My 1978 911 targa SC is a problem to start when hot. Cold starting is fine and hot starts if left for just a short time is fine. But when it’s been run and left for about 15mins its a problem to start. The engine has just been rebuilt runs and pulls excellent and has good oil pressure. Runs in normal range on temp gauge. Any thoughts on the hot start problem?
Hot starting problem 911 SC
- Thread starter Brian S
- Start date
Hi Brian,
I had a similar problem with my 1978 SC. I tracked it down to the fuel pump valve loosing pressure.
I put a new pump on and had no issues after that.
The only way I could get my SC to go was to hold the throttle to the floor and crank the engine.
It would eventually start.
Cheers,
Rich
I had a similar problem with my 1978 SC. I tracked it down to the fuel pump valve loosing pressure.
I put a new pump on and had no issues after that.
The only way I could get my SC to go was to hold the throttle to the floor and crank the engine.
It would eventually start.
Cheers,
Rich
do these older models have a crank position sensor?
nerosdad
New member
Hi, the truth is there is a number of things it could be. A good suggestion above about fuel pressure (there is a valve called the accumulator) next to the fuel filter. If that's had it can be very tough to start but you'd typically see issues starting period if that was gone.
In answer to the last post, no there is no crank sensor (it all predates that kind of magic
)
Before you jump in a buy things, I'd be tempted in asking the folks who rebuilt the engine to take a look, you'll need fuel pressure gauges to check for fuel related issues and maybe she needs tuning / air mixture setup etc.
Good luck
In answer to the last post, no there is no crank sensor (it all predates that kind of magic
) Before you jump in a buy things, I'd be tempted in asking the folks who rebuilt the engine to take a look, you'll need fuel pressure gauges to check for fuel related issues and maybe she needs tuning / air mixture setup etc.
Good luck
I haven't had a hot start problem myself (mines usually cold starting) but I've read a failing fuel accumulator can also cause hot start issues - Haynes/workshop manual also mentions it as a likely cause.
Have heard it's so easy and expensive to end up randomly replacing each component bit by bit to try to cure and a fuel pressure gauge set to help diagnose is the ideal - for a leak down test in this case. Should still hold a certain minimum system pressure after 20mins. I assume you'd have to try and clamp the fuel pump line to prove if pump valve or accumulator?
If you get a rich/flooded mixture after starting from hot - i understand you would also have to suspect one of the six CIS cylinder injectors leaking and not closing fully.
Have heard it's so easy and expensive to end up randomly replacing each component bit by bit to try to cure and a fuel pressure gauge set to help diagnose is the ideal - for a leak down test in this case. Should still hold a certain minimum system pressure after 20mins. I assume you'd have to try and clamp the fuel pump line to prove if pump valve or accumulator?
If you get a rich/flooded mixture after starting from hot - i understand you would also have to suspect one of the six CIS cylinder injectors leaking and not closing fully.
Make sure you get someone who knows how to test the fuel pressure. I did not and am now the owner of a rebuilt fuel distribution unit, fuel accumulator, couple other bits. Still did not hot start and turns out it was the fuel pump. Probably wasted £2000 chasing the problem because there was no one local to me (France) who knew what to look for. Ended up trailering the car back to UK and fixed in one day.
vitesse
Active member
Its the same Bosch Continuous Fuel Injection system as used by the 924,928, BMW ,Mercedes ,Audi vehicles of the same era-Bosch do a very good service manual which you should be able to download.
I have a copy which I found invaluable when I re-commissioned a friends 1976 924.
Main problem is the Fuel Distributor which has a central sliding needle which can stick plus a pressure relief valve which can need adjustment with shim washers.
All items need the help of a fuel pressure test gauge device which I made for around £32 and with a multimeter,one can test the ancillary items -cold start valve etc.
However it can make sense to drain the fuel tank ,remove the take-off closure plate & check for C**P which can completely ruin the Distributor unit .
I rebuilt the 1st one successfully but unbeknown to me his fuel tank had half a handful of tealeaf like c**p piled on the outlet which then totally contaminated the distributor.
I have a copy which I found invaluable when I re-commissioned a friends 1976 924.
Main problem is the Fuel Distributor which has a central sliding needle which can stick plus a pressure relief valve which can need adjustment with shim washers.
