86 Carrera - cold start issue question

73pcar · 11-26-2017, 06:34 AM

I have owned 911s of almost every decade up to the current year, but the 80s vintage is new to me with my recent purchase of an 86 Carrera. Very, very happy with it....nice original unmolested car that runs perfectly...but at cold start (not touching the accelerator and just turning the key) it immediately fires but just as quickly turns off. It will continue this behavior unless I 'catch' it by keeping the rpms up with a bit of accelerator fluttering (as if trying to keep a non-chocked carb car running when cold). It only takes about 20 seconds of keeping the idle up at around 1K rpms and then it idles fine. I am guessing some 'cold start' component has failed and would appreciate some education on what I am experiencing and a fix. Thank you!

cabmandone · cabmandone's Garage

The only true cold start component I can think of is the Cylinder Head Temperature sensor. I suppose if it was sensing the car was warm it wouldn't enrich the mixture for cold start.

gomezoneill · gomezoneill's Garage

Quote:

Originally Posted by cabmando

The only true cold start component I can think of is the Cylinder Head Temperature sensor. I suppose if it was sensing the car was warm it wouldn't enrich the mixture for cold start.

This^^^^^^^^^^^^^

steely · 07:37 AM

I would also check the ICV - they can get some gum in them - there are threads you can search for to clean it, pretty simple to squirt some carb cleaner in there.

I would also suggest looking for any vacuum leaks which could introduce unmetered air and have a leaning effect on the air/fuel mix. This sometimes rears it's head as temps drop.

The CHT if open, usually defaults rich.

Your warm idle should be 800 if you want to check that and adjust if reqd if you're too low to begin with. (and a low idle is a symptom of a vac leak)

76FJ55 · 11-26-2017, 07:49 AM

For the temp sensor, if you disconnect it, the computer will default to cold start enrichment. So to test if it is a cylinder head temp issue, you can just unplug the sensor before trying to start it. It should start and idle time initially, but will be extremely rich as the engine warms up.

If you've got a vacuum leak that can also cause cold start issues. One test would be with the engine warm and at idle, remove the oil cap. If the car dies, it may indicate a vacuum leak. Or there is also the possibility that the oil vent tube restrictor is no longer in place, we can go into those details if necessary.

73pcar · 11-26-2017, 08:26 AM

Thank you all for your prompt and helpful suggestions. Again, because of my unfamiliarity with this engine, can one of you point me to where the cylinder head temp sensor is? As mentioned in my OP, it takes a nice idle (around 900-1000) after holding the idle up by the accelerator for about 20 second after starting it, and therefore it is still a 'cold' engine. Fully warm the idle is virtually the same. Re a possible vacuum leak, when I was finishing up topping off the oil after I changed it the other day, I started the car and then removed the oil cap it the RPMs did drop but not significantly...but audibly lower. Is that slight RPM drop normal when the engine cap is removed while running, or does ANY drop suggest a vacuum leak? Thank you again for your help and patience in helping me.

gomezoneill · gomezoneill's Garage

It's normal for the idle to drop when the cap is removed.

The CHT is located on the left side of the engine. Not easy to change if you're a novice. You might want to see if there's a local Pelican on this board that can assist you lot's of us are willing to help out a local if we can. The connection for the sensor is on top of the engine tin mounted in a bracket with two other plugs for the speed and reference sensors. To troubleshoot you'll need a OHM/Voltage tester. I'm not sure what the specs are but I'm sure someone here can tell you off the top of their heads.If I were you I'd fix it right away it's not good to crank your engine until the rev's catch up.

I would purchase a Bentley manual this would help you sort things out.

73pcar · 11-26-2017, 09:22 AM

Quote:

Originally Posted by gomezoneill

It's normal for the idle to drop when the cap is removed.

The CHT is located on the left side of the engine. Not easy to change if you're a novice. You might want to see if there's a local Pelican on this board that can assist you lot's of us are willing to help out a local if we can. The connection for the sensor is on top of the engine tin mounted in a bracket with two other plugs for the speed and reference sensors. To troubleshoot you'll need a OHM/Voltage tester. I'm not sure what the specs are but I'm sure someone here can tell you off the top of their heads.If I were you I'd fix it right away it's not good to crank your engine until the rev's catch up.

I would purchase a Bentley manual this would help you sort things out.

Thanks again. Just to clarify, there is no excess cranking of the engine needed...it fires immediately and I just need to hold the idle up for about 20-30 seconds and then it will hold a normal idle. If I just turn the key when cold it will also fire immediately but die just as quickly if I stay away from the accelerator...hope this clear up the symptom. Thank you for mentioning the possible availability of folks to help. I'm actually not a novice at all...just unfamiliar with this critter and its fuel injection system. I've attached a photo of what I think you are referring to re the (3) connectors...is that what you were mentioning, and if so, which of the three plug in connections is the CHT one?

cabmandone · cabmandone's Garage

Top connector (white one) is your CHT.
Are you saying it idles rough when warmed up? Or does the idle hold around 900? If you're stumbling once warm I'd check vacuum lines and your Idle Air Control valve which mounts in front of the throttle body and has two hoses connected.
The fact that you noticed an audible drop in idle with the oil cap removed would seem to indicate your vacuum system is in good shape as that is normal when you remove the oil cap. If you didn't notice much drop you might want to visually inspect your vacuum lines for cracking.

73pcar · 11-26-2017, 10:49 AM

Thank you for confirming the CHT connector location so I will disconnect it before the next cold start and see what happens. I guess I'm not doing a good job at describing my condition. Car runs perfectly cold or hot and with consistent idle. My question stems from it not wanting to stay running when starting it cold...it immediately starts with a touch of the key then stalls and would keep doing this if I just kept trying to start it without touching the accelerator. So, I just hold the accelerator pedal down a bit or flutter it with some easy revs and it won't stall. After keeping the idle up for about 20-30 seconds, the car idles and runs perfectly....hence my initial thought about something in the 'cold start' system.

cabmandone · cabmandone's Garage

I think I'd start with verifying the CHT.
It was probably me not understanding exactly what you were saying more than you not describing the issue properly. If the idle is consistent and the car isn't idle searching I'd say your IACV is functioning.

73pcar · 11-26-2017, 12:30 PM

Thanks again. I have it unplugged with a reminder note in the car and will try a cold restart tomorrow and report back. Again...thank you all for your help...very much appreciated as I become familiar with my latest PCAR.

proporsche · proporsche's Garage

As the boys said it could be as easy as to clean you idle control valve.Remove it ,clean it with carb cleaner .Also what is your idle when warn.I would turn your idle screw a little out if your idle is around 800...
Ivan