Misfire on Cylinders 1 and 6

[PNW LSC]

Hey everyone, I've got a '98 XLT with the 4.0 SOHC that has an ongoing misfire issue. When it's cold it runs amazing, with absolutely no problems, but as it warms up it begins losing power and misfiring to the point that the MIL flashes. I've ran codes and found that there was a misfire on cylinders 1 and 6. I've replaced spark plugs, wires, coil pack, injectors, primary oxygen sensors, fuel pump, and fuel filter. So far nothing has helped. I've actually found a few people having this problem, but no one had an answer. Any ideas?

 

[ExploringNC]

100% problem is intake manifold gaskets. More info here.

 

[2000StreetRod]

Misfires

The PCM computes the power of each cylinder and when one is low reports a misfire for that specific cylinder. Typical misfire sources are ignition, fuel injection and compression.

Ignition problems could be due to a bad plug, wire or coil. You've replaced all of these.

Fuel injection problems could be due to a bad injector, pressure regulator, fuel filter or pump. The only problem that is cylinder specific is a bad injector. You've replaced injectors and fuel filter.

Low compression could be due to worn/broken rings, worn or sticking valves, blown head gasket or cracked head. The cylinders that are misfiring have different heads which would make a blown head gasket or cracked head unlikely.

When you replaced the plugs did you notice anything unusual about the plugs from cylinders 1 and 6?

I suggest that you do a compression test before replacing anything else. Don't forget to keep the throttle wide open when you do the compression test.

 

[PNW LSC]

Thanks for the replies. I figured a compression test would be next, just to see what 1 and 6 are doing. The plugs from both were clean, showing only normal color. The intake gaskets were replaced when I did the injectors, in fact the gasket around #6 was pinched when I removed it, but the new one didn't help. It's amazing how well it runs when it's cold, incredibly strong and smooth with no idle issues. There's actually no idle issues when it starts misfiring even.

 

[2000StreetRod]

ECT sensor

Here's another idea but not likely the problem. The PCM enriches the fuel mixture when the engine is cold based on the ECT sensor. If the sensor is bad and reads cold when the engine is warm then the mixture will be too rich and the engine will run poorly. However, if that were the case then the plugs should show signs of fouling and the exhaust should be black. Also, DTCs should get set indicating running too rich.

 

[PNW LSC]

I suspected the ECT, however with it only showing a misfire on two cylinders I ruled it out. I might switch it with a known good sensor from a friends car to test it.