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[ted von Kampen]

I have been chasing a fuel pressure/hot start problem for some time. I have changed the accumulator and rebuilt the fuel distributor. The engine will start and run, fuel mixture can be set ok, system operating pressure after warmup is about 50 psi, system pressure is 70 psi after warm up.

When the engine is shut off, the fuel pressure drops to about 10psi and slowly leaks down to zero in a few minutes.

As I understand, the fuel pressure drop is a big part of the hot start problem. Shortly after shut down and the fuel pressure drops to zero the engine will not start until it cools off.

To chase the cause of fuel pressure drop I have done and found the following.
1. Blocked off fuel distributor and pressure gauge on just the incoming fuel
line. Turned fuel pump on and then off. Pressure fell to 30psi and it held
overnight. So the fuel pump check valve is working as it should.
2. Removed the banjo bolts on the fuel pump top and replaced with just
bolts and copper gaskets. That disconnects the injectors. Turned pump on
and pressure rose to system pressure but on pump turn off it dropped to
about 10 psi and bleed down to zero in a few minutes.

So what is left to check? I have readjusted the pressure regulator and put another new o ring on the tip and re-shimmed to get the system pressure up to 70 psi.
I am open to comments and suggestions on where to go next. The car runs okay after it starts but after it warms up the hot start problem emerges. Thanks for any help

[Jim d]

There are ten potential causes for Residual pressure to drop too quickly. Here’s the list as well a test method to rule out what might be an issue.

1) Warm-up Regulator (WUR)
Close the valve that is part of the fuel pressure gauge between the WUR and the fuel distributor. After re-pressurizing the system, if the Residual pressure is now within spec, then the either the WUR or the gas cap is at fault. If after checking the “Leaky gas cap” step and the cap is determined to be OK, then the WUR is at issue. The WUR will need to be refurbished

2) Leaky gas cap
Visually inspect the gasket on the gas cap for any cracks or tears in the gasket. If gasket is compromised either replace the gasket or the gas cap and then re-run test #1 to confirm the residual pressure is within spec.

3) Bad or leaking fuel accumulator(s)
If the diaphragms in the accumulator(s) fail, they are designed to create an external gas leak. Remove the small screw(s) at the bottom of the accumulator(s). Visually inspect for any leak immediately after turning off the fuel pump.
Replace is there are any signs of leaking.

4) Fuel pump check valve leaking
Depending on year & model the check valve is either internal to the fuel pump or external to the pump. The check valve is designed to prevent flow retuning the to the gas tank when the pump is turned off. After turning off the fuel pump, immediately clamp the hose on the outlet of the fuel pump using something like a long nose Vise Grip. If the Residual pressures improve, then the check valve is bad. If the car has an external check valve then it needs to be replaced. If check valve is internal to the pump, then replaced the fuel pump or add an external check valve (Bosch 1-587-010-536)

5) Air/fuel adjustment set to be too rich
If the air/fuel adjustment is set too rich, then the slits in the distributor plunger will be opened when the engine is turned off which will reduce the Residual pressure. Remove the banjo bolt screw for anyone of the fuel injector line from the fuel distributor. With the fuel pump manually bypassed to run, if there is fuel coming out of the distributor outlet then the air/Fuel adjustment was set too rich. Turn the air/fuel adjustment screw CCW until the flow stops. Then turn the air/fuel adjustment screw an additional 180 deg CCW.

