As someone who has owned, raced and worked on a fair number of small and big block Chevrolets I just have to disagree with the idea that this is some mysterious problem beyond an experienced mechanic. On a sbc/bbc with low oil pressure symptoms coupled with deep engine vibration the very FIRST place i would look would be the bottom end. A properly working s/bbc oiling system is pretty robust. No i would not trust the light. I would do what my 1966 factory service manual said. If low oil pressure is suspected, Install a temporary mechanical gauge and ascertain system pressure". When i saw the 6-7lbs oil pressure at idle, that would be it. Bottom end trouble. And yes, I can guarantee you I would spot a missing main cap - even laying under the car. I have a 427 and a 454 out in the shop right now and if you buy the gaskets and pay the labor I'll be glad to shoot the demo photo and post it up.
The missing bearing shell WOULD be hard to see - but i wouldnt find it by looking for it anyway. See, the low oil pressure and the vibration would scream to me "main and/or rod bearing issue" so it wouldn't matter WHAT the issue was. It would be found in the tear down - because any small or big block Chevrolet with a deep engine vibration and low oil pressure's got a bottom end problem and that babies coming out!
Of course I don't just post about them - I actually work on them.
Thanks for giving us the chance to exercise our little grey cells, as Detective Poirot would have put it. Brain teasers have always been with us as the following story will show.
Years ago I saw this story in a 1910s-1920s era automotive (maybe SAE) magazine. I apologize for not being able to give full attribution, but I believe it to be true.
It seems this auto manufacturer was developing a new engine for a new model car. The prototype engine was set up on a test stand to run it in before dyno testing. The order was given to fire it up and the engine seemed to crank and crank until it fired up. It ran fine for a while, then huffed and puffed and died. This happened two or three more times and the enginers and mechanics went into their full diagnostic mode just as we have all tried to do in xti's mystery. It was to no avail as the engine did its crank & crank, fire up & run, and puff & die show.
The order was given to tear down the engine. All parts seemed to be in spec, and while head-scratching was going on, a mechanic started to pick up and look at, and measure, and touch the parts. After some time, he discovered that the camshaft gear had extra teeth so that there was no longer a 2:1 ratio between the crankshaft and camshaft. After cranking for some time, the cam would come into correct phasing with the crank, and the engine would fire and run. It ran long enough to run itself out of phase and the engine would die. A new camshaft gear with the right tooth count solved the problem!
Best regards to all and enjoy the Memorial Day weekend,
Myron – I once worked on a mid 60's Pontiac Parisienne (the Canadian version of a Bonneville, but it was actually just an Impala with different sheet metal). It was owned by one of my neighbours - she was an Avon Lady (hint). It had incredibly low mileage, as she only drove it from one house to the next one, and then eventually, drove it home.
It was a 275HP 327 with a QuadraJet carb. The car had NO power.
I started it up in her driveway, and it was virtually silent. I wasn't absolutely sure that it was even running. About the only way you could tell it was running was listening to the squeak of the fanbelt, and noticing that the fan was turning, albeit VERY slowly.
If you opened the throttle, you were rewarded with a sigh, and a barely perceptible increase in engine RPM.
For some reason, I decided to pull the valve covers, and started the engine again. The rocker arms were, well, wiggling. Sort of. Most were moving up and down no more than 1/4", some not at all.
I pulled the intake manifold, and saw what appeared to be the most perfectly crafted valley cover I had ever seen. It was an exact duplicate of the underside of the intake manifold.
This was a phenomenal find, as I had never seen one like it before, at least not one this well crafted. I had no idea that GM had ever made anything like it, and was very curious as to why they would bother.
It fit so perfectly into the block that there were no spaces around it to even lift it out.
I took a screwdriver and attempted to insert it between the block and the "cover".
It wasn't a cover at all. It was BLACK GOO 6" thick.
She had never changed the oil in the car since it was new – just added more. The car had probably never reached operating temperature even once in its entire life due to her driving habits.
The cam had no lobes on it at all. Some of the lifters were worn so badly that the valves inside the lifter were literally hanging out the bottom, as the bottom of the lifter was completely worn away.
