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Old 06-25-2004, 01:34 PM
Posts: n/a
Had rough idle, now hard cold start

I have been hunting down a rough idle problem for some days now, using the board along the way. I have changed the plugs, the airfilters, cleaned the Idle valve ( it works, I hear it click when i run 12v on it, at idle seems to have 3.9v current ), ran Techron through, changed the motor mounts along the way. Changed some of the vacum rubbers, i did the lines some months back. Adjusted the timing.

However during this period all of a sudden the car has a real
problem starting in the morning, this just happened.
As soon as the car is hot, it runs ok, still fluctuation on the idle but not too bad.

I have no clue why all of a sudden cold start would be a problem, I have to give it gas to start, I have to rev it up to
get it going.

Any ideas ???

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Old 06-26-2004, 08:17 AM
mike65's Avatar
Registered User
Join Date: Oct 2003
Location: Ireland
Posts: 166
Faulty Cold start valve or sensor or connections between the two?


1990 Euro 190e 2.0l Petrol Now Sold Maybe a w116 280se next
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Old 06-26-2004, 02:03 PM
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Yea, the best guess I heard so far, I guess thats the one beneath the airfilter housing. I will check that one out.
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Old 06-26-2004, 07:06 PM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Could also be the warm-up compensator, or it's power supply.

But if you have to push the gas to start it, I wonder if you are getting any air through the idle valve at start. Maybe the idle control unit is failing, and keeping the valve closed until the coolant sensor opens.

Next time you try a cold start, pull the connector off the coolant sensor - rear of right cylinder head, under the throttle linkage - and see if that makes a difference. That will simulate a warm engine for the idle control unit. The idle will be low but the engine should run.

Also try a cold start with the idle valve unplugged. If you get the same problem, that would eliminate the idle system as a problem. You should get an "idle" around 1100 RPM.

If that doesn't make a difference, try unplugging the frequency valve before a cold start. If that makes a difference, then the lambda system may have gone off the deep end and is trying to lean the mixture at start. The valve is on the intake manifold near the valve cover on the left side. On my SL, it has a yellow connector with two wires.

Good luck.
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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Old 06-26-2004, 07:38 PM
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Chuck, a lot of info there to work with, big thanks. I will have plenty to work on, the car only gets cold over night, so it will take me some time to trouble shoot.

Btw, I was reading your signature, as it happens I just moved to Falls Church myself a few months back.

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