Piston Slap: Deffo Not Your Father's Oldsmobile! (Part II)

Sajeev Mehta
by Sajeev Mehta

Paul writes:

Hello Sajeev,

This is my second time writing in about my Oldsmobile. I solved the cooling problem with a mechanical fan, however now I am having another problem. As you may recall I swapped in a ZZ4 GM Performance 350 CI motor, and now it will “diesel” for awhile after I shut it off. It only does this after it has had a chance to warm up. Do you have any ideas for fixing this?



Sajeev answers:

Dieseling is a common problem with carburetor equipped vehicles from yesteryear. If you’ve owned a vehicle with one of these glorified toilet bowls for an extended period of time, odds are you’ve experienced this. I did before, and I have again: our Ford Sierra project car and it’s 2BBL carb just dieseled last week!

Honestly, the five well-written causes for dieseling in the Wikipedia article (first sentence, paragraph above) does a pretty fantastic job addressing the issues. I assume your Olds, like most not-totally-complete project cars, isn’t driven on a daily basis: meaning that carbon build up isn’t a concern. Perhaps the idle speed is too high. Since the ZZ4 has a fairly mild cam profile, keep it around 800rpm. Wikipedia also mentioned timing: make sure that ZZ4 is set to the correct specs ( 10 degree BTDC @ 800 rpm 32 degree total), but I doubt that’s the problem here.

The remaining problems are my concern, and they all point to the condition/tune of the carb. How is the accelerator pump doing? Are its seals in tip-top shape? Is the carb tuned too lean and still running a bit too hot? Fatten up the mixture a little and address any more cooling issues. I hope you still don’t have cooling issues!

If the carb is some old pile you had lying around (or got for cheap) perhaps this is a good time to consider a stand-alone EFI swap. Man, they are dirt cheap these days, and would really add the element of modern luxury to one of the nicer luxury rides of all time. Of all time?

Don’t believe me? Just go sit in a 9th generation Olds 98 Regency (or Buick Electra/Park Avenue cousin) and get back to me. Plenty of old world Detroit luxury with a bit of modern production values stemming from the 1977 downsizing of these monsters. And introduction of gee-whiz tech goodies in the 1980s, natch. These are just as nice as a Caddy without being ostentatious, and leagues ahead of any Panther. Oh yeah, I just said that: 1980s C-body for the win.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

Sajeev Mehta
Sajeev Mehta

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5 of 47 comments
  • MaxHedrm MaxHedrm on Sep 09, 2013

    Just leave it in gear when you shut it off. Problem solved. ;^)

    • See 2 previous
    • 69firebird 69firebird on Sep 09, 2013

      @burnbomber I had a 1969 with a 350 4bbl that wouldn't stop.I punched the throttle once,had a loud boom and turned the mufflers into This ^ kind of shape.Never tried that again.

  • Scribe39 Scribe39 on Sep 10, 2013

    Another question to take us back to the bad old days. I had nearly put dieseling out of my mind before reading this. I recall it well, and the most common fix was, as noted about, to shut it off in gear. I belileve the shutdown solenoids came after that.

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