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engine rough idle

Started by Rob Smith, 25 May 2022, 07:22 PM

Rob Smith

I'm having an issue with my 1973 450se. When started cold, idle is really rough sounds like a few plugs not firing and #4 and #8 plugs sooty. The temp gauge is erratic and doesn't seem to move any.   I haven't let it warm up because it's too rough. Any help would be appreciated.


Rob Smith


You're probably wondering why the dirth of responses?
The answer is not straight forward.
This topic has been dealt with a dozen times over the years.
It always leads to the same thing. You cannot ask why is it "missing" when the answer is 5 paragraphs minimum.

For a start...the temp gauge.
Is it actually getting a reliable and constant source of coolant flowing past it? OR
Most likely a bad connection where the sender unit in the head meets the terminal ..make sure that contact is 101%. 
Is the sender dodgey? Remove from cyl head and test it.
All ok?
What about the continuity of the wire from the sender to the gauge?
Test for continuity.
All good to this point?
 Must be the gauge. Remove speedo binacle..a career in itself. Test gauge. No go?..must be the gauge.
Send it for yourself..replace as appropriate.

The miss when cold. Oh boy how long is a piece of string.?

Start with electricals..the basics. 

Distributor cap?........plug leads.? spark plugs. All good? Are you sure? how have you gone about testing these?
Don't waste your 8 new plugs.

Now fuel. you are lucky. You have D jetronic.

You seem to have isolated  a couple of cyls. They are sooty. So they are not working. This points to electrical BUT they could be a couple of injectors running rich. 
Swap the injectors with a couple that are clean.

Either the same cyl will clag up which case there is an electrical supply to the plugs issue OR
the cylinders that bomb out will be the ones that the injectors were swapped with.

Note: I'm not super sure about D jectronic  but don't try cleaning the plugs. Once they've fouled they are cactus.
Report in.

Rob Smith