Interesting Problem - can I stump this forum

[74goldtarga]

I ran my 951 at the racetrack this weekend in a PCA DE. I have run this same car at the same track many times without a problem. Background on the car - it has 134,000 miles with about one-third of the last 10k being track miles - I haven't touched the engine other than timing belt and water pump (and tune-ups/oil changes) since I have owned it. The engine has Weltmeister chips, wastegate shims and a Lindsey Boost Enhancer.

Here's what happened - I drove the car to the track - 140 miles no problems. In the first session I'm going down the main straight shifting through the gears with full-throttle applied after shifts and running to redline. In 4th gear at like 3K rpm and the dash boost gauge reading 2bar the engine stops pulling - it doesn't buck, the rpm stays stable but my foot is to the floor and there's no acceleration. If I lift the throttle the boost falls off (obviously) and I can then floor it again and it will accelerate to redline like normal. This happened multiple times in a long sweeper where I short shift to 4th (so I don't have to shift mid corner) and then feather in the gas mid-corner, at corner exit I put my foot to the floor - usually this is where the car would basically stop accelerating. This happened multiple times on the main straight as well, sometimes I would have to lift off the throttle a couple of times.

My mechanic thinks it might be a throttle switch, doesn't sound like the clutch because the rpms stayed stable. What do YOU think?

[74goldtarga]

I didn't mention , although it should be obvious, I probably can't reproduce this symptom on the street although it would be fun to try, but possibly on a dyno if need be. I could try some full-throttle runs on the on ramps and such but on the track I don't think it happened below 4th gear.

[chally72]

Maybe the throttle position switch has a dead spot high in the range. There are ways to test this using a multimeter.

[74goldtarga]

Chally - is this dead spot something that would suddenly develop? I have no experience with these, I'm used to things just failing altogether, is it strange that it only happens in higher gears? That is what is making me doubt the throttle switch. I am planning on getting a spare and trying it out, but I think the car will probably run just as it does now - fine . . . except when I am going past 90mph in a corner or 115 on a straight. Thanks.

[chally72]

Usually they wear out in a particular range. Since you aren't often romping on the throttle that high up, it might be that oil, etc, is blowing by the dried-up o-rings for the throttle shaft and screwing with things inside.

I recently took apart my switch, and there was a good deal of oil inside from exactly this. I had excess oil in the intake from a bad turbo, but if you have bad seals for a while I'm sure you aren't doing the TPS any favors.

[74goldtarga]

Thanks, excellent, that's like 3 votes for the throttle switch. Interested if others agree or disagree. I'll probably try to pull the switch later this week and check it out.

[running_cold924]

stock airbox? maybe sucked up some rubber at the track?

[74goldtarga]

Interesting thought but the air filter should have caught any stray rubber I would think. Thanks.

[running_cold924]

not saying it got into the engine......but BIG pieces of rubber the get sucked up into the intake and lay on top of the air filter and hinder flow. if it can't flow enough air, it can't boost properly.

its like trying to run a marathon with your hand over your mouth.

there is a post on rennlist, i'll see if I can find it

[running_cold924]

this is what someone found in their intake:

its off of a post on rennlist called "turbo issue" under the 944 turbo and turbo S forum

[74goldtarga]

That's crazy , definitely worth a look.

[Flatsixhead]

Check all of your vacuum lines.. Just in case.

[KEV951]

how old is your fuel pump?