samurai_k

I have been trying to sort out an idle problem. Did the obvious/easy stuff such as new fuel/techron flush. New plugs, wires, caps/rotors. New fuel/air filters. Cleaned the ISV/MAF. New 02 sensors. Checked the cylinder head temperature sensor- ok-> resistance reading with in spec when the engine is cold vs. hot. Checked the vacuum system to hold vacuum- ok -> Start the engine to build vacuum. Turn off the engine, then ignition back on after several minutes. Watched all three vacuum actuators move in the engine and able to move the vacuum actuator behind the CCU by pushing the recirculate button as well.

Finally today I put positive manifold pressure on the intake to check if there is air being sucked in post the MAF.

Found a few leaks. Wondering if there are any experts here to comments on solutions.

What I did to identify the vacuum leaks was with a DIY pressure cap I made with a tire valve stem and 3" rubber pipe cap. Clamped it on the MAF and pumped in 5psi of air with an air compressor.

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Once there was air in the intake, I could easily hear a hiss from the Varioram resonance flap area. Sprayed some soapy water in the area and could easily see big bubbles being created. The seal has perished as I can easily stick a small screw driver in the gap. Ordered a new seal and o-ring so I think I am good to go for this leak.

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Sprayed the fuel injector bases- no leaks.

Sprayed the intake runner rubber gaskets and where it connects to the cylinder heads- no leaks

Sprayed the left and right varioram flaps on top of the intake- small leaks

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Sprayed the FPR (fuel pressure regulator)- small leak

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Can these leaks on the FPR and the varioram flaps be addressed? New FPR? New o-rings for the varioram flaps?

Thanks!

TheOtherEric

I’d be amazed if the amount of air that can leak in through these areas would have any material impact on how the car runs. Every 993 leaks at the resonance flap seal, for example. Unavoidable,

I suppose if the leak at the FPR were bad enough it could cause problems but there’s a Repair Manual procedure for checking fuel pressure.

Endoman

Have a look at the check valve and attached rubber distribution elbow. Even with those small manifold leaks the vac system should be ok. The leaking resonance flap seal will have dumped oil mist over the top of the engine shroud.
https://www.porscherepair.us/porsche...schematic.html

TheOtherEric

I agree (the FPR isn’t tied into the vacuum system), but i think his concern is that bypass air is getting in, after the MAF, thus causing havoc. So the intake is pulling in air through the FPR leak, through its vacuum tube and into the intake. It would be a pretty small amount of air though.

samurai_k

Thanks guys.

On my car the idle is rough. Engine just about dies and then I think the ECU catches it and saves it from stalling. Goes through the cycle every few seconds consistently. When I am on the gas no problems. Engine pulls strong once I apply gas to redline.

I looked at the resonance valve area and it appears a section of the seal has perished as I can stick a small screw driver between the plastic flange and intake manifold. Hoping this is the only thing I need to sort out...

k722070

pretty neat rubber pipe cap strategy for finding the leaks in varioram.
but I'll still spend your money freely and suggest getting a mityvac mv8500, makes life super easy when checking the vacuum system.
and try other crazy things while the car is idling,
like unplug the isv, or pull the vacuum line off the fuel pressure regulator.
try to reset the throttle position sensor, I can't remember if durametric will do that.
or how about this, is it possible the battery was disconnected for a day or more,
and then after reconnecting and start up the throttle cable was too tight?
or you accidentally pressed the throttle pedal?
either of those will screw up the throttle position sensor as the ecu comes online making idle difficult.

samurai_k

Credit goes to @TheOtherEric who shared his version of it here: https://rennlist.com/forums/993-foru...rformance.html

Tonight I read the factory service manual and they reference the test I did in Section 24 page 30 of the Fuel System, Electronic Injection: 24 Checking components of injection system for leaks



I actually have one on my bench and on my do list. Autozone loans the tool which is convenient. I haven't had a chance to do the "leak down" test on the vacuum system yet as I did the visual test before getting the tool.

Didn't think about messing with the ISV/FPR and resetting the ECU... Thanks

ToreB

I can't comment on the engine behaviour.
However, when discussing the vacuum system: By my experience, vacuum leaks are commonly seen in the rear air intake actuator. The diaphragm fails and leaks. Most components work, but the reduced vacuum at engine revving will make actuators to fail to operate.
A typical problem is a "sagging" resirk flap when you accelerate, causing a whooomphfff-like sound and reduced HVAC air flow.
Cheers,
Tore

TheOtherEric

Since you have the Repair Manual, you should work through the various test points that can cause "Erratic Idling." See the big chart in the diagnostic section: Test Points 2,3, etc. That's definitely where you want to focus your attention.

pp000830

When using positive pressure at the airflow intake I would use care to use very little pressure as positive pressure will tend to exhibit leaks from pressuring sealed components and hose overlaps open where during normal operation vacuum would tend to suck those same potential leak points closed or sealed. I would think the oxygen sensors can adjust for quite a bit of air leaks before it results in a lean misfire so I would look for gross leaks only as small leaks on the intake, such as on the manifold, probably are not the root cause. Also, pressure in an area where vacuum is only applied during normal running could potentially blow an o-ring or some other sealing mechanism out of place,
Andy

jrmdir

This is a shot in the dark FWIW. I should probably know this, but do our cars have a basic EGR system with separate EGR valve? On my '77 Targa, that valve was not seating and introduced exhaust gas at low RPM's when it was not supposed to be open. Had a very definite negative impact on idle quality. Again, FWIW.

Ron

samurai_k

Yes. Mine had a layer of oil covering the actuator. I cleaned it and I think removing the gunk may have caused the problems I am facing. Inititially I thought it was the o ring. The intake pressure test confirmed it was actually the seal between the valve and intake manifold.

samurai_k

Thanks. I read through the factory manual again and there are some good content as it explains not only what the procedure is to correct the issue, but also how the component works and why it would cause an error code or issue. Much better than the Bentley service manual!

samurai_k

Thanks. Leak down vacuum system test in order!

TheOtherEric

Vac system is not the problem, so don't waste your time. You can unhook the vacuum reservoir completely thus disabling the vacuum system and the car should still idle just fine. I know cuz I've done it. Granted it will cause some bypass air to get pulled in, but that won’t affect idling.