1976 Surging Idle
 

My '76 911 has had the 2.7 replaced with an '81 3.0. When I start the car the idle will surge up and down and when I take the car out of the garage initially, the RPMs will go from 1K - 2K and there is little I can do to control it. once I start driving around a bit the problem seems to go away.

I was noticing some grinding while putting the car into 1st gear from a standstill and I thought it was a transmission issue but then I noticed that sometimes when I'm stopped at a light the RPMs are stuck at 2k. Sometimes if I give it some gas it will settle back down to 1k.

Why would the RPMs stay "stuck" at 2k? Why does it surge on a cold start?

Thanks in advance.

1K is your base idle? That seems pretty high. Did you or PO set the idle that high? I believe your system will cause havoc when idle is bumped up that high.

As far as grinding, that could be the gear box because your engine speed is too high for the gear's synchro bands to catch up.

Again sounds like a AFR is out of wack.

Please confirm your year's base idle settings

I've had the car for about 3 weeks now and I'm just now learning its tendencies. Before that, it was owned by my Step Dad so we can talk about what the car is doing regularly. He did get the car appraised to do a major overhaul and he does suspect that the guys might have messed with some idle settings or mixture.

I don't know what a normal idle is for it or where to find it, I'm basically going off of sound and feel.

Your tach should be working, try lowering the idle to 900, spec should be +/-50 RPMs.

BTW, Welcome aboard, I highly recommend a Bentley Manual or a Haynes at a minimum, that is where I got your idle specs. Adjust after she is plenty warm as the car when cold will have a warm up period at higher idle. Do NOT sit and idle, drive off immediately and keep her under 3500RPMs until warmed up while driving.

Thanks, I'll look into setting the idle a little lower and I really do need to pick up that literature.

My big concern is why would the idle sticks at 2k when I'm doing normal driving in the city when I come to a light and shift to neutral?

1981 CIS system.
Could be a vacuum leak or decel valve or a bad O2 sender or it is set to run overly rich. Or the WUR or the AAR or, or...... guessing and fiddling with it doesn't usually work.

You need to have a CIS fuel pressure tester and a CO meter and some other test equipment to test the system methodically and then repair it.

Here is a good read for you to start with:
http://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies.html

Could be as simple as your accelerator cable sticking.

If engine is cold, that is the car warming up mode but it should decrease after 10-15 minutes.

+1, go thru the thread Timmy2 posted, I am sure it will give you an idea.

Bad bushings will do it. Weak spring, broken cruise control cable too.

I'd fuel pressure test it first, then smoke test it for air leaks to see what kind of condition your CIS is in. If it gets better after it warms up I'd put my money on an out of spec WUR during the warm up cycle. Maybe is a rich surge when it's cold. A fuel pressure test will confirm this.

Like all other guys have said you have to make sure the CIS is airtight and all the components are up to spec first. It's generally a number of small issues as the system ages.

Sounds like your idle air regulator (the device located between #5 and #6 intake manifold) is out of wack and needs to be adjusted. Or you could always substitute the regulator out of the 2.7 engine and be done with it. And while you have both in your hand, you might take a peek inside to compare the difference in the openings that would explain the problem.


Cheers,

Joe

That could very well be the AAR not closing properly or another air leak bypassing the CIS sensor plate like a leaky decel valve or leaky aux air valve, air box leaks, engine idle timing out of adjustment, idle bypass on the throttle body out of adjustment, you just have to go through it all. When I had big air leaks in my CIS it would stumble and stall at stops rather than a high idle. When you remove your oil cap with the engine running does the idle drop? If not then that usually indicates a significant vacuum leak somewhere in the system.

This is the go to thread for CIS diagnosis:

http://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies-2.html