Hot restart problem CIS Problem

[JIMF.911S]

Porsche is 1977 2.7L 911S, 62K miles. I've replaced the fuel accumulator, the fuel filter and the injectors with Bosch parts. I replaced the fuel pump two years ago with Bosch part number 911-608-102-00-M14.
Left [at operating temperature] overnight the engine won't start the next day. Lifting the sensor plate making the pump run for a few seconds will get the engine going.
When I've left the engine bay lid open it usually starts.
A few posts mention the fuel pump check valve as a possible problem area.
I'm seeing Bosch # 893-906-093 M-14 as a check valve that fits my car. I don't recall seeing it when I replaced the pump. Did the original pump have an integral check valve?


Thanks in advance for your help with this aggravating problem.

[coomo]

Originally Posted by JIMF.911S

Porsche is 1977 2.7L 911S, 62K miles. I've replaced the fuel accumulator, the fuel filter and the injectors with Bosch parts. I replaced the fuel pump two years ago with Bosch part number 911-608-102-00-M14.
Left [at operating temperature] overnight the engine won't start the next day. Lifting the sensor plate making the pump run for a few seconds will get the engine going.
When I've left the engine bay lid open it usually starts.
A few posts mention the fuel pump check valve as a possible problem area.
I'm seeing Bosch # 893-906-093 M-14 as a check valve that fits my car. I don't recall seeing it when I replaced the pump. Did the original pump have an integral check valve?


Thanks in advance for your help with this aggravating problem.

You need to carry out a pressure test.your likely losing pressure.You can carry on replacing parts forever.

[Hx911]

Originally Posted by JIMF.911S

Porsche is 1977 2.7L 911S, 62K miles. I've replaced the fuel accumulator, the fuel filter and the injectors with Bosch parts. I replaced the fuel pump two years ago with Bosch part number 911-608-102-00-M14.
..........

did the problem start then (2 years ago) or just recently?

[JIMF.911S]

@coomo
The fuel pump was replaced because it was dead after sitting for at least five years.
The other parts were replaced because they were 43 year old original parts.
For approximitly US$400 and two hours time I've eliminated the what should have been the issue related to vapor lock. I'm not sure about the check valve. Is it internal to the pump or is somewhere else in the fuel system?

[JIMF.911S]

@Hx911
This started recently. Last month after sitting for 10 months[!] I cranked the starter and after a few attempts the engine fired up. Drove it to operating temperature[less than 200f.] Next day it would not start.
The engine idles like a Swiss watch and the car runs like bat outta hell.
It will restart immediately just not when left for an extended time.

[Walt Fricke]

What you are describing doesn't make sense. You say your engine is warm after being off over night. That is a cold start, not a hot start. And it is normal for the residual fuel pressure to be all gone after sitting over night.

The check valve and residual pressure specifications are, indeed, designed to keep some pressure in the system while the engine is hot, which prevents vapor lock.

On the other hand, lifting the sensor plate is the time honored way of dealing with vapor lock, which is a hot start issue, and often caused by the lack of residual pressure.

So how does your engine remain hot or warm (above ambient temperature) over night?

[JIMF.911S]

Walt
I ran the car early this afternoon. After a few hours with the engine lid closed and the engine still warm it would not start. Cranked six or eight times for more than thirty seconds ......nothing.
Lift the sensor plate, hear the injectors whine, turn the key and it fires right up.

[kach22i]

The brake booster operates off vac pressure from the intake manifold. There could be a leak in this line.

Until I can replace the line via partial engine drop I put some goop on the large flexible hose running along the firewall that connects under the car to a metal junction. My leak was near the connection under the car.

This improved both cold and hot starting.

No leaks using starting fluid method at intake manifold gasket.

I have never seen a thread on this topic.

Hose may be weak and collapsing.

You can order new hose from Pelican and cut to length.

[kach22i]

Pictures




I also readjusted the fuel/air mixture for cool weather prior to this, and think that helped.

[boyt911sc]

Originally Posted by JIMF.911S

Walt
I ran the car early this afternoon. After a few hours with the engine lid closed and the engine still warm it would not start. Cranked six or eight times for more than thirty seconds ......nothing.
Lift the sensor plate, hear the injectors whine, turn the key and it fires right up.

Jim,

You have a serious fuel delivery problem. Use a fuel pressure gauge to test and diagnose this problem. And I will bet some money that you have unmetered air going into the system and low residual pressure. Your fuel pump could deliver more than 100 psi. of fuel pressure and could expel any vapor lock people like to claim. Vapor lock occurs in carb system because they operate at less than 5 psi. system pressure. CIS particularly a 930/turbo uses more than 100 psi. of system pressure. Your CIS has 65~75 psi. system pressure.

How much pressure do you thing a vapor lock could generate to prevent a 100 psi. system pressure from flowing. The biggest problem in solving CIS problems is the lack of understanding about the system.

