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Old 10-03-2007, 06:49 PM
Join Date: Oct 2007
Location: Charlotte, N.C
Posts: 2
82 380 SL idle problems

The vehicle in question has been brought to my shop today after visiting the local Mercedes dealer and another repair facility due to the very poor idle. The vehicle runs very rich at idle and will leave soot marks on the concrete pad outside of my shop after a few minutes. It has 92,500 plus miles on the odometer.
While at the Dealership, the following items were replaced, and the vehicle idle still remains poor. (very rough) which smoothes out at increased rpms.
1. The warm up regulator was replaced
2. All of the fuel injectors were replaced
3. The catalytic converter and 02 sensor(s) were replaced
4. The frequency valve appears to have been replaced as well

The shop that sent the vehicle to me had performed the following in order to diagnose the vehicle:
1. Tested the engine compression -170psi across the board
2. Pulled the valve covers to check the cam timing
3. Replaced the spark plugs as one bank's plugs were fuel fouled
4. Checked the battery voltage- 13.2-13.8 volts dc.
Its been a while since I jumped into diagnosing a CIS system and I had the following thoughts:
1. Check the spark plug wire routing to ensure that they are routed correctly
2. Check the function of the over voltage relay
3. Smoke test the vacuum system to see if there are any vacuum leaks. (this would help to explain why the engine smooths out after the rpms are increased as the vacuum diminishes.
I was thinking that there could be a leaking cold start valve but I did not notice any leaks during my examination of the engine compartment. Now I realize that these symptoms might be the result of a bad fuel pressure regulator, a bad water temp sensor or several other reasons (each more unlikely than the next) however I don't know where to start because of the unknown history of the vehicle. If I use the diagnosis by exclusion strategy I'm afraid that the customer may be facing hours of diagnostic time until I can establish a baseline.
Any and all suggestions on what strategy might be the most productive and what tests I may run would certainly be appreciated. Thank you.
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Old 10-09-2007, 08:04 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
First thing I would try is adjusting the fuel mixture. Try leaning out the mixture to see if it is too rich.

The CIS injection is all about fuel pressure differential. Measuring the system pressure VS the control pressure when both cold & warm would be the next step IF the fuel mixture makes a difference.

You can verify TDC by removing #1 cylinder spark plug & seeing if that piston is truly at top when the crank reads TDC.
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Old 10-10-2007, 02:27 PM
Join Date: Oct 2007
Location: Charlotte, N.C
Posts: 2
Thank You.

Thanks M.B. Doc for the reply.I sincerely appreciate the response. I had checked the CO level and it is at an acceptable level. I did find however, that the passenger side cam timing is off. Apparently the owner had brought the vehicle to another shop that installed a new timing chain, guides and tensioner. Instead of using the bigger timing mark (as on the drivers side), they used the smaller mark instead. This adversley affected the engine timing and consequently the manifold vacuum (which was approximately 14").
I'll re-time the engine and then get back to you regarding its performance.
Thanks again,
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Old 10-10-2007, 04:36 PM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Nice wrench!
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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