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Old 02-01-2007, 12:46 AM
250 Coupe's Avatar
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Join Date: Mar 2004
Location: Des Moines, WA
Posts: 743
Wrenched on my 450SEL yesterday

I worked on my poor running '76 450SEL yesterday.

It's had a lumpy idle, sometimes backfired when pulling away from a stop and would run on when shut off. Warm starts were hit or miss.

The timing is at 5BTDC. I've fixed all the vacuum leaks I can find except one in the climate control behind the dash. The master cylinder was leaking between the booster and the cylinder. There was a big leak in the fuel vapor recovery system. I've pulled plug wires one at a time looking for a misfire and found nothing.

I finally had a chance to get out my new fuel pressure tester and the CO meter yesterday.

Cold control pressure was .8 bar and ramped right up as the heater element warmed up but it stopped at 2.5 bar. Disconnecting the full load enrichment vacuum line would change it a bit but I couldn't get it anywhere near the 3.4 - 3.8 bar spec. By closing the pressure tester valve I can read system pressure which is 6.2 bar, a bit higher than spec but I don't think it's a problem.

A quick check of CO showed 8.5% at idle. Yikes! No wonder it runs like crap. By throttling the pressure tester valve I could simulate a control pressure of 3.8 bar which would bring the CO to spec (0.5% - 1.5% at idle). Since I don't have a spare fuel filter or warmup regulator, I adjusted the mixture screw to bring it to 1.5% at idle. WOW, what a difference. Smooth idle, maybe a bit more power. Starts right up warm or cold.

I haven't done a fuel flow test but assume I should change the fuel filter because I don't have enough flow to make proper control pressure.

Michael
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1971 250C 114.023 Sadly, it's been sold. I will miss it.
1976 450SEL 116.033 Maybe not so sadly, it's also gone. It was beginning fall apart more than I was willing to put it back together.
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Old 02-03-2007, 12:18 PM
Milt
 
Join Date: Jul 2002
Location: N.W. Arkansas
Posts: 172
Michael: That was a good recap of your fuel pressure testing. In fact I printed it out as I will be doing the same thing sometime. Having never done it it helps to have some idea of what would or should be encountered. Milt.
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76 450 S.E.L.
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Old 02-22-2007, 01:29 PM
250 Coupe's Avatar
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Join Date: Mar 2004
Location: Des Moines, WA
Posts: 743
Update

I changed the fuel filter on the 10th and didn't notice much of a difference.

Yesterday I hooked up the pressure tester and CO meter again and started investigating. Long story short, it turned out that the vacuum lines to the warmup regulator were backwards. There was a change in the regulator in 1979 and I must have a later regulator. I had them connected for a 1976 regulator.

When the vacuum line is pulled, the control pressure should go down, making the mixture richer. Mine was opposite. Lot of head scratching about that until I found a picture that showed the difference.

Michael
__________________
1971 250C 114.023 Sadly, it's been sold. I will miss it.
1976 450SEL 116.033 Maybe not so sadly, it's also gone. It was beginning fall apart more than I was willing to put it back together.
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