Mysterious WOT surging X-brand

More X-brand mysteries... this time my 04 cobra

Car ran fine for couple years with this setup, took crank pulley off over winter to rebuild and now is down on power with some surges of acceleration that 'almost' feels like it used to. I checked pulley alignment and found an offset between the crank and the other pulleys, corrected the pulley offset and it changed nothing. Brought it to a dyno for some pulls and also to verify the A/Fs on passenger side (my wideband is on drivers side) are still good.

Well the A/Fs are fine so thank god don't have a lean cylinder on my non-monitored side, but graphs say it all, low power and surging wildly. Below is graph and below that is datalog from SCT during the pass, wideband on dyno is passenger side pipe, wideband in SCT is drivers side.


Full symptoms:
-Low power overall except "sometimes"
-Airflow maybe slightly less? Hard to judge with the high temps we've been having
-Car idles lower and smoother, almost like there's more load on the engine
-Idle vacuum also lower, about 17.5 inches at 850ish (20ish normally, but also used to idle at like 900-950ish due to single blade TB)
-Idle vacuum does recover to 20 or slightly more with SC belt off
A/Fs at WOT are the same, short terms are right where they used to be

To date:

- I've run a compression test, all cylinders are decent, #1 is down by maybe 10ish psi
- We did a dyno pull with the cap off my catch can and blow-by was negligible
- Re-gapped my NGK TR6s to .028 from .032
- Swapped TR6s for brand new NGK BR7EF gapped at .028
- Swapped MSD coil pack set in the car for spare OEM set
- Swapped BACK to TR6s gapped all the way down to .022 to verify not a spark issue
- Re-aligned crank pulley as described earlier
- Had several different belts on it


0 changes from any of this. The car doesn't break up when it surges, it just feels like someones holding the brakes and letting off randomly, this only happens at WOT. Car runs great, runs as cool as usual, doesn't use a drop of coolant or push any out of the overflow.

Next things to do:
-Maybe refine crank pulley alignment as there is some angular mis-alignment between it and the SC
pulley
-Would like to pull valve covers and make sure no broken springs or popped off rocker arms...

Have you guys seen valve springs cause this issue? Or considering it's a 4 valve engine, maybe 1 rocker arm popped off or 1 broken valve spring cause this?

Does it still have cats? If so I would check to see if they are starting to fail/clog.


Can put a pressure gauge in place of a primary o2 sensor to test backpressure.

Where the Haze ?

No cats... so no exhaust restriction

California storage unit...

Belt slipping at all? Is it making the same boost as before?

Is timing steady or does it follow the surging?

Did you get kicked off the Ford sites? We all love tech but if I had a small block mopar problem id not go to another brands sites members for the fix, Id go to ones who know the platform.

Why not get the haze out and get back to mopar?

I'm thinking (and hoping) it's belt slip, boost seems a tiny lower than normal but it's not dipping or surging significantly, you can see it in the dyno graph.

Timing is rock solid, at least according to the ecu

I did post on the various ford places... gotta say particularly the mustang crowd is the lowest IQ crowd I've dealt with. Mostly crickets, some people that know what they're talking about but has most people baffled, so I'm putting it out to all my resources to try and see if anyone has ran into something like this.

Cuz the haze is a massive and expensive project...

Lean

Nope, have a look at the A/F traces on the datalog and dyno graph, solid low to mid 11s

What is the IAT?

Joe

90s

It was perfect until “took crank pulley off for rebuild.” ? If so I’d retrace that. What is a crank pulley rebuild? Could diameter change? I don’t know, just asking to understand. Do you mean like a dampener rebuild? I’d be looking around that area for a loose connector. Anything you could’ve bumped to cause an issue. Recent work is a great clue when chasing down an issue.

F or C?
I would think a Ford Forum would help you a lot more than on here
Ish nex verstain un sprekineze Ford
and I can't spell German words worth a hoot either

Ya that's where I've been focusing, first on pulley alignment but I might have a good lead now... balancer might not have been pressed on fully causing wobbly reluctor...

https://www.youtube.com/watch?v=MX-WySODB14

That makes sense, so the timing really isn't steady if the crank position sensor isn't reading the reluctor wheel correctly. I don't know if there is an external timing mark on your balancer but I assume you could draw a TDC line on it anyways and throw a timing light on it if you made a little coil on plug jumper for it.

Had the same idea :P But also... alot of work

Sounds like you’ve got a good lead. I was trying to think back about the 4.6 fords I’ve had and I thought they have a crank sensor that read off the dampener. Been quite a few years for me. Sure seems like that could be your issue.

Just wanted to post a resolution to this:

Root cause was stupid ATI balancer had too much press, did not seat against the reluctor causing it to rattle and wallow easily a new keyway width in both directions... so during accel probably had significantly retarded timing until it bounced forward and gave some advance. Also giving ford [censored] for this too as who uses a stupid stamped steel reluctor that engages on the keyway anyways, this is why other modern v8s like the LS's and Hemis have a large pressed on reluctor as part of the crank.

Nice .