This one had me scratching my head too. Nothing we tried seemed to be getting us any closer to a solution.
After 'exhausting' every other possibility, the only things left that we couldn't absolutely verify were the fuel level in the float bowl under load, and the remote possibility of a restriction somewhere in the exhaust system.
QuadraJet carbs have a composite float, and I have seen them soak up fuel and sink too low, so we put in a new one to be safe. No change.
I decided to remove the mufflers and fire up the engine to see if the mufflers were somehow plugged up.
The sound that came out of the theoretically 'open pipes' was anything but what I expected.
There was a weird whistling / hissing sound along with the expected open exhaust sound.
As the engine was revved, the hissing sound nearly drowned out the open exhaust sound.
I shut down the motor, crawled under the car, and took a very close look at the two roughly 5' long pipes between the headers and the mufflers.
I realized that one was dead round for it's entire length, but the other one was slightly oval for about 12" in the middle of the pipe. It was only about at most, 1/2" out of round, but it was noticeable.
I grabbed a hacksaw and cut the pipe off about 1/2 way through the 12" long slightly oval section of the pipe.
What I saw completely dumbfounded me.
The pipe was standard mid 60's-mid 80's GM double wall construction (a fact that I knew, but discounted as having anything to do with the problem).
Even though the outside wall of the pipe was virtually untouched, the inside pipe was completely crushed flat. As an example, take a 3" diameter x 1/4" wide rubber band, and put it inside a similar 1 1/2" diameter rubber band. The loop it forms looks exactly like what the inside of my pipe looked like.
Except my pipe was much worse.
Evidently, I had at some point driven through a deep water puddle. The sudden cold shock on the outside pipe had caused it to contract. The inside pipe was still hot, and had nowhere to go, so it buckled and separated slightly from the outside pipe. When the pipe cooled overnight, there was a small gap left between the walls of the pipe. This meant that a slight vacuum formed, which of course pulled in moisture-laden air to fill it back up.
The next morning, the moisture flashed to steam and, since it was impossible to stretch the outside wall, it further enlarged the space by crushing the inner pipe a little bit more.
Every time the car was parked, the cycle would repeat. As the space between the pipes grew, larger and larger volumes of 'wet' air were drawn in, and when it flashed into steam, the inner pipe was rapidly crushed until it was completely flattened against the outer wall.
The only reason the engine ran at all was the fact that I had re-used the stock intake manifold and used stock gaskets to put it back on. These manifolds have a heat riser passage that connects one exhaust port of each head together. Normally, there was a thermostatically controlled valve on the passenger side exhaust manifold that forced all the exhaust on one side of the motor to go through the intake manifold to the other side when the engine was cold. This was intended to prevent icing of the carburetor in cold weather.
Ironically, if I had used an aftermarket aluminum manifold, the engine wouldn't have run at all, as they usually don't have a heat riser passage in them.
Thank you all for trying to wrap your heads around this. For me, it was simultaneously one of the most frustrating, but ultimately rewarding diagnostic sessions I have ever experienced.
I may have missed this in the various emails, but did you lads check the camshaft? Gradual power loss may be the result of camshaft/lifter incompatibility (camshaft was changed, as I recall) resulting in cam lobes so worn that they can't open the valves.
Also, was cam and crank timing checked? A worn timing chain may result in the chain jumping a tooth or more resulting in retarded cam timing. The result is reduced low-end performance, but noticeably perky high rpm performance. You would find that the distributor would have to be advanced every time the chain jumped a tooth. After a while the cam timing would become so retarded that the engine would not start or run very weakly if it did.
I had first hand experience of these very symptoms on a 1971 Dodge with a 318 ci V-8 that used a plastic-coated cam gear that started to degenerate at the 125,000 mile mark. I presume this coating was for quiet operation, like the infamous Flathead Ford molded fiber cam gear.
Another thought here; was there good oil flow to the lifters? Low oil pressure could result in collapsed lifters and again, valves don't open or open a very small amount. Forget this idea though, if you changed to solid lifters with the camshaft change.
