Hi everyone. I have a 1992 f250 with a 351 Windsor and it has an intermittent misfire. Let me start off and say the truck will run fine then act like it drops 3 cylinders. I just did head gaskets so I have replaced a lot of items. They are plugs, wires, cap, rotor, ignition control module, of course all other gaskets related to the heads being removed. I first thought it was a fuel problem because it would run strong then misfire so I dropped the rear tank and all was fine. Checked the fuel pressure and it was within limits.It will misfire on the front or rear tank. So I replaced the fuel filter. Still same problem. I've checked spark, and compression all good. It doesn't matter weather it's hot or cold. Been running for 5 minuets or 2 days. Also when this happens it starts to smoke real bad and smells like fuel. Once it runs fine there is no smoke. I have made sure the plug wires were put back in the same place to prevent crossfire. I've also tried the old plug wires and still have the same problem. There is no fuel smell from the vacuum line on the pressure regulator and the fuel pressure is good when its misfiring. Any help would be appreciated? Thanks Alan.

 

i had a simlar problem in my bronco the egr valve got plug and the o2 sensor came apart and the little jumper for adjustimg the timimg came apart some how caused to to load up and miss

 

O2 sensors are good and all the ignition components are connected properly and in place. I'll have to check the egr. I herd it cycling but it might be cycling at the wrong time. But this misfires so bad it almost doesn't want to stay running. I don't think the egr would make it misfire that bad.

 

Yes it can. If the EGR is open when it is not supposed to be then it creates a lean condition. A lean fuel mixture can result in misfires.

Have you checked for codes? You can have a stored code or other condition without triggering a Check Engine Light (CEL).

 

Ok checked the egr and it was working. I even manually activated it and it was not the problem I was experiencing. It had a rough idle but it would even out when I gave it gas. The problem I have is it is like it dropped 4 cylinders and it doesn't matter if you are on the gas. I did the test to get the codes and they were 173 hego shows system always rich, 327 evp or dpfe circuit below minimum voltage, 328 egr closed voltage higher than expected, 334 egr closed voltage higher than expected, and 634 mop sensor voltage out of self test range a/c on.

 

These two things are probably related, the MAP is a critical component in this EFI system and it will throw the computer for a complete loop if it's out of range so swap in a new one and make sure the vacuum line is connected and supplying a constant vacuum with the engine running.

All those EGR codes means you probably have an erg position sensor(EVP) fault.

 

I unplugged the map sensor while the truck is running and it just changed the idle and ran a little rough. Nothing like the problem I was having. Of course I can't get the truck to do it now so ill have to keep trying.

 

I tried to wrap my head around this problem when i had a 351 swap in a mustang, my "miss" was actually a heavy surge in the motor due to a bad MAF and a sticky valve..