924 Hot start issue!
08-18-2021, 08:22 AM
#1
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924 Hot start issue!
So I bought my first car, a Porsche 924 1985. I drove it home about 10 km and it worked fine. After getting home we took it to the highway. When we got home the car wouldn’t turn on anymore. We checked for spark and it was ok. Then we did some research and couple hours later checked fuel pump relay and cleaned it. After putting it back in the car turned on fine. In reality it turned on because it got to cool down.
The car cold starts fine but after the engine has warmed up the car doesn’t start. To be exact I can let the car sit for 8 minutes and it turns on fine but any longer than that it will not start, the starter will just spin. After letting the car cool down for couple of hours it will turn on fine.
The engine gets spark but not gas? After checking the relay we thought that it might be the fuel accumulator. Unscrewing the bolt at the bottom of the old accumulator dripped some fuel so we decided to change it. Sadly changing it didn’t help. We have checked also the engine temperature sensor and it seems to be good. Unplugging it doesn’t help the car start.
When changing the accumulator we noticed that the fuel pump had been replaced and seemed like that the original pump at the bottom of the tank wasn’t in use. Could the aftermarket pump cause this issue?
What should we try next to get this beautiful car to work properly?
TLDR; The car cold starts fine but it wont start after letting it sit over 8 minutes after turning it off. The car starts after letting it cool for 1-2 hours. Fuel pump relay is good and fuel accumulator has been changed. Engine temperature sensor seems good too. Any tips or help?
Link to album of photos and a video to get better idea of the situation: https://photos.app.goo.gl/JHu8X6QGCMKGp4tx5. I will add a video of the car not starting later!
-Tuomas (from Finland)
The car cold starts fine but after the engine has warmed up the car doesn’t start. To be exact I can let the car sit for 8 minutes and it turns on fine but any longer than that it will not start, the starter will just spin. After letting the car cool down for couple of hours it will turn on fine.
The engine gets spark but not gas? After checking the relay we thought that it might be the fuel accumulator. Unscrewing the bolt at the bottom of the old accumulator dripped some fuel so we decided to change it. Sadly changing it didn’t help. We have checked also the engine temperature sensor and it seems to be good. Unplugging it doesn’t help the car start.
When changing the accumulator we noticed that the fuel pump had been replaced and seemed like that the original pump at the bottom of the tank wasn’t in use. Could the aftermarket pump cause this issue?
What should we try next to get this beautiful car to work properly?
TLDR; The car cold starts fine but it wont start after letting it sit over 8 minutes after turning it off. The car starts after letting it cool for 1-2 hours. Fuel pump relay is good and fuel accumulator has been changed. Engine temperature sensor seems good too. Any tips or help?
Link to album of photos and a video to get better idea of the situation: https://photos.app.goo.gl/JHu8X6QGCMKGp4tx5. I will add a video of the car not starting later!
-Tuomas (from Finland)
08-19-2021, 10:01 AM
#2
I had the exact same situation. I replaced the fuel filter and fuel pressure regulator, at the same time. Problem was solved. I can't say which one of those two was faulty!
08-19-2021, 10:29 AM
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I will find the filter and try it first! Thank you! 😄
08-19-2021, 01:24 PM
#5
Three Wheelin'
The issue is residual fuel pressure.. your check valve is bad.
when the car is hot and you shut it off the fuel is boiling because it looses pressure. This creates air pockets which the injectors can’t overcome… my guess is that when it’s hot and won’t start you could force it to start by tossing some fuel or either in the intake…
when the car is hot and you shut it off the fuel is boiling because it looses pressure. This creates air pockets which the injectors can’t overcome… my guess is that when it’s hot and won’t start you could force it to start by tossing some fuel or either in the intake…
08-19-2021, 05:30 PM
#6
Three Wheelin'
The issue is residual fuel pressure.. your check valve is bad.
when the car is hot and you shut it off the fuel is boiling because it looses pressure. This creates air pockets which the injectors can’t overcome… my guess is that when it’s hot and won’t start you could force it to start by tossing some fuel or either in the intake…
when the car is hot and you shut it off the fuel is boiling because it looses pressure. This creates air pockets which the injectors can’t overcome… my guess is that when it’s hot and won’t start you could force it to start by tossing some fuel or either in the intake…
08-19-2021, 07:26 PM
#7
The 924 2.0 engine suffered from hot start issues and I went through the same pain with mine. First you need to get or make a fuel pressure gauge set, this plumbs in between the top of the fuel distributor head and the line that goes to the warm up regulator which lives on the back of the inlet manifold.
This will give you the clues you need to figure out what's going on and show the fuel pressure when cold, warm, system pressure and in this case residual fuel pressure - what happens to the pressure when you switch the engine off. It should hold for at least 20 minutes but I suspect yours is dropping straight down if you can't start the car after 8 minutes.
Basic overview: The fuel pump should have a one way valve in it which stops the fuel flowing back to the tank via the supply line when you turn the engine off, the accumulator holds a bit of pressure to keep the lines pressurised after shut down and there's a valve in the metering head which should stop the fuel simply flowing back down the return line. If any of these are faulty the fuel drains back quickly leaving empty lines and you can't start a car on air.
There is a bodge to add a switch in to ground the cold start valve (injector at the back of the inlet manifold) and make it fire to give a squirt of fuel to help the car start, this would give you a get out of jail card but shouldn't be needed when everything is working.
