Smithmotorworks

1986 951 , aftermarket chips (cyntex, I think), manual boost controller, open exhaust, stock MAF, stock FPR, turbo S turbo,

Here's the problem: At 15lbs of boost at about 5k rpm the car cuts out. its like someone turns the key off then back on ,etc. - Fuel cut?

The fuel pressure and volume are good - rises correctly with boost
The air/fuel stays in the 11's
Flywheel sensors are new and I verified the air gap is adjusted correctly.
I have checked the TPS (new) and the MAF ( tested,and swapped with known good just to be sure) and Knock sensor. I have checked all grounds and harness. There are no boost leaks. I also swapped DME and KLR with Known good computers. However same chips.

So...

Is it possible to "over run" the stock MAF ?
Don't all aftermarket chips remove the overboost code?
Am I just seeing fuel cut from overboost?
And if so would a different set of chips cure this?
Am I just missing something?

Thanks

Cory9584

mine used to do something similar when the diverter valve wasnt holding pressure well

Smithmotorworks

Forgot to mention that

Checked, working correctly, but replaced with billet diverter just to be safe.

Thanks

xsboost90

my car did that w/ the stock chips- you need a klr chip that gets rid of the boost cut- the engine starts pulling too much air through the afm to make boost and tells the car to cut boost. I put in my 951max chips and i no longer have that. Ran 18.4psi today w/ no problems and perfect air fuel.

PorscheDoc

I would pull the DME and KLR and verify if it has chips. That does sound like overboost protection kicking in, and the RPM range is about where it happens.

TheRealLefty

Agreed with Doc. Symptomic of the stock chip overboost fuel cut off, which usually doesn't happen until about 5K fourth gear because, at full throttle, you're usually shifting out of first, second and third before the safety mode is triggered.

xsboost90

mine would hit that thing in 2-3-4 getting on the highway...guess i drive too fast to work. It would not do it on the track except for certain times usually in the middle of a sweeping corner or the end of a long straight- just die then come back on w/ no boost. Nothing instills confidence in your instructor like reaching up and turning the ignition switch on/off during a fast run.

RKD in OKC

Doesn't the overboost keep you from making boost for about 30 seconds after the overboost cut out?

A loose engine ground wire will cause the key on/off thing under hard acceleration and when you get in the throttle on right hand turn exits. But comes right back on to full throttle full boost.

xsboost90

the over boost would cut out then only let you run up to like 5psi boost after that until you reset the ignition switch. I could get 15psi for a couple seconds then i had to let off or it would cut off completely then only 5psi again.

eniac

First thing is to verify what chips you have exactly. With unknown chips you really should not be going over stock boost in the first place.

Second, it's the DME chip that controls the boost cut off not the KLR.

When you turn the boost down to 13 psi, does it still cutout? If not then it would seem to be a function of the DME chip, aftermarket or not. For stock chips this is normal behavior and not all aftermarket chips raise or remove the cutout.

xsboost90

really...car i just worked on has a killer cut off around 4k when boost comes on- if you let off the gas it acts like nothing happened. Loose ground wire huh. I did just have the bellhousing off but i thought i cranked that puppy down tight.

Van

Dan, did you ever solve this? It sounds similar to what I'm seeing.

fast951

The Overboost protection is part of the DME. The DME signals the KLR to take action! (We use the stock KLR and run as much boost as you want!, The KLR is not the issue if you are hitting overboost).

The stock DME NEVER sees actual boost as (psi), it uses a calculated load value based on air flow. Basically, you can trigger the overboost at the same boost level (x psi) if you have a high flowing turbo.

If you go back to the mid-late 1990s, the problem you describe with the hardware/software you are using is a well known problem.

Once the overboost kicks in, the car will stumble if you hit the gas. It clears itself if you recycle the ignition or around 60sec after it happens.

Turn down your boost, it should clear up. Email if I can assit!

Spidey944

My car is currently doing the same thing. High RPM, full TPS signal, Full boost. Goes right back to normal if throttle is let up, but if RPM's are increased to same level again, same boost, same issue. I have never got it to cycle into overboost protection at all. I have tried with 12 PSI and 14 PSI with no change. I guess I could go lower....

Looks like it's time to call John....

fast951

If I read your description correctly, you are having issues at a certain RPM vs. boost. (assuming no boost leaks), it might be a issue with the ignition system, which I would check as well. Bad plugs, cap/rotor, plug wires, Speed/Ref flywheel sensors can be problematic.

Of course, if you lower the boost to 10psi, and the problem is solved it may not be your ignition system, but a boost related problem. (Ignition can still be a issue as under boost the ignition system must work harder).