1983 911sc hot start issue

[JB630]

When trying to hot start recently, the engine would fluctuate and struggle to idle but wouldn't die and after a few minutes it would resolve. Now when trying to hot start, it'll jump to idle ~1000rpm then die. I can overcome this by applying throttle but once I let off the gas the car will die. I can hold throttle, get it in gear and drive it for a few blocks then the engine will run with little to no issue. I am planning to bring it to my mechanic but wondering if anyone had thoughts?

[Schulisco]

Could be several reasons. To determine which part(s) causing the issue the following steps
has to be performed to narrow it down.

1. How is the cold start? Instantly on almost the first crank or does it takes some seconds?
2. Check the control pressure cold and warm and report it here.
3. How is the residual pressure?
4. Check the system for tightness with a smoke generator
5. How quick does the system pressure build up when starting cold and warm?

Thomas

[JB630]

Thanks Schulisco, cold starts take about 5 seconds to turn over which has been the case historically and is consistent now. I believe the cause for the extended cold start is the engine isn't very tight (in need of a rebuild) and I believe air is seeping in. The accumulator is also new within the last 500 miles. I'll have to record it again next time I take out the car but I believe it pops up to around 4 bar on the cold start and hovers between 1-2 when at operating temp. My initial thoughts are either the accumulator or the FP check valve. I am admittedly a novice at best mechanically (very generous).

[JB630]

Confirmed the car jumps to 4 bar on cold start and over the course of about 5 minutes of driving comes down to 1 at idle. Currently I can't hot start without throttle so it is reading higher than it would at idle, roughly 3-4 immediately after starting then down to 1 after driving a couple blocks to the point that the car can idle with no throttle. Likely just bringing it to the mechanic at this point but trying to understand likely issues as best I can ahead of time.

[Schulisco]

CP at 4bar when cranking and then falling back to 1-2 bar? Really? This is not normal! The cp must not jump up to 4bars when starting the engine. There's sth going wrong. First step is to check the WUR when manages the control pressure.
Then also urgently make smoke test the whole CIS! To me it looks that the system is not tight any more. Vacuum leaks can cause serious issues on a CIS and they're the most likely reasons for problems. Without being sure that there's no vacuum leak any further investigation makes absolutely no sense.
This is the pressure / temperature curve of the lambda controlled late US SCs (81-83) should look like:


Thomas

[JB630]

Thanks Thomas, this is really helpful. I think the consensus is that I will need to do a top end rebuild sooner than originally planned.

[boyt911sc]

Originally Posted by JB630

Thanks Thomas, this is really helpful. I think the consensus is that I will need to do a top end rebuild sooner than originally planned.

JB,

Why would you need an engine rebuild? Are you burning oil when you drive your car? Any leak down and compression tests done earlier? How many hundred thousand miles on your odometer? You know more about your car than anyone else.

Tony

[JB630]

There are just over 100k miles showing but I purchased the car with aftermarket wheels with tires that are a hair smaller than factory spec so it is over clocking by about 15%. The car burns a lot of oil. I'll typically add .25-.5 quarts each fuel fill up though shockingly the engine isn't leaking. There is evidence of bore scoring and a poor leakdown on one of the cylinders definitely evidence of worn valve guides. I've been slowly having the car mechanically restored as the budget allowed over the last year, it has definitely been a black box.

[Schulisco]

Mandatory literature:
https://www.pelicanparts.com/catalog/accessories/rebuilding-porsche-911-engines.htm
Also the "Porsche 911 Workshop Manual" by Porsche itself, available on the web as PDF.

Thomas