I'm in the UK so no emissions kit on my 924 which may change things a small bit depending on where you are.
This will give you the clues you need to figure out what's going on and show the fuel pressure when cold, warm, system pressure and in this case residual fuel pressure - what happens to the pressure when you switch the engine off. It should hold for at least 20 minutes but I suspect yours is dropping straight down if you can't start the car after 8 minutes.
Basic overview: The fuel pump should have a one way valve in it which stops the fuel flowing back to the tank via the supply line when you turn the engine off, the accumulator holds a bit of pressure to keep the lines pressurised after shut down and there's a valve in the metering head which should stop the fuel simply flowing back down the return line. If any of these are faulty the fuel drains back quickly leaving empty lines and you can't start a car on air.
There is a bodge to add a switch in to ground the cold start valve (injector at the back of the inlet manifold) and make it fire to give a squirt of fuel to help the car start, this would give you a get out of jail card but shouldn't be needed when everything is working.
I'm in the UK so no emissions kit on my 924 which may change things a small bit depending on where you are.
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08-22-2021, 01:32 PM
#8
Instructor
+3 on the residual fuel pressure issue. I had the same hot start issue on another one of my cars, replaced the fuel pump and haven't had a problem in 3 years.
08-23-2021, 02:50 PM
#9
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Thank you everyone for your comments! Changing the fuel filter did not help, which was expected.
As russmw said, adding a cable between the red and white cable of the cold start valve and the ground of the battery did help and the car started up normally. This is only a fix for the symptoms and not the original cause.
As GPA951s said, The problem is most likely a bad check valve. Unfortunately we are having problems finding tech check valve even with the Haynes guide. The Haynes guide says that the check valve is next to the throttle body and connected to the air flow manifold. Some models have the check valve in the fuel pump. Does anyone have information to help us find the check valve on this specific car. This car is a 85 924 with the 2.0 L engine.
Thanks for your help this far!
As russmw said, adding a cable between the red and white cable of the cold start valve and the ground of the battery did help and the car started up normally. This is only a fix for the symptoms and not the original cause.
As GPA951s said, The problem is most likely a bad check valve. Unfortunately we are having problems finding tech check valve even with the Haynes guide. The Haynes guide says that the check valve is next to the throttle body and connected to the air flow manifold. Some models have the check valve in the fuel pump. Does anyone have information to help us find the check valve on this specific car. This car is a 85 924 with the 2.0 L engine.
Thanks for your help this far!
08-23-2021, 03:33 PM
#10
Three Wheelin'
On 44s it’s just on the other side of the fuel pump. It’s purely mechanical.. on audis it was also on the outlet side of the fuel pump although it was in the tank.. it will not be in the engine compartment anywhere… it’s just a hex shaped part threads onto fuel pump and banjo bolt in it.. not really that familiar with 924,s layout..
08-23-2021, 03:40 PM
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On 44s it’s just on the other side of the fuel pump. It’s purely mechanical.. on audis it was also on the outlet side of the fuel pump although it was in the tank.. it will not be in the engine compartment anywhere… it’s just a hex shaped part threads onto fuel pump and banjo bolt in it.. not really that familiar with 924,s layout..
08-28-2021, 08:13 AM
#13
This is what screws into the outlet side of the fuel pump and has a little ball and spring in it which closes when that pump stops.
https://www.design911.co.uk/fu/prod1...e-94460895101/
https://www.design911.co.uk/fu/prod1...e-94460895101/
06-28-2024, 07:43 PM
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Thank you everyone for your comments! Changing the fuel filter did not help, which was expected.
As russmw said, adding a cable between the red and white cable of the cold start valve and the ground of the battery did help and the car started up normally. This is only a fix for the symptoms and not the original cause.
As GPA951s said, The problem is most likely a bad check valve. Unfortunately we are having problems finding tech check valve even with the Haynes guide. The Haynes guide says that the check valve is next to the throttle body and connected to the air flow manifold. Some models have the check valve in the fuel pump. Does anyone have information to help us find the check valve on this specific car. This car is a 85 924 with the 2.0 L engine.
Thanks for your help this far!
As russmw said, adding a cable between the red and white cable of the cold start valve and the ground of the battery did help and the car started up normally. This is only a fix for the symptoms and not the original cause.
As GPA951s said, The problem is most likely a bad check valve. Unfortunately we are having problems finding tech check valve even with the Haynes guide. The Haynes guide says that the check valve is next to the throttle body and connected to the air flow manifold. Some models have the check valve in the fuel pump. Does anyone have information to help us find the check valve on this specific car. This car is a 85 924 with the 2.0 L engine.
Thanks for your help this far!
06-29-2024, 12:45 AM
#15
+4 on loss of fuel pressure. I had the same issue. I fixed it with a brand new Lindsey Racing fuel system and check valve. I now have a loss of fuel pressure again... Regulator or Check valve for me.
Little tip I used to make it to class on time, unplug 4 injectors and try to start the car. Give it some cranks. Then plug in the injectors in. While giving it throttle try to start the car. This is good for if your stuck somewhere with a vapor lock issue.
Little tip I used to make it to class on time, unplug 4 injectors and try to start the car. Give it some cranks. Then plug in the injectors in. While giving it throttle try to start the car. This is good for if your stuck somewhere with a vapor lock issue.
