3.2 Carrera Smoke Test

[tx63conv]

Earlier this year, I had an issue where all of a sudden, my car would stumble past 1100 rpm. Idles fine at 900. Searched and searched and eventually had it sent to a local air cooled shop. After additional hours of searching, mechanic found a cracked vacuum hose.

Last Wednesday, took the car on its weekly trip--no issues. Thursday evening started stumbling again. Barely got it up the driveway. Started again over the weekend. Same issue--idles fine but stumbles when revving.

In an effort to avoid another costly bill, I would like to try a smoke test first. Where is the best place to introduce smoke into these motors? Any other thoughts/suggestions?

I've watched a couple of videos and read some items but I've never performed this kind of test.

[NotStock]

Not sure what is best, but I'll tell you what worked for me. I highly recommend smoke testing. I found a bunch of leaks on my 3.2, some in very unexpected places (crack in thermo valve thing on back of throttle body for example).

Anyway, I pulled the vac line for the fuel pressure regulator and fed smoke into that. I found that the air box had a few tiny leaks that were making it difficult to be 100% sure I'd found all the problems so I yanked the air box and made a 1/8 aluminum block off plate and bolted it where the air box mounts.

Don't pull the brake booster line and feed smoke in there. Mine leaked really badly at the connection to the tunnel line and I wouldn't have found the problem if I'd used the booster line port.

[sparkysvega]

Check the idle switch . It sounds like it’s randomly working , wich will cause the stumble .

[HaroldMHedge]

I made an adapter that mounted to the mass air flow meter using the 928-110-201-01 gasket as a guide.

With the air filter/cleaner housing removed and injecting the smoke just before the flow meter you test the whole system.

[Spetrow]

Fuel regulator and fuel damper vac lines are easy but if you have cruse control, the smaller vac can be an excellent place to connect to.

[NotStock]

Originally Posted by HaroldMHedge

I made an adapter that mounted to the mass air flow meter using the 928-110-201-01 gasket as a guide.

With the air filter/cleaner housing removed and injecting the smoke just before the flow meter you test the whole system.

I like this idea a lot. If I had been thinking it would have been really easy to drill and tap the aluminum block off plate I referenced for a hose barb. I did the same thing and traced the gasket.

[tx63conv]

Thanks for all the suggestions. Any recommendations on smoke tester or did you make your own? Were each of you chasing down problems or was this more preventative--I assume you were chasing down problems.

Originally Posted by sparkysvega

Check the idle switch . It sounds like it’s randomly working , wich will cause the stumble .

The car idles fine all day. The stumbling is when I rev it above 1100 or so....I thought the ICV only controlled the idle circuit.

[Spetrow]

I used https://www.amazon.com/gp/product/B07JVT1NG2/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&th=1 at LT $100 it works great. No flow meter and cannot change smoke pressure. I blocked off MAF with and electrical 4x4 plate drilled with hole pattern from gasket. Pressurizing MAF is the best.

[sparkysvega]

Not the ICV . The idle micro switch tells the dme when the throttle body opens and comes off of idle . Mine was sticking closed intermittently and the car stumbled so bad it was undrivable in the rpm range you are experiencing. To check it , you just need a multimeter . Disconnect it and check that it makes and breaks contiinuity when you open and close the throttle body . You should also here it click when it makes and breaks contact . Procedure is in the Bentley manual .

[tx63conv]

Originally Posted by sparkysvega

Not the ICV . The idle micro switch tells the dme when the throttle body opens and comes off of idle . Mine was sticking closed intermittently and the car stumbled so bad it was undrivable in the rpm range you are experiencing. To check it , you just need a multimeter . Disconnect it and check that it makes and breaks contiinuity when you open and close the throttle body . You should also here it click when it makes and breaks contact . Procedure is in the Bentley manual .

Ahh....thank you! I will check that first.

[tx63conv]

I havent had a chance to test the idle control switch--doing that tonight.

Just to 100% confirm on the suggestion above about using the Air flow meter as the injection point for the smoke test--The gasket between the air filter box and the flow meter was used as a template on a sheet of aluminum. This gasket:

https://www.pelicanparts.com/More_Info/92811020101.htm?pn=928-110-201-01-OEM&SVSVSI=911M

This mates to the air filter box, correct.

[NotStock]

Looks right, but you can just use the one that is on the car now. I only used the gasket for the hole locations.

You don't have to make the block off plate, its just a nice idea to make the test as good as possible. My first few rounds of smoke test were done with a rubber glove over the snorkel thing on the air box with the smoke plugged into the fuel reg vac line.

[tx63conv]

Originally Posted by sparkysvega

Not the ICV . The idle micro switch tells the dme when the throttle body opens and comes off of idle . Mine was sticking closed intermittently and the car stumbled so bad it was undrivable in the rpm range you are experiencing. To check it , you just need a multimeter . Disconnect it and check that it makes and breaks contiinuity when you open and close the throttle body . You should also here it click when it makes and breaks contact . Procedure is in the Bentley manual .

I think it might be this. I am getting constant continuity when I open the throttle--even wide open. The procedure mentions engine off but doesnt say anything about having the key on.

Looks like this is the part:
https://www.pelicanparts.com/search/?q=porsche+throttle+micro+switch&redir=yes&host=search

Also looks like there is some adjustment---I am going to guess that if I still have continuity (switch is closed) at wide open on the throttle, then the switch is bad.

If the switch is disconnected, how would that impact the throttle while the engine is running? I would like to test this another way before I replace.

[wazzz]

If you try to crank the engine with the switch already disconnected, that may confuse the ECU and make the engine hard to start.
Start the engine first, let it warm up a bit, then when idling drops to 900-1000, disconnect the switch. Idle speed should climb up to 1500 or 2000 rpm. From there you can drive test your car and see if it still stumbling. It should not, but be aware that engine speed won't drop back to idle when releasing the accelerator. You may need to brake harder because there is no help from the engine in this situation.
But at least you will know you've found the culprit.

[tx63conv]

Thanks. I hope that is the culprit.

Wazzz....did exactly as you said it would. Let it idle till warm. Stumble on throttle. Removed plug, revved freely. Thanks everyone!