Intermittent Random Misfires, 10%+ LTFT at idle

Joined
Dec 10, 2018
Messages
293
Location
New York City
I've been having this issue for a while. Basically I keep having 10-20 misfires on cylinders 3,4,5 (Northstar V8). The other cylinders have 0 or 1 misfires. They seem to happen at high load. I've tried combinations of new OEM spark plugs and coils, but nothing really changed.

I was also looking at my Long Term Fuel Trims and at idle, They hover between +9% and +11%. When I'm driving at highway speeds or have the pedal pushed down, the long term trims go down to 0 and +5%

I then changed the MAF sensor with a Delphi brand. My throttle response did improve, but I still have the misfires.

Is it the fuel injectors? I looked for vacuum leaks in the obvious spots, but found nothing.
 
Since the fuel trims get much better at higher engine speeds would indicate a vacuum leak somewhere which is causing a lean miss at lower engine speeds.
 
I've been having this issue for a while. Basically I keep having 10-20 misfires on cylinders 3,4,5 (Northstar V8). The other cylinders have 0 or 1 misfires. They seem to happen at high load. I've tried combinations of new OEM spark plugs and coils, but nothing really changed.

I was also looking at my Long Term Fuel Trims and at idle, They hover between +9% and +11%. When I'm driving at highway speeds or have the pedal pushed down, the long term trims go down to 0 and +5%

I then changed the MAF sensor with a Delphi brand. My throttle response did improve, but I still have the misfires.

Is it the fuel injectors? I looked for vacuum leaks in the obvious spots, but found nothing.
I would be looking for air leaks between the air cleaner and throttle body, if it has an accordion those tend to develop cracks in between. You can use a pair of hose pinch off pliers to pinch vacuum hoses one at a time and see if there is a change.
Make sure the throttle body is tight, you can use a propane torch with a hose turned on full but not lit to move over where gaskets seal and vacuum hose fittings, the engine must be one and cooling off if you dont have a smoke tester which is the best thing to use.

The other thing is you may have gotten a bad new MAF, I just recently went through this crap with a brand new Delphi, it failed right out of box (6 miles to be exact), I wouldnt trust that thing as far as I could throw my Volkswagen. Use OE only from the dealer of a known good one, your old one is possibly fine.
 
In addition to the other steps suggested, I would spray test for leaks around the upper intake plenum. Sounds very much like a leaking gasket or seal(s)
 
Interesting given Avoca is a town of 2,264. Might have 500 cars in the town.
I guess he has been getting a lot of calls from Youtube people wanting free advice or trying to book something and then just causing the phone to ring constantly. He says he has plenty of local business.
 
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