Bogging Down on Cold Start?

[BrianVA]

I have a somewhat still new to me 83 911sc targa (155k miles) and I am hoping you all can troubleshoot an issue I am starting to see more frequently. It isn't a daily, but I try and get it out at least once or twice a week. Lately I have noted that on cold start ups, its starts right up but the RPMs stay under 1k until things warm up and then it idles at 1k. My biggest concern though is that by the time I get it backed out of the driveway and into 1st gear, I will give it more gas and it seem to struggle for a second or two and then it either gets enough fuel or whatever the issue is corrects itself and off I go without issue.

I have also noticed the tach bump on random occasion, which I believe is a sign the alternator is on its way out but not sure if the two are related.

So the big question is, is the bogging on the cold starts related to the injectors, the alternator, vacuum leak, fuel pump, or something else? Any info or ideas would be appreciated. Thanks.

[Schulisco]

Welcome to the club of aircooled Porsche fanatics Pretty nice car! The Targa is always a good choice and for me, behind the coupe the most desired model.

I own practically technically the same model and from my experience when being new to the car I always expect that those cars, if they have some running issues, not only a single problem is in place. Mostly it's a combination of worn parts and vacuum leaks. This means you have to check almost everything on the CIS system.

Because you criticize the cold running phase first and foremost the WUR (warm up regulator) is suspicious, also vacuum leaks and the mixture setup in general. Mostly some or, if things went bad, almost all parts are partly worn and might be just in spec or little out of spec. This is mostly not a problen on a single part, but if there are more than only one is out of spec, the problems summarize and are more and more hard to identify.
I don't want to blame anybody, but my experience is, that such cars with running problems often have been maintained by people who are not very familiar with the CIS system. And because of the presence of few adjustment points they "tune" the CIS to a point the car will run somewhat good, but even not perfect, mostly with reduced performance and poor fuel economy etc. If the owner is confident with that, I'm fine with that too. If it's my car, I'm not. And that's the point, where the quest to fix a bad running CIS begins...

What would I do?

1. Check for vacuum leaks (with a smoke generator only! Neither brake cleaner, nor cigarette smoke)
2. Check the control pressure of the WUR (cold and warm), also check the electrical connection if it's supplied properly! You need a CIS pressure measuring unit.
3. Check the AAR if it opens up on cold and closes (almost) being warmed up, also check the electrical connection if it's supplied properly!
4. Check the mixture with a exhaust analyzer and make a new ground setup if necessary

Also have a look on the superb videos from Curt from Klassikats.com: https://www.youtube.com/playlist?list=PLkWFofpgz2OgF9FdliKnlzuY0pygqRkMj

Thomas

[AlKidd]

Im no expert but on cold start the idle should be high and then come down as it warms up. What kind of temps are you considering cold? Air cooled engines do like a warm up before driving. How long are you letting it warm up? Some more details on the intake in your car will help those more knowledgeable answer. EG CIS, MS fuel injection etc.

[Schulisco]

Originally Posted by AlKidd

on cold start the idle should be high and then come down as it warms up.

Correct.

Originally Posted by AlKidd

Air cooled engines do like a warm up before driving. How long are you letting it warm up?

Sorry, absolutely not correct.
A proper working CIS engine runs when cold and even when warmed up. That's why Bosch/Porsche added such gizmos like the CSV, WUR, AAV etc. Drive it carefully as long the engine is cold with no high revs over 3500 until it's warmed up. It heats up much quicker when being on the street...warming up on idle is far morebad for the engine because it takes longer and more of unburned fuel gets into the engine oil and this washes the oil from cylinderbore if it's getting too much. Also you waste fuel for nothing...even in these days....

As you wrote: Higher idle rpms when cold about 1200, less than 1000rpms when warmed up depending on the specs, Carreras idling lower because of the quicker and fully electronic EC. The mostly mechanic injection (with a flavour of electronic).SCs idle around 900-950rpms. Mine - with another cams likes more to idle around 1000, because the CIS is too laggy to control the mixture to idle on lower rpms.

Thomas

[yelcab1]

When I had my 80SC, I was able to make the WUR adjustable so that I could deal with the cold start fuel pressure. As for warming it up, I never did. Started the car and drove off.

[theiceman]

start and drive , if you cant, something is wrong , never let ANY car idle to warm up.

[Johner]

Hi Brian

Nice looking car! Congratulations

Another thing to throw into the mix is the lambda system (assuming it's a US car). I'm not familiar with it, but there may be some simple checks you can make to confirm that's operating correctly

Other than that, follow Thomas' advice and get set up to check for vacuum leaks and fuel pressures. You can get set up for these tests for less than $200 and if you plan to keep the car long term they are a good investment. The tests themselves are pretty strsightforward, lots of threads on here describing the set up.

While you wait for those you can do a simple test for vacuum leaks by removing the oil cap whilst idling (warm) to check if revs drop. That won't be conclusive, but if they don't change then it gives you a clue that you have a vacuum leak. Checking timing is another thing you can do independently if you have access to a timing light.

John

[BrianVA]

wow guys thats for all the input, great stuff here. I def need to do more research on the issues but will do the oil cap test while idling and start there. I never let it really warm up when starting it as I was always told that wasn't great for air cooled. I don't start it up and immediately tear out of the garage, but I think I am typically started and done within a minute at most. I will keep you all posted on this.

83 911 has always been my dream car (I was born in 83) so I def plan on keeping this one for awhile.

[Schulisco]

Brian,

because you're new to the aircooled 911s....
Lucky man...I was born in the early 70s, and the Carrera RS is far beyond my budget...

There's a lot of literature available, even online and offline.

I want to recommend some literature to you:

Online: https://forums.pelicanparts.com/porsche-911-technical-forum/725532-sc-service-manual.html

Offline:

1. https://haynes.com/en-us/porsche/911/1965-1989 (the Haynes) Not briefly. but better exploded drawings than the Porsche Workshop manuals
2. Porsche 911 SC Service Manual from Bentley (the Bentley) https://www.amazon.com/dp/0837617057
3. Bruce Andersons 101 projects on Porsche 911 (https://www.amazon.com/dp/0760308535/)
4. Paul Frere - Porsche 911 development history (THE bible to the background knowledge for the 911), a must read for any 911 enthuasiast
https://www.amazon.com/dp/1859608396/

There's a lot more, but those are some og most important books from my perspective...

Watch those videos from Curt / KlassikATS.com, they explain more than googleing around for hours, after that you can start reading to get the complete picture!

Have fun!

Thomas

[boyt911sc]

Brian,

Before you start your car in the morning do this test. Pull out the electrical plug to the WUR and measure the heater resistance (Ohms) using a multi-tester meter. Post the value. You should have a WUR-090 for a US spec ‘83 SC. Keep us posted.

Tony

[AlKidd]

I didnt say let it idle till warm, I said air cooled engines like a warm up which is what happens when you drive gentle till the gauge starts to climb. Remember these are oil cooled engines and the air cools the oil. There were no details in the initial post as to what is cold, where he lives etc. I let it warm up a little longer at 40F than I do today at 100F.