side and that leak causes the MAF and MAP to not read correctly which causes it to run like this. If you would like to have this rough running condition looked at, consider having a YourMechanic certified techniciancan come to your home or office to diagnose why it is running rough.

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