Running a little rough at low speed & backfiring -280SL AUTO

[Mike K]

I went out for a long drive in the country today, open road no traffic. The car was running beautifully, averaged between 45-70MPH, however from time to time I picked up a bit of a popping/back firing noise.

When I slowed down at an intersection, she was running a little rough almost as if not firing on all cylinders. As I accelerated the popping/backfiring came back and then went away as I got up to speed.  At the next intersection the same thing happened. I put her into neutral and floored the pedal, she initially coughed a bit, no smoke out of the exhaust.

I got home, removed and checked the spark plugs, they were a perfect colour, rusty-grey, no soot. I took her out for a drive again and it seemed a little better, but on the home stretch there was a bit of roughness again as I decelerated down the driveway.

I’m not sure where to next, ignition leads?

Any input will be appreciated.

Thanks,
Mike

[Cees Klumper]

Lots of possible causes: plugs, wires, points if you still have them, coil or ballast resistor(s), loose linkages, fuel supply impeded. Best check everything methodically one by one as per the linkage tour and other guidance.

[Mike K]

Thanks for the input Cees. I'll go through everything as you've suggested.
I've got a 123 ignition which was installed about 3 years ago. I drive the car almost daily, and yesterday was the first time this happened.

Best,
Mike

[Mike K]

Following Cees' advice above, I checked the following:
1. Linkage is correctly adjusted.
2. No fuel flow issues.

3. Checked & replaced spark plugs- I'm using NGK BP5, the plugs were replaced about 5, 000 miles ago. These plugs which I took out yesterday were rusty grey, no soot perfect colour.

Which leads to me believe it's an electrical/spark issue- wires or coil? Am I correct?
I have a 123 ignition with Bosch red coil for the past 3 years with no issues.

With the new plugs, the car ran beautifully, but after 20 minutes it started running rough again, sounds & feels like it's not running on all cylinders, however no back firing or popping this time.

It appears that once it's settled into operating temperature and after 20 to 30 mins. it starts running rough.
This rough running/popping & backfiring all started suddenly the other day after about 35 minutes of driving on the open road.

Next step?
I think I'll replace spark plug wires & check coil?
What else should I be looking at?

Any advice will be welcome.

Thanks,
Mike

[kampala]

If temperature seems to have an effect, I would be tempted to rule out two temperature sensitive items.  The WRD and CSV.   

WRD - When at operating temp, confirm that that the Air stops sucking.  For me it was best unscrew the small can air filter at the WRD and feel directly at the inlet for sucking.   

CSV - After warm up, turn car off and disconnect the fuel line at the CSV and plug so it is sealed.   Then take for a long drive.

Good luck.

[Mike K]

Thanks Oz. Will do and report back.

Best,
Mike

[Mike K]

It appears that the problem is solved:
After running through everything, I read the Pagoda Notes  Vol.3 no.4 on the 123 system. This led me to check the vacuum lines which run from the throttle body to the emission control set up, and from there to the 123 distributor.

I noticed that the vacuum pipe running from the emission control unit to the distributor was disconnected from the brass nipple on the 123 housing. I reconnected this and immediately the car's running normally again. I've now cable tied the vacuum pipe into place.

Invariably it's most often the small stuff that gets you!

Thanks Cees and Oz for your input.

Best,
Mike