After rereading your hints about the deep puddles and the exhaust, did you perhaps park with the tailpipes immersed in a puddle, and when you shut her down maybe it sucked a bunch of water into the muffler, causing restriction? But I'd think any trapped water would just push out with a few revs of the engine. Just grasping at straws based on your hint.
If water has nothing to do with it, I'd go with the stretched timing chain / skipped teeth. Been there, done that!
Ok, I'll take a stab at this. You've said it's not a fuel or ignition problem, so I'll assume timing is ok too. Compression was good, and from what you're telling us it sounds like exhaust restriction was to spec. A cam lobe may be slowing wearing away, but this would only affect one cylinder (never heard of all the lobes going flat) and I doubt one cylinder down on power would have this much of an effect.
The timing chain could be stretching too causing valve (and hence ignition) timing to be retarded more and more, and it's finally slipped a tooth or two? I think you mentioned it had a "racing" type double roller timing chain didn't you? I hate those things - they might be stronger than a stock factory chain and maybe the chain doesn't stretch as much, but the teeth depth is much shallower, so just a little bit of stretch will make those type of chain sets skip a tooth. Yes, I know that from experience - the durability on those double rollers is lousy. I've seen them fail in 10K-20K miles A stock factory chain & gear has really deep teeth, and the chain has to stretch a lot (and usually the nylon on the nylon coated ones has to be mostly chipped off) before it'll finally skip a tooth.
You hint that water intrusion might be the culprit. I've experienced this on a high performance fuel injected car (86-87 Buick intercooled turbo) where the owner had installed a high flow cold air intake which sucked in air from under the bumper and had driven through pouring rain. The air filter was soaked and was throwing water on the mass airflow sensor causing it to read inaccurately. However, since this is an old school carburated engine with an old school air filter above the carb, I can't imagine it getting soaked or being able to throw enough water down the carb to cause the issues you're seeing.
But, staying with your water intrusion hint, the factory air cleaner housing had a Thermac didn't it? This is the thing with a flap that redirected the air intake from the normal path to sucking air off the exhaust manifold when the engine was cold (to try to get warmer air into the carb). So maybe the Thermac was corroded so that it was stuck in that position, and maybe the air hose to the exhaust manifold was crushed or blocked with debris and the engine was starving for air?
That's my best guess given the available info. Looking forward to hearing what the answer is!
I've seen several people noting that the mechanics would have noticed a missing main bearing cap if the pan were pulled.
All the main bearing caps were in place. Only the center main bearing lower shell was missing. This part is only about 1/8" thick, and is completely hidden by the cap.
The only way to see if it was installed would be to remove all the main bearing caps.
But this infers that the mechanics should have expected this type of problem.
I highly doubt that more than a couple of the more than 1 million small block Chevy engines built ever suffered from this error.
To have a mechanic consider a literal 'once in a million' chance as an occurrence that should be investigated first seems to be asking a bit much.
How many of us would have connected a slight low oil pressure issue with a vibration problem anyway?
It would seem far more likely to me that some out of balance rotating part would be the culprit, as the oil pressure variance was evidently within reasonable tolerances. With no lower shell to hold it in place, the upper bearing shell may have rotated enough to partially block the oil passage in the block, making the loss of pressure relatively small. Maybe they cured the light by increasing the idle speed slightly.
The oil pressure might have been considered a separate minor problem that would be figured out after they cured the vibration.
What if the oil pressure light was burned out? Then how would you diagnose it? Who would even consider checking oil pressure when looking for a vibration?
I can hear it now -
Customer: "I have a vibration".
Service Tech: "OK. We'll tear the motor apart and look for missing bearing shells. I've never heard of it happening, but let's look there anyway - I'm sure GM will understand and pay us for our time."
Haven't read through all the posts but I would have thought the oil pressure light coming on would have been a clue. One reason I like a pressure gage versus a light (or too late light) is that you can monitor the actual pressure.
I suspect if the oil pressure was checked you would have seen low oil pressure throughout the rpm range.
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