Ky944TurboNewbie

What would a change in brake pedal feel tell me.

Van

Problem with the check valve to the brake booster.

Van

Once, on the track, I had the rubber boot that connects to the throttle body blow off (hose clamp wasn't tight enough). Under boost it would blow apart enough to be a major air leak, wouldn't build boost or run right - it was a total loss of power.

However, at idle, the suction would pull it all together, so it ran OK. Up to 1 bar absolute, it would run OK, but then under boost it would blow apart again.

Oddjob

Similar symptoms w/ a split on the bottom side of the turbo outlet hose. Split on the opening lap of a club race. As soon a boost came up it fell on its face, coughing, stumbling, belching black smoke. But ran, drove fine off boost. Have seen that same thing happen to other 951s too.

Ky944TurboNewbie

I just did a little cursory look at the car and noticed the clamp holding the blow off valve was loose. I was able to get about a quarter inch of play up and down. I tightened it down and started the car up for a quick test drive.

Seemed to run much better cold that it has in a long while. I might have solved my problem with needing to warm the engine a few minutes before feeling like it won't die quickly. However, I was still having the problem on boost.

This time I was able to get on the highway and trigger the problem. It didn't happen in 1st gear but did in second at about 5200 rpm's. Once it happened, I kept the car running off boost and timed it. After exactly 1 minute, it would allow me to boost again. I triggered the problem again after getting boost back. This time in 3rd gear at about 4500 rpm's. After it is triggered, when I approch 1 bar on the gauge, it stumbles. Below about .8 or .9 bar, it is fine. Does this mean I am back in some sort of overload protection thing again?

Van

You did check *all* of the other hose clamps, right?

Ky944TurboNewbie

So far I have only checked the blow off valve. I am trying to do this between finding a babysitter for tonight and helping my daughter with her school project for black history month.

With a little more looking, I discovered the hose clamps on the manifold that go to the ISV and the brake booster were a little loose. The hoses to the intercooler near the radiator could use a little tightening but weren't too bad. There were some clamps by the fuel vapor purge system that were a little loose also.

I'll take it for another ride in a few minutes. Fingers are crossed.

Ky944TurboNewbie

I haven't found the problem yet. Still cutting off at boost.

Chipster

I had this problem a few years ago. Same indications. If you haven't done so, replace ALL of the vacuum lines you can with new (inexpensive kits are available) including the BOV (they go bad).

Worked for me.

F18Rep

This is a good thread bc I've had a similar thing happen too and never really understood all the things that happen with an overboost. I've seen the blown off clamps and split hoses but there were times when I thought I was overboosting and hitting the protect. One RL'er suggested that the protect was accompanied by a blink code but I never had a chance to confirm that. I made up the little LED reader with a 2 foot pair of wires and then just slipped it under the ws wiper and watched for the event. Sorry I can't say that this works though, didn't for me.
Another idea would be to make up a boost adapter (2" copper sweat cap with a schrader valve threaded into to). It goes into the intake hose for the IC (can't test the turbo hose this way). This is a no engine running test - mostly to identify leaks. Once I fixed my leaks and kept my boost below 15, I didn't see the problem again. I also tried two sets of DME chips and had the situation with both stock and autoauthority... It sure would be nice to see the actual software pseudocode that handles this....Bruce

Rogue_Ant

Start_overboost_pseudocode:

load = Airflow / RPM
overboost_threshold = overboost_array[rpm]
if (load >= overboost_threshold)
then fuel_cut = yes

End_overboost_pseudocode


As you can see, it is not overly complicated... What is important is that the DME (factory software) does not actually know boost pressure. Rather it derives a guess from calculating current airflow and RPMs. So, if there is a false or incorrect AFM signal, the DME could trigger overboost erroneously. A weak AFM spring could do this, as can boost pressure leaks. Furthermore, improving engine volumetric efficiency (or displacement) will also cause the DME's boost "guess" to be incorrect - again possibly causing an overboost situation.

Ky944TurboNewbie

Joshua,

Does the overboost reset after 60 seconds? In my case, it is the engine cuts out and cannot exceed atmospheric pressure (according to my stock gauge) for 60 seconds and then it is normal again.

Oddjob

Yes, that is consistent w/ the factory DME overboost protection. Once it is triggered, it will cut/dump fuel everytime the throttle is opened (more than 75% or so) for a short time period, before resetting itself. 1 minute sounds about right.

Rogue_Ant

60sec sounds about right...

The limitations and draconian response of the factory overboost protection is what prompted me to change it.

944hal

You just described the problem I had to a T. Look on the internet and get a inexpensive aftermarket DME chip and try that. Solved the problem. I had replace the TPS, air box, fuel pump, reference sensors, and had every wire back to the computer checked.

Mine was stock except for 3" exhaust and Lindsey wastegate.