It's a little hard to describe but under full throttle my engine flutters, like it has a slight miss/constant power hesitation. It pulls hard but the acceleration isn't smooth, fluttering is the best word I can think to describe it. There may be a "tingy" noise as well, but I'm not sure what I'm hearing.

I don't think it is fuel related as I'm running 93 octane and octane boost, plus by HP tuners shows 0 knock retard.

My gut feel is that it is spark related. I have GMP Red wires and iridium plugs so I suspect it might have something to do with a coil pack(s)?

I'd like to tune full throttle but if its spark related I'm unlikely getting complete burn in a cylinder(s) so its going to show up extra rich. I'm intentionally rich (~10.0) as I have a supercharger (I had this problem before the supercharger) but I don't want to try to dial it in until this problem is resolved (obviously).

This problem is more pronounced sometimes than others and sometimes it has gone away completely (rarely)

My engine mods:
98 A4
heads/cam/intake/exhaust/TB/supercharger.
injectors/plugs/wires are new, coils are old.

The car does seem to have a slight miss, so its very likely related. How should I go about diagnosing this issue? Changing plugs and wires are one thing, but replacing the coil packs could be expensive...but I'd do it to get this problem to go away.

Thanks for the help!

 

Any thoughts? Could it be that one or more of my coils are failing under load at high rpm?

A better way to describe it is a light surging under load at higher rpm.

I have dart heads with beehive springs is it possible I'm getting some valve float? 5000 rpm and up?

Any thoughts would be much appreciated.

 

Anybody?

 

I'm not sure valve float would be an issue at only 5k rpm. You said you put in new injectors, do they flow enough to support the S/C? Possible they could be going static at high RPM, starving the engine. In my experience, I've only once solved an issue by changing a coil on a car, the post was badly corroded and it had issues all through the engine range.

I don't have much experience at the level you've modded, I'm just a lowly shade tree mechanic, but I thought I'd give you my stab in the dark since nobody has responded yet. Good luck, those little quirks are really irritating and often hard to track down.

 

Thanks for the response. Yeah, I was hoping it was a problem with coil(s) but from my reading it sounds like those rarely fail.

The injectors are 60# injectors and the pump is a LPE pump. I didn't suspect the injectors because the problem existed prior to the new injectors...but I suppose the problem could be the same (going static at high rpm).

If its not the injectors, coils, or valve float what could it be. The light surging/flutter is throwing my AFR off and its useless and likely dangerous to attempt to tune WOT AFR....for example, if I'm dragging a dead cylinder at high rpm (no spark) then trying to dial in the AFR would result in a extra lean condition, even though the AFR is being reported as 11.5...as the dead cylinder is influencing that number.

If a coil is failing at high rpm under load would I get any codes?

 

 

I'd say you should start by testing the ignition, injectors, and coil underhood fuses to make sure you are getting the correct voltage. Per Bill Curlee:

Turn ON the ignition and check the voltage on the ignition/coil/injector fuses and make sure that its FULL battery voltage

If its a lot less,, thats an issue with the IGNITION SWITCH output

 

I haven't had the chance to test the voltage on the items you mentioned. However, does it sound like its an electrical issue?

It doesn't always surge under WOT but it does more often then not. It had this behavior prior to the Intake/TB/injector/SC install as well, but after the Heads/Cam install.

One thing I'd like to mention is that my screen was removed from the MAF for additional flow...could this be it?

The light flutter/surge under WOT is the best way I can think to describe the behavior. You don't have to concentrate to feel it happen but its definitely not a holy WTF sensation either.

When the behavior is more pronounced my AFR is more rich, not sure if this helps isolate which bank is responsible (as the WB is on the driver side bank).

Is it possible I have a weak valve spring or a lifter that isn't bleeding off fast enough and holding a valve open?

I appreciate all the help on this!

 

Another lowly shade tree mech checkin in. The presence of a supercharger scrambles my thought process for what ails you, however, could it be something as simple as during install of new injectors, plugs and wires, you may have disconnected a vacuum line? sam grasping at the cheapest straw

 

There are a few number of vacuum lines on the motor,I'll take a look as see if any of them may have come disconnected. Although I might suspect an idle issue if that was the case...but you never know.

 

Well, I have read numerous posts regarding this issue. Removing the MAF screen can cause the same type of symptoms that you describe. However, I don't have any experience with SC cars. I am assuming that you are running the OEM MAF? Correct? I have heard nothing but bad things about the aftermarket ones.

If I were you, I would try swapping out the MAF with one that has the screen. Hopefully, you can track one down just for testing purposes. If it solves your problem, then you can buy a new one. They aren't very expensive.

 

The reason I didn't think of the MAF at first was because some model years don't have screens in the MAF housing. I wonder if the design is different for the oem screen less MAF?

 

I’ve had a similar issues with motorcycles, both were grounding issues. Also once with my 68 Camaro, fuel filter leak. If it seems to have a slight miss, couldn’t hurt to go over the grounds again.

I’m new to the corvette world, so don’t judge for the stupid suggestion. Can you easily put a fuel pressure gauge on and monitor it during high rpm to see if it starts to taper off?
If the supercharger is belt driven, could it be slipping?
Does it do it only on acceleration in high rpm? Or just high rpm?

Stupid question: Have you determined that it occurs as a function of engine speed and not vehicle speed?

 

An ignition problem can cause the hesitation you describe. It's an easy check that costs nothing to perform, so why not rule out the easy stuff first?

 

Yep, I have a fuel pressure gauge. No loss of pressure (58psi through redline).

Its the pro charger bracket so its likely the belt is slipping, however, this problem existed before the supercharger.

The problem seems to only occur at high RPM at WOT, so its happening under hard acceleration...at least thats only when I notice it happening.

Function of engine speed..under load.

 

I'll test it tonight.

 

Voltage checks out.

 

Were it me, and I knew that the fuel system was good, I'd have a look at the wires first since they do see a lot of heat. Are any of the wires touching the headers? Could have damaged the insulation and arcs to the header under heavy load (might be the noise you're hearing). If there's nothing obvious wrong, I would grab a known good wire and swap one at a time out and test the car. I would also pull the plugs and have a good look at them. You might want to do the same with the plugs and swap one at a time out.

If there's nothing found there, I would think about getting a single coil and swap them out one at a time.

Are you not getting any codes thrown? Seems an intermittent miss would be detected.

My daughter's Ranger PU (I know it ain't a vette, high performance or supercharged) threw a code. Turned out to be a plug with one of the iridium tips missing. Guess she got lucky and the tip got spit out the exhaust.

 

I'm guessing you are not seeing any p300 codes for misfires. You mentioned this happened right after the heads and cam install. If it was right after that then recheck all the wiring and grounds that you took off on the swap. If all that checks out you very well might have valve float if you didn't change springs as well when you did the cam.

 

Scan the current misfire counters. As some "tuners" disable the P0300 misfire code. And if the car is tuned in speed density mode, the misfire counters just stay at 0.

The tingy noise is probably spark knock as I've seen some "tuners" disable knock retard.

Russ Kemp