Mystery Engine Diagnosis

Discussion in 'Jeeps with the Buick Heartbeat' started by theroundbug, Dec 3, 2020.

  1. theroundbug

    theroundbug Well-Known Member

    Putting this here since"Jeeps with the Buick heartbeat" are usually my 231 odd-fire. I'm at my wits end here with this motor. I have suspicions but I don't know how to confirm them. Here's the situation

    Since I've owned my Century it's had this issue where under load, especially at WOT at low rpm, the engine will backfire out the carburetor primaries (it's a dull grey backfire not burning) and the car will stumble on its face until throttle is let up. Sometimes, it wouldn't do it at all and I would drive 50 miles with no hiccups. Other days like today I can barely get to the gas station without treating the throttle like a baby. I've done so much work on this thing I don't know what else to do short of have the entire motor rebuilt from the ground up. Here's a list of things and their effects

    -Buick 231 Odd-Fire v6-
    Replaced plugs and wires multiple times - passenger plugs consistently lean, number 2 white white white
    Brand new rebuilt Delco HEI distributor with a Blue Streak coil
    Had the 2GC carburetor rebuilt
    Added fuel pressure regulator
    Blocked off EGR and all other emissions
    Rebuilt TH350 transmission
    New PCV valve
    Confirmed timing 12' at 900 rpm idle

    No change so I started getting a little more crazy - I thought maybe intake manifold leak between ports SO
    Replaced intake manifold with a 4 barrel Offy and a Slayer 450 carb - old pan gasket was NASTY
    Runs GREAT but still backfiring with lean plugs on 2,4,6
    Jetted up bit by bit from 55/60 to 65/70 - 10 sizes up is quite a lot, engine liked it every time I went up
    Found a 4 barrel snorkel air cleaner hooked that up for Thermac in the cold mornings - engine liked that on the highway a lot
    Every time I would jet up it became harder to backfire I climbed a pretty long hill and almost teared up when it did it without jerking until today it just fell flat on its face and barely accelerates at all.

    No mechanic has been able to diagnose this issue or is to lazy to do so and just wastes my money. They all just say "it's a problem with the carb" Well I have a brand new fuel delivery system so obviously that wasn't it.

    Here's my diagnostic data
    Vacuum at idle 18-19 in, perfect vacuum response across range, quick little shake between 18/19
    Fuel pressure 6.5 (per carb manual) and stays that way during backfires
    Sparks appear strong
    Compression is good 140ish one cylinder was at 128 still within spec per factory manual
    Rockers all moving smoothly nothing seems stuck
    No external vacuum leaks that I can find.
    No coolant/oil consumption or leaks.
    All ports were pretty carbon-caked up when I had the intake off, cleaned those up

    Any ideas? I'm too deep in to this thing to "put a 350/455 in it" so don't suggest that please - been down that road. Everything I'm reading either says it's running lean - but I've already jetted up 10 sizes - or something with a sticking valve, worn timing chain, piston issue all of which would require some serious work.
    Last edited: Dec 3, 2020
    69GS&M21 likes this.
  2. pbr400

    pbr400 68GS400

    Sloppy timing chain would cause that, and it’s easy to check.
    69GS&M21 likes this.
  3. theroundbug

    theroundbug Well-Known Member

    Thank you for reading! I had my suspicions about timing since I set timing at 12' a month ago and checked it yesterday and its at 8' somehow, correcting back to 12 didn't do any favors lol

    How do I confirm if it's the timing chain? I will try the crankshaft test on my next day off, but is there any way to check if it has slipped and how much?

    edit: I forgot to add because I didn't think much of it, but the timing mark dances around quite a bit, like 3 degrees when I shine a light on it. This would add to the possibility of a worn chain I think?
    Last edited: Dec 4, 2020
    6769RIV likes this.
  4. Jim Weise

    Jim Weise 1000+HP

    If there is excessive slack in the timing chain, then it will have retarded cam timing. The looser it is, the farther it is retarded.

    If it skipped a tooth, it would barely run.

    Do the dist cap off, crank rotate back and forth test, and see how loose it is.

    69GS&M21 likes this.
  5. theroundbug

    theroundbug Well-Known Member

    So it’s definitely not tight. I can move the pulley a bit before the rotor moves. I measured the distance on the alternator fan blades. I think this engine has the original chain and gears which mean it’s probably sheared teeth off... that’s what Pat Ganahls book is saying happens with stock gears on the 231.

    Is that a normal amount of play? Or should it be fully synchronized. I also just hooked up a tach and found that it won’t rev past 3800 rpm and the belts start squeaking. The redline for this engine is around 5200 or 5400 stock

    Attached Files:

    Last edited: Dec 6, 2020
  6. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Based on your symptoms the first thing I would do is install a fuel pressure gauge and monitor it while driving it hard.

    If it won't rev past 3800, I'd check for a flat cam.

    The alternator pulley isn't a great unit of measure for the timing chain slack. You have to remember the alternator is overdriven nearly 3x over the crank pulley. You'd have to measure the pulleys and math it out. Bring the engine to TDC and then use the timing tab as your gauge. Much easier to measure that way.
  7. theroundbug

    theroundbug Well-Known Member

    Fuel pressure was measured already as originally posted, no changes at all even during backfiring. Timing mark dancing around 5 degrees is normal?

