Cold start problem

I'm looking for suggestions on tracing down a cold starting problem on my '79sc. The engine fires right up even when cold but stalls out after only a few seconds. Usually on the 3rd try it will hold its own and run very rough for about 30 seconds before it smoothes out. I'm worried that it will get worse as the weather gets colder. What are the most common causes for this problem??

thanks.

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Dan

This seems to be a very common problem. The fact that it fires up right away, suggests that the thermotime switch and the cold start valve are working properly. The next thing that should happens is that the engine should idle at about 1800 rpm, and then slowly reduce to 950 rpm in about 4 or 5 minutes. When your engine finally idles without stalling, what is the idle speed? If it's not increased over normal, that suggests that the aux. air valve is not opening when cold.
Also during cold starting, the warm up regulator enrichens the mixture by decreasing the control pressure to the fuel distributor. The warm up regulator may be the trouble. The CIS system has lots of little components all over the engine connected by fuel lines and vacuum hoses. There are good books on Bosch fuel injection that do a good job of describing the different CIS systems, and have procedures for troubleshooting each component.

I'm still working on a very similar cold start problem on my 79SC (as are many other people).



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Bill Krause
'79 911SC Euro

Check to make sure all wires are connect.

I found a couple of loose ones on my 82 SC engine. What a difference!!!

Now starts and idles very nicely when cold


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Smoke Daddy

We all have 79 SCs ...funny...
Must be a common thing because mine does the exact same thing you all describe. I would suggest trying the thermotime switch first as it is a hell of alot cheaper than the WUR.
Good luck!

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Leland Pate

___79 SC Targa

Could be a variety of problems. Bill's comments are highly accurate as are Jim's. I've also seen similar problems that I have attributed to the all-too-familiar residual fuel pressure leakdown problem. Typically the car has the most trouble starting when hot. the cold start valve will spray since it takes littel pressure, but the cart then stalls after a few seconds when the CSV quits spraying. The cylinder's injectors don't spray until the fuel pressure comes up a bit. If this doesn't happen soon, the car starts and stalls several times before running, poorly on just a few cylinders, then slowly adds cylinders as injectors open to the rising fuel pressure.

I confess I don't have a fuel pressure testing set but have always wanted one. They are essential to CIS trouble shooting. ESSENTIAL. And now they are much less expensive I hear.

Anyway, it could be a variety of things and there are procedures for isolating them one by one. For the home mechanic this is far less expensive than guessing. For the commercial mechanic, guessing is the more lucrative option.

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'83 SC

I bought the fuel pressure tester from JC Whitney ($65). I just have not had a chance to use it yet. I'm pretty convinced that my problem is the WUR, and hopefully the pressure tester will show that. If it is the WUR, I'm going to open it up and see if I can see any obvious problem (debris, broken diaphram etc). My 79SC is a ROW version. I think the WUR on it is different from the US, as is the fuel distributor. Hopefully I'll be able to fix it myself, or find a rebuilt unit with the right part number. Does anybody know the specs for a ROW warm up regulator? What should the system pressure be, what should the control pressure be when cold, and when warm?



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Bill Krause
'79 911SC Euro

The Fuel accumulator retains a residual amount of fuel and pressure for the initial start up--both cold and warm--but especially when warm. The tell tale sign of it going bad is HARD starts.I'll say your cold start valve is doing it's job too. Your 2-3 attempts to start might be the system building up operating pressure--a job for the accumulator.
Try breaking the line going from the accum. to the engine. Do this hours after it ran-or up to a day later--if you get a quick spritz of gas then your accum. is ok. It should have an ample amount of pressure stored in the diaphram from the last time it ran.
you could do some research yourself.Get the BOSCH Text for european vehicles--damn near college bound--and extremely detailed on all the J-K-L and Motronic Jet systems.



[This message has been edited by Tlook (edited 09-27-2000).]

[This message has been edited by Tlook (edited 09-27-2000).]

I think the accumulator's job is to hold system pressure, not to build it up. That would be the fuel pump's job. I had disconnected the air flow detect switch, which will cause the fuel pump to come on before you start the engine. I would let the fuel pump run for 5 to 10 seconds (plenty of time to get pressure), but it would still cold start in the same way. The only difference was that instead of a 1/2 second lag before the engine fires, it would fire immediately. It would still settle into a very rough low idle and sometimes stall.

What's the complete name of the Bosch text that you mentioned. My Bosch book is a good primer on all Bosch systems, but lacks specific details of my K-Jetronic system.



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Bill Krause
'79 911SC Euro

I have the same problem with my 78 911 SC 3.0. But, I didn't have that problem until I adjusted the airflow sensor plate. The car would start up immediately and ramp up to 2000 rpms and then idle down to 1000 after it warmed up.

I was trying to lean out the fuel distributor and I adjusted the airflow sensor in the process. So, I would suggest doing a fine adjustment to it. Be careful and not adjust it too much because it can cause the engine to backfire. Use the Haynes manual as a starting point and adjust up or down to see if it corrects the problem.

Steve

I think you can compensate for the cold start problem by setting your mixture to run rich all the time. So when you set your mixture properly, the cold start problem shows up. All this still points to the WUR not richening up the mixture when cold. I'm going to check my fuel pressures this weekend and get this damn problem solved.



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Bill Krause
'79 911SC Euro

Another good reason to avoid a lean mixture. It may not need to be "rich," but lean is probably not the way to go. Fiddling wiht CIS systems incurs the risk of airbox explosions. Be sure your Popoff is ready for that.

Air flow sensor plates are tricky to unbolt and reset. If you do, here's a trick: To make sure the plate is centered, wrap a long piece of paper around the circumference of the plate. This shoudl hold it equidistant from the edges of the cavity it sets in.

