High Idle (1800-1900)on start up but 950 w/in a few min.
 

Car(79SC) runs like a top ,except for the high idle on start up.Engine was rebuilt top to bottom approx 1500-2000 mi ago to stock specs except DC 15 cam grindand ARP hardware.
I worry about brg. wear for thoes few minutes upon start up for a few minutes until it warms up settles into its 950 idle .

I have not baselined the CIS in a while.
Should I be concerned about this high idle (1800-1900) on start up? and does it impact rod/main brg wear ??

Is this recent, maybe along with a weather change? Have you checked that the CO2 is set at a comfortable 3.5%?

Jdub,
No,it is not a recent change ,but seems to be a bit of idle creep since the rebuild .
The mix was set by the sniff test , and plugs are a very very light brn.
Damn car runs great. But, is the hi idle a concern upon start up as far as the brgs (wear) are concerned??
I should add that it hunts a bit before it settles in to the 950 idle.

Cis is a challenge for me , but I can live with the high start up idle if the hign spin on start is not contributing to brg wear.
I do know that below 350 rpm the brgs do not maintain a good oil film on the brgs , but what about 1800 revs on cold brgs as the oil pressure builds and things get warm???

Is it a Euro...they are supposed to act in that manner when cold only, not a hot start. I'm not sure about the US models. On a dead cold overnite start mine goes to 15-1600 rpm then settles down. 930-10...it runs fantastically And burns no oil...ok, maybe 8 oz. in 3k miles.
That won't hurt/cause wear issues.

Reiver, Not the euro, ;( ,but still a strong runner .My high idle is also only on cold start.
I think I better get the Bently out and do a baseline CIS analysis.
The start procedure is dead nuts reliable as well,crank over once,(pressure build?) and she fires on the second crank ,always.

Sorry not to be of more help - just thinking of something relatively simple (read: cheap) that would help. I think you are right to study the system since knowledge is power after all!

SC's jump up in RPM for cold start per design. Duration of high idle is a function of the aux air regulator if all else is perfect.

You won't find "baseline" CIS info as in how long the idle should remain in the high 1000's in Bentley. I have never seen this info documented in a diagnostic format. (I have been wrong plenty of times but pretty sure this is the case).

It can be argued you may have a small false air leak delivering "extra" air impacting cold start. During normal running that leak may have been balanced off with the idle air bypass screw? Mine goes to 1,700 for whatever that duration is. That duration will change with outside temperature.

With respect to oil pressure, watch your gauge. Dead cold start should be plenty of bars right away.

I think you are quite ok save a little something or other?

Thanks Bob,
I cleared the AAR a no of yrs back ,and it passed the freezer test ,but I am sure it is time to do it again .
I have a ultra sonic cleaner I use for bing carb parts, and I wonder if that is better than squirting brake cleaner on each end ????

Yeah, my 83 sc has done this same thing for the 17 years i have had it... my warm up cycle takes much longer in cold weather than in warm weather.

mine got higher and higher and then when it was at 2100 i took the CIS off and found a cracked airbox. might not be your issue but they're all going to crack eventually.
I never heard a backfire prior to this either. I think it just died a natural death....

thats way too high and annoying i might add. you need to remove the AAR- the one with an electrical plug- and adjust it. there is a movable bushing in the rear that the heating element attaches to. get a hammer and punch or a small press and knock the bushing in the direction that closes the air passage a bit. it might take a few tries to get it where you want it. i have mine set for 1100 cold idle and it seamlessly settles to 900 in a minute or so. there is no reason for the engine to idle that high when cold.

oh and the hunting during warmup is most probably that the cold control pressure is too low and therefore too rich.
rich=hunting

dont squirt anything in there because it wont fix anything.
and finally a cracked airbox will cause a lean condition- misfire, not a high idle

One thing that can be done is to let it warm up and screw the big idle air bypass screw in all the way clockwise. The engine should die or barley chug over if most all the air leaks are eliminated.

If it runs at 500 rpm you have an air leak.

Two minute test post warm up. Worth your time.

I don't know enough about what a rich mixture will do to cold idle rpm but it's best to push back on tweaking the air fuel mixture until you're certain everything else is in spec.

Does it stink rich once warm?

Have you tweaked the CO screw ever? Not looking to point a finger at all. Interested if you could cheat a little bit (before every other stone is turned over) just to see. I know my discipline would be weak in this situation.

Did they advance the dist timing from stock?

Good info.

I have seen different air leaks do different things when hit with carb cleaner.

I am not aware of a rule that says false air = high idle. However, I have suggested it plenty of times.

jason, thoughts on the more aggressive cam with respect to the high idle? I got nothing from lack of experience. Thinking it would maybe be slightly rougher but not higher.

Also, mine is 1,700 rpm for a couple of minutes. Always thought this was normal. Is 1,800 / 1,900 radically higher than my perceived "ok" 1,700? Or is 1,700 up there as well?

You can slow it down by closing the valve in the auxilliary air regulator a bit. Take it off and look through it to see the air gap. Put a 6mm socket against the smaller of the two aluminum plugs on each side at the end. Squeeze in a vise and watch with a flashlight as the air gap gets smaller. Move it just a bit and reinstall to test when the engine is cold, like in the morning. Go too far and you will need to remove the larger aluminum plug and push it back closer to the original position.

I had this Problem a while as well. In my case it was cracked original injector boots and way past their useful life injector O-rings. Take a look at this as you are troubleshooting as well. Good luck!

as far as the effect of the cam i can not comment on, but i think it has little to do with the high idle.

IDK maybe its just me but i cant stand when i start up the car and wham boom the idle is in the stars and all the noise that comes with high idle and a cold high mile air cooled engine. ive owned 4sc's and 1 carrera and none of them cold idled above 1200 or so. i would say 1700 is way too high.
the mod ive described is really easy to do. you might have to crack open the AAR to be sure which way to move the bushing, and a little movement goes a long way.
for the longest time i thought- air leak=high idle. that was until i had a real air leak in the form of a cracked airbox and leaking intake runner gaskets. thats when my aha moment happened, im slow. there are 2 types of air leaks, one in the metered section and the other being UNmetered. sure if you have a leak somewhere in the aux air plumbing the idle will increase because that air has been metered therefore the appropriate amount of fuel will be delivered for that air. if the air leak is UNmetered- intake gaskets,airbox- it will be lean and misfire

Wow, interesting. Over the years I have had more than a few very reputable porsche shops work on my car in So Cal, and they never mentioned a high idle (1500+) at startup as being abnormal. Go figure... Thanks for the great information!
cw

First thing that entered my mind was jason is in southern California (warm). You are further south than him.

4 SC's in his stable across time. Great reference data.

Commentary about metered air is a little confusing, though. Everything downstream of the air sensor is metered air. That includes a crack in the airbox that allow air to enter the box innards and most definitely an intake runner to head or rubber tube thingie leak where the runner connects to the box.

Four SC's is almost a statistical mass. :D

Guys , I second the thanks and appreciation, I think I am at the cusp of the Ahaaa moment with your excellent feedback.
The air box is brand new. I "don't think" there are any air leaks ,since I reinstalled the intake and new box after the rebuild ,but I will check and verify again.

I Hope to dig into the aar this weekend for peace of mind and to ensure happy non stressed bearings on start up.I have a de at Pocono next week and would like to put this task in the finished column before then.
Thanks much

Nice. Really nice.

I live in Akron, Ohio.

Our PCA DE's are at the Hartville Hardware flea market parking lot.

You can get up to 37 mph between cones. :)