Ockham; Part of my comment was that a missing main bearing cap would be a rare occurence and not an obvious problem to look for. Since you believe the missing part would have been obvious, please forward a picture looking up into the engine from below with the oil pan off.
I've seen a variant on this theme with rebuilt engines where somebody align bores one of the crankshaft main journals off center from the others, and again you get a crank that flexes on every rotation. The ultimate failure is similarly spectacular.
For a few years now folks have been using the horizontally opposed six engine for the Chevy Corvair in aircraft applications, and crankshaft failure is a similar concerne there, partially because the crank and bearings weren't designed for thrust loading (which is what a propeller will put on it) and because there's a pretty big moment arm involved when you're swinging 75" of propeller instead of turning 24" of transmission flywheel.
After a few crank failures in that application, it wasn't long before somebody went back and designed an engine add-on to solve the problem:
Sounds like the same problem my folks had with a 1967 Buick LeSabre, one section of the exhaust was double wall pipe and gradually the inner wall collapsed (rusted) enough to restrict airflow. It was a insiduous problem that none of the dealer mechanics had ever seen before. It took a long time to track down that problem.
GlennA says "I doubt they would have noticed it (missing main bearing cap) even if they dropped the oil pan"
Really Glenn? did you mean "I'm SURE they would have noticed a missing main bearing cap if they had dropped the oil pan."
...because I absolutely cannot imagine a competent mechanic pulling an oil pan and not noticing a missing main bearing cap! That's like going to a dentist complaining of "finding some blood in my mouth and a sore place" and him not noticing that you have a tooth knocked out.
Thanks for staying in the game. Interesting guess. The TH350 transmission does have a vacuum modulator line that goes from the intake manifold to the transmission. It basically changes the shift points and shift 'ferocity' according to engine vacuum. The lower the vacuum, the later the shifts, and the harder the shifts. The thinking was that if the manifold vacuum is low, then you probably have your foot in it so to speak, and you would probably want it to shift harder and later.
But it's only a one-way system, and the line has a very small orifice at the end of it to limit the speed of the vacuum signal. Even if the line came off the transmission, it wouldn't affect anything other than to make the transmission shift very hard and very late.
If it came off at the motor, it would create a vacuum leak that might affect the engine's operation slightly, but it wouldn't change over time.
Good try, though.
If you re-read my last post, the answer was actually correctly guessed, but I'd like to give our group of shadetree mechanics a fling at a final diagnosis of cause and effect before I cap off the story.
It's back to what, when and where.
What & when is a mid 70's GM vehicle with a basically stock exhaust system that shows no outward signs of trouble. Where - is Vancouver, BC - famous for our rain. And the attendent puddles it generates. Some of them can get big. And deep.
So how did these things conspire to gradually kill our car?
The saga will end tomorrow night - even if nobody figures it out by then.
xti; This is now beyond my depth. But I do have a guess. You didn't say whether the engine rev's when in drive, or what happens with the rear wheels off the ground. You did say the transmission, torque converter, and differential were okay.
My guess is that there is a vacuum line between the transmission and the engine that reduces engine power during transmission shifting. In neutral the engine doesn't care, but in drive it would need that handshake. So that vacuum line was blocked, pinched, or disconnected, leaking, so the engine was at 'no-power' thinking the transmission was shifting (forever).
Nice to see some activity here. Thanks for taking the time to think about it. I should mention that I will take the suggestions as they come in and try to relate them to what we did at the time to solve the problem. It`s likely that someone will (or maybe even has) come up with the cause of the problem, but I`d like to expand on exactly how the potential cause should be diagnosed as well, as some tests that might have helped us diagnose it faster were either difficult or impossible to perform. (At least for us at the time, being backyard mechanics with limited access to cool test stuff)
GlennA: Aha! Another Cleese fan, I see. Nice!
