366 Big Block, 1991, Missing/Backfiring/Blown Exhaust Manifold

[danjoh13]

I'm hoping to get some experienced opinions on what I might try next on a 1991 366 Chevy bus engine that's developed some issues. The bus is used as a "team" bus for a yearly bike ride across Iowa, so the thing sits most of the year and then for a week gets heavily used to haul a bunch of adults and their bikes, gear, and drinks for a thousand miles or so. This past year we did our usual amateur mechanic work on it, replacing the plugs, wires, oil, filters, etc. This time, though, we accidentally got two of the ignition wires crossed and the bus ran for a couple of dozen miles that way before we got it straightened out. That didn't seem to cause any major issues, although it did start to miss and have backfires every once in a while, for some reason mostly in third gear when accelerating up to highway speed. Once at highway speed it seemed to run fine, even when loaded down with a lot of weight and lugging up hills. But partway across the state, the engine suddenly started missing a lot more, and on its way up the first big hill of the day it blew an exhaust manifold. The thing limped home really underpowered, missing a lot and of course running really loudly. A retired school bus mechanic that happened to live next door to the place where we were loading up passed along his thought on it - a worn camshaft. What do you fine folks here think? Should we take off the valve covers and see if we can visually tell if the valves are lifting the way they ought to? Any other suggestions on relatively simple things we could do to try to understand what we're dealing with? Thanks.

[ifixedit]

Before you start tearing into it I'd first verify for sure you do actually have all the wires in their correct position. Firing order is 1-8-4-3-6-5-7-2. Start out with terminal #1 on the distributor cap (cylinder #1 is on the drivers side of the engine) and go around the distributor cap clockwise. Cylinders 1-3-5-7 are on the drivers side, 2-4-6-8 are on the passenger side. When you say it blew the exhaust manifold, what exactly do you mean? Did the exhaust manifold just crack, did it come apart in pieces or what? If it was bad enough to do serious damage to the manifold then something serious could have happened to the engine as well and yes I'd suspect some internal damage of some kind. I'd be looking to see if you broke a rocker arm next. Once you get the valve covers off it will be readily evident if that's what happened, you'll have a rocker arm just laying there, usually what happens is the cup where it pivots breaks. I'd also be looking to see if you have a few bent push-rods, you could very well have a few in pieces laying in the valley, I've seen that happen due to having the wires mixed up. How many miles on it?

[danjoh13]

Thanks, those are some really good thoughts. I've gone over the firing order a couple of times but I'll do that again to be really sure it's now right. I should have been more clear about the exhaust manifold - it was just the gasket that blew, and I assumed it was due to backfiring. But I can't tell you how many miles are on it. After an engine rebuild in ~2002 it's only been driven a couple thousand miles a year for things like bachelor parties and our annual bike ride.

 

Checking rocker arms and bent push-rods will be high on my list of things to look into. It makes sense to me that they'd take a beating with sparks happening at the wrong time. Thanks again for the suggestions!

[ifixedit]

No problem, hopefully things didn't get too ugly inside but I'm betting you'll either find a broken rocker arm or two or a few bent / broken push rods (most likely). Either way it's not going to be that big of a deal to fix. Once you get the valve covers off if there are no broken push rods, some could still be bent. Rotate the engine through its cycle clockwise by hand if you can using a breaker bar / large ratchet and a socket (taking the spark plugs out makes this much easier) on the crankshaft bolt on the balancer and have the cam lobes off the lifters / push rods / rockers (i.e. no tension, valves closed) as you check each cylinder and push rod. With valve fully closed you should be able to spin the push rods with your fingers. If they spin freely without any visible wobble or they don't catch on anything as you spin them, then that one is fine and you can move to the next. Any push rod you find that has any deviation from being totally straight will need to be replaced. If in doubt take the push rod out and roll it on a flat surface, any wobble and it's a goner. If you need to replace any, be absolutely sure to get the correct length replacement! Bring along one of the good ones for reference. You can probably rob some out of an engine at a junkyard. The 366 is the "small" big block or the "big" small block whichever you prefer to call it. They were most common in the C60 and C65 trucks from the 1970's, so if you want to find used parts you're going to need to contact a place that has a few of these trucks hanging around.

 

Good luck and let us know what you find!