TAC Tech => Mechanical => Topic started by: red79phoenix on April 24, 2021, 03:48:01 PM
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Backstory: exiting the highway my car backfired out of the carburetor and died. That was about 10 years ago, I have not been able to get it going again since.
I had another thread where we had tested multiple things, I know I am getting fuel and spark. Today I did a compression test using a $20 amazon tester. On a cold engine (because it won't start) it read 55 100 85 90 35 50 40 30. From my understanding the compression is likely my problem? That high variance is a no no and the 30/35 is really low right?
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Backstory: exiting the highway my car backfired out of the carburetor and died. That was about 10 years ago, I have not been able to get it going again since.
I had another thread where we had tested multiple things, I know I am getting fuel and spark. Today I did a compression test using a $20 amazon tester. On a cold engine (because it won't start) it read 55 100 85 90 35 50 40 30. From my understanding the compression is likely my problem? That high variance is a no no and the 30/35 is really low right?
Yes, that is very low, even for a low compression smog era motor. And that degree of variation is a problem, too. An engine won't fire without at least 80-90 PSI in the cylinder. What you could try doing is filling the cylinders with a little bit of oil to add compression. If it fires up and runs, that's how you know your problem is compression related. The oil is effectively "filling in" the lost space, temporarily adding compression.
Time for a rebuild, man.
I'd also consider trying a different compression tester. Good ones use schrader valves.
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I would pop a valve cover to make sure the valves are working. Could be a broken timing chain. That would hold a valve or two open and show odd compression on multiple cylinders.
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Yes, that is very low, even for a low compression smog era motor. And that degree of variation is a problem, too. An engine won't fire without at least 80-90 PSI in the cylinder. What you could try doing is filling the cylinders with a little bit of oil to add compression. If it fires up and runs, that's how you know your problem is compression related. The oil is effectively "filling in" the lost space, temporarily adding compression.
Time for a rebuild, man.
I'd also consider trying a different compression tester. Good ones use schrader valves.
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The short block was rebuilt less than 5000 miles ago. Heads were not touched. Possible it is just a valve problem? Any idea what it would cost to do a valve job on the heads? I was planning on selling it once I got it running, but maybe it is worth more not running vs putting money into it?
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i agree sounds like timing chain issue, possibly sprocket has come off or maybe broken cam
symptoms dont fit worn motor, never would have started to make that trip
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Verify that the valve train is operational before contemplating repair costs of unknown problems.
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Yes, that is very low, even for a low compression smog era motor. And that degree of variation is a problem, too. An engine won't fire without at least 80-90 PSI in the cylinder. What you could try doing is filling the cylinders with a little bit of oil to add compression. If it fires up and runs, that's how you know your problem is compression related. The oil is effectively "filling in" the lost space, temporarily adding compression.
Time for a rebuild, man.
I'd also consider trying a different compression tester. Good ones use schrader valves.
The short block was rebuilt less than 5000 miles ago. Heads were not touched. Possible it is just a valve problem? Any idea what it would cost to do a valve job on the heads? I was planning on selling it once I got it running, but maybe it is worth more not running vs putting money into it?
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Yes, the suggestions on the timing chain and valves are also sound. I suggested the oil thing because it's easy. Lol But so is the checking the valves/timing chain.
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Pull a valve cover and turn the engine over. No valves going up or down means the timing chain is broke. The clue is the backfire and died. The engine didnt lose that much compassion just like that.
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Pull a valve cover and turn the engine over. No valves going up or down means the timing chain is broke. The clue is the backfire and died. The engine didnt lose that much compassion just like that.
Both valve covers are off. All valves appear to be moving up and down.
Yes, the suggestions on the timing chain and valves are also sound. I suggested the oil thing because it's easy. Lol But so is the checking the valves/timing chain.
I'll give the oil a shot.
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Olds are NOTORIUS for jumping timing at 100K miles.
My 69 Cutlass manual had a simple test for finding out simply by pulling the valve covers and measuring.
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Next step would be to pull the timing cover and check that the chain didn't jump some teeth.
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I pulled down a '77 350 like that, also had plastic teeth throughout and chain had slack on one side and signs of slipping.Factory oil solidified in intake crossover and heads. But had date coded leads and everything untouched. Everytime I read about an all original details car going for big money, I imagine that engine in it lol. Originality also means never maintained a thing.
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I am a little confused why people think the timing chain has jumped, wouldn't that be reflected on all cylinders and not some of them? Short block including timing chain was replaced ~ 5,000 miles ago.
Haven't had a chance to put oil in the cylinders yet, planning on doing that when I get back in town.
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Ok, so new short block, assuming it was put together correctly eliminates the bottom end...mostly. The symptoms you are describing and the low compression indicates a problem with the valve timing or the valves themselves. You will most likely need to verify the valve timing is correct. The correct way to do this is to remove the timing chain cover and verify the position of the crank and cam gears is correct. It's also possible you have multiple damaged valves that aren't seating correctly, but if that was the case I would expect the compression values to bleed off quickly. Most likely the valve timing is off.
Did the short block already have the timing chain and cam installed? Other possibilities are wrong pushrod length or improper valve lash.
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Quadrajet power valve popped on mine yrs ago. Replaced & good to go.
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Olds motors do NOT like valve train geometry changes as its non adjustable stock. Yiour lifter pre-load is probably WAY off since you took motor apart and head gaskets changed and that isn’t easy to fix. Pull heads. Get them checked. Convert to adjustable valve train and set the lifter pre-load one at a time. Something tells me you have much bigger problems with compression that low. If you are not keeping the car then above isn’t much help. This is where I would suggest not putting any money in stock 4A heads and using early 350 heads. Those would wake up compression, but still need a lot of work and will cause other issues with carb as motor will need more fuel.
Run a leak down test.
Good luck