Basic question about fuel consumption

[driveasintended]

Hi all
I recently bought a 1982 911 sc. Top end was rebuilt just 20K ago. When I got car it ran like a dream. I spoe to previous owner who said it was probably due for a valve adjustment.
I found a guy, supposedly reputable, he did the Valve adjustment.
Since then, it hunts at cold idle for about a minute, then smooths out.
Biggest issue, car was getting 20MPG now it's getting between 10 and 12.
What could be causing the fuel consumption to be so drastically increased?
Thanks in advance for any ideas.

[stlrj]

Originally Posted by driveasintended

Hi all
I found a guy, supposedly reputable, he did the Valve adjustment.
Since then, it hunts at cold idle for about a minute, then smooths out.
Biggest issue, car was getting 20MPG now it's getting between 10 and 12.
What could be causing the fuel consumption to be so drastically increased?
Thanks in advance for any ideas.

Mixture!

More than likely he dailed it too rich which is what wrenches are trained to do on CIS engines.

[driveasintended]

So if I lean it out a little, that should bring the MPG's back up? And eliminate the rough idle at start / warm up?

[Joe Bob]

Use an exhaust analyzer....

[stlrj]

Originally Posted by driveasintended

So if I lean it out a little, that should bring the MPG's back up? And eliminate the rough idle at start / warm up?

Yes.

Do you have the 2mm allen for the mixture adjustment?

[driveasintended]

I do have the wrench. Thanks.
There's some other issues. When I lean it out, it doesn't run as well hot. When it's richer, it runs great hot, but has that surging idle when cold. And the fuel economy is atrocious.

[ossiblue]

If this was my car, here's what I'd do.

First, take Joe Bob's suggestion and set the mixture correctly using a gas analyzer. When dealing with CIS, tweaking the mixture is usually left for last.

Second, using a fuel pressure gauge set, I'd get a reading of my fuel pressures at cold, and hot to see if they're in spec. A too low cold control pressure will cause hunting at idle, as you describe, that will disappear when the engine warms up. I'd report the pressures to the board here for interpretation if I didn't have the spec charts available.

[kach22i]

Check for air leaks at all the hoses and the base of the big rubber boot at the top of the engine.

Vacuum leaks will cause the idle to vary when cold along with incorrect mixture as others have mentioned.

[Joe Bob]

Low flow propane or quick start sprayed at possible leak areas will make the idle change.

[81 SC Ray]

I believe that is a 3mm allen wrench for the mixture control - not 2mm.

[ossiblue]

You have received good advice from all posters regarding different fault areas common to CIS. My advice would be to formulate a step-by-step plan and stick to it.

You began with high fuel consumption and a hunting idle both of which are consistent with an overly rich mixture. IMO, it would make sense to check the specs of mixture and fuel pressure first as it is easy to do and has a bearing on your problem. If you aren't able to check these easily yourself, at the very least you might call the mechanic and ask what, exactly, he did besides the valve adjustment or, better yet, specifically ask if he adjusted the fuel mixture screw. Many rough running problems, possibly from air leaks, are masked by a richer fuel mixture.

Certainly you may wish to check out the entire CIS system for air leaks which cause lean running conditions, but from your posts it sounds like the car still runs "like a dream" when warm. Again, your problem arose when the car came back from the mechanic and are symptomatic of rich running rather than lean. If you establish that the mixture and pressures are in spec and the car does not run/idle well, then is the time to pursue other areas such as air leaks.


Do what you feel comfortable doing, but think it out first, make a plan, and follow it logically. You'll get lots of good advice from this board and you'll soon have this problem solved.

[euro911sc]

Where are you located? There could be several of us Pelicans around to help out sorting your CIS or at least lending some of the more specific tools like gas analyzers Fill out your profile!

-Michael

[jwakil]

If you don't have an oxygen sensor, then your mixture sitting will only be good at the conditions you used to set. So when outside temps get hotter or colder, you will run richer or leaner. I've gotten to the point where depending on the weather forcast I tweek the mixture about a quarter turn or less. Otherwise, you either end up stalling and idle hunting when it's hot, or hesitating when it's cold.

[stlrj]

Originally Posted by driveasintended

When I lean it out, it doesn't run as well hot. .

That's because you probably have some vacuum leaks.

[ossiblue]

Originally Posted by driveasintended

I do have the wrench. Thanks.
There's some other issues. When I lean it out, it doesn't run as well hot. When it's richer, it runs great hot, but has that surging idle when cold. And the fuel economy is atrocious.

Originally Posted by stlrj

That's because you probably have some vacuum leaks.

+1! You essentially undid what the wrench had done which was to enrich the mixture to overcome the lean condition instead of finding out what was causing the lean condition. Most likely air leaks.

Oh yeah, I second Michael's suggestion of filling in your profile more completely. In the long run, it's a tremendous help to you and those who want to offer help.

[driveasintended]

Thanks all.
I now have a clearer vision of what path to take.
And I added some info to my profile.
I'll let you all know how it's going as I proceed. And I very well may have more questions. I am grateful for this forum.
AC

[ossiblue]

Originally Posted by driveasintended

Thanks all.
I now have a clearer vision of what path to take.
And I added some info to my profile.
I'll let you all know how it's going as I proceed. And I very well may have more questions. I am grateful for this forum.
AC

Lots of help here, waiting in the wings. Glad to see the added info in your profile and you might wish to add a signature line which includes your car identification--saves having to post it every time you have a problem to discuss.