tucker.1234

Hi all, I have been trying to diagnose a really weird problem with my 1987 944 Turbo ever since I bought it about a year ago.

Randomly, the car will start backfiring at high RPMs when given WOT or anything close. When this happens, only partial throttle can be applied, otherwise the car will stall. When WOT is given at a higher RPM, it seems like everything dies (I hear no combustion), however the turbo seems to stay spooled (I can hear it whirring) and boost is still applied on the dash gauge. If WOT or something close is given at lower RPMs, the car will reach around 4k RPMs, and then start backfiring (flames coming out of the exhaust every half second or so). The car can be revved out fully if partial throttle is applied all the way through. This happens in every gear. If I turn the key off and back on again, the problem disappears for a couple minutes, and then returns.

I have replaced all my vacuum lines, re-sealed the throttle body, smoke tested the intake system for any leaks, replaced my spark plugs, replaced my fuel pump and filter, replaced the O2 sensor, checked timing, and checked fuel pressure (car was off for this, dme relay jumped). Additionally, I tested the AFM according to the Clark's garage manual, and the voltages / resistances checked out. Also, not too sure if this is relevant, but on my old fuel pump, I could hear a whining noise coming from it after driving for an extended period of time, which is why I replaced it.

One factor that could possibly be an issue is that I'm pretty sure this car has a 1986 engine swapped into it. When I was replacing my belts, I noticed the car had the old style tensioner on it. Also, when I purchased the car, the tach and boost gauge was inoperative. This was due to there being the new style (8 pin) T1 connector coming from the gauge cluster, and the old 4 pin red connector coming from the DME. Due to there being the old 4 pin red connector, I would believe that the DME and KLR are from an '86 as well. I'm so confused at this point, so I'm hoping someone has had a similar issue and resolved it.

tucker.1234

bump

alex_cristocea

did you try to see if you get an error code?

heliolps2

Hey There,

Check you turbo cycling valve.

944M3

Since you mention it’s RPM related, could be bad reference or speed sensors. Check if they are gapped correctly.

Could also be bad wiring. Weird things can happen with these old harnesses. Clean all your grounds. Visually inspect harness for any cracks.

Good luck. Sounds like one of those phantom issues that pop up with these old cars.

944M3

Forgot to mention, since also related to WOT, check throttle position sensor. Clark has instructions on how to test.

Mine was bad and it would randomly over rev.

whalenlg

+1 on the cycling valve.

Mine had been removed in favor of a Lindsey dual port waste gate and manual boost controller. Despite all my efforts, boost would spike to 20+psi until I added the cycling valve, removed the boost controller and routed the waste gate back to a single port.

Now my gauge peaks out at 15psi under wot (I have Huntley stage 1 cheater turbo installed by the previous owner).

heliolps2

Also check your spark plug gaps, I 've had this issue before when i got on boost and I would get blow out from to much gap. It happened on a dyno pull. I didnt know it was a thing.

Oddjob

How much boost are you running, and do you still have the factory DME chip installed?

Toolbelt0

I would pressurize the intake and check for air leaks first. This would be the first step imo.