Tony

[stlrj]

Originally Posted by JIMF.911S

Left [at operating temperature] overnight the engine won't start the next day. Lifting the sensor plate making the pump run for a few seconds will get the engine going.
When I've left the engine bay lid open it usually starts.

This is very confusing. Are you talking about a cold or warm start problem? How can your engine be at operating temp overnight?

[Walt Fricke]

Tony - I've always wondered why the pump wouldn't push the air through promptly. But vapor lock (or at least what is conventionally called vapor lock) is real, at least in some 911 models - the ones with the fuel pump mounted on the rear torsion tube. I've seen it happen with some frequency on, for instance, the 1974 models at the track. One driver even installed a plastic funnel in his engine compartment so he could pour ice water down directly onto his fuel pump to get his car to restart after a track session. The higher the pressure in the fuel line, the higher the boiling temperature of gasoline.

The 930s high pressure pump is back in that same place, isn't it?

Perhaps the difference is that air in the pump will prevent it drawing fuel from the tank? As well as preventing whatever air pressure is created by the pump from pushing fuel out the injectors? Though if you pump your overheated brakes long enough, you usually get some useful braking from them? Pressure is pressure.

But doesn't he have the classic symptom of failed residual pressure? Since the engine is hot or at least warm, the WUR isn't enriching the mixture? And the CSV isn't allowing that system to function, because the points in the sensor are open due to the heat? Even his mention, which baffled me in his first recounting, of not having this problem when he left his engine lid open, could point to the engine having cooled off more under those circumstances. In theory, his engine should start fine when warm/hot, because warm control pressure will restart such an engine just fine absent something wrong.

And when cold, it starts just fine. And when he raises the air flow plate, it starts fine - as did the '74s at the track when you did that. Raising the air flow plate is the equivalent of enriching the mixture, as with the CSV, is it not? You wouldn't expect richening a mixture which shouldn't need to be enriched (as with a warm WUR) would help? But it does, so something is going on here. If his relatively new fuel pump (up front) is failing, why isn't he having cold start as well as hot running problems?

Of course, if he would just do a standard CIS pressure checks, the problem could most likely be pinpointed. But he doesn't have that instrument.

The stock 1977 2.7 fuel pump has an integral reverse pressure valve, as I recall. When you look down the outlet opening of the pump you see what looks like a white plastic cross, which is the valve. Comes with the pump.

[JIMF.911S]

Thanks to all the members who have responded.
I will try and clarify my original post. When I try to start the engine from cold it will not start.
After lifting the sensor it fires right up. Idles rock steady at 950 RPMs. After a drive and reaching operating temperature I could shut down and the car would restart immediately. After a very few hours restarting would be difficult. Left for an extended time I'm back to having to lift the sensor plate.
The sound of the injectors changes after a few seconds. Could this sound be gas displacing vapor in lines from the fuel distributor?
I also checked all the hoses for tightness including the brake hose underneath.
I'm ordering a fuel pressure gauge today.
Again, thanks to all.

[JIMF.911S]

UPDATE
After a spirited drive and an operating temperature of +/-200 the car was garaged 4PM FRI.
I left the engine lid open. Car started this morning immediately.

[targa80]

There have been many many posts on warm start problems on this forum. All of the Pelicans who have had this issue (me included) and took the time to educate themselves on the CIS system for their model year and have resolved said problem with troubleshooting logic and proper test equipment. Naturally, you have to have the desire to expend the time and effort along with support from this forum. Look for the post on CIS for dummies written by Tirwin. That post is about as good as it gets when you need to educate yourself on the CIS.

Now, I’ll get off the soap box and try to provide some useful info based on my own experience. When I purchased my 1980SC in 1991 car would fire right up cold and run great. After warm up shut off the engine and wait 15 minutes car would crank and crank for along time before starting. This was due to the loss of residual fuel pressure in the system.

1. The car starts right up cold because of the cold start valve which only injects fuel into the intakes during a cold start . This masks the loss of residual fuel pressure.

2. Trying to start a warm engine with no residual fuel pressure and no activation of the cold start valve will crank for a long time until residual pressure is built up.

3. Fuel pump will either have an internal or external check valve based on model year. This is the first part that maintains residual fuel pressure.

5.The fuel filter may have a leak at one of the fittings.

6.The fuel accumulator has a spring loaded diaphragm that maintains a constant fuel pressure provided by the running fuel pump and for a long period of time after the pump is shutoff. If the diagram is ruptured or leaking the fuel will leak back to the fuel tank through a return line on the accumulator or leak out of the accumulator if it doesn’t have a return line.

To test these components you need a fuel gauge test rig to isolate the each of the components. In my own case I started with the fuel pump and found the internal check valve was shot. Purchased an external valve and resolved the warm start problem. That was over 29 years ago. The car has not had that issue since. Now to fix the recent blown air box.