What Fails On The WUR And Can It...? - PeachParts Mercedes-Benz Forum

knarf · #1 04-22-2006, 11:17 AM

Greetings Forum Members,

I think that I found the problem causing the poor running of my 1985 280sl, sooty tail pipe deposits and spark plugs, as discussed in previous posts.
HERE'S WHAT I'VE GOT;
Dead head fuel pump pressure over 8 bar.
Fuel system pressure 5.3 bar.
System holding pressure after engine shut down approximately 2 bar over 30+ minutes.
Steady system vacuum.
The WUR is receiving 13V from the plug.
The WUR electrical receptical resistance is 28 ohms
BUT my fuel control pressure reading does not rise higher than .7 bar after warm up.
It used to go to 2 bar before I screwed the WUR adjustment bolt in a couple of turns not realizing that this would reduce not raise the control pressure.
I have concluded that I need a new WUR or fix the existing one.
Can they be repaired without replacing components?
Exactly what fails in these units? Do the springs eventually fatigue and loose pressure over time?
Will tapping the pressed in pin holding the heated bimetal spring revive my control pressure readings?
We're getting top down weather here in the NE now and I'd like to be able to enjoy it driving, not just parked.
Opinions please, a new WUR isn't cheap!

Thanks,

Knarf

Matt L · #2 04-22-2006, 12:14 PM

The regulator shouldn't fail on its own, but several possibilities come to mind.

You could have crud lodged in the valve in the WUR, preventing it from fully closing off the fuel flow and thus properly regulating the pressure in the control line. I've seen crud in the WUR on a Saab, but it had the opposite effect of limiting flow, causing the control pressure to always be too high. But dismantling the WUR is easy enough, and they always seemed to work fine for me upon reassembly. I've yet to have to replace anything inside of one. Seems to be a very robust design.

You could also have crud lodged in the fuel distributor, in the tiny hole that feeds the control line. That's a much bigger issue, as you can't dismantle the fuel distributor and expect it to seal, as the seal is made by precisely-machined metal-on-metal contact. But since you have some control pressure, it would seem if the WUR completely blocked all flow, the control pressure should reach system pressure quite quickly.

Richard Wooldridge · #3 04-22-2006, 02:24 PM

Hi Knarf,
Are you sure you understand how the warm-up compensator works? From your post it seems you may not. Here's what should happen: As the warm-up compensator's valve is pushed more closed, either by the bimetallic spring being released when hot, or the adjustment spring by turning it in (clockwise), the closure causes MORE control pressure to be presented at the top of the control piston in the fuel distributor. This causes the piston to lower, leaning out the fuel mixture. NOTE that the pressure source is from an orifice within the fuel distributor to the control pressure line, and this pressure is only REDUCED by the warm-up compensator - the return line from the compensator goes to the fuel return line that returns fuel to the gas tank. The factory manual says the pressure is variable from 0.5 bar to 3.5 bar.

When the engine is cold, the bimetallic spring pushes against the warm-up compensator springs, lessening the pressure against the compensator's valve, allowing more fuel to bleed off to the return line, lowering the control pressure at the top of the control piston.

So, turning the adjustment in a few turns should have leaned out your mixture somewhat. I turned the adjustment on my 450SL's WUC in .050 to correct a too-rich mixture at idle. After "messing" with the WUC it is necessary to readjust the idle mixture using the 3mm allen wrench.

Regards,