'99 5.0L Misfire Mystery

[Zeleske]

Hi all, I've gotten help too many times to count by reading other posts in this forum, but now I'm stumped.

5.0L '99 Explorer, 219k

I have misfires on cylinders 2 and 3. The fuel trim is quite high on bank 1 (where the misfires are) - hovers in the 20's consistently, I've seen it as high as the 40's in the cold with some gas. Bank 2 stays well centered around zero.

What would cause such a high fuel trim on bank 1? With the fuel trim history cleared, and fresh spark plugs it seems to run fine until the trim rises and then it seems to misfire rich.


Here's what I've been through so far:
New plugs
New plug wires
Destroyed a coil pack while goofing around, so a a new coil pack too

Then called a buddy certified master mechanic who throws out ideas without much action:
Compression check passed
Fuel pressure was in spec
Spark check was weak, plug seemed fouled. A new plug sparked strong for a short time until it got fouled by so much fuel
Replaced upper and lower intake manifold gaskets

Today I switched the upstream O2 sensors between bank 1 and bank 2 (thanks to the post about pulling the center console), problem stayed on bank 1 - so not the O2 sensor.


I'm stumped. All I'm thinking is that maybe that lower gasket didn't go on quite right, or there is another vacuum leak somewhere that would touch only bank 1?

 

[pcam13]

I would say vac leak. Try spraying around with carb cleaner anywhere you replaced gaskets as a positive fuel trim indicates lean and the PCM is adding extra fuel to compensate.

 

[delexploder]

Any codes?

 

[SoHK_alumni]

underfueling/clogged injectors?

Try getting a set of injector o-rings and (maybe) a upper intake gasket.

Swap the nos 1 and 4 injectors with say 5 and 6 and reassemble.

See if the problem follows the injectors.

 

[Zeleske]

Forgot to mention, replaced injectors on bank 1. Didn't really want to, but got frustrated and got some refurbished ones.

I'll try the carb cleaner again. I could never got much of a reaction out of it, but I'll give it another go.

Thanks guys.

 

[SoHK_alumni]

Time for a noid light kit. Make sure injectors 1& 4 are getting triggered open.

 

[blakshukvw]

I know when I did my lower manifold gaskets the clips that lock each injector connector to the injector broke when squeezed to release so I had to zip tie every one of them to the injector body to ensure they don't come off.

 

[Zeleske]

Yeah Blank, all mine broke too. I also zip tied, great minds!

I'll look into the noid tester. Would an injector not firing, or firing inconsistently, drive the lean code and then push up the fuel trim so other cylinders misfire rich?

 

[boominXplorer]

Plug wires? The reason the fuel trims are rising is because of the unburnt fuel coming from the cylinders.

 

[Turdle]

I wonder if there is a slight amount of coolant getting to the spark plugs and fouling them. Any coolant loss at all?

 

[Zeleske]

Not a noticeable amount of coolant loss before. I haven't driven it much in the last month, so I can't be sure now. If it was, it wasn't much - doesn't coolant burn a characteristic color (know I read this but didn't regoogle just now).

Any likely place the coolant could get in? Lower intake gasket?

Boomin - I replaced the plug wires. With a spark tester the spark was quite weak, then I replaced that plug and it was nice and strong, soon faded.

I might get some time to take a look at it again on Wednesday...