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Old 02-08-2009, 04:50 PM
Strife's Avatar
General Purpose Geek
 
Join Date: Apr 2004
Location: KY USA
Posts: 2,238
Weird 116 No-Start

85 380SL, 120K miles, good and recent tune

It just dropped dead after being parked for four hours and wouldn't start. Here's what I did:

1. Checked wiring at fuel pump, checked motor of fuel pump, pump spins
2. Substituted FP relay with "probably good" one, no result
3. Jumpered pins at FP relay (30/87), FP operates
4. I can hear the FP run after failed start cranking for about 1 second with the relay in.
5. I know I am getting a spark, verified with timing light.

Questions:

My 86 560SL FP operates in RUN even with the engine not running, I can hear it. Does the 85 operate in START (crank signal?) or RPM signal ONLY? BTW, I know that the FP relay is getting power at terminal 15 (fused, hot in START or RUN) and at 30 (always). The 85 doesn't run the motor in RUN when you first turn the key, and I don't remember if it did or not.

Can a fuel pump go "kinda bad", where it pumps but doesn't deliver the fuel? I have pressure gauges if you think this is worthwhile. This pump is not a Bosch (yeah, I know, a mistake) and it has 20K miles on it.

My next step is to substitute the fuel pump (with a Bosch). An oddity, I have spare 560SL fuel pumps, which look the same but have different part numbers, and the resistance of the 560SL pump motors is noticeably higher than the installed 380 pump. Maybe this pump is drawing too much power and pumping slowly?

This is driving me crazy, because an "easy" problem, which I was prepared for, would have been found by now!
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  #2  
Old 02-08-2009, 05:22 PM
Pooka
 
Join Date: Sep 2005
Posts: 664
When you put the key in RUN the fuel pump should run.

I think the proper pressure for the pump is about 42 psi.

Fuel pumps can go bad in that they will run but not pump.

You may also have a bad fuel pressure regulator that will not allow the pressure to reach the correct pressure.

I am not up on this system enough to know this, but all Bosch system have a return line so the unusued fuel can return to the tank. Is you fuel pressure regulator bad and letting the fuel pressure drop below what it should be?

Pooka
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  #3  
Old 02-08-2009, 06:53 PM
Luther
 
Join Date: Dec 2008
Location: St Petersburg, Fl
Posts: 100
I talked to an import mechanic here in St Pete a few weeks ago letting him know that I clean fuels tanks and his comment was due to the ethanol fuel he replaces two fuel pumps a week. The new fuel (ethanol) is damaging the rubber hoses, diaphragms, o-rings, pump impellors. Another issue with ethanol is it is a great cleaning solution, it will remove all of the varnish from the walls of your tank and relocate it in to the fuel lines, fuel filters or any end user.

From a person that cleans tanks, I love ethanol.

Good Luck...
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  #4  
Old 02-08-2009, 08:53 PM
Strife's Avatar
General Purpose Geek
 
Join Date: Apr 2004
Location: KY USA
Posts: 2,238
Well, it's beginning to look like the relay (AND my spare relay) are bad. It does seem odd that the fuel pump doesn't run in the RUN position. I could swear that it did (it does on the 86, but that system is different, the fuel pump relay is different and there are two fuel pumps).
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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm
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