LoKi; MyronB; We checked the fuel system and the filter in the QJet. Pressure was OK in the shop, but we had no way to test it under load, as it is a steel fuel line all the way to the carb, and I didn`t want to cut it to install a fuel pressure gauge that could be used on the road. The thought of running a rubber fuel line to a gauge on the outside of the car didn't thrill me very much, either.
I tried to think of some way to test the fuel pressure under load, and realized it was really only a matter of trying to figure out whether the float bowl was full or not. Holley carbs have little sight plug holes in the bowls to check fuel level, but no such hole in QJets, and the bowls are buried too deep in the carb to drill any.
I thought about swapping the QJet for a Holley (I had about 50 of them at the time), but I couldn't find any of my Holley to QJet adapters for some reason.
The accel pump was working. I had rebuilt the carb when I did the engine work, and I did (past tense) know my way around a QJet. Secondaries were fine, and the vacuum pot was functioning correctly.
I have to admit to removing all the evap stuff, so that wouldn`t have been a problem. Good call on the gas cap. But we swapped it for a new one (of the correct model), but it made no difference.
We shall say for the purposes of this discussion that the fuel and ignition systems were not the cause of the problem.
GlennA; Good call on the transmission. I did have a car that exhibited similar problems, and it turned out to be the torque converter. But not on this car. The trans, converter and rear end were all fine. Fluid was clean and didn't show any sign of burn.
CRITIC: We checked the compression, it was fine across all cylinders. We considered exhaust problems, so we pulled the mufflers, shook them, and peered inside them. There were no strange noises coming from them, and no evidence of visible problems, but without cutting them up, there was really no way of being certain.
The pipes appeared to be fine, and were very solid when hit with a hammer. The outside of the pipes had some surface rust, but were in great condition, so we moved on. The headers were inspected, and showed no signs of damage. (I have seen low hanging headers crushed before, but these ones had no such damage).
We looked up inside the pipe to the header, and didn't see any problems, but we couldn't see past the first bend in the pipe.
Being a 1973, it didn't have any cats. We considered trying to measure backpressure, but were unable to think of any way to do it, and weren't sure what measurements to expect even if we could figure out a way to do it.
I'd like to expand on this thought though. How would you test for restriction? We will assume the mufflers are OK. The car has true FACTORY dual exhaust with no crossover pipe. All components seem to be in slightly rusty, but otherwise excellent condition. No visible dents or holes anywhere from the headers to the tips of the tailpipes. If memory serves me correctly, the system had 2 1/4" pipes. Might have been 2" though.
What might explain the cause of such a restriction? Remember, this was a gradual loss of power. It didn't happen overnight. It took between several weeks and a month to kill the car. There was only a barely discernible difference from one day to the next, and it would always start and run fine at idle. It did seem to deteriorate much faster as it approached the end, however. Sort of an exponential curve.
It should be mentioned that we had retained the stock cast iron intake manifold when we changed the cam, and used new factory gaskets when we re-installed it.
I sprayed some ether around the edges of the manifold, but the RPM didn't change, so we discounted manifold leaks as the problem. We also figured that any leak big enough to kill the engine would have killed the idle as well.
Bob Salter - I had a 1987 IROC Camaro that was running crappy. I asked the dealer how they went about figuring out what the problem was (remember, these cars had computers, but with very limited diagnostic capabilities).
Their answer? "We just keep throwing new parts at it until the problem goes away". At which point I asked - who pays for the parts that didn't fix the problem? Answer - "You do".
This story does the heart good of this do-it-yourself and tinkerer when Mr. Goodwrench can't figure out the cause of a problem and who's diagnostic is to throw parts at the problem hoping one of them will be the solution. This makes me wonder how many times this method was applied to cars out of warranty and the owner was charged for every single part plus labor.
I think that Myronb is on the right track, the problem may be with the fuel system. Check to see if there is water in the gas. Pull the main fuel filter and dump it into a container, water should show up readily.
Check to see if the accel pump in the Qjunk carb is functioning.
I would check for vacuum leaks, it sounds as if it may be running lean under load. Are the secondaries opening too soon on the carb?
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