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  #1  
Old 10-23-2007, 03:01 AM
Tony 12's Avatar
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Location: Barossa Valley, South Australia
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W123 280E Warm Up Compensator

My '79 280E is getting horrible fuel economy. It runs fine stone cold, gets rough a minute later but idles ok again when fully hot. Except after a very long run in hot weather. Then it gets a misfire when returning to idle and on first take off. If you put your foot down it and give it a rev, it clears up, which seems like the plugs may be fouling???
I took the warm up compensator off this afternoon for a look. Pulled it all apart and cleaned it out. There was a great lump of something behind the thin metal diaphragm thingy. When I applied 12V and it warmed up ok.
I assume it should get 12V, but under what conditions. I turned the ignition on and got nothing across the terminals of the connector. Nothing either when using a separate earth and probing each terminal individually. Yes, it was cold.
I've yet to try it again as I'd like to know if I should be chasing an electrical problem here first.
Many thanks in advance for any advice.

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76 W116 280SE
74 W107 450SLC
88 W124 300E
82 W123 280TE
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  #2  
Old 10-23-2007, 08:17 AM
t walgamuth's Avatar
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Join Date: Apr 2005
Location: Lafayette Indiana
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My warm up compensator went bad on my 280e euro. It ran really really rich until I got it fixed. ten mpg.

Tom W
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #3  
Old 10-23-2007, 09:52 AM
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You need 12 volts to it, or the bi-metallic arm will not bend and you will be stuck with low control pressure = rich mixture until the engine is fully warm. Once the engine is warm, the engine heat will cause the bi-metallic arm to bend.

But the WUR is powered by the fuel pump relay, pin 87, not the ignition circuit, so you need to jumper the relay to test. If your car has the safety switch on top of the fuel distributor, you can pull that off to get power to the WUR.

The lump on the plate was probably the grease used to hold the pin in place during assembly.

But something else may be going on with the fuel economy. A first step would be to check control pressure and get a CO reading.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Last edited by ctaylor738; 10-23-2007 at 09:59 AM.
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  #4  
Old 10-23-2007, 05:02 PM
Tony 12's Avatar
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The lump of dirt was on the inside of the metal diaphragm so it wasn't the grease.
Regardless, that is all excellent advice. Particularly on the electrical side.
Thank you for the help.
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76 W116 280SE
74 W107 450SLC
88 W124 300E
82 W123 280TE
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  #5  
Old 10-24-2007, 05:48 AM
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Well after the "service" the car definitely starts up better and no longer needs and throttle at any point during first start up or at all until fully warm. It also seems to run more smoothly. The true test will come with my fuel economy, but at least that bit of work has made some improvement.
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76 W116 280SE
74 W107 450SLC
88 W124 300E
82 W123 280TE
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  #6  
Old 10-24-2007, 10:03 AM
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Join Date: Apr 2000
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Glad to hear it's feeling better.

Out of curiosity. I have seen three flavors of WUR valves:

- two thin metal sheets with holes
- a very thin bottom area machined into the plate with the four screws
- an compressible o-ring

Which does your car have?

Thanks,
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #7  
Old 10-24-2007, 05:08 PM
Tony 12's Avatar
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Location: Barossa Valley, South Australia
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Hi Chuck
Yes, it certainly drives better, but I have yet to confirm an improvement in fuel consumtion.
The WUR on my car it has a very thin metal diapragm under the plate with the four screws.
I didn't know there was more than one type.

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76 W116 280SE
74 W107 450SLC
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82 W123 280TE
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