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Old 08-06-2003, 09:19 AM
ctaylor738 ctaylor738 is offline
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,315
EHA Current Measurement Results - Evaluation?

My 1986 300E has an intermittent rough idle and stalling problem. It usually occurs shortly after reaching operating temperature. It may suddenly die at idle, be hard to restart, and then miss under load at lower RPM. Or it will start missing under load, refuse to idle, etc. These episodes may go on for half an hour and persist if the car is shut off and restarted. Then it will clear up and run well for some time.

Under normal conditions, the car also has a slightly rough and slightly surging idle.

At idle, the on/off ratio fluctuates around 50%. But it will drop as the idle decreases and then increase as the idle speed increases. I cannot tell which is the cause and which is the effect.

I have rounded up the usual suspects: cap, rotor, and plugs are new, and the plug wires were replaced about 20K miles ago.

A couple of days ago, my EHA test harness arrived, so today I installed it and hooked it up to a DMM with extended leads so I could watch it while driving. I was hoping to catch it in the act, but no luck. But I did observe the readings - in all cases there was constant fluctuation:

Idle: -.6 to +.4 warmup, -2.1 to -1.2 hot idle with AC on

Normal driving, including highway: -2 to .8

Hard acceleration +8

Full deceleration -64

Is there anything in these readings to suggest what the problem might be? Any other driving conditions to be tested?

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; 08-06-2003 at 09:27 AM.
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