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'83 SC

I think I can understand how a bad accumulator could affect cold start. If the diaphram is ruptured, the accumulator might take a while to fill up with gas, which would keep system pressure from coming up to the correct amount. Maybe fuel pressure is enough for the cold start injector to work, but not enough for the regular injectors. As soon as the accumulator has filled up (through the ruptured diaphram), then fuel pressure goes up. I'm not sure if this can explain my problem, but it should easy to test if the accumulator is holding pressure.

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Bill Krause
'79 911SC Euro

FUEL ACCUMULATOR: Damps fuel-pressure surges and holds residual pressure in the system when the engine is shut off. Damping action protects fuel distributor from the rapid build up of fuel-pressure during start up. When the engine stops, the accumulator holds fuel pressure to help prevent vapor lock, and improve hot starts.
If your thinking the mix should be richer on cold start, then examine the control pressure regulator..it has the bi-metal arm and electrical winding in it that is manipulating (or should be) the mix in the first place. It's located on the engine block-left I think-next to one of the intake runners--square. Originally called the WUR-warm up regulator.
These are quotes from the book I mentioned
"how to understand, service, and Modify--
BOSCH FUEL INJECTION & ENGINE MANAGMENT
by Charles O Probst, SAE
Bentley publishers
/http://www.rb.com

I've had my eye on the WUR for a long time. Unfortunately, it's not a cheap thing to replace on a hunch. I've ruled out just about everything but the WUR and the accumulator.

I purchased a CIS tester, and will be able to fully test the WUR, and the accumulator. Hopefully it will be cold enough here in Minnesota this weekend for the symptoms to show.



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Bill Krause
'79 911SC Euro

my 79sc has done this from day one. I'v made a point never to give it any gas at start up and once the idle drops (with in seconds) I'm on my way. I believe the problem gets worse in the Winter months because, cold thick 20/50 oil, it slows everything down at start-up.

I switch to synthetic 5W30 when the weather gets cold. That helps cold starts. I've been living with this problem for a long time too. The only time it really bugs me is that sometimes rather than recovering from the rough idle, it will stall, and then backfire when I try to restart it. I've got a pop-off valve, but still I don't like backfires. There must be something unique to the 78 or 79 SC's that is causing this problem. Maybe the WUR just isn't a very good match for the engine. I've seen the articles on this website about modifying your WUR to make it adjustable. Maybe that's what it will take.

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Bill Krause
'79 911SC Euro

Bill -

I don't think this is unique to the 78s and 79s. I believe my 80 acts exactly as yours does on cold starts. Turn the key and WHAM! it fires instantly, and then dies a second later. Then try again, and it cranks and acts exactly like fuel pressure is building up. After a few seconds, it catches and runs. But then mine runs slowly while warming up, which makes me think that part of it is related to the WUR. So my hunch is that I need to deal with the check valve/accumulator and the WUR in order to cure these behaviors.

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Greg Slater
1980 SC
gslater@uswest.net

I finally got my CIS fuel injection test hooked up and was able to take some measurements. System pressure was 74 psi, control pressure (at 36F) was about 15 psi (with power to the WUR disconnected). I connected the WUR, and control pressure ramped up slowly to about 50 psi after about five minutes. Looks like the WUR is doing it's thing correctly. Also, the residual pressure held about 20 psi for over 30 minutes, so that's well within spec also.

I let the car sit over night. With the pressure gauge still connected, residual pressure was down to 0, air temp was a cool 26F. I had my wife start the car so I could watch the control pressure while the car rough idled, but instead, it started up and ran just fine. Fired up, and went right to a 1900 rpm idle and then slowly idled down to 900 rpm.

I guess connecting the pressure gauge fixed it???? I'm thinking maybe I had some debri either in the WUR or in the fuel distributor that maybe came loose. I'm going to try this a few more times before I disconnect the pressure gauge.

Any ideas?


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Bill Krause
'79 911SC Euro

Cold Start saga continues.

I opened up and cleaned my WUR yesterday. It didn't look bad at all, but I don't think it would take much to gum up the inside of it.

Today, I cleaned my fuel distributor. But, as I was taking the air boot off, I saw that the inside of the air funnel, and the inside of the throttle body were covered with a black sludge (oil overflow no doubt). I've had the car nine years, and have never over flowed the oil, so this stuff has been in there for a very long time. I cleaned it all up to a nice shine with carb cleaner, and proceeded to clean the fuel distributor (it wasn't bad looking).

After it was all back together, I started the car up. It ran, but very rough. I had my CIS tester connected, and the control pressure looked OK. I reached into the airbox with my hand and gently lifted the air flow plate, and the engine started running smooth at a high idle.

THEORY: All of that sludge on the inside of the air funnel caused the mixture to be rich because the obstruction of the sludge would cause the sensor plate to rise more. My previous mechanics, rather than cleaning the funnel, simply adjusted the mixture leaner. Now that the sludge is gone, the mixture is way too lean. I believe this has been the cause of my cold start problems, because the sludge also messed with the geometry of the funnel, that causes enrichment of the mixture at idle.

QUESTION: I want to roughly adjust my mixture. What is the proper tool to adjust the mixture with? Looks like a thin, no handled screwdriver or something like that would work.



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Bill Krause
'79 911SC Euro

It's a 3mm allen head. Turn clockwise to richen. Only turn it a tiny bit, what seems like barely anything at a time, until it idles OK. A 1/64th of aturn is probably all that is necessary. Anything more than a 1/8th turn is too much. After you turn it a bit, blip the throttle and let it stabilize. The screw you are turning is mounted directly on the sensor plate arm so don't push down on it, just let the weight of the tool drop it into its spot. And like I said, don't get crazy with the adjustment.