Idle to high..

ufool · Post by **ufool** » Sat Jan 28, 2006 11:49 am

Yip..I'm in the same boat as everyone else seems to have been in! Done searches on pelican but still not come up with the solution.
My problem..car starts fine, idles around 900 rpm, pulls off fine, runs fine but then once it gets warmed up the idle speed jumps upto 2K (ish). This can be brought back down using the clutch but if I blip the throttle it will go back up and stay there for way too long. Makes city driving 'interesting'. Any thoughts...air leak? WUR? too lean? Cheers
M

Smiles · Post by **Smiles** » Sat Jan 28, 2006 12:36 pm

Sounds like an air leak making the mixture too lean. You can find where the leak is by using a handheld flower sprayer on mist setting and when the engine is running, direct it at different points. You know you have the right spot when the engine idle drops.

Ive got the same problem on my 912 - my rebuilt carbs should be back from AJP very soon (at great cost!)

You runnning carbs?

domtoni · Post by **domtoni** » Sat Jan 28, 2006 12:39 pm

Is this car an MFI or carburated vehicle?

Thanks
Dom

sladey · Post by **sladey** » Sat Jan 28, 2006 8:41 pm

I've had something very similar on my 3.2 - tried lot of things but stupidly it came down to a sticking throttle linkage - bit of lube & it was good as new.

I sussed it out by the fact that I could drop the revs by pulling the throttle up instead of down, and also when I did the same sort of throttle movement using the throttle on top of the engine (as opposed to using the pedals) the revs fell back to 800 straight away.

Hope it's as simple for you - it was a big relief to me

Don't know where the linkage is on the early cars but presume it's the same place - sticking out of the side of the gearbox/engine - right underneath the car where it can get filled with crap and seize up

Cheers

Mark

ufool · Post by **ufool** » Sun Jan 29, 2006 8:43 am

Cheers for your replies. The car's a Carrera3.0 with a standard CIS system. Recently rebuilt. It's not the throttle linkage...checked that and it's all smooth. An air leak sounds more like it. I'll check with the plant sprayer method. The strange thing is this only happens AFTER the car has warmed up after a good run. So how can an air leak not be there when it's cold but then appear once it's hot..if you see what I mean? I guess things expand and somethings perhaps opening up once hot. The fun continues...
Mark
PS. I've just posted my finished project on the Pelican site if you want to look at it. Post called 'Finally..after 2 and a half years'

Garrett · Post by **Garrett** » Sun Jan 29, 2006 2:19 pm

Might be the adjustment of the screw contacting the shut off solenoid microswitch. Check your idle stop screw, it might not be making the switch when throttle is closed/off. The exact same symptoms were cured on mine by adjusting the idle stop screw, admittedly MFI, but as far as I know, the CIS system has a microswitch too. Its described on p67 of the haynes manual (911 1965 thru 1989: Haynes 80020 (264))

KS · Post by KS » Sun Jan 29, 2006 9:21 pm

ufool wrote:So how can an air leak not be there when it's cold but then appear once it's hot..if you see what I mean?

Because the cold-start enrichment system will compensate for the lean condition caused by the leak. When the engine warms up, the mixture leans out and Bingo! there's your problem...

ufool · Post by **ufool** » Mon Jan 30, 2006 11:17 pm

Ah..now that makes sense! Just out of interest what does this micro-switch connected to the throttle butterfly intake do? Seems to be connected to the fuel pump relay via the alarm system (which I don't have)?

Garrett · Post by **Garrett** » Tue Jan 31, 2006 10:17 pm

I've just checked the Porsche workshop manual. The microswitch is in parallel to the thermoswitch and, properly adjusted, makes when the hand throttle is open, for cold running, pumping supplemental fuel. I don't know whether the main throttle linkages cause it to switch, but if they do, then, if it isn't set right, it could cause the problems you have experienced... easily checked and adjusted.

Garrett