Hard to Start When Warm

Hard to Start When Warm

Car is hard to start after sitting for a few hours. Starts fine, when I hit the accelerator while cranking. It also starts after a four-five seconds after sitting overnight. But will not start after sitting an hour or so.

Is this the check valve or something else? Where is the checkvalve?

1989 928 S4

I experienced similar problems a few months ago. Cold starts O.K. but hot starts after sitting for a while definitely dodgy. The cause was largely or totally due to failure of the diaphragm[s] in the fuel dampers and/or pressure regulator. Either way, if one has gone the chances are the other two are about to go and so I replaced the lot at a cost equivalent to $200.

The problem can also be caused by faulty fuel pump check valve which sits on the fuel pump discharge connection. Either way if the system loses fuel pressure it simply does not fire up when hot. You can test for this by fitting a pressure gauge on the end of the fuel rail, switch off the engine and watch to see what happens. If any of the above are faulty, then pressure will drop off quite quickly with the engine switched off. Easy if you are a useful home mechanic.

A simple test is to remove the vacuum connection on top of the fuel pressure regulator and the two fuel pressure dampers. Push the pad of your finger tip hard onto the nipple and if you can smell petrol on your finger tip bingo [assuming you haven't got petrol there to start with]. I could smell petrol on two of mine.

Good luck
Fred R
1990 S4 auto

I had this problem with my 87 S4. Replaced the check valve, which did not cure the problem (by the way, the check valve is located with the external fuel pump aft of the fuel tank). I then replaced the fuel filter and the problem was solved. For pictures and procedures see Dave Roberts Website at Porsche 928 SPECIALISTS
http://www.928gt.com/ , also for a simple test of the check valve see Greg Nichols' Website....
Nichols Home Page

Hope this helps....

Ted Childs
87 S4 5sp Grandprixweiss, RMB

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