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Need help troubleshooting an engine issue: (vibration/miss at WOT, low rpm)

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Phoenix, Az
I can't figure this one out (and yes, I know, this is a coyote/roadrunner hybrid). Since I last installed this motor (I've had it out once for rebuild, and once again after the build to adjust a number of things), I've had this weird vibration/hesitation/miss at WOT in higher gears at lower RPM ranges (anything sub 3000 and presumably higher but I'm afraid to stay in it). Car is smooth otherwise, including high RPM operation.

I've checked the following:
-Verify that the motor mounts and trans mount were tight
-Unplugged knock sensors to see if it was timing being pulled (I have monitoring gear, timing did not change with the sensors unplugged) Its possible the banks are flipped due to the harness, but it seemed like it only went on one way for the two sensors
-Switched tune revisions, and try a previous tune from another vendor
-Tested on two throttle bodies

During this time the spark advance is usually between 15-17 degrees, AFR around 12.2. No codes, which is frustrating, coolant temp of 179. Car drives fine all day long outside this issue. No abnormalities on the highway when cruising.

I did find a leak in the inlet tube after the maf and before the throttle body, but I've resolved it and while the engine runs more smoothly at wot, its still breaking up.

The builder believes this could be the clutch having a weird wear issue (and there's merit to the thought as I used a mcleod RXT on a stock flywheel, which wears the overhang 1" or so on the inside of the flywheel disc). However the more I test this, the more I feel like its engine management or something in the motor. It also happens close to wot, but I believe these cars consider over 80% as wot? I am not terribly educated on the subject but wouldn't any sort of misbalance in the clutch or any part of the rotating assembly show itself at more than just wot?

I'm going to clear the memory on the car, reprogram it with the latest tune I have from AED, and do some datalogging. Additionally, I'm going to feed the car some 100 octane (as I just use 91 per the tune and availability) in a blend to see if this is knock-retard.

After logging and sending the logs off, the next step for me should be to take the car to a shop or dealer to have them run it through their diagnostic processes as I have no idea what could cause this.

Anyone have thoughts? The engine build is below:

MMR 1000 shortblock, 11.5:1 compression
Boss 302 heads
Boss 302R exhaust cams (the factory boss 302 cam)
Boss 302 intake
JLT Cold air intake
Accufab throttle body
fresh o2 sensors
stock ford plugs (gap unmodified from box, though I thought the instructions say not to alter those?)
stock coils
AED tuning
stock exhaust manifolds
CJ competition pulse ring
jegs cat deletes
stock mounts

I'm at my wit's end, just can't figure this out. Any help is appreciated, this is the last thing between me and enjoying my car.


My understanding is that any AFR above 12 at WOT is not good. This is what my tuner told me and he got his info from a Ford guy in Detroit. Mine is set to 11.9. Can you read the PIDS for the AFR (commanded) vs. AFR (measured) under WOT?
captdistraction said:
Since I last installed this motor (I've had it out once for rebuild, and once again after the build to adjust a number of things), I've had this weird vibration/hesitation/miss at WOT in higher gears at lower RPM ranges (anything sub 3000 and presumably higher but I'm afraid to stay in it).

Was this problem evident after the first install, or did it develop only after the second install? If it wasn't there the first time, my vote would be something not connected or grounded properly and not tune related.


Too Hot for the Boss!
Couple of good ideas in this thread already:

1. Intake / vacuum leaks. If the intake was off, check all of the gaskets at the intake to head as well as all lines in & out.
2. Spark Plugs - Don't usually see gaps causing issues on NA cars, but if you are going to pull them, check for cracks as well as that can lead to misfires.
3. Lean condition at WOT (dropped / clogged injector? Loose injector harness)
4. Can you swap the JLT intake for the OEM intake? Wondering if the MAF position is getting turbulence or feedback
5. Grounds... check EVERYTHING in the engine compartment as far as grounds and connections are concerned
6. I'm assuming no obvious OBD2 codes... right?

I've seen clutches go bad, and they usually present themselves throughout the RPM range. Same with flywheels.

I really hate these kinds of drivability issues, and they can become very costly if you have to take it somewhere for diagnostics.
Sounds like multiple issues
A vibration is normally caused by a bamance issue
Although a miss can cause what can be considered a vibration also

I would think that if it was i inbalance in the short block or driveline
It would have been obvious from the start

U indicate that it seems to have just shown up after pulling the motor for
Some repairs i would look at the repairs you did
And start there isnt that entire clutch system balanced?
Thats why they supply flywheels when u get them now


Phoenix, Az
a stumble is a good word for it. I can't imagine it being a driveline/rotating issue unless its just something really weird like a pinion angle (but odd given how long I've had my car lowered)

AED wants me to take it to a shop with equipment for looking at misfire counts and check the coils etc.

I have new plugs, I'll check them and install tomorrow and get some video for you guys along with what data I can get from the SCT datalogger software.


Phoenix, Az
So the new plugs come out of the box at 0.051" gap. How does that sound? Should I gap them closer or leave them alone and install as-is? I haven't removed the existing plugs yet.

Here's an interesting excerpt from the Boss302S manual:
Spark Plugs
BR3E-12405-DA (Heat Range 1) or M-12405-M50 (Heat range 0)
GAP= 0.04 (1.0 mm)
Replace after every 6 hours of usage


Because racecar.
Arizona, USA
Note for anyone buying plugs--both RockAuto and Tousley sell the Motorcraft SP519s for around $5.50 each, about half of where I can see them anywhere else.
I'm no expert but those look perfect to me.


Cracked plug. Could be the culprit, but the ECU wasn't throwing any misfire or timing codes so....not sure.

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