Odd misfire - EGR Ports

[tomj76]

I've had this add misfire problem, and possibly have resolved it.

The vehicle is '96 Windstar with 3.8L engine.

The misfire occurs when driving on a level or slight downhill highway, usually at ~60 mph. I doesn't occur if traveling 70 mph in the same situation. The misfire could not be felt but did cause the CEL to flash. Once the misfire happens it does not reoccur for a relatively long time.

The misfire code is P0301, and no other codes. The freeze frame data says that the engine is running slightly rich (negative LTFT). There is a small load on the engine (<25%) and the throttle is nearly closed.

This started to occur a few months ago at the end of long trips (400-500 miles). During the last trip I took it happened very consistently under the above circumstances plus the misfire started immediately (within the first 10 miles). I did notice what seemed to be a lack of power immediately after the CEL began to flash.

I checked the fuel pressure on the fuel rail (to determine if an injector is leaking) and it maintained pressure for >15 minutes. I also checked the #1 spark plug for gap and fouling. The gap was a bit large, although it matched the other plugs and appeared clean. I reset the gaps on the plugs that I pulled.

Given that the #1 plug is difficult to access w/o removing the upper intake manifold, and knowing that the EGR ports can clog with hard carbon, I removed it to check the plugs. The EGR ports were thoroughly clogged so I cleaned them.

After re-assembly there was no misfire on the test drive, which was designed to drive very consistently at 60 mph.

I have experience misfire due to clogged EGR ports before, but this normally occurred when climbing a slight grade ~1-3% and the misfire could be felt.

While I don't know for certain that the problem has been eliminated, I wanted document this odd one for forum readers.

[wiswind]

Sadly, the only way to clean EGR ports is to remove the upper intake manifold and physically clean them.
One TEMPORARY test that you can do is to remove the vacuum hose from the EGR valve and see if that solves the problem.....this will cause the CEL to come on.
If the problem goes away, you know that the EGR system is the cause.....ports or the EGR flow Sensor (EGR valve very rarely fails).
Of course, you have to wait until the problem gets bad enough in order to do this....
Do NOT leave the EGR valve disabled.....this is ONLY a test......so don't forget to re-connect the vacuum line to the EGR valve.

If this does not clear the problem, dirty fuel injector (Berryman's Chemtool) or coil pack......if the spark plugs and wires are fine.

[12Ounce]

My 465k mile '99 SEL has pretty-much clog-free EGR ports ... because I did a bit of re-engineering on the old girl long ago.

The modification: Mind you, this is only possible with the earlier two-piece plastic upper intake manifolds. The plastic upper is separable by un-clipping the inner manifold away from the plastic shell-cover. I separated the two pieces then broke away all plastic clips ... both male and female. Cleaning all surfaces thoroughly, and using grey RTV, the plastic inner manifold was bonded and sealed to the plastic center intake manifold.

This modification keeps oil from dripping so freely down against the EGR ports and gumming them up. The ports remain accessible for the very rare cleaning that they will now require. BTW, access to all bolts below is maintained with this modification.