Idle / Running issue
Thread Starter
Rennlist Member
Joined: Dec 2009
Posts: 482
Likes: 32
From: Central Ohio
Idle / Running issue
Hello,
MY1981 5-speed.
I've researched the list and online and don't find an answer to my issue so I need a bit of help.
I have an idle/running issue. Un-metered air is the problem. I hear a high pitched whistle when idling.
Completed a top end refresh recently. This included all rubber fuel and vacuum lines. I did replace the spark plugs when the spiders were off but did not include any other 'projects/replacement' as I wanted to limit issues. The refresh went very well.
Started it up and allowed it to reach temp. Idled well enough. Not perfect but well considering the amount of work preformed. Started test driving it. Worked through a couple issues (throttle micro-switches crossed, OOPS). Finally trusted it enough to attempt a short drive. Drove approximately 20 miles. Performed nicely but during the trip the idle went south. I now have a vacuum leak. Smoked it and discovered a leak on the right side of the engine. The vacuum limiter appeared to be leaking massively. Preformed the procedure outlined in the WSM, as listed below:
WSM 24-19
CHECKING VACUUM CONTROL
Run engine at idle speed.
Pull conrrol hose for decel valve off of branch
and connect hand vacuum pump US 8026.
Decel valve will open and idle speed rise when vacuum is sufficient.
Okay. This was not conclusive.
I pulled the unit off and static tested it. Vacuum applied to the small port results in a vacuum of 1 bar (15lbs or so) that starts dropping immediately and returns to 0 bar in a few seconds. Is this proper operation?
I'm not convinced the limiter (Porsche p/n 930 110 173 00) is bad but do not find any procedure outling definitive tests.
My question is: What does this limiter do exactly? How do I verify the part is serviceable or not?
MY1981 5-speed.
I've researched the list and online and don't find an answer to my issue so I need a bit of help.
I have an idle/running issue. Un-metered air is the problem. I hear a high pitched whistle when idling.
Completed a top end refresh recently. This included all rubber fuel and vacuum lines. I did replace the spark plugs when the spiders were off but did not include any other 'projects/replacement' as I wanted to limit issues. The refresh went very well.
Started it up and allowed it to reach temp. Idled well enough. Not perfect but well considering the amount of work preformed. Started test driving it. Worked through a couple issues (throttle micro-switches crossed, OOPS). Finally trusted it enough to attempt a short drive. Drove approximately 20 miles. Performed nicely but during the trip the idle went south. I now have a vacuum leak. Smoked it and discovered a leak on the right side of the engine. The vacuum limiter appeared to be leaking massively. Preformed the procedure outlined in the WSM, as listed below:
WSM 24-19
CHECKING VACUUM CONTROL
Run engine at idle speed.
Pull conrrol hose for decel valve off of branch
and connect hand vacuum pump US 8026.
Decel valve will open and idle speed rise when vacuum is sufficient.
Okay. This was not conclusive.
I pulled the unit off and static tested it. Vacuum applied to the small port results in a vacuum of 1 bar (15lbs or so) that starts dropping immediately and returns to 0 bar in a few seconds. Is this proper operation?
I'm not convinced the limiter (Porsche p/n 930 110 173 00) is bad but do not find any procedure outling definitive tests.
My question is: What does this limiter do exactly? How do I verify the part is serviceable or not?
Rennlist Member
Joined: Apr 2006
Posts: 35,695
Likes: 511
It's not unusual to find one or more of the intake running rubber couplers to be folded and not clamped good underneath. Get some brake cleaner and spitz it along the runners and see if you get a change in idle.
Three Wheelin'
Joined: Jul 2010
Posts: 1,774
Likes: 406
From: Bowling Green, KY
You can disconnect the idle switch and cap the vacuum feed to temporarily bypass the decel valve. This should just be used for diagnostics on 5-speed cars. The auto cars don't actually need the decel valve, and Porsche quit using them for the later L-jet M28.20 engine/trans combo.
