MC951

I am looking for some help on a long standing issue with my '86 944 turbo. I have owned the car for 8 years and have been battling this issue since I bought the car.

The car setup is a stock motor/turbo with the factory afm, 3" exhaust, no cat and currently running 951max chips for the setup and a lindsey boost enhancer. The motor is fresh off a rebuild after a blown head gasket (had 180,000 miles on the original). The rebuild was done keeping things stock except for a few reliability items...widefire head gasket, steam vent kit, baffled oil pan and that's about it. The engine runs great and dyno'd at 245 hp at the whells with 15lbs boost. The car is street registered but mostly used on the track in DEs and TTs.

During the rebuild, all hoses that needed replacing where done, and the wiring harness recieved a good going over but was not replaced. It did show some spots around some of the sensors that needed some tlc, but everything seemed ok with it.

Here's the issue...occasionally, under wide open throttle, at around 4500 - 5000 rpm the car hits what seems like a boost cut off although with the chips I'm running 15 lbs should not be causing an overbooost condition. I have replaced the chips (originally autothority stage 2), tried stock (with stock boost), and even bolted a known working maf with chips on and still get the same condition. I recently borrowed a known working dme and klr, tried those and same result. The strange thing is that it doesn't happen all of the time. When allowed to go through the revs, the car is awesome and just as I want it.

Yesterday on a test run, cruising in 3rd gear, I floored it, ran up to about 6000 rpm, shifted to 4th and on to about 5000 rpm when it cut off. This after 4 or 5 successful tests without it cutting out. I immedietly backed off and rolled into the throttle at around 4000 without much boost on and it cut out a number of times just easing on the throttle. I backed off, shifted into 3rd and tried it again. This time the car reved right through to 6000rpm in third and on through to 5500rpm in 4th without a cut out.

I'm certain the the chips and the dme/klr are not the issue as i've tried numerous combos without any change. There are no boost leaks.

What else should I be looking at? I love the way the car gets along when it doesn't encounter this issue.

One thing that I've noticed on the track is that it might cut on the first lap out, but once the engine gets hot it goes away.

I've had more than one qualified mechanic look at things with no resolution yet and I don't want to just through parts at it.



Thanks for looking

mudbuddha

This may not be of any help but it sounded like an electrical wiring issue and my other thoughts would be fuel related so may be new fuel fiilter and/or fuel pump? There was a thread or two some time ago somewhere regarding random boost cut out etc...

MC951

fuel filter and pump are new...I'm thinking electrical/wiring too but wondering where to start on that monster. I've searched other threads and they haven't lead me to a solution.

mudbuddha

Fuel injectors are good? Is the battery in good shape, strapped down tight and connections are good...sometimes under heavy accel or pulling some g...the battery would move just enough to cause a drop in voltage. Could be a ground problem too. Also, does the car gets stored in or out doors---moisture could also be an issue. I am just pointing out some areas that can be examine further and hopefully someone else can chime in with a solution.

JET951

How old is the Throttle position switch? We have has TPS do very similar symptoms
Sean

User 52121

Well a quick 'n dirty way to rule out for sure if it's overboost cut-out would be to simply disconnect and plug the vac line that runs to the KLR. If the KLR sees no boost, then it won't activate overboost cut.

Before you do that - do you have a real boost gauge in the car, so you can be sure of how much boost you're actually getting?

Jfrahm

What does the tach show? Can you rig up a noid light and a test lamp on the coil to look for fuel or spark cutout?

Rogue_Ant

Overboost fuel-cut is a DME operation.
The KLRs overboost response is only to lower boost.

MC951

I haven't checked the injectors and they are one item that is the same since ownership.

The battery is not the issue, it's newer, tight and connections are good. The car is stored in doors.

The TPS has been replaced once during my ownership a couple of years ago.

Where should I be checking for ground issues? I talked to my mechanic about that possibility and he thought that they would have noticed any ground issues when they had the motor out.

User 52121

Interesting. So boost cut still happens by inferred boost levels then? (RPM vs. AFM signal)?

I thought the KLR triggered it. I guess I stand corrected!

MC951

yes


The tach falls off. I had a fuel pressure gauge on the rail one time when on a dyno and it showed fuel pressure falling on its face when it cut out.

Rogue_Ant

Yes, for stock & stock styled software, overboost is calculated in the DME from an airflow / rpm function.

eman930

What about your knock sensor? Is it good, Is the wiring good, Is it torqued crrrectly?

MC951

one item that I thought of while reviewing the RogueTuning.com site is that I'm using the MSD blaster coil which according to their site you should change the charge-time data in the DME software to avoid being needlessly harsh on the DME transistor and wires...could this be something to look at?

michaelmount123

There's a big clue. Suggest you follow that.