Mercedes repair

[Txakura]

1986 300E (gas) mpfi

 

so I got called to look at a dead Mercedes, but after it sat overnight it fired right up

then died again 2 days later

 

it seems like, there is a fuel pressure regulator that is not passing fuel to the distribution manifold

 

there is the inlet from the pump, a line to the fuel distribution block, a small return line and a vacuum line to the air cleaner housing

 

I THINK the return line is for any fuel in the regulator when the throttle is snapped shut or demand for fuel drops off - it has somewhere to go and does not keep the valve in the regulator open

 

I THINK the small 'vacuum line' is actually for any leakage past the spool valve seals, not to move the valve - more of an emissions thing and it creates a small negative pressure on the opposite side of the valve

 

here's the kicker, Mr.Bosch fuel regulator is $900 !!!

 

the next logical step is to check the fuel pressure being supplied to the regulator - if the pump is weak it wouldn't unseat the valve and allow fuel to flow to the manifold - hence no fuel getting past the regulator

 

but it occured to me I have an extra Subaru FI fuel pump... is the world ready for a Mercedes GL Wagon hybrid?

[muffman]

I think that the fuel pressure regulator allows fuel to flow back to the tank when there is too much fuel pressure. There should be a feed on the fuel rail from a fuel filter, and that is where the fuel should come in. Fuel pressure regulators normally make your car run really rich when they stop working. I tried the diagnose/replace parts thing for a while on an early 80's volvo, it would be much cheaper to have a mechanic figure out the problem. It is really amazing how much easier stuff with obd/obdII is to diagnose.

[muffman]

Oh, does that thing have mechanical fuel injection? Working on that was a nightmare for me. I was thinking the kind of fuel pressure regulator most electronic fuel injection has, the systems are pretty different. Please forgive my pointless rambling.

 

Subarus do have a very bosch-like fuel injection, so the fuel pump might work.

[Txakura]

we got it, it was a fuel pump relay that (apparently has many functions) was the culprit... the relay was around $300, but I found a re-flashed one from "programa'' for $79

 

that was very very educational, I am glad my venerable ea82 is still a 'mechanical car' to a large part, and not as crazy with the electronics, relays and such - sure we have a few items but...

 

sort of like an A-10 versus an F-22 both very good at their jobs, just two different ways in going about it