66 cutlass convertible. 350 Motor was rebuilt in 2003, maybe 10k miles on it. All new rebuild, .030 over pistons, performer cam and intake., heads rebuilt, lower end. holley 600 vacuum secondaries.HEI was replaced last summer. Ran good. Last week here in NJ weather was in 80s and sunny, drove down to the Beach. Heading home at night, pulled onto highway after getting gas, punched it and car took off nice merging onto road. Then boom, loud back fire. stumbled a little. Pulled off road, car was idling rough still occassionally backfiring. Had good oil pressure and temp was 190. Didnt see anything obvius under hood so i decided to drive it home,70 miles staying at 55mph. wasnt making full power and was still intermittently backfiring. I figured a new rebuild was on order.

Highway had mile marker signs and first problem happened at exit 70. i took it easy for next 40 miles, under 55 mph,still backfiring and hesitating. then all of a sudden it ran fine. For next 30 miles back to house it ran ok, still not 100% because i felt some minor vibrations but made it back home.

Carb was untouched for 14 years so i ordered a rebuild kit, but after that i still had some roughness/ vibrations.

Did compression test, #5 was only bad cylinder, had under 50psi, pulled valve cover and found this .

 

trying to add pics....

 

 

then after a further inspection realized exaust push rod gone

 

 

Pulled off intake, both lifters still in their bore, Broken 2 halves of exhaust pushrod laying on top of lifter galley. Swapped out two other rods/rockers from another cylinder and saw good pressure on #5 cylinder. Used a scope to look in cylinder, piston and valves appear ok. just order new set of pushrods and rocker assemblies for motor

 

Anyone ever experience this? Im assuming initial backfiring problems were due to losing exaust push rod? Then after 40 miles i lost the intake rocker and then proceeded on basically 7 cylinders

 

i actually had replaced valve cover gaskets last month and checked that all bolts were torqued.

 

I did a thread on this back in late August early September. Mine broke in the center on the #3 exhaust valve and the intake valve was stressed. I didn't pull all the others. I pulled two off the 350 and ordered a set of roller type from Comp Cams. They kind of set up like Chevy's and you have to measure for new push rods but haven't gotten that far yet. I'll handle it this winter. Talked to a friend of mine who's been a performance mechanic for years and said it's common on Oldsmobiles. He told me to order a better grade stock appearing set from Summit, but I'd already ordered the Comp Cams set. No one seemed to experience it on CL though.

Olds 455 motor with about 3K on a fresh rebuild.

 

I've definitely seen the pushrod through the rocker arm problem before. The punched oil hole in the pushrod cup in the rocker can initiate a crack, causing a fatigue failure. Can't say that I've seen pushrods break, however.

 

Thats what we're trying to figure out what happened, and when. Im guessing exaust push rod broke first, causing backfiring. But why? Then that led to intake pushrod breaking rocker.? Maybe overheated since i did drive another 40 miles. Looking in spark plug hole with a scope i didnt see any valve or piston damage. Doesnt look like vale hit piston.
It was a good feeling driving car home. and not having to get a tow. I really figured motor was shot at that point.

 

You don't know that the pushrod broke first. As I said, metal fatigue is a primary cause of the rocker arm failure.

 

But the exaust rocker that had the snapped-in-half push rod looked ok, the socket didn't looked stressed out or distorted.

The intake rocker that was split, that push rod appeared fine, wasnt bent at all.
Had to be one reason why i had two failed components on one cylinder.
I plan on replacing all pushrods/rockers and bridges

 

The broken rocker happened once on my buddy's 460 ford, no obvious reason so we just replaced the rocker w a used one and never had a problem again in the next 3 years of opereration that I had access to the engine