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Bosch CIS Hot Start Problem


One of the most common questions on with the Bosch CIS fuel system concerns the so-called “hot start” problem and how to fix it. Unfortunately, there is not a single cause but rather as many as 10 possible causes and it can be easy to spend a lot of time replacing different parts and getting nowhere. For this reason, a common modification, known as the “hot start fix” is often applied to work around the problem without actually solving it; however, this isn’t always the best idea. I’d recommend reading the Bosch K Jet-tronic “Workshop Manual” for Porsche cars first if you are unfamiliar with the operation & components that make up the CIS system.


In a nutshell, the car won’t start when it’s hot. It starts fine when it’s cold and runs perfectly until the engine is turned off and but when you try to restart the car after a period of time, it turns over on the starter but refuses to fire. It may start after turning over for some time, or it may have to be left to cool down altogether before it’ll start again. It can be an extremely frustrating problem.


The basic cause is very simple: vapor in the fuel supply to the engine. The fuel system is designed to keep gasoline in liquid form until it reaches the injectors and enters the inlet manifold, and the various components of the system regulate the air/fuel mixture to keep it at the correct ratio to run the engine. If the fuel is allowed to vaporize too early, the regulation cannot be maintained and the engine doesn’t run. Until this vapor is replaced by liquid fuel, the engine won’t start.

Under normal circumstances, the fuel system uses a simple technique to stop the fuel turning into vapor: pressure. When running, the pressures in various parts of the system range from about 30 psi to 75 psi, more than enough to keep the fuel in liquid form.

However, if the engine is switched off and the residual pressure is not maintained, the heat inside the engine bay warms up the fuel and allows it to expand into pockets of vapor.

Confirmation that this is indeed the cause of the problem can be determined by measuring the Residual Pressure (the fuel pressure in the system after the fuel pump is turned off) in the fuel system. The Porsche specifications are after turning off the car for 10 minutes the residual pressure should be above 25psig, after 20 minutes the pressure should be above 22 psig and after 60 minutes the pressure should be above 14.5 psig.


The intent of this document is to provide a systematic approach to identify which component(s) is the root cause via test methods.    

There are ten potential causes:

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

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.   

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.

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 returning 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.  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 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)

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.

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.  

Leaking injectors

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

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.

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

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.