All items need the help of a fuel pressure test gauge device which I made for around £32 and with a multimeter,one can test the ancillary items -cold start valve etc.
However it can make sense to drain the fuel tank ,remove the take-off closure plate & check for C**P which can completely ruin the Distributor unit .
I rebuilt the 1st one successfully but unbeknown to me his fuel tank had half a handful of tealeaf like c**p piled on the outlet which then totally contaminated the distributor.
Thank you all for such insightful comments and help on my hot start issue.
i have ordered a new fuel pump as that appears to have been the issue that a few of you have referred to. It’s only a £200 part and easy to fit.
if this doesn’t cure the problem then it’s off to a specialist garage for a full pressure test.
i have ordered a new fuel pump as that appears to have been the issue that a few of you have referred to. It’s only a £200 part and easy to fit.
if this doesn’t cure the problem then it’s off to a specialist garage for a full pressure test.
Hi,
Having gone through this issue with my 924 a few times over the last 10 years Vitesse covers things well. A fuel pressure gauge will give you a better idea of what’s happening and what your residual fuel pressure is doing (dropping like a stone would be my guess). It normally drains back through the pump if the non return valve is faulty or draining down to the tank if the pressure bleeds out - all this can be identified with the gauges.
As said above finding a garage who understand CIS injection which might be a struggle will save you hundreds in replacing random parts which are fine.
thanks
Having gone through this issue with my 924 a few times over the last 10 years Vitesse covers things well. A fuel pressure gauge will give you a better idea of what’s happening and what your residual fuel pressure is doing (dropping like a stone would be my guess). It normally drains back through the pump if the non return valve is faulty or draining down to the tank if the pressure bleeds out - all this can be identified with the gauges.
As said above finding a garage who understand CIS injection which might be a struggle will save you hundreds in replacing random parts which are fine.
thanks
Hi.
Does anyone know what the system residual pressure is supposed to do "long term"?
All details seem to only state a minimum pressure that must be maintained after 20minutes (only) - which mine does no problem. The following morning on testing, it's starting from 0psi/bar. I can't find if thats as expected or not.
Then when cranking from cold, my system/control pressure only creeps up in 2 to 3 stages of 5-10 psi each time - until something like 30 psi is reached - then it fires - that's about 3-4 cranks.
I can't find any reference to this test and how quickly system and control pressure should pick up from cold cranking and if my slow build up I'm seeing is an indicator to anything.
Best regards
Neil
Does anyone know what the system residual pressure is supposed to do "long term"?
All details seem to only state a minimum pressure that must be maintained after 20minutes (only) - which mine does no problem. The following morning on testing, it's starting from 0psi/bar. I can't find if thats as expected or not.
Then when cranking from cold, my system/control pressure only creeps up in 2 to 3 stages of 5-10 psi each time - until something like 30 psi is reached - then it fires - that's about 3-4 cranks.
I can't find any reference to this test and how quickly system and control pressure should pick up from cold cranking and if my slow build up I'm seeing is an indicator to anything.
Best regards
Neil
Evening,
Residual pressure will naturally fall as the system cools and the wur reduces pressure etc (mine holds minimum pressure for about 45 minutes then drops to zero in another 10 minutes).
Cold pressure will start at zero but should build up to the correct cold control pressure quickly (which is related to ambient temp) as the fuel pump starts then start increasing as the wur heats up over a few minutes to the hot control pressure value. If your cold control pressure isn’t getting built up quickly it sounds like something isnt quite right.
I’ve only worked on mine which is a 924 and there are a few differences between the various CIS systems Porsche used so someone else may have other experience
thanks
Residual pressure will naturally fall as the system cools and the wur reduces pressure etc (mine holds minimum pressure for about 45 minutes then drops to zero in another 10 minutes).
Cold pressure will start at zero but should build up to the correct cold control pressure quickly (which is related to ambient temp) as the fuel pump starts then start increasing as the wur heats up over a few minutes to the hot control pressure value. If your cold control pressure isn’t getting built up quickly it sounds like something isnt quite right.
I’ve only worked on mine which is a 924 and there are a few differences between the various CIS systems Porsche used so someone else may have other experience
thanks
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