By on September 9, 2013

CapturePaul 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.

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47 Comments on “Piston Slap: Deffo Not Your Father’s Oldsmobile! (Part II)...”


  • avatar
    danio3834

    ZZ4 in a 98 Pregnancy? Sign me up! Fantastic value in a stout crate motor.

    Like has been said, try resetting your ignition timing, or backing it down a few degrees to see if that helps. Before adjusting, make sure the vacuum advance is working correctly though. I assume you’re running an HEI distributor (why wouldn’t you be?).

    Out of curiosity, what kind of carb are you running?

    • 0 avatar
      69firebird

      I have to say you’re running too much advance for the gas/w/ethanol we have today.Especially if you’re running regular.Back off your timing 2 degrees,wait till you’re near empty and try a better grade of gas,and see how it runs.

  • avatar
    -Nate

    Um;

    _NO_ vacuum signal to dizzy at idle ! .

    I’d also suggest adding a throttle kicker solenoid , GM used them from 1968 onwards , you set the base hot idle to 400 RPM’s then actuate the solenoid and set the running idle speed to 800 RPM’s and VIOLA ! never any dieseling even when tuned to easily pass SMOG test .

    -Nate

  • avatar
    -Nate

    OBTW : is aftermarket F.I. really cheap these days ?

    I’d love to add F.I.to my ’63 ~ ’89 Chevy Inline 6 banger…..

    It’s a three port head .

    TIA ,

    -Nate

    • 0 avatar
      bumpy ii

      EFI kits are easy enough to find for the popular V8s. Looks like you’d have to homebrew one for the CHevy I6, though, unless there was some marine TBI setup you could borrow parts from?

      • 0 avatar
        Jeff Weimer

        There’s a number of semi drop-in (you have to drill and tap for an O@ sensor in the exhaust manifolds) Throttle-Body injection systems that bolt right up to the carburetor mount plate, with a Holley pattern for sure, and probably Quadrajet, too.

        Not Cheap – They’re about $2000, give or take.

    • 0 avatar

      http://www.megamanual.com/index.html

      Totally worth a read.

    • 0 avatar
      Scoutdude

      If you want to do it cheap you get the complete TBI system from a late 80’s early 90’s Chevy 4.3. While you are at the wrecking yard grab a frame mounted fuel pump from an 80’s Ford truck, van or Fox and an HEI from a 70’s Chevy straight 6. All up you can get the parts for $200 or so. Add in an adapter to mount the TBI to the existing manifold bolt on and go. If you really want to optimize it you can get a chip burner to tune it perfectly but most of the time it will run way better than a carb w/o a tune. Even with the stuff to tune it you can still be on the road for less than $500.

      • 0 avatar
        -Nate

        @ ScoutDude : ”

        If you want to do it cheap you get the complete TBI system from a late 80′s early 90′s Chevy 4.3. While you are at the wrecking yard grab a frame mounted fuel pump from an 80′s Ford truck, van or Fox and an HEI from a 70′s Chevy straight 6. All up you can get the parts for $200 or so. Add in an adapter to mount the TBI to the existing manifold bolt on and go. If you really want to optimize it you can get a chip burner to tune it perfectly but most of the time it will run way better than a carb w/o a tune. Even with the stuff to tune it you can still be on the road for less than $500.”

        Thanx but I want a three injector setup , I have so many on going projects now I don’t want to spend too much time cobbling one up .

        -Nate

  • avatar
    bumpy ii

    Maybe it’s just me, but I saw the ad with the suits up top and immediately pictured them as a group of ’70s GM executives chuckling about how the “Jap car” fad will never last.

  • avatar
    cwallace

    A homebrew EFI sounds like a lot of trouble and corking off of vacuum lines. What about just ordering a professionally rebuilt carb? I got one for my smogged-to-death Nissan pickup for 300 bucks online, and it was a weird Hitachi model so you might find a cheaper one. It came with a new cutoff solenoid that solved my dieseling problem, and didn’t require any special effort on my part to dial in. With all the emissions craziness that is built into engines of this vintage, it might be an easier way to go.

