Fast idle on an SC

[Adelgigs]

Guys,

Since I've had my SC it will not return to idle until the clutch disengages. So if approaching traffic lights and I put the clutch in, the engine will maintain the same PRM until I let the clutch out. Then she drops down and idles without a problem.

Another example is if I blip the accellorator whilst changing down the revs won't return to idle. Very annoying.

Is this typical, or better still, WHAT'S THE ANSWER ?????

Thanks, Simon
'81 Euro SC
www.adelgigs.com/911sc.htm

[Joe Bob]

Have you checked the throttle body arm position when this happens? If it is not on the stop and it can drop down to normal idle when you push it to the stop, sounds like a throttle cable needs replacement.

If not, could be an intake leak someplace.

[Adelgigs]

Thanks Mike, but how would letting the clutch out have a bearing on these points ?

[Joe Bob]

Yep....it puts a load on the engine and it snaps the throtle back....

[Glasgow 911SC]

This sounds very similar to my problem. After driving the revs stay quite high (about 2K). Sliping the clutch settles them back down. I've been told that the deceleration valve could be to blame.

[porsche_virus]

Heh...I just picked up another 911 last night...a '76 Targa that had been sitting by the curb for months. It did the same thing...super-high idle after pushing in the clutch and at stop lights...very embarrassing and annoying, but also very simple. The throttle pedal wasn't returning back to dead stop, even though my foot thought that it was. Reach down with your hand at a stoping point when it's running high and pull the pedal back towards yourself and see if it goes away. I know it sounds like it would be obvious, and you may have already tried it, but I fixed mine by shooting WD-40 throughout the entire throttle linkage system, but mostly on the throttle arm bushing at the pedal cluster. It is made of plastic (bushing) and gets really gunked up with rust and old grease, making it hard to even move by hand.

The first thing I did to diagnose it was pop off the ball links at the tranny throttle bellcrank and up front at the pedal cluster. That allowed me to move just the main throttle linkage freely back and forth to verify it wasn't stuck somewhere in the path. Then I grabbed the throttle arm at the pedal cluster and could barely move it by hand. I soaked it in WD-40, and an hour later, it was like new. Also make sure that BOTH throttle return springs in the back on the throttle body are ON and in good shape. You'll be able to tell once you disconnect the ball-link from the throttle bellcrank at the tranny and try to move the funky looking throttle link that comes from the engine bay.

OR, it could be the decel valve

[78Jason]

I have the same problem - on a 78 sc - ONLY when hot, though.... and it went away when I *connected* the intake vacuum line on the Fuel Pressure Regulator or warm up regulator as it's called in the books (yes, someone had disconnected it for SOME reason and plugged the rubber hose.) However, the other day, I advanced the timing to about 5 deg BTDC (you don't want to know what it was) and all performance went up - except my high idle only when HOT came back. It idles high fine when cold, and normal idle 950 at warm. Is this the Fuel Pressure regulator or the cable do you think?? I was ready to shell out $250 for the regulator.

And what is more likely it - the deceleration valve or the warm up (fuel pressure) regulator - assuming it's not the throttle cable?

[jason2guy]

it sounds like either your basic idle setting, or your AAV.
how high does it idle when cold?

[78Jason]

When cold, it starts right up and idle jumps up fast to 1800. After 2-5 minutes, it slows down to 1100, and eventually settles down at 950 and stays rock steady at 950 until the temp is at operating temp (needle is at 270 degrees on the guage - or 9 o'clock position). Until I adjusted the timing it was behaving more normally - ie idle never stayed at 1800 for more than a .5 sec. Now, it will hang there, sometimes brought down by the clutch trick, sometimes not.

I *just* ran out of work to try pulling the the peddle back and it seems like it might be part of the problem - but not all of it.

Since another member suggested here it might be the deceleration valve - and I've been told it's possibly the warm up regulator (fuel press. reg) and now you say it could be the AAV.......

What is the relationship between those three units as it relates to heat, and pressure and of course the end result - idle???

How can I test those three units to be sure it's not one of or all of them? It is 25 years old, after all.

[adamred]

I know a million people are going to jump in here but 270 deg is way too hot, I get nervous when my SC stays in the 210-220 region for too long, I'd get that checked!

Adam

[78Jason]

oops! sorry - I was using degrees as if you laid the temp guage on a compass.... sorry - i'm a pilot! 270 degrees is the same as the little hand at the 9 o'clock position - or simply perfectly level pointing to the left. I believe that the level position correlates to the 220 degrees Farenheit temp - but I can't be sure till I can get a thermal reader from somebody!


However, I have been working on bringing the temp down more - i'm adding an oil cooler thin tubular (30 or so) type real soon. I've heard 220 is normal operating temp.

[1fastredsc]

When i first purchased my sc, it would idle like crap at about 800 rpm when cold. After warming up it would idle at about 1,400 rpm. Come to find out, the Auxilary Air Regulator (aav) was stuck closed and not opening during cold start up like it's supposed to. Therefore the previous owner raised the static idle to compensate for it. After replacing the aav, and then resetting the idle, all things are fixed. Although possible i doubt the decel valve is bad, it's just a diaphram that's spring loaded to blurp air toward the engine during fast decel to prevent that popping sound in the exhaust (which i think is cool personally).

[78Jason]

yea fastredsc - I think the guy who had mine had some problems, and found disconnecting the WUR fixed his problem so-so.... those are the bugs i'm trying to hammer out now. So what i'm gathering is:

- AAV - used during cold start. my cold start is ok, idle is normal (1800).

- deceleration valve - that controls popping in exhaust when letting off the gas and decel.... I don't have that noise.

-WUR - used during all temps to control the fuel pressure. So at low temps and warm temps - it adds or takes away pressure. At hot engine temp (220 deg F) - what does it do??? Could this be the problem? Do I have to replace it to test it at hot or can I measure vacuum or something else to test it when hot ???

So what I'm guessing is it's either the WUR or the Throttle Cable.

Know how to test WUR?

[Adelgigs]

It could be the de-acceleration valve, but I don't understand how the valve would know what position the clutch was in ?

Are there any sensors in the gearbox / bell housing that detects what the clutch is doing ?

Any other ideas ?