Checked my OM and found both W9DC and W9DC0 are listed o.k. for plugs. Thus, I put in a set of new W9DC and found the old ones were W9DC0.

I "feel" the car (91 560 SEL) is running better except at idle. The longer I wait at the light the worst it gets. It seems the idle is going down from 550 RPM to around 450 and I could feel it start to shake.

What is the difference betwen a W9DC0 and a W9DC? Could the new plugs require a new timing?

Not sure about the plug type difs but when my 190e started doing that at stop lights it ended up being the motor mounts causing the excess vibration.

__________________
89 190E
2.6L
Arctic white
Grey leather
Sunroof
Pirelli P400 rubber
Smoked corners

The 0 indicates a bigger center electrode. They are supposed to be compatible but if you search you will find that many have had problems with factory recommended plugs.

$8.00 to switch back is a pretty good price to rule it out, if it is the problem, you'll know what plugs to get in the future.

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html

Called dealer and they only carrys W9DC0 for 560 SEL. Went to NAPA and studied the cross reference chart for 10 minutes. W9DC0 is what came with the car and W9DC is listed as o.k. for replacement. However, W9DC0 uses 0.8 mm gap but W9DC uses a 0.9 mm gap.

Got home and regap all the W9DC to 0.9 mm. The heavy shake is gone but it still not as smooth as I like. Will drive it for a day to see if it smoothes out at operating temp. If not, I have got a set of W9DC0 from dealer at $2.5 each.

Changing from the OE HD electrode to the standard electrode will NOT cause the idle issues you are reporting. Both should be gapped the same. The difference is that the HD version has a wider center and ground electrode. You have another problem!

Check that the throttle position switch on the end of the thottle valve shaft is sending the idle signal. As they get old the contacts can misbehave and cause idle problems. If the ECU does not get the idle signal from the TPS it will not go into idle control mode to maintain idle speed/stability.

Duke

Hello Duke, are U sure about W9DC and W9DCO need to be gapped the same? They printed on the package that W9DC has 0.9 mm gap and W9DCO has 0.8 mm. Once I got all W9DC gapped at 0.9 mm , the car runs fine. However, I do notice a much higher gas consumption. Instead of 200 miles, I got only 120 miles per 1/2 tank of Exxon 91 gas.

Could a large gap cost me more gas?