Surging Idle '88 911

[FA-18C]

If there is no change when you take the oil cap off, I believe you have a vacuum leak somewhere. The hose that runs off the oil filler tube can get cracks, and highly recommend using a mirror and feel for all of the connections to rule that out, if you have not.

[kyngfish]

Anecdotal but I’ve never had the car idle correctly since putting in a Steve Wong chip. Been 7 years or so. At first it would surge until engine was slightly warm. Now it will surge on startup until I blip the throttle. Then it’s fine. When I initially got the chip I contacted Steve and followed his instructions on setting idle to the letter. No change.

I’ve checked 11 different ways for a vac leak and never found anything. Idle drops accordingly when I pull the oil cap. Checked hoses cleaned idle control valve - replaced CHT sensor after testing. Replaced O2 sensor. Cleaned out throttle body. Nada. Someone I trust commented that the S Wong chip doesn’t always play nice with the idle valve. Couldn’t confirm or deny without replacing the valve which I just never bothered doing since it stated idling fine after blip.


If I had to make a bet - it’s the valve and I need to get a new one since the engine has been rebuilt now and it still does the same thing. Long ago when I went on a mission to find a new valve I found out that older Volvos used the same part but at a fraction of the price, still couldn’t find one OEM but I found the part using the Bosch part number instead of the Porsche one. They sell cheap ones now aftermarket. It’s a simple enough part that I would for sure run that risk if I felt like it. Can’t remember which part number is used tho.

[mysocal911]

Originally Posted by kyngfish

Anecdotal but I’ve never had the car idle correctly since putting in a Steve Wong chip. Been 7 years or so. At first it would surge until engine was slightly warm. Now it will surge on startup until I blip the throttle. Then it’s fine. When I initially got the chip I contacted Steve and followed his instructions on setting idle to the letter. No change.

I’ve checked 11 different ways for a vac leak and never found anything. Idle drops accordingly when I pull the oil cap. Checked hoses cleaned idle control valve - replaced CHT sensor after testing. Replaced O2 sensor. Cleaned out throttle body. Nada. Someone I trust commented that the S Wong chip doesn’t always play nice with the idle valve. Couldn’t confirm or deny without replacing the valve which I just never bothered doing since it stated idling fine after blip.


If I had to make a bet - it’s the valve and I need to get a new one since the engine has been rebuilt now and it still does the same thing. Long ago when I went on a mission to find a new valve I found out that older Volvos used the same part but at a fraction of the price, still couldn’t find one OEM but I found the part using the Bosch part number instead of the Porsche one. They sell cheap ones now aftermarket. It’s a simple enough part that I would for sure run that risk if I felt like it. Can’t remember which part number is used tho.

Numerous Porsche independent shops have reported this too! The idle typically "hangs" above 1000-1200 on decel.

[kyngfish]

Originally Posted by mysocal911

Numerous Porsche independent shops have reported this too! The idle typically "hangs" above 1000-1200 on decel.

Just want to clarify that by surge I mean that it bounces from like 600-1200. I’ve also had the injectors flow checked cleaned and rebuilt.

[mysocal911]

Originally Posted by kyngfish

Just want to clarify that by surge I mean that it bounces from like 600-1200. I’ve also had the injectors flow checked cleaned and rebuilt.

Basically the same as what others have stated, i.e. surging/hunting around 900. When replaced with a stock chip, no more problems.

[cliyde]

I wouldn't bother setting the base idle yet. Eliminate other potential issues FIRST, fix it, then fine tune your base idle mixture.

That said, if RPM's didn't change with the filler cap off AND you didn't feel any Vacuum with your hand over the filler neck opening, you likely have a vacuum leak somewhere else. (i.e. oil breather hoses, to and from oil tank, crank case underneath intake via filler neck or PCV valve, etc.)

Recommend the AFM swap NEXT if you rule out vacuum leaks.

Cheers.

Originally Posted by gcredantporsche

I didn't notice any changes in RPMs with the oil filler cap off, which was a surprise. I bypassed the ICV with the engine cold and set the idle then. Should i set the idle when the engine is warm?

I'll check the flywheel speed and reference sensors. I haven't done that yet.

[cliyde]

Originally Posted by FA-18C

If there is no change when you take the oil cap off, I believe you have a vacuum leak somewhere. The hose that runs off the oil filler tube can get cracks, and highly recommend using a mirror and feel for all of the connections to rule that out, if you have not.

What he said

[gcredantporsche]

Originally Posted by cliyde

What he said

I tried replacing the AFM with another from a 3.2 that was working fine, and that did not change the surging idle. I did another smoke test and noticed a tiny amount of smoke coming from behind the rubber boot below where the rubber Y joint is, but for the life of me I could not pinpoint it because even with a mirror and light it was hard to see. So I replaced the rubber y-joint that has the two small hard lines that come out of it going to the fuel damper and the other side (regulator?) which was easier to get to with everything removed. I then put everything back together and also tightened up everything I could in terms of vacuum hoses and clamps. I then drove the car around until it was warm, and noticed that the warm idle was running at about 1100-1200 rpm. When I got home, I jumpered the B / C to bypass the ICV and the engine was slightly surging. I reduced the idle down to 850 RPMs and then drove it around again, and it ran great. I also removed the oil cap and noticed the idle changed when it was off (which wasn't the case previously). I let the car cool down for at least an hour and when I started it up, there was no more surging. Hopefully I solved the vacuum leak! Thanks everyone for your help and advice.

I also had a thread on rennlist which was also helpful.

[almostordinary]

Awesome! Glad you found your issue. I just did a smoke test on mine, I actually use the hard line from the FPR to run the smoke, on my last test this morning my brand new oil cap is leaking... Going to replace the yellow 87 style w the Bayonette style since it has a much larger O-Ring surface.

Never thought to put clamps on those vacuum lines to the FPR and FPD. May look and put nylon zip ties for good measure.

Congrats on the solve!

-Ian

[mysocal911]

Originally Posted by gcredantporsche

I tried replacing the AFM with another from a 3.2 that was working fine, and that did not change the surging idle. I did another smoke test and noticed a tiny amount of smoke coming from behind the rubber boot below where the rubber Y joint is, but for the life of me I could not pinpoint it because even with a mirror and light it was hard to see. So I replaced the rubber y-joint that has the two small hard lines that come out of it going to the fuel damper and the other side (regulator?) which was easier to get to with everything removed. I then put everything back together and also tightened up everything I could in terms of vacuum hoses and clamps. I then drove the car around until it was warm, and noticed that the warm idle was running at about 1100-1200 rpm. When I got home, I jumpered the B / C to bypass the ICV and the engine was slightly surging. I reduced the idle down to 850 RPMs and then drove it around again, and it ran great. I also removed the oil cap and noticed the idle changed when it was off (which wasn't the case previously). I let the car cool down for at least an hour and when I started it up, there was no more surging. Hopefully I solved the vacuum leak! Thanks everyone for your help and advice.

I also had a thread on rennlist which was also helpful.

Glad you found the air leak, as noted per post #15.