    • 0 avatar
      28-Cars-Later

      I don’t disagree, but I’d point out most states exempt vehicles of this vintage. I can’t wait till my Volvo his 25 and I can put the antique plate on, all of the anti-smog stuff is going in.

      • 0 avatar
        Ryoku75

        What year is it? With most cars of the 80’s I’d be careful about removing smog and “cats”, I had a Mustang with all that removed and it never ran right as the timing and idle were never re-adjusted.

        But then again, old Volvos are some of the most persistant cars you’ll ever find.

        • 0 avatar
          28-Cars-Later

          MY93. I’m hoping to start by converting it to be more euro spec with the cam and emissions nonsense, supposedly to meet US compliance Volvo just detuned the engine a bit. I’d hope to wake it up a little which I’m sure runs afoul of the EPA/DEP killjoys. Supposedly adding the 242 turbo exhaust (to my non turbo 244) bumps hp a little bit, if I end up doing that I prob won’t be putting in the cat, we’ll see.

          • 0 avatar
            Ryoku75

            Adding a turbo exaust will net you a 6hp gain, running Euro equipment will bump the hp up to around 130 so expect a good 20hp increase.

            A Euro computer might be required to handle the adjustments, I would suggest adjusting the engines timing if necessary as well.

            If I had the money and resources I’d probably do the same thing myself, as is theres still enough power to pass up slow pokes during rush hour.

          • 0 avatar
            FuzzyPlushroom

            Yeah, the factory ‘M’ cam (Volvo stamped letters into the ends of the cams) is just depressing. Some earlier and Canadian models came with moderately warmer cams (‘A’ or ‘B’), which are easy enough to find on various Volvo enthusiast sites or on eBay; the factory turbo cam (‘T’) is a slight improvement as well. As far as I know, the engine’s still non-interference with all of ‘em; only the lumpiest aftermarket ones affect clearance that much.

            Freer-flowing exhaust certainly wouldn’t hurt, though it won’t make a huge improvement, either. It’s not really cost-effective to try to make substantial power without turbocharging the car; fortunately, that’s easy enough to do, even with the existing long block, if you drag home a wrecked 740/760/940 Turbo. Those cars made ~162 HP and ~192 lb-ft from the factory, I believe, with fairly conservative boost (9 lbs, iirc) from the fairly petite turbo. Pre-’90 cars had Garrett T3 turbos; ’90+ used Mitsubishi TD04-13C units, which are commonly upgraded to ‘reclocked’ 15G turbos from later FWD models.

            All that said, boosting the car (particularly if it’s an automatic) will have an adverse effect on fuel economy. My automatic 244 (non-turbo, all stock) routinely got 23-24 MPG in mixed driving; my similarly stock automatic 745 Turbo, albeit in rougher shape, never achieved better than 19 when I tried, and usually got 17ish. Turbo Volvos got stronger automatics, but even the AW70 used in N/A 240s holds up reasonably well; the same was true with the manual gearboxes, in which case the true five-speed (designated ‘M47′) is weaker than the iron-cased ‘M46′ (four-speed plus OD), and isn’t a good bet if you want much more than stock turbo-car power out of it.

            Hope that novel helps. I’d suggest the Turbobricks performance section – they’re cool people, though if you venture down into off-topic, it gets seriously weird at times.

  • avatar
    Vulpine

    Personally a fan of the Cutlass of the early ’70s myself; anywhere from ’69 through ’75 though the one I liked the most (and one of two I owned from those vintages) is the ’75. The “Rocket 350″ under the hood had good power and reasonable economy for its day–at least as good as most modern trucks as long as you stayed out of the extra two barrels on the carb. Bringing it up to a more modern suspension would be nice, but NOT any of these low-rider “squashed” looks. Big enough for Caddy-level luxury without the Caddy ostentatiousness.

  • avatar
    TR4

    To those who suggest changing or checking the ignition timing/vacuum advance: how could this possibly help? When you have dieseling the ignition is OFF, so its timing is irrelevant, isn’t it?

