I have a friend who does not get on the internet much but has a problem with his stroker A4 C5.

When he goes WOT the car stalls out and then backfires. When he restarts the car everything is fine, <b>NO CODES</b>. The car idles fine and will perform normally at cruise or 75% of wot, but at WOT it stalls out and backfires. At the first occurrence, I thought it was the timing chain letting go, but that is not the case. The plug wires are fine and the valvetrain sounds good. ZERO codes have been found. The only thing identified so far is that one bank may leaner than the other. I do not know how lean as of yet, but I was told it was a significant amount, not the .87 versus .90 stuff. It may be .85 and .99+. The problem occurs everytime the engine hits WOT now. The engine has been professionally tuned (put out about 480rwhp) and has been performing well for about 1000+ miles.

Any ideas????

thanks for any opinions

 

Have you monitored the fuel pressure during WOT? sounds like it could be going to low.

 

not yet, I will remember that

ttt

 

ttt

 

Sounds like what my car was doing last year.

I bet right after this happens, the car starts right up and it's fine, no SES codes or anything. You experiment by finding a long stretch of road and try to see how much throttle you can give it before it happens... for me it was like 5500 rpms and higher. Sometimes when this happens, the car is still running but that's rare. It does not happen all the time. There is a split second between the moment the motor cuts out and the car backfires... Sound like your buddy's car?

I had the same problem.

It's one of 3-4 things, but they are all related.

1) Crank sensor is damaged. I replaced this twice.
2) Crank sensor wire is burned by the #4/#6 header primaries and the crank sensor is grounding out. I redid this.
3) Reluctor wheel on the crankshaft has lost it's crush and is moving around.
4) Too much crank endplay (highly unlikely). I checked this.

Let me continue. Over the last two years I only found two other folks who had this problem. One guy cured it by swapping out his engine control harness. They think that he had a pulled wired to the crank sensor. Other guy had a bad reluctor wheel.

I had a bad reluctor wheel, it had .030 sideplay and .125 endplay. It had lost it's crush and when the car went WOT the reluctor wheel would move and move out of range of the crank sensor. When the sensor doesn't get a signal for a few seconds in a row it will shut the engine off.

Now, you can try replacing the crank sensor. You can also buy a new pigtail connector from GM and redo the crank wire from the pcm to the sensor. I did both of those things twice with help from my friend Russ. I have an extra pigtail connector and sensor if you want them.

If the car continues to do it it's more than likely mechanical, ie. the reluctor wheel. The reluctor wheel can loose it's crush if it was removed and reinstalled, or if it comes in contact with something inside the motor and it gets kicked loose.

Other things I would check:

-Loose grounds, the 3 in the back of the driver's head, the big one off the alternator. I redid them all.
-Bad regulator/ alternator... Not likely the cause unless the battery is practically dead. The engine will run with substantially low voltage based on my research.
-Crank endplay... Not likely. GM specs call for .002 - .008 endplay, but most of the Lunati aftermarket crank assemblies are in the .005 - .015 range for endplay. I found cars with .020 - .022 endplay that ran fine. Worst case the thrust bearing will be worn, maybe in a stick shift car.
-Losing major fuel pressure. My research said that you'd have to have a huge drop in fuel pressure to shut the car off, like from 58 psi to like 20. I found folks who's fuel pressure would get into the 30's and the cars would not shut off they would nose over and run lean. I changed this too.


If you want to call me to discuss this, feel free, just email me. I can give you the cell phone number of a guy in Louisiana who spent a few hours with me trying to chase this problem down, and he's a Caddy service manager to boot. He was the first example I gave, they changed the harness out.

I did try that but the problem would come back. I can only surmise that the reluctor wheel was getting progressively worse.

This problem sucks and I threw a lot of money at it, and there were like a ton of a folks who tried to help me god bless them.

prostockjohn@hotmail.com

 

Thank you very much for your detailed response John, it is very much appreicated. I will pass this helpful info along.

Thanks again