6) Fuel distributor plunger O-rings leaking
The plunger has O-rings around the slits openings. If the O-rings fail to seal, then fuel pressure can escape, causing low Residual pressure. Remove all of the injectors, leaving the fuel lines attached. Place the injectors into individual clear plastic containers. Manually bypassed the fuel pump to run and after 30 seconds, then turn the pump off. Fuel should stop flowing from the injectors very quickly after the pump is turned off. If none of the injectors are leaking then both the distributor and/or the injectors are not causing the issue.
If they all are leaking then make sure the air/fuel adjustment is not set too rich before proceeding.
If some of the injectors are leaking and some are not, then jot down which are. Say for example injector for #1-cylinder was leaking and for #2-cylinder was not. Swap fuel lines at the fuel distributor. Connect the #1-cylinder line to the distributor port that originally feed #2-cylinder. Connect the #2-cylinder line to the distributor port that originally feed cylinder #1. Watch again what happens again after turning the fuel pump on & off again. If the leaking injector is now that one connected to the distributor port that originally feed #1-cylinder, then the O-ring inside the fuel distributor needs to be replaced. The fuel distributor should be refurbished.

7) Leaking injectors
If the leak moved to the line that is now connected to the distributor port that originally feed #2-cylinder, then the injector is stuck open and will need to be replaced.

8 Cold start injector leaking
Remove the cold start injector from the intake manifold with the fuel line attached but electrical plug disconnected. Place the outlet of the valve in a clear plastic container. Connect 12v directly from the battery to the valve. The valve is polarity insensitive so it doesn’t matter where the + and – is connected. With the fuel pump manually bypassed to run. There will be fuel coming out of the valve. Remove the 12v power to the valve, and fuel flow from the valve should immediately stop. If the fuel continues to leak out the valve then it will need to be replaced.

9) System pressure regulator O-ring leak
Internal to the fuel distributor is the system pressure regulator. It has an O-ring that seals to prevent flow back to the fuel tank. If the O-ring leaks then Residual pressure will be low. Turn the fuel pump off and immediately clamp the hose that feds back to the gas tank using something like a long nose Vise Grip. WARNING: DO NOT CLAMP THE HOSE WHILE THE FUEL PUMP IS RUNNING. A HIGH FUEL PRESSURE SAFETY HAZARD WOULD EXIST. If the Residual pressures improve then replace the O-ring

10) Supplementary start valve
If this valve is leaking it will bypass flow around the WUR to the return line going to the gas tank. This will reduce the residual pressure. It will also reduce the cold & hot control pressures making the car run rich. Unfortunately the lines connected to the SSV are rigid lines so there is no easy method to verify that when closed its not leaking. If both cold & hot control pressures are within spec, then the SSV most likely would not be an issue.
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Jim delToro
1977 NA
CNY PCA member

[Fasteddie313]

That’s a pretty good article..
https://cnypca.org/bosch-cis-hot-start-problem/

Excellent information..


Except one thing..
Think they might be yanking your chain a bit about the gas cap..

For one, the fuel tank shouldn’t be holding any pressure, much less residual pressure..
For two, this would cause you to have a hot start problem every time you refueled you car as taking the gas cap off would lose all of your residual pressure, which I would think to be quite a design fault..
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80 Turbo - Slightly Modified

[Jim d]

[Fasteddie313]

Very interesting, you very well could be correct, as I do not 100% know the complexities of the evap system on these cars, as it’s not something that terribly sparks my interest..

But..
Shouldn’t the fuel tank, when engine is off, be open to the charcoal canister, and the charcoal canister be open to atmosphere if pressure, therefore allowing you to fill the tank with fuel, and the displaced air/fuel vapors to travel through the charcoal canister, thereby trapping the fuel vapors displaced within the activated carbon, to later be sucked back into the engine, when the purge valves later reverse when the engine is ready for it?


Another thing..
All those large braided cloth hoses that are part of the evap, I’m 95% positive all of those leak like a sieve, judging by the condition of the smaller like/same ones that actually matter for the engines vacuum system..
If not all replaced, which would include dropping the tank I believe..


Stick around more man..
I love CIS theory!
Have you seen any of my FrankenCIS tuning stuff?
I bet you know a lot about the operations of the lambda system and the effects of distributor diaphragm differential pressure
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80 Turbo - Slightly Modified

[Jim d]

[Fasteddie313]

Sent you a PM with links..
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80 Turbo - Slightly Modified