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· Registered
109 Posts
Discussion Starter · #1 ·
I recently started having problems while trying to boost above 0.5bar. At that point the car has started to stutter and doesn't accelerate nicely anymore. The problem has gotten progressively worse during last ~500-1000 miles.

The car now also seems to idle quite badly sometimes (warm & cold). When it doesn't idle well, it sounds as if only 5/6 cylinders were working so one cylinder may be constantly missing for a few seconds, then it works again, and then not ...

I started to solve the problem by changing spark plugs, which didn't help. Next step was testing if I have boost leaks. I did the test by blocking several pipes as described in this picture: and pressurizing the entire system up to 1.2bar.

During testing I found out two things:
1) my cam covers pressurized -- compressed air started leaking out from oil filler cap
2) there is a leak somewhere in the underside of IAC valve, maybe its gasget is leaking. The leaking sound becomes easily noticeable at 0.5 bar, but the leak seems very small (while tested by wet hand).

For 1), I suspect that I have leaky valve stem seals. Last summer my car would puff smoke from time to time while idling. After changing oil this spring, I haven't seen any smoke puffs, however.

What else might allow pressure to get into head covers? Do you think it is possible that some of my valve stem seals is in such a bad shape that it is causing all of my problem symptoms?

Anyone else ever had IAC valve gasget leak? It might also be something else near bottom of the IAC valve.

The car is a 1994 6-speed Targa with 105500 miles and:
- new stock turbos, changed ~2000 miles ago (no problems in first 1000 miles after the change)
- hks vpc (on default settings)
- apexi s-afc2 (on default settings)
- hks dli ignition
- GReddy 3-ROW front mount intercooler
- K&N air filter
- Blitz Dual Drive Blow Off Valve
- Lightweight Flywheel
- Blitz Turbo Timer

I already ordered new valve stem seals and IAC-to-intake manifod gasget. Any suggestions what to do/troubleshoot next other than changing both of these?

Thanks! -tomi

· Alpine Hardtopper
1,306 Posts
It's always good to replace teh stem seals if they need it. But that would have to be a HUGE leak to cause boost problems. Like"spyhunter" smokescreen kind of huge leak. I would recommend breaking the test area in to a few chunks. The common problem is leaks at the IC or IC piping. Make a plug that fits on the turbo outlet and another that fits on the throttle fitting and pressurize that section. If everything is fine there, look elsewhere.

You could also do a compression test, or a leakdown test on each cylinder to see if you're getting excessive blowby in to the crankcase.

I can't tell you anything about the leak by the IAC valve. It doesn't sound good though. Definitely get the new gasket in there.

Another totally different place to check would be the coils and sparkplugs. Supra coils have a tendency to go out, and any time there's higher boost it's a lot harder on the coils.

· SupraForums Sponsor
1,461 Posts
Tomi, please email me pictures of your engine bay, particularly the intake manifold area and vacuum hose routing. I want to see the hose configuration for the VPC MAP sensor. Also, scan your ECU trouble codes and let me know what you find.
The vacuum leak needs to be fixed, and if it's coming from under your IAC valve it could be either one of the two hoses going to the IAC valve, one of the two hoses going to the pressure tank, or one of the two hoses going to the EVAP VSV, as they are all in that general area.
Make sure that you have new coil pack wiring harnesses/clips so the coil packs have a good connection, change the fuel filter if you haven't already, and use a bottle of fuel injector cleaner with about 3-4 gallons of fuel (even if it isn't the problem, these things cause common problems and should be done regardless.)
Give me a shout once you've done these things, and we'll figure the problem out. See ya buddy.

· Registered
109 Posts
Discussion Starter · #4 ·
Thanks to you both! Before you replied (Jeremy), I had forgot your earlier advice to check the 1200OHM resistor that's related with the VPC install.

What I know now:
- I've read codes 24 and 42 from the engine
* 24 afaik means that ECU doesn't get the measured amount of air flow from the VPC
- I measured the 1200OHM resistor and it was ok
* tried to make a better contact for it to the wire harness a few times but that didn't help

My best quess is that the stuttering starts when my ECU switches to open loop mode (=when it starts ignoring lambda and tries to use amout of air flow). Do you have any advices on how to troubleshoot the VPC system further?

Maybe I should measure the resistance from ECU side of the wiring harness next? And probably try to get some voltages out from VPC & S-AFC II?
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