Rough Start - Good Plugs and Relay - PeachParts Mercedes-Benz Forum

#1 06-12-2003, 08:33 AM

I am experiencing a rough start, actually it's my car, although mornings for me lately have been no picnic... but I digress.
While the car starts almost effortlessly, she is missing on a couple of cylinders. Any revving results in shaking and whitish-bluish smoke until she is warm and then all is smooth. I tested the glow plugs and the ohm meter shows that all are good and strong. The relay is giving just above 12 volts to each plug during warm up. So, either something is disturbing the tranfer of energy from the relay to the plugs or some other cause is effecting the process (injectors?). What are the most likely diagnostic courses that will lead me to the facts?
Thanks.
Mike

P.E.Haiges · #2 06-12-2003, 11:07 AM

Mccan,

1. Check valve clearances.

2. Try isolating miss to certain cylinders by loosening pipe nuts on injectors. If you find the bad cylinder(s), switch the injectors with good cylinder(s) and see if the misfiring follows the injector switch.

3. Do a compression test.

P E H

Rick Miley · #3 06-12-2003, 01:25 PM

Wow, I'm surprised that PEH accepted your glow plug test and went on to harder stuff. It has been reported here a number of times that glow plugs can test good and still not work. The ultimate test is to remove them from the engine, put 12 volts to them and see if they glow red hot within a few seconds. If you want to do PEH's injector line test, it will tell you which plugs to pull.

#4 06-12-2003, 01:30 PM

Here's the deal. I cleaned off the posts and reamed out the plugs at the relay and this corrected the problem. I was figuroing out what might all the variables be and which would be the easiest to address. Contact points seemed a reasonable palce to start. I got lucky.

#5 06-12-2003, 04:37 PM

I had the same problem with my '76 300D. I did as you then changed the fuel filter. It starts great now and idles good. I am starting to get a little nailing in the injectors, so I guess it's about time to pull them and clean them.

Rick Miley · #6 06-12-2003, 04:49 PM

You guys come back and help during October, which is glow plug month. Didn't you know June is cooling system month?

P.E.Haiges · #7 06-12-2003, 06:32 PM

Rick,

I thought about testing th GP as you said but I thought the misfiring was occurring after the engine was running longer because Mccan said it missed until warm.

Usually a bad GP will make it miss after starting but the engine will run with out missing in a very short time.

I agree that the best test of a GP is as you described unless the GP read open circuit. Then you know they are bad unless there is a wiring or connection problem.

P E H