    I have a new cam I was going to have reground by Isky and put in with a timing set, so that would fix a flat cam. I’m just worried about burnt valves or bad rings etc. Putting a cam/timing set in is much cheaper than a full rebuild

    Here is a slowmotion video of the passenger rockers, none seem to be moving less than the others so I'm not so sure a flat cam is the culprit

    Degrees on the crankshaft pulley were hard to measure, I'd estimate maybe 10-15 degrees?
    Last edited: Dec 6, 2020
  8. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    It was unclear whether you drove the car with the gauge on it or just revved it.
  9. theroundbug

    theroundbug Well-Known Member

  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Do you have a voltmeter? Check the voltage across the battery with the engine running. Probably has nothing to do with your problem, but a while back, the regulator inside my alternator went, and the voltage spiked to 18 volts. First indication was belt squeal when revving the engine. Looked down at my voltmeter gauge and it was pegged at idle.
    69GS&M21 likes this.
  11. theroundbug

    theroundbug Well-Known Member

    :0 I'll go check.

    Voltage across battery terminals is 14.5 while running

    I'll add for anyone wondering - the engine idles wonderfully, and revs relatively nicely albeit with what feels like a slight miss but it is an odd-fire so it's hard to tell. It's only when it needs power that it starts to choke and jerk around. I'm sure once this issue is resolved it will idle even better and rev even better but idling there is nothing obvious that someone would look at and say "there is something wrong"

    Another thing - the engine has always ran perfectly cool, even in 110' heat. Never overheats. New 3 core brass radiator kicks ass.
    Last edited: Dec 7, 2020
  12. theroundbug

    theroundbug Well-Known Member

    So I've ordered timing chain, gears, cam bearings, full gasket set, timing cover, dizzie hold down, timing indicator, lifters, springs, cam gear bolt/washer from TA Performance. Am I missing anything? I have a new water pump so that can remain.

    Do I need a new tensioner? If so how would I know if it's left or right?
  13. 69GS&M21

    69GS&M21 Silver Level contributor

    If none of that solves your problem, you know what you need to do:

    Pull that 6-banger and install a reliable Buick V-8 :)

    (Or at least a 3.8 from a Regal)
    6769RIV likes this.
  14. theroundbug

    theroundbug Well-Known Member

    My mechanic said he'll also check my springs while he's in there, I have new ones standing by. The cam is on hold because Isky, Shneider AND TA Performance are all somehow unaware of the .420" maximum lift issue with stock heads. Blows my mind. I'm talking with the owner of Kenne Bell right now to see if he can send his Mark 1L profile to one of them so they can do the Kenne Bell profile grind for a stock engine since, as you know, Kenne Bell does not do that type of work anymore.

    If putting in a new timing set doesn't fix it the problem I will have spent significantly more than a full rebuild would have cost due to constant mis-diagnosis by many mechanics. It will probably sit covered for years until I can garage it and do a swap myself, most likely with a modern crate engine, and be a forever "project" instead of the daily it was supposed to be.
  15. theroundbug

    theroundbug Well-Known Member

    Well I had the timing chain/gears changed, a TA distributor put in and had the timing verified. No change. That was a waste of money
  16. 6769RIV

    6769RIV Platinum Level Contributor

    Not really. That all needed doing anyway. [​IMG]
  17. theroundbug

    theroundbug Well-Known Member

    It would have been done anyway during a valve job or rebuild, so in essence it was a very expensive diagnostic check. I gambled and lost. Anyway I can't keep throwing money at this right now. Time to find a beater until I can

    With that said, speculations include weak valve springs, burned valve or bad seats. Compression was good so I don't think there is anything wrong with the cylinders, pistons or rings and problems with those would have much more obvious symptoms.
    Last edited: Dec 19, 2020
  18. theroundbug

    theroundbug Well-Known Member

    So my mechanic felt bad doing so much work not knowing exactly what the problem is so he did a thorough diagnosis and found intake valve on cylinder 4 is leaking 30-40 psi.

    As for the timing that did fix the fluctuations in the timing mark and vacuum gauge. It does run smoother minus the backfiring under load and the new shiny components are great to look at :p

    TA Performance distributor is night and day to a rebuilt Delco, even if recurved. Well worth the money.

    Think I'll change the valve springs and if that doesn't fix the leak I'll have Valley Heads do a Smokey rebuild to the heads. Thanks for the suggestions everyone
  19. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    A compression, or leak down test figures it out. It turns out that the basics are always important. Glad you found the problem.
  20. No Lift

    No Lift Platinum Level Contributor

    Late to the story on this. Are you saying that he did a leakdown test? On a used engine 30% leakdown is not unusual and the car will work fine. IMO a badly burned exhaust valve will cause a much lower leakdown number but it would allow exhaust to leak back into the intake probably causing popping fairly easily. Did you ever actually post a video of the actual noise? If you do get rid of the air cleaner so it can really cry out.

    I looked at the video of your car running and the car itself looks nice. A Special no less. Completely unrelated question but does your car have a front swaybar?

    I know it has been changed but those rotors in the odd-fire engines always looked shakey to me.

    What exhaust are you running?(maybe I missed it?) Does it still have a cat. converter? A semi-blocked exhaust (converter) may flow enough air at idle and light cruising but the higher throttle opening could cause it to start to back up the exhaust into the intake and we all know that those V-6's take a lot of throttle to get going. You can plumb a 1/8" pipe fitting into the exhaust right before the converter fairly easily and then measure the pressure with a fuel pressure gauge. It should not be more than a few lbs at WOT. May be easier, unbolt the exhaust at the Y-pie before the converter and take it for a test run and see what it does.

    No carb should be have to jetted up 10 sizes from what they came from the factory with as long as the jest weren't changed previously. I can only assume that the valve springs have been examined and none are actually broken. That would have been easy to spot when the cam was swapped.
    theroundbug likes this.

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