Why do I have a hard hot restart?

I replaced my fuel pump, fuel distributor, fuel injectors, air box check valve, fuel filter and fuel accumulator and I still have a serious issue with restarting a hotnormal Temp. engine. Now thinking about replacing the car. Can anyone please advise. Frustrated 1979, 911SC, 3.0L, owner.



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This post was auto-generated based upon a question asked on our tech article page here: Pelican Technical Article: Fuel Filter and Accumulator Replacement - 911 (1965-89) - 930 Turbo (1975-89)

[Nick at Pelican Parts]

In between the fuel distributor and the air intake below is a 3mm adjustment to fuel mixture that may need adjustment since you replace the fuel distributor. Also there should be a warm up regulator, cold start injector and frequency valve that all have an effect on how much fuel is injected on a hot restart. Are you lean on the restart (will it start easier with starter fluid) or are you flooding out (do you need to open the throttle all the way to get it started. Pull a spark plug and see if it is clean or black and full of soot.

- Nick

[rubing]

Hey Nick! I have a similar problem with my 3.0sc on warm start (after leaving sit for a few hours). My issue as you describe above is that i need to open throttle all the way to get it started. Start right up in either cold or after just driving...only have this issue after sitting a few hours.

[boyt911sc]

Find someone who knows and understands CIS in and out. Not all good mechanics understand this old and antiquated fuel injection system. Good luck.

Tony

[stlrj]

Try grounding the red/black terminal connected to the thermo time switch and see if it starts.



Cheers,
Joe

[chris_seven]

Have you checked to see if the system is holding pressure?

[T77911S]

check valve at the fuel pump.

start with fuel pressures.
timing, not a big one but check it.
mixture setting.

how do the plugs and ignition look.
what plugs?

[pensacola mike]

Don't sell the car! I had a similar problem, after I replaced the fuel lines I didn't have anymore hard start issues after running the car. The fuel lines were bad shape, cracks and eventually leaking and had to be replaced. I wish you the best.

[Alansglide]

Had this exact same problem .
After months of dealing with it and being stranded multiple times I took it into the shop.

Final solution , a faulty cold start injector

[T77911S]

hot start issue.

could also be FP is not running in start position. you would think this would also effect cold starts but the CSV works here. with hot start CIS needs fuel pressure to operate. residual may not be enough.

try removing the connector from the top rear of the AFM. this will run the pump with the key on and engine off.

[al lkosmal]

Lots of new parts, which is a good thing, but has the system been checked to verify that it is within spec?

A good place to start is testing the warm/cold control & system pressures. This will verify if the CIS is working to spec.

I also find it very handy to have an o2 sensor hooked up to monitor/verify the A/F ratio while testing.

The cold start valve only operates when it is cold...I.E. for cold starts. The thermotime switch activates the cold start valve..based on temp, so should not be an issue with warm starting.

Is there a check valve installed on your fuel pump? If not, this can cause your problem.



Where are you located? There may be a Pelican close by that can help with the system testing and debugging.

[ericbash]

If you put in a Pierburg fuel pump, you may need to get creative with a check valve. Here's how I did it on my 75 CIS 911. See post #104.

Born in '75. Me and my 'new' 911s.

[aoncurly]

Can someone answer if Ethanol in all gases has an effect? I've heard that vapor lock occurs more readily because of Ethanol in the gases nowadays. I have this same problem with hot starts.

[T77911S]

no, there would be a LOT more people complaining of hot starts.

[boyt911sc]

Originally Posted by aoncurly

Can someone answer if Ethanol in all gases has an effect? I've heard that vapor lock occurs more readily because of Ethanol in the gases nowadays. I have this same problem with hot starts.

Aoncurly,

Gasoline and alcohol are incompressible fluids. Vapor lock problem common in carburator (low pressure) system will not happen in high pressure fuel injection system like CIS. Even if you are able to vaporize the fuel inside the fuel line, you can not stop or prevent the fuel from flowing through the lines with 75 psi. (100 psi. for 930) system pressure. The hot start problem you are having is not caused by the fuel per se, but your engine set up. Test and confirm your residual fuel pressure, ignition, fuel mixture, and unmetered air. Don't blame today's fuel unless it is contaminated. We all use the same fuel in our cars. How come you are having problem and we don't. If fuel is the culprit, we would be all having similar experiences. My two-cents.

Tony

[rrichar]

Very likely the check valve for the fuel pump, a common hot start problem for CIS systems.

[stlrj]

Originally Posted by al lkosmal

The cold start valve only operates when it is cold...I.E. for cold starts. The thermotime switch activates the cold start valve..based on temp, so should not be an issue with warm starting.

So what is the purpose of the thermo time switch that has 45*C stamped on it? Is that really that cold?

Joe

[boyt911sc]

Originally Posted by stlrj

So what is the purpose of the thermo time switch that has 45*C stamped on it? Is that really that cold?

Joe

Joe,

You have been fascinated with the TTS and CSV operation and keeps suggesting about these with regards to hot start problem. Several times I called your attention about your misconception and flawed understanding how they work. The operating temp. for the TTS is 45°C (113°F). Below 113°F it is closed (grounded) and above 113°F it is (open). The only time the CSV could be energized is when the TTS is grounded to complete the circuit 113°F and below with the ignition switch @ START position.

Above 113°F, the TTS is open and the CSV is disabled because of the opened circuit. Why 113°F? Porsche did not set this value. Bosch determined it and used through out the years since the introduction of CIS in the auto world. This is a Technical Forum and we are just discussing the technical aspects of things. We all make mistakes and I made a lot of them too.

Tony

[mysocal911]

Originally Posted by boyt911sc

Joe,

You have been fascinated with the TTS and CSV operation and keeps suggesting about these with regards to hot start problem. Several times I called your attention about your misconception and flawed understanding how they work. The operating temp. for the TTS is 45°C (113°F). Below 113°F it is closed (grounded) and above 113°F it is (open). The only time the CSV could be energized is when the TTS is grounded to complete the circuit 113°F and below with the ignition switch @ START position.

Above 113°F, the TTS is open and the CSV is disabled because of the opened circuit. Why 113°F? Porsche did not set this value. Bosch determined it and used through out the years since the introduction of CIS in the auto world. This is a Technical Forum and we are just discussing the technical aspects of things. We all make mistakes and I made a lot of them too.

Tony

Good post, as many seem to not understand when the CSV contributes to starting a 911 CIS engine.

[al lkosmal]

Originally Posted by stlrj

So what is the purpose of the thermo time switch that has 45*C stamped on it? Is that really that cold?

Joe

It is monitoring engine/oil temps and it does not take long for your engine/oil temps to reach that temperature (45C/113F), after starting, and it takes a long time for your engine to cool off after running at 180 F- 200+F for a while.

Thermotime switch typically will be grounded and activate the cold start valve when the engine is cold and 1st started....but the engine will quickly reach 45C and higher and then the Cold start valve will not be activated....

This seems to be in alignment with your experience.....fires up ok when cold...but not when the engine is warm...........hence the name....Cold Start Valve.

Do you have a check valve on your fuel pump????

regards,
al