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

by Sajeev Mehta

Updated: May 16th, 2014 Published: September 9th, 2013

piston slap deffo not your father s oldsmobile part ii

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?

Thanks,

Paul

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.

#305 #307 #350 #5.0 #5.7 #98 #BroHam #Brougham #Chevy #CrateMotor #Oldsmobile #OldsmobileNinety-EightRegency #Regency #SmallBlock #ZZ4 #GMPerformance

Sajeev Mehta

More by Sajeev Mehta

Comments

Join the conversation

5 of 47 comments

MaxHedrm on Sep 09, 2013

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

Like Reply
- See 2 previous
- Burnbomber on Sep 09, 2013
  
  Beat me to it. Did that on my Olds Cutlass, 2bb 350 V8. Turned it off in gear to kill the dieseling.
  
  Like
- 69firebird on Sep 09, 2013
  
  Yes.Lots of them on GM's quit in short order.Or they didn't have the bolt-kick adjusted out far enough.They were supposed to pop out upon turning off the ignition,and goose the throttle linkage at the carb a bit.I remember it being a problem during early emissions adaptation,and when Octane ratings took a dive in the early 70's,about that same time,the solenoid deal came into play. Yeah....I'm that old.I worked in a gas station when I was a kid selling Sunoco 260,walking around outside with a fat wad of cash in my pocket most of the day.See that much nowadays? Those solenoids never worked that great,even when they operated correctly.It was a half-assed measure,which pretty much sums up how they did emissions control back then.
  
  Like
- 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.
  
  Like
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.

Like Reply

Recent Comments

Jeff I don't believe that a teenager should have a brand new vehicle nor should they be driving a really old vehicle. Most teenagers will not fully appreciate being given a brand new vehicle and need to learn that there is a responsibility to owing and driving a vehicle. An older vehicle especially one that is very old lacks the safety equipment that newer ones have.
Dale It's not for me but it looks nice for someone else.
Ted VoGhost, I am looking for 350-400 mile range with 5 minute full recharging that is available like today’s gas stations. My personal threshold has more to do with recharge availability and times than the 350 mile range. I’d be ready to switch at a dependable 250 mile range and 10 minute charge times.
V8fairy I am starting to see some red emerging on the roads lately. We also have a lot of used import Toyota Aquas and some of them are in really cool colours, like orange, electric blue and bright yellow. What I really miss is colourful interiors, I'd love a bordello red crushed velvet interior, that'd sell me on an EV
Ted Bryant Agree with Mikey.Manual. Any manual transmission car. 1) can't text-and-drive, 2) forces driver to pay attention, 3) perfect security because most thieves can't drive stick, 4) fun to drive, 5) friends won't drive the car (they can't drive stick), 6) compression start -- never get stuck, and 7) will always be able to drive any vehicle anywhere. Did this for both kids -- after a couple weeks of complaining, they finally got to it, and now only drive manual. And they are both great drivers. But their friends do poke them for driving stick -- "oh neat - a manual. do you bake your own bread and sew your own clothes too..."