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Join Date: Aug 2014
Posts: 2
Cold start issue

1983 4.7 928S auto euro (UK) on cold start car will fire straight away then dies , does this may be up to 8times then fires and runs , if you try to feather throttle car dies and wont fire leave 5 mins and fires and dies, once running all fine any help appreciated warm start no issue
Old 02-24-2018, 09:19 AM
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JK McDonald's Avatar
Join Date: Oct 2003
Location: Texas
Posts: 886
Residual Fuel Pressure -

Hey Jonn - There could be other factors but your symptoms sound like your fuel system has bled off the residual pressure needed to restart the motor after your car has been parked a while. Over time this is a very common issue with our 928's. There is a one-way check valve (about the size of your thumb) installed on the output side of the fuel pump that keeps a residual pressure on the injectors when shut down. The closed injectors & pressure regulators hold this pressure at the other end of the fuel system. This static residual pressure is important to supplement the fuel supplied by the fuel pump to initially fire the motor during a restart.

There are a couple of corrective steps you can take:

1. Replace the Fuel Pump Outlet Check Valve - Make sure the injectors aren't leaking as well when parked.

2. If you have the L-Jet Fuel System - Turn the key from "OFF" to the "ON" Position several times prior to trying to restart the motor. The solid-state fuel pump control relay automatically runs the fuel pump in a "Pre-Start Mode" for several seconds each time the key is turned on. This OFF-ON cycling of the key will help to replenish the injector pressure prior to a cold restart.

If you have the CIS Fuel System - the easiest way to reprime the fuel system is to add a cabin toggle switch (spring loaded to OFF position) to the air flow switch wire on top of the fuel distributor unit. This air flow switch is the ground wire for the fuel pump circuit. By adding a spring loaded toggle switch you can safely force the fuel pump to run after an extended period of being parked.

3. For the L-Jet Fuel System - Wire in an additional over ride switch/relay to manually run the fuel pump. This is what I did to pressurize my injectors before trying to start "Miss Purdy" if she has has been parked for a while. The additional relay was installed in my fuse panel and the control switch (Spring Loaded To Off Position) is mounted in the storage shelf by my right knee.

4. My wife has suggested that I remove the radiator cap and drive a Toyota Camry under it.

Good Luck, Michael
1984 928S - "Miss Purdy"
1987 911SC - "Frau Helga"
1986 930 - "Well Hung"

Last edited by JK McDonald; 02-24-2018 at 05:47 PM..
Old 02-24-2018, 05:14 PM
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Join Date: Aug 2014
Posts: 2
Thanks Michael, will check these out
Old 02-25-2018, 07:35 AM
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