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 Post subject: Vapor Lock on my 740
PostPosted: Sun May 15, 2011 10:46 pm 
Bone stock

Joined: Fri Jul 30, 2010 10:55 pm
Posts: 6
Hi Guys : My 88 740(no turbo) sometimes will not start when it is hot. Stopped for gas today and then it wouldn't start .Vapor Lock? If I come back to it an hour later it will start. Any suggestions?


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 Post subject: Re: Vapor Lock on my 740
PostPosted: Mon May 16, 2011 3:34 am 
Haha, I just built a W24 Octo-Turbo, now what?!
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Joined: Wed Dec 21, 2005 8:40 pm
Posts: 3492
Location: Calgary, Ab
"would not start" meaning? No cranking? Crank and no fire? Crank and bucking/farting but no running? Or running but stalling?

I assume you mean that it cranks but doesn't do more than that. If so, can you hear the fuel pump running for half a second when you first turn the key to #2 (run)? Can you hear it during cranking? If the fuel pump is running both times, it could be the Radio Suppression Relay, or RSR. There is a black relay, about 1"x2"x3", just in front of your passenger strut tower. There will be a second one just like it, either right next to it or in front of the driver's strut tower. Swap one relay for the other and see what happens. If the engine runs and your air conditioning stops working, you've found your problem. If your A/C wasn't working before, all bets are off... This could havebeen your problem there too. These relays fail regularly when things get hot, and stop providing power to the fuel injectors. Carry a spare.

If you can't hear your fuel pump working, it could be the fuel pump relay. This is a white relay about the same size as that black one, on the relay panel attached to the fuse panel under the radio. It could also be the ignition switch, which may provide power to the computers in position #2 but not in #3 (start). It might be the ignition module (little black & silver box bolted to the inner fender behind the driver's headlight) too. If it's only not starting when hot, I think we can rule out grounds (I hope!)


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 Post subject: Re: Vapor Lock on my 740
PostPosted: Wed May 25, 2011 11:02 am 
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Joined: Fri Jul 30, 2010 10:55 pm
Posts: 6
Thanks for the suggestions. I will pick up some extra relays and swap one out at a time to see if it solves things for when it will not fire. The fuel pump relay was changed when it will not fire so we are off to the the relays.


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