Dropping Idle

[7783911]

I have been having an issue with my idle speed when the car is just started and warming up..

Cold start is instant and idle is fine...however, until 100% warmed up...when you lift off the gas (or clutch in) the idle drops very quickly to 350 and bounces back up or when really cold, it drops and stalls before it can bounce back up

Once warmed up...there is still a rapid drop but it only drops to 650 and instantly recovers to normal idle speed of 900

I am not sure where to start my investigation

I have a 1983 SC engine (77 Targa) implant. The car did sit in storage for many years before I purchased but its been almost 1 year and other then this issue (and somewhat suspect mileage) it is tight and responsive

any thoughts?

bc

[DRACO A5OG]

Check for vacuum leaks, a cracked rubber hose will expand as it the engine bay heats up helping to seal up the suspect leak. Open the oil cap cold and hot see if there is a difference.

After that I would check your fuel filter.

BTW, Welcome a board! She is Purty

[boyt911sc]

BC,

You did not specify if the '83 SC engine still has the OEM CIS on it. If it does, you need to know the following:
Fuel pressures (control and system)
Absence of any significant source of vacuum leak
Fuel mixture (do not tinker yet).

What's the WUR number? What's the heater resistance (Ohms) at room temp. Is the engine a USA/ CAN or Row model? These are some of the basic information we need to know before we could give a meaningful advise. Keep us posted.

Tony

[tirwin]

Welcome!

+1 to what Draco said.

Your mixture is too lean. Check for a vac leak.

There are two mistakes people tend to do when encountering this problem. The first is they jump straight to adjusting the mixture. That is the LAST thing you do. The second is they replace the fuel accumulator. Usually doesn't help and it's just a guess instead of actual troubleshooting.

Read this: CIS Troubleshooting for Dummies

[7783911]

It has the K Jetronic Injection...

I have previously suspected a vacuum leak only because I did the oil cap on/off test and there was no change in idle as a result..

I may take it somewhere for a proper leak test and that makes a logical first stop in the trouble shooting..thanks

[DRACO A5OG]

If you have a cigar, you can locate your leak 😁

[nene]

Draco,

You are going to turn us all into smokers....buy it will be worth it, as I still need to do this!!

[DRACO A5OG]

Guilty, I am a Bad Influence

[T77911S]

its too rich.
when it is rich if you blip the throttle and the RPMs will drop then come back up.
i have not seen it do this when lean.
this is how i set the mixture by ear. set it rich until the RPM;s drop then back it off to where thye dont.

this could be a sign the WUR is not heating up due to bad WUR or no power to it

[gazzerr]

My 82SC would do this and stall at stop signs. It was sucking unmetered air from numerous holes in the CIS Pope's hat (surprised it even ran) and the vac line from the throttle body to the WUR was very loose.

[jwakil]

Seems we have the same problem. (See my very recent post). Unfortunately I can't do the oil cap test on my 930 as it is vented, unlike the SC, but other things should be similar. Let me know what you find and I'll do the same.

[7783911]

I contacted my Porsche guru (ex mechanic from Porsche Race team in NA) and he will take a look, he believes (as T77911S suggests) that this is a mixture issue but he wants to see the car himself. If the weather is good..i am for a drive tomorrow..but without the Targa top since its in NJ being rebuilt as we speak

[T77911S]

he sounds like a doctor.

doctor, i have a headache. " you need to come in so i can get paid to give you an asprin".

[jwakil]

Originally Posted by 7783911

I contacted my Porsche guru (ex mechanic from Porsche Race team in NA) and he will take a look, he believes (as T77911S suggests) that this is a mixture issue but he wants to see the car himself. If the weather is good..i am for a drive tomorrow..but without the Targa top since its in NJ being rebuilt as we speak

A mixture issue would also be evident when engine is warm, and mixture is set it when engine is warm. It does not explain why we have problems only in the warm up stage. If you mess with your mixture, it might not run as well when warm.

[7783911]

no vacuum leak....(he also states removing oil cap wont work on older models)...the main culprit is the lack of back pressure from my race exhaust...he adjusted the rate at which the rpm drops via the decel valve and its running perfect now...

next task...slow windows..tried graphite powder in runners..didn't really help...looks like doors need to come apart for a thorough look

[Bob Kontak]

Originally Posted by T77911S

its too rich.
when it is rich if you blip the throttle and the RPMs will drop then come back up.
i have not seen it do this when lean.
this is how i set the mixture by ear. set it rich until the RPM;s drop then back it off to where thye dont.

this could be a sign the WUR is not heating up due to bad WUR or no power to it

I have to think this is very possible.

I would remove vacuum leaks first as he has some. Once sealed and if rich, symptoms may get worse. That would be a good thing - meaning closer to the answer.

[Bob Kontak]

Originally Posted by 7783911

no vacuum leak....(he also states removing oil cap wont work on older models)...

Yes - can't remember the cut-off year but it was close to the start of the SC.

Slow windows.....Yes, pull the door panel. It's slow going on the 911 with all the pieces/parts. Find the terminal numbers on the back of the window switch and associate with the wire colors and fully remove the switch for ease of access.

Find the big arced gear that moves the regulator and put grease on some of the teeth. Best to reinstall the switch after the panel is out of the way so you can move the big gear to where you can apply the grease to three of four sections. Then test. I don't think you need a bunch of grease.