trouble with 79 911SC CIS

[fredmeister]

I have a problem with both cold start and warm start on my 1979 911SC with CIS injection that I could use a hand solving. Here is a description of what is happening and the results of system pressure readings I took th eother day:
Upon trying to start the car after sitting overnight....(COLD START) the car will fire momentarily for a half second and then immediately quit. After trying to start it this way for 4-5 times it will finally run at high idle 2000 rpm and gradually settle at 1000 rpm after 30 seconds or so as it warms up.
It will idle this way normally for however long I let it run, however, if I shut it off and IMMMEDIATELY TRY TO RESTART (WARM START) IT IT WILL NOT EVEN KICK.

Now I hooked up CIS pressure gages and got the follwoing readings:
Cold pressure= 2.3bar
Warm pressure= 3.2bar
Control pressure (cold, engine off)= 2.8bar
Rest pressure after shut down= 1.8bar
Rest pressure after shut down (20 minutes later)= 1.4bar

NOW HERE ARE MY QUESTIONS:
1) ARE THESE PRESSURES NORMAL OR OK?
2) With the gages I was able to shut the flow valve in the line between the warm up regulator and the fuel distributor when it was running and I SAW NO CHANGE IN THE PRESSURE OF 3.2BAR WHEN I OPENED AND CLOSED THE VALVE, IS THIS NORMAL?
3) WHAT WOULD CAUSE THIS COLD AND HARD STARTING PROBLEM?

Any input is really appreciated as this hass got me at wits end.

Fred

[kplackmeyer]

Fred,

Some clarification of your pressures, please

Cold pressure= 2.3bar (Is this COLD SYSTEM PRESSURE, OR CONTROL PRESSURE?)Warm pressure= 3.2bar (Again, is this SYSTEM or CONTROL?)


--System pressures are not temp dependant, only Control pressures.
--Interesting that you say if you shut the flow valve between the WUR and the DIST you saw no change in the pressure. That is how you check SYS vs CONT...how were you able to get these readings any other way?

I have attached the pressure specs from the porsche shop manual, but I have some doubts as to weather you obtained the correct readings.

Make sure your guages are connected the proper way...it matters which end of the hose is on the WUR and which is on the DIST.

[boyt911sc]

Fred,

Your residual pressure seems to be holding up. But your warm control pressure is on the low side of the spectrum. Without the system pressure to base on, it will be a wild quess. Could you measure your system pressure and see if you get 65+ psi?

At this point, I will make sure that the fuel pump is delivering sufficient fuel pressure to have a good baseline data. Looking at your numbers, the warm control pressure is low and an indication of a WUR or FP going bad.

Lastly, when you closed the valve between WUR and FD, the fuel pressure should change and go up. This part, I'm a little bit confused.

Tony

[DelPico]

My car started lousy and started warm lousy if I let it sit for a while. Replaced fuel accumulator, all bettor.

Should have done it five years ago.

[rwolak@verizon.]

Originally Posted by DelPico

My car started lousy and started warm lousy if I let it sit for a while. Replaced fuel accumulator, all bettor.

Should have done it five years ago.

you're lucky. My '83 has cold start problems (seems like its starving for gas---if I keep revs up around 2K for about it minute, it then runs fine). Hot starts are no problems. I've changed both the accumulator and fuel pump check valve. Still have the problem. I'm starting to lean towards either the cold start valve or the thermal timing switch. All I have to do is figure out how to test them while still in the car.

[fredmeister]

Kerry,
I did have the pressure gages correctly hooked up, that is pretty obvious if you understand the flow of the system. I thought it was obvious from the post that I was talking control pressure when I listed cold and warm readings. Thanks for the chart.
That being said, my question
2) With the gages I was able to shut the flow valve in the line between the warm up regulator and the fuel distributor when it was running and I SAW NO CHANGE IN THE PRESSURE OF 3.2BAR WHEN I OPENED AND CLOSED THE VALVE, IS THIS NORMAL?
This basically means that the system pressure and control pressure were the same value when it was running warm (system pressure being read when you close the valve). I expected it to jump to system pressure values when I did this so it should have gone from 3.2bar with valve open to 4.7bar when valve closed (tell me if this is correct thinking?).
I think this is what Tony is saying.
From everyone's responses so far, I am tending to think the fuel pump may be going bad, however I installed an aftermarket Pierburg fuel pump 5 years ago when the Bosch unit failed. Is this normal for a fuel pump to last only 5 years, seems a little short to me?

[stealthn]

Fred,

Might be a red herring, but I had almost the exact same problems on my 79. It turned out to be the starter. Replaced with new one and been fine ever since.

Good luck.

[boyt911sc]

Fred,

The delivery pressure of your fuel pump @ 3.2 bar is too low for an efficient start-up. You need a FP that can deliver 65 - 70 psi consistently. A used FP is OK to use as long as the delivery pressure is around 65+ psi.

In your case, when your engine is cold, the CSV (cold start valve) injects additional fuel and after multiple starts, the engine runs. This is an indication of low fuel pressure supply to open up the 6 injectors with good spray pattern. Hot start will be more of a challenge because the CSV will not be adding extra fuel during start.

Get a good working FP and I believe your car will run better. Good luck.

Tony

[Porsche_monkey]

Originally Posted by boyt911sc

Fred,

In your case, when your engine is cold, the CSV (cold start valve) injects additional fuel and after multiple starts, the engine runs. This is an indication of low fuel pressure supply to open up the 6 injectors with good spray pattern. Hot start will be more of a challenge because the CSV will not be adding extra fuel during start.
Tony

My thoughts exactly.

[T77911S]

the high idle sounds like an air leak. could be the decel valve or the aux. air valve ( dont get this confused with the aux. air reg.) the aux. air reg. (AAR) is controlled by 12v, the aux air valve (AAV) is closed byh vacuum as soon as the car starts. one of these could be "slow" to close.
i will send you my pressure readings when i get home.

[Jim Williams]

fredmeister,

Maybe i'm mis-reading something, but if your gauge is connected properly, and you are reading 3.2 bar or so, and you close the valve, you will see system pressure. When you do this you will effectively be causing the control pressure to rise to 5.2 Bar (system pressure). If the engine is running, it will die. the higher pressure in the control circuit will hold down the air flow sensor and starve the engine for any fuel.

If you are not seeing system pressure of 5.2 Bar or so with the valve closed, gauge connected correctly(engine running or not), you have a fuel pump problem, or a regulator problem in the fuel distributor.

[fredmeister]

Wanted to update everyone on this problem.
It was the FUEL PUMP after all. I replaced it with a rebuilt unit and it started right up and restarts after shutdown as well ......no problems.
Note: The idle speed now has increased slightly from where it used to run before I started seeing this trouble, so perhaps this pump has been failing progressively over time. The "bad" pump was still pumping fuel obviously because I used it to drain the gas tank before replacing it and the flow seemed strong, just was failing to provide enuff system pressure I guess.
Thanks for all the ideas everyone.

Fred

[azasadny]

Fred,
Excellent!!