    Checking the idle speed and adjusting it slower if practical is a cheap and easy first step. The throttle kicker solenoid sounds like a good idea. IIRC, Volkswagen had an ignition switch activated solenoid for the idle jet to fix dieseling in their later air-cooled engines.

  • avatar
    -Nate

    Whoops ~ topic drift , sorry .

    FWIW , the totally worn out 250 C.I.D. I6 engine in my 1969 Chevy C/10 Shop Truck runs fine , really it does , no dieseling , gets 20 + MPG can chirp the tires and so on but I know that adding F.I. to it will improve every aspect in every way , it’s basic 7th grade physics at work here .

    Capping off vacuum lines etc. isn’t what I want to do .

    I was just hoping there’s a simple , co$t effective solution like the Jeep F.I. up grade , I don’t want to cobble up a frankeninjection system .

    I just scored a 1977 292 C.I.D. I6 for it and would like to improve a bit if it’s simple .

    If not , I know how to keep an InLiner engine sharply tuned better than most .

    I’ve had very good luck with ‘ malaise era ‘ engines retaining most of the smog crap ~ I disable the air injection (who wants deliberately warped valves ?) and the EGR, some times I allow the EGR to remain but modify it to only come into play after the engine is fully warmed up and above 2,000 RPM , then you’ll never notice it atall and yes , it does reduce those evil nitrous oxide emissions .

    -Nate

    • 0 avatar
      Jeff Weimer

      You know, it may just need an adapter plate machined to adapt the throttle body – It’s feedback EFI, so it will adjust the F/A ratio based upon the O2 sensor.

  • avatar
    -Nate

    @ ” TR4

    To those who suggest changing or checking the ignition timing/vacuum advance: how could this possibly help? When you have dieseling the ignition is OFF, so its timing is irrelevant, isn’t it? ” .

    No because when the ignition timing is retarded for smog reduction reasons , it causes hotspots in the combustion chambers so the very first thing to do always is : check and adjust the ignition timing .

    Likewise , if there’s any vacuum signal to the dizzy @ idle , the advance curve is all screwed up causing the engine to work harder , this causes it to run hotter .

    90 + % of the time when a Customer brings me a poorly running and / or dieseling engine , it’s basic tune up stuff like this that most
    ” mechanics ” (feh) are far too lazy to bother checking .

    The first thing any good Mechanic learns is ” K.I.S.S. ” : Keep It Simple Silly because when you know all those myriad things that can go wrong , your mind tends to jump ahead and imagine complex problems and solutions when the reality is quite simple .

    -Nate

  • avatar
    mikey

    Wow! What a wealth of info “Nat” and “danio 3834″ are. I used to own an 84 Caprice with a 305 quadrojet My mechanic went with danio’s fix?

    It worked

    • 0 avatar
      danio3834

      I originally made a boo boo before nate corrected me, you will only have a vacuum signal at the dizzy at idle if someone connected it to manifold vacuum instead of port vacuum (very common mistake). nate is right, you should have none.

      nate is bang on with this one though, the general small “tune up” things should be gone after first. My experience of resetting the ignition timing has resolved a lot of these as it can afftect combustion chamber temperature and vacuum a lot, and could even cause the throttle blades to crack open a bit. So yeah, that’s where I’d start.

    • 0 avatar
      olddavid

      Vacuum and carburetors. Alchemy. Years of attempting to synchronize Hudson Twin-H and 240’s have caused me to believe some things are better left to those who innately get these devices. I bow to anyone with this skill set.

  • avatar

    Don’t forget the idle circuit shut off solenoid. If it has one (many cars of the era did) if it is gummed up and stuck open it will contribute to the problem. Ahh the old Hitachi carbs on Toyotas and Honda’s with them. Get stuck open and suddenly diesel like pigs. Chryslers too. Also when stuck closed or not getting voltage with the key on. No or Crappy idle to boot, that is normally what happens, but I have seen them stick both ways.

    • 0 avatar
      Scoutdude

      As mentioned above American cars use anti-dieseling solenoids or “throttle kicker” to close off air to the engine rather than fuel like most of the imports of the era did. If it has an aftermarket carb it would have neither.

