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Old 12-24-2006, 12:09 PM
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Join Date: Jan 2006
Posts: 11
126 1991 300SEL starting problems

I have a 1991 300SEL with ~170000 miles. Recently I have been having a tough time on cold starts. I crank and crank and it just does not catch. After about a minute of cranking, it catches. The first two minutes are rough with the engine really struggling. If I wait a few minutes and then give it some gas, the engine runs well. Once the engine is warm it runs fine. I don't have problems with warm starts.

Things I have done:
Changed fuel injectors
Changed air filter
Fuel filter recently changed
Fuel pump sounds good and changed 15K ago
Oil change

Any ideas??

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Old 12-24-2006, 04:29 PM
Secret Squirrel's Avatar
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Join Date: Mar 2005
Location: Commerce City, CO
Posts: 74
Hi, this should be an easy fix:

First do the water temp sensor (located on the driver's side, top rear of the cylinder head):

If that seems to help, but it still doesn't seem quite right, do the OVP:

These are the first two parts I changed on my first 300 SEL and it was behaving exactly like yours. It starts like new now.
(2) 1988 300 SEL W126
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Old 12-24-2006, 04:36 PM
vwbuge's Avatar
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Join Date: Sep 2001
Location: Johnstown, Pennsylvania
Posts: 2,106
New plugs?
'85 300SD (formerly california emissions)
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'68/'69 Empi Imp body on '65 Beetle pan
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Old 12-24-2006, 05:59 PM
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Join Date: Jan 2006
Posts: 11
plugs < 10K miles ago
so how does the water sensor and OVP lead to my problem?? Also, someone said I should change the bi-metallic choke spring. Any thoughts?
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Old 12-25-2006, 10:20 AM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
The sensor tells the injection system's computer that it's cold and that it should activate the cold start valve, and for how long, and that it should richen the mixture until the engine warms up.

Bad readings from the sensor = hard start, poor warm-up.

The OVP relay powers the computer and other electronics. If it, or its fuse is bad, then you have the same situation as above.

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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