Author Topic: Car died on the highway. Compression test results... 1979 403  (Read 1824 times)

red79phoenix

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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?
"I took the restrictor plate off to give the red dragon a little more juice, but lets keep that on the down low it isn't exactly street legal."

nUcLeArEnVoY

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #1 on: April 24, 2021, 04:38:26 PM »
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.
« Last Edit: April 24, 2021, 04:44:01 PM by nUcLeArEnVoY »
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

FormTA

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #2 on: April 24, 2021, 05:38:45 PM »
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.
79 Trans am low buck LS swapped
79 Formula 301 (Work in progress)
67 RS Camaro (waiting it's turn)
69 Dodge charger on late model charger chassis
49 Ford F1 on a 2003 Chevy ZR2 Chassis (current project)
Names, Luke. If I hear anyone telling me they're my father....

red79phoenix

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #3 on: April 24, 2021, 08:27:55 PM »

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.
[/quote]

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?
"I took the restrictor plate off to give the red dragon a little more juice, but lets keep that on the down low it isn't exactly street legal."

aussieta

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #4 on: April 25, 2021, 02:32:57 AM »
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
1978 Y84 W72 WS6
A Camaro looks like it could kick your ass.
A Trans Am looks like it's coming over to do it

Re: Car died on the highway. Compression test results... 1979 403
« Reply #4 on: April 25, 2021, 02:32:57 AM »

FormTA

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #5 on: April 25, 2021, 06:32:03 AM »
Verify that the valve train is operational before contemplating repair costs of unknown problems.
79 Trans am low buck LS swapped
79 Formula 301 (Work in progress)
67 RS Camaro (waiting it's turn)
69 Dodge charger on late model charger chassis
49 Ford F1 on a 2003 Chevy ZR2 Chassis (current project)
Names, Luke. If I hear anyone telling me they're my father....

nUcLeArEnVoY

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #6 on: April 25, 2021, 08:44:26 AM »

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?
[/quote]

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.
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

tiggershark

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #7 on: April 25, 2021, 11:08:55 AM »
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.

red79phoenix

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #8 on: April 25, 2021, 11:46:59 AM »
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.
"I took the restrictor plate off to give the red dragon a little more juice, but lets keep that on the down low it isn't exactly street legal."

scarebird

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #9 on: April 25, 2021, 12:30:36 PM »
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.

FormTA

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #10 on: April 25, 2021, 12:37:29 PM »
Next step would be to pull the timing cover and check that the chain didn't jump some teeth.
79 Trans am low buck LS swapped
79 Formula 301 (Work in progress)
67 RS Camaro (waiting it's turn)
69 Dodge charger on late model charger chassis
49 Ford F1 on a 2003 Chevy ZR2 Chassis (current project)
Names, Luke. If I hear anyone telling me they're my father....

Wallington

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #11 on: April 25, 2021, 07:36:02 PM »
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.

red79phoenix

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #12 on: May 06, 2021, 09:20:01 AM »
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.
"I took the restrictor plate off to give the red dragon a little more juice, but lets keep that on the down low it isn't exactly street legal."

chief poncho

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #13 on: May 06, 2021, 03:48:48 PM »
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.
« Last Edit: May 06, 2021, 03:55:14 PM by chief poncho »
1971 Lucerne Blue Trans Am (455HO, Automatic) - currently my project car.
Previously Owned TA's/Musclecars: 2002 Pewter WS6/M6 Trans Am, 1968 Dodge Superbee 383/4speed,  1975 TA 455/4speed, 1989 Mustang LX 5.0/5speed, 1980 TA Indy Pace Car, 1977 TA 400/Auto, 1989 GTA 350TPI, 1990 GTA 350TPI, 1986 IROC 305TPI, 1989 Mustang LX 5.0/Auto, 1993 Mitsu 3000GT VR4 (and probably a few I've forgotten about)

highway star

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #14 on: May 07, 2021, 11:51:41 AM »
Quadrajet power valve popped on mine yrs ago. Replaced & good to go.

Re: Car died on the highway. Compression test results... 1979 403
« Reply #14 on: May 07, 2021, 11:51:41 AM »

olds403

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Re: Car died on the highway. Compression test results... 1979 403
« Reply #15 on: May 08, 2021, 09:23:47 AM »
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
1979 Trans Am (Red)
1979 10th Anniversary Trans Ams (Dads)
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Re: Car died on the highway. Compression test results... 1979 403
« Reply #15 on: May 08, 2021, 09:23:47 AM »
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