A couple of weeks ago my car became hard to start when warm. First thing in the morning it starts just fine but once the temp gauge moves off the bottom anything longer than a 5 minute stop and the car takes about 15 seconds of cranking before it will fire up.
I changed the fuel check valve as a WIAT while changing the fuel filter about a year ago and I have tried 2 LH units (don't get me started on that theory again!!!). I've tried a spare ignition unit and I have swapped the temp 2 sensor. Both fuel dampers have been swapped but the car is exactly the same.
Once it is running it doesn't miss a beat its just hard to get it started when its warm.


After making a fuel pressure gauge it appears that as soon as I switch the engine off I am losing fuel pressure. With the engine running it is steady at 3,5 bar but it drops to zero within 30 secs of the engine stopping.
When I watch the pressure gauge while someone else cranks the engine I can see the pressure rise again and even predict the exact moment that the engine will start. I have checked all the usual suspects and there appears to be nothing wrong. Is there an easy way to check the injectors (even though I have swapped them and the problem is still there).
According to Bosch and the Porsche WSM the pressure should not have fallen below 1 bar after 20 mins.
I have 0 bar after 20 seconds


Your pressure loss may be misleading...
If an injector leaks into a hot engine, the engine will be "flooded" - that is, there will be a VERY rich mixture. This will cause hard starting. After the engine gets cold, the fuel will have evaporated - and a cold engine likes a very rich mixture better than does a hot engine.

Wally Plumley
928 Specialists


Here's what I've done today:
Clamped the rubber hose by the fuel pump to eliminate the fuel check valve..........pressure still drops instantly when engine shut off.
Fitted a rubber hose in place of the metal return line on the fuel pressure regulator and clamped it.......ditto Checked fuel dampers (again).......still the same. It has to be one or more leaky injectors as there's nowhere else for the fuel to go. If it is then that's pretty unlucky to swap 8 for another 8 with the same problem. Injectors coming out again tomorrow (got it down to a fine art now)



There are only 13 places that fuel pressure can go without making a puddle:

Into the intake manifold
- Eight injectors.
- Three diaphragms.

Into the tank
- Fuel pressure regulator
- Fuel pump check valve

If the engine doesn't crank more quickly after pulling and replacing the pump fuse, then it doesn't sound as if fuel is leaking into the intake.

Remove the cover over the fuel tank in the hatch area. Wrap the jaws of a pair of Vise Grips with tape so as to not damage the hose. Have someone cut the ignition switch, and as the pump dies, instantly clamp the return line shut.

If the fuel pressure drops, you have just eliminated the fuel pressure regulator. If the fuel pressure holds for thirty minutes, you have found your problem.

If the pressure drops, sounds like the most likely problem is the fuel pump check valve. You might can do a similar test to check it...

It can be really frustrating when you replace everything that should be the problem, but still have the problem.

Wally Plumley
928 Specialists


Our 928 L and LH-Jetronic fuel injector lines are "gang wired" together, i.e. the fuel injectors are all on or off at the same time. The 12V "activation source" is routed to the injectors and the LH module provides the switching current path to ground/earth. So, when the ignition is turned off, the activation voltage is removed from the injectors by the appropriate relay and even if the LH was defective and kept the injectors "turned on" there isn't a chance of the injectors staying open.

Rich (with my Electronik Repair hat on)

Is it possible that your dreaded LH is holding an injector open? In other words if you disconnected all the injector signals and power and then pumped up the system with the pump only then shut the pump off would it hold? Is it possible for there to be an electrical short that would hold an injector open?



I've had a warm start problem for a while. Car starts fine when cold, but if it is warm it needs to be turned over quite a bit to get her going and I need to depress the gas pedal for a little extra help and then she gulps and gasps for a bit until the idle smoothes out.

I have already changed the check valve on the fuel pump with no luck.....the front damper vacuum line was removed and inspected, no gas present and a nice HISSSSSS sound upon removal of the elbow connector. The rear damper vacuum fitting is on so snug I did not want to mess with it. BUT when I went to look at the fuel pressure regulator the vacuum elbow fitting slipped right off and gasoline dripped out an onto my finger......THIS IS BAD RIGHT?

From what I have read (I'm not an experienced mechanic) if there is gasoline present in the regulator or dampers that means they are shot and need to be replaced.....??

What are the arguments for going with the stock regulator vs. an aftermarket one that is adjustable?

Take care, Dan

87 5 speed black/black with RMB & Cat by-pass


The warm start problem on my 87 5 speed has been solved. As it turns out it was nothing too unordinary. Both my rear damper and fuel pressure regulator were toast. They both allowed fuel into the vacuum lines and held little or no pressure after a short time. When I opened up the fuel lines just a little gas dribbled out, not the shower of gas that usually sprays all over the place. The front damper was replaced as well along with the Temp II Sensor for good measure.

Now the car fires up immediately.

Thanks to all who helped out ....special thanks to Dan Brindle "Dan the Pod Guy" for his descriptive and helpful e-mails.

Dan 87 5 speed , black/black , w/RMB & Cat By-Pass



Hot start problems are normally caused by fuel pressure leakdown. The most common reason is a bad check valve which lets the fuel back up into the tank. Another not uncommon reason is leaking injectors. I currently have that problem with my 78 US. Diagnosis is easy. If it won't start hot, put the accelerator to the floor. It will then start and miss on one or more cylinders until the excess fuel is burned off.

Another reason for hot start problems not associated with leakdowns is due to a bad thermotime switch. If shorted it can continuously activate the cold start valve which will flood the engine during hot starts.