Hot Start Problem Fix

by David Russell

The Problem

First some background information.  The hot start problem common to many Porsche 924 models, both normally aspirated and turbocharged is usually due to early timeout of the thermo-time switch.   It is not due to the fuel pump, or the fuel accumulator, as many people believe. 

During all start conditions, cold or hot, the thermo-time switch provides ground to the cold start valve, and the ignition switch provides battery voltage to the other side of the cold start valve for as long as the ignition switch is held in the start position.  As long as both the ground and the battery voltage are applied to the valve it will remain open, and spray extra fuel into the intake manifold.  This extra fuel is necessary to aid in starting the engine. 

During a cold start, the time the ground is supplied by the thermo-time switch can be as much as ten seconds.  After that the thermo-time switch "times-out," and the ground is removed.  Even if you continue cranking the engine after the ten-second time-out period, only battery voltage is supplied to the valve, the ground is removed.  Without the ground, the valve will close, and the extra fuel flow is cut off.  This is to prevent flooding of the engine.   After the thermo-time switch has cooled down, the ground is once again applied to the valve during cranking, and the normal start cycle is in place.

During a hot start, the time that the thermo-time switch applies ground to the cold start valve is significantly reduced.  Various Bosch publications indicate that it may be as short as three seconds, however my experience suggests that it is often much shorter than this.  So, when you go to start your hot engine, particularly on a hot day, very little extra fuel is injected into the engine.  On an older engine, this is often not enough to start the engine.  To compound the problem, the thermo-time switch has now timed out,  and must cool before it will supply the necessary ground to the cold start valve to get the extra fuel to the engine for a start.  On a hot day this can be as much as 20 minutes.  Too long to wait, if you need to get somewhere.  This kit is designed to get around this problem, by supplying ground to the cold start valve through a push button switch that can be depressed while cranking, for as long as is necessary, thereby providing the extra fuel needed for even a hot start.

The Kit

The kit consists of seven pieces.  A push button switch, two lengths of wire, one is one foot long, and the other is seven feet long, two wire splices, and two terminal studs (stake connectors).  Check to be sure that you have everything before starting.


The following instructions were developed for a 1980 924 Turbo.  I do not have access to a normally aspirated 924, however the procedures should be very similar.  I would appreciate your feedback so that I can improve the instructions for all Porsche 924 models.

1.        Locate the cold start valve.  It is attached to the end of the air distributor (intake manifold) near the firewall.

2.        Slit open the insulation on the cable attached to the cold start valve for about two inches.

3.        Locate the two red wires inside the insulation.

4.        Select the wire with the white stripe, this is the ground wire that you will splice into.  The wire with the black stripe supplies the battery voltage from the ignition switch, and should not be disturbed.

5.        Place one of the splice connectors over the ground wire, so that the wire runs all of the way through the open slot of the splice connector.

6.        Place one end of the long ( 7 ft.) wire supplied with the kit, in the other slot of the splice connector (the slot that does not go all of the way through), and be sure that the wire is inserted all of the way up against the stop.

7.        Use a standard pair of pliers to squeeze the metal stake of the splice connector all of the way down through both wires.  The top of the metal should now be even with the plastic housing.

8.        Route the long wire along the existing wire loom on the engine fire wall, and feed it through the small  fuel line grommet on the fire wall into cowl section located between the fire wall and the windshield.  If this grommet is not available on you car (NA or Euro version, etc.), find another convenient grommet or drill a small hole through the firewall.

9.        Strip off about one quarter of an inch of the insulation from the other end of the long wire and apply one of the terminal studs over the striped portion of the wire, and either crimp or solder in place.  If you do not have either a soldering iron, or a wire crimping tool, the wire can just be striped back a little farther, and later connected to the switch without the terminal connector.

10.     Using a standard screwdriver, pry the large grommet (about one and one-half inch in diameter) from the top of the hump above the footwell.  This is the grommet that the windshield wiper cable is coming out of, from the inside of the car.

11.     Route the long wire through the opening in the grommet along side of the windshield wiper cable, and begin to feed it down through the grommet opening.

12.     Using a flash light, look down into the grommet opening, and you will see a slot (about one half inch wide) at the bottom of the compartment formed by the hump.

13.     Route the long wire down through this slot into the inside of the car.

14.     After you have put enough of the wire through the opening that it should touch the floor of the car, look under the dash and locate the wire.

15.     Look at the bottom of the dash, below the steering column.  You will see a round hole, of about one half-inch diameter, just to the drivers side of the steering column.  This is the hole that you will mount the push button switch in.

16.     The long wire must be routed up under the dash, and down through this hole, prior to connecting it to the switch.  The easiest way to accomplish this is to use a piece of safety wire, or single strand copper wire to "fish" the long wire through.  Route the safety wire up through the switch hole, underneath the steering column, and toward the firewall, until you can see it.  Connect the safety wire to the long wire via the terminal connector, and gently pull the long wire down through the switch hole.  Do not connect the long wire to the switch at this time.

17.     Using a small standard screwdriver, gently pry the headlight switch from the dash.

18.     Disconnect the plug from the back of the headlight switch, and lay the switch aside.

19.     Locate the brown wire going to the back of the headlight switch plug.  This is a good ground point for the pushbutton switch that you are about to install.

20.     Strip one end of the short wire supplied with your kit, and crimp the second terminal stud on to this wire.

21.     In the same method used to splice the long wire to the ground wire on the cold start valve, splice the other end of the short wire to the brown ground wire on the headlight switch plug.  As an alternative, a good chassis ground can be used.

22.     Using the safety wire, fish the terminal lug end of the short wire under the dash and down through the switch hole.  The opening made by the ignition switch will aid in this effort.

23.     Connect both the short and the long wire to the screw terminals on the push button switch.

24.     Push the push button switch into the hole in the dash.  If this hole is worn, you may have to use some tape on the switch to make a tight fit.

25.     Reinstall the headlight switch to the plug, and press it gently back into the dash.

26.     Reinstall the rubber grommet on top of the footwell, using a small screwdriver to pry the lower ring on the grommet into place.

27.     You are finished with the installation.


Your installation has not interfered with the normal operation of the thermo-time switch, or the cold start valve.  Always try to start your car normally.  However, if it fails to start normally, then depress the push button switch while cranking the car with the key, and it will start right