  • avatar

    Dieseling is almost always caused by idle speed being too high, a vacuum leak, float level too high, timing is off, or grossly rich idle mixture

    In reality, it’s probably a vacuum leak under the carb, or the idle speed is a touch high. My 77 Chevelle only diesels when the A/C kick-up solenoid is mis-adjusted for too high an idle speed, and even then it only coughs once or twice. All this on a bone stock and unopened original 305 with probably 150,000 miles on it (no friggin clue on mileage but based on all the original parts it’s probably gone around once)

    Then again, our 84 Delta 88 managed to evade all the best diagnosis of dieseling and we just learned to floor it when shutting it off. – wow forgotten about that till now!

  • avatar
    CoreyDL

    I saw a ~95 Ninety-Eight Regency Elite Brougham go by yesterday as I was in the yard. Beige on brown!

  • avatar
    PrincipalDan

    Nothing to contribute, just enjoying this thread.

    God if I’d had a ZZ4 crate motor in my 87 Cutlass Supreme Brougham I’d have lost my license before leaving college.

  • avatar
    stuart

    Back in the ’70s I saw an informative article about dieseling in Popular Mechanics (or something similar). The author researched the issue in his ’60s muscle car, and determined that it wasn’t “dieseling”; there was a weak spark!

    He traced it to the alternator light (!!). When the car is on and the alternator is not turning, the ignition switch supplies 12V to one side of the ALT lamp, and the other side grounds through the alternator; the lamp lights. When the alternator is spinning, it pushes 12V back up that wire, so the lamp sees 0V (12V on both inputs); the lamp goes out.

    When the ignition is OFF, but the alternator is still spinning, it will push 12V back to the ALT lamp, and maybe half of that voltage will pass through the lamp and continue to the ignition circuit. If the engine is warm and in reasonable tune, even a weak spark will keep it going. This was his “dieseling”.

    The Fix: he added a diode in series with his ALT lamp, so that 12V from the alternator couldn’t reach the ignition wiring. This fixed his “dieseling” problem.

    Quick Test: disconnect your ALT lamp wiring (or remove the bulb). Does it still “diesel” ?

    YMMV,

    stuart

  • avatar
    GS 455

    Sajeev can you tell us exactly how an 80s C body is leagues better than a Panther? How would you compare the 88 Town Car Signature to the Olds 98 Regency?

  • avatar
    Searcher

    Dieseling is the result of too much air entering the engine when shut off. Why that would be can be many things, from a vacuum leak to a poor tune.

    I do note that in this case we have a performance engine installed in car that didn’t come with it and presumably against an automatic with an unknown converter stall speed. I’ve seen it lots of times where a engine with some cam in it is hooked to a tight converter making it so that a high free-idle was needed to get a barely tolerable idle speed in gear. The worst I recall was a late Seventies Corvette with the proverbial “potato” cam that had to have an 1800 rpm free-idle just to idle at 600 or so in gear. We tried everything but the cam/converter mismatch was just too bad and we had to send him away to get something done about that first.

  • avatar
    hands of lunchmeat

    Theres no indication of what manifold/carb hes running, But itd be a shame if he was still running the original q-jet in this.

    As for aftermarket EFi, its tempting, but still a bit pricey. The dropin kits for SBC’s are absurdly easy to install and setup, it makes me wish sometimes i was more of a american car guy. Literally bolt it in, and drive. Tunes itself. Coming from days when programmable EFI was basically a box with a bunch of wires coming out of it, and zero basemaps, the stuff out nowadays is pretty incredible.

    If you were to go fuelly, you literally ditch a lions share of the vacuums lines that the old carb motor uses. One fr brake booster, one for fuel pressure regulator, and one for the distributor, if youre keeping your old one. thats it. the rest can hit the skip basically.

  • avatar

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

    • 0 avatar
      burnbomber

      Beat me to it.

      Did that on my Olds Cutlass, 2bb 350 V8. Turned it off in gear to kill the dieseling.

      • 0 avatar
        69firebird

        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.

    • 0 avatar
      69firebird

      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.

  • avatar
    Scribe39

    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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