Mercruiser 260 Backfire Through Exhaust

Kola16

Kola16

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Hello all (again). I have a 5.7L Mercruiser 260 with electronic ignition and a Holley Sniper throttle body EFI. The engine recently overheated due to throwing the water pump belt and it blew the head gasket.

I have now replaced both head gaskets and all spark plugs, wires, cap, and rotor. There is no water getting into the cylinders and it is running at the correct temperature. Unfortunately now it is backfiring through the exhaust.

I checked timing and it is at 10 degrees. I also adjusted it up and down a little by turning the distributor to see if the dial on my timing light was off, but that did not change the backfire.

The AFR is reading correctly around 13.5 or so.

I then realized I might have lashed the valves wrong so I relashed the valves and did find that I lashed a few too tight. When I lashed them again, I am pretty sure I did them right.

So I am still stuck wondering why I am getting this backfire. Could I have damaged one of my flat-tappet hydraulic lifters or something? Please help. Thanks all!
 
Lou C

Lou C

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I'd be concerned about all that white smoke! Did you have that before the overheat?
When you did the HGs did you have the cyl heads checked out by a machine shop?
I had a similar problem (overheat, followed by blown HGs a few seasons later) a few years back and the machine shop found that the heads had cracks in the exhaust valve seats, I wound up replacing both heads with reman units. If you have an overheat bad enough to blow HGs I think it makes sense to get the heads checked out since they can suffer damage that may show up right away or later on...
 
Q

QBhoy

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Sounds and looks like water in fuel or water getting in somewhere.
 
Kola16

Kola16

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Update. You are all correct. There was water getting in. I removed all the plugs and did a water pressure test through the heat exchanger. Water was clearly leaking into cylinder number 1. Also, plugs 6 and 8 were wet. Additionally, I had switched plug wires 2 and 4, which is probably the reason for the backfire. I am going to have to disassemble and test the heads at a machine shop instead of visual inspection since I am not sure that I am qualified to asses the heads. I am assuming I need the block tested too? I am not sure what I would use to plug all those coolant passages either. Seems like I have an early end to my fishing season since I do not want to fork out money for a new longblock. Aluminum heads sound appeasing though since my boat is pretty rear heavy.
 
Lou C

Lou C

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Well I can't say what happens in most cases but my block was fine when both my HGs blew and both heads had cracks. My overheat was BAD.... You can get the shop to check out your heads, if they are OK do a valve job at the same time. For the block if you're not wanting to pull it, I think there's a way you can check for cracks by using dye, I'd find out about that. If it passes then use a mechanic's straightedge and feeler gauges to measure flatness all across the block deck. Should be flat & level to less than .003". Mine was, no cracks, so the next thing you have to do is to get a thread chaser and clean out all the cyl bolt holes in the block. They often have crud in them and that will make your torque readings inaccurate. If the head bolts are rusty, get new ones and use sealer on the threads. Doing this I had no leaks, still good 5 years later.
I would not use alu heads, they are less durable and more expensive than plain old cast iron.

Also there is a way to air pressure the cooling passages, before taking it all apart, do a search for that it has been posted a few times.
 
nola mike

nola mike

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Lou C said:
Also there is a way to air pressure the cooling passages, before taking it all apart, do a search for that it has been posted a few times.
Click to expand...
He already pressurized them, that's how he knows it's leaking. I think he'd need to remove the heads, install block off plates over the water passages/top of the block, and pressure test just the block.
 
Kola16

Kola16

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nola mike said:
He already pressurized them, that's how he knows it's leaking. I think he'd need to remove the heads, install block off plates over the water passages/top of the block, and pressure test just the block.
Click to expand...
Oooh I looked up "block blockoff plates" and that is exactly what I need. I think I can make my own out of all the aluminum plates I have. So my plan is to take the heads into a machine shop to get tested, and then make blockoff plates for the block, so I can test the block. Seems like a big chore, but then I don't have to pull the engine. Do I need to pressure test the intake manifold too?
 
Scott06

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Kola16 said:
Oooh I looked up "block blockoff plates" and that is exactly what I need. I think I can make my own out of all the aluminum plates I have. So my plan is to take the heads into a machine shop to get tested, and then make blockoff plates for the block, so I can test the block. Seems like a big chore, but then I don't have to pull the engine. Do I need to pressure test the intake manifold too?
Click to expand...
Did you check the block and head surfaces for flatness could have warped either and be the source of leaking. Normally need to use a machinist straightedge and feeler gauge
 
Kola16

Kola16

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Scott06 said:
Did you check the block and head surfaces for flatness could have warped either and be the source of leaking. Normally need to use a machinist straightedge and feeler gauge
Click to expand...
I did not. I will have to look up how to do that. I have feeler gauges, but I am not sure that I have a compatible straight edge.
 
Kola16

Kola16

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Another update. I pressure tested the port side exhaust manifold. It leaks. They are about 4-5 years old so it was time to change them anyway. Another thing to check when you boil all the water out of your engine. I'm still going to test the block and heads.
 
Scott06

Scott06

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Kola16 said:
Another update. I pressure tested the port side exhaust manifold. It leaks. They are about 4-5 years old so it was time to change them anyway. Another thing to check when you boil all the water out of your engine. I'm still going to test the block and heads.
Click to expand...
If the heads are still on leave them and pressure test the block and heads together.
 
Kola16

Kola16

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Scott was correct I had only tested the one. Tested the other tonight and it was good, but still going to replace both. I also tested the block with the heads still on. There was a leak somewhere. I could here something in cylinder 1. I could not get my ears close enough to hear the starboard side cylinders (even numbers) due to the exhaust manifold being in the way, but I guess it is irrelevant since I am going to have to take the heads in anyway. There was water in the starboard side cylinders so Mike was right too. So heads are going to the machine shop and I'm going to have to get new manifolds....wooooh haha
 
Scott06

Scott06

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Kola16 said:
Scott was correct I had only tested the one. Tested the other tonight and it was good, but still going to replace both. I also tested the block with the heads still on. There was a leak somewhere. I could here something in cylinder 1. I could not get my ears close enough to hear the starboard side cylinders (even numbers) due to the exhaust manifold being in the way, but I guess it is irrelevant since I am going to have to take the heads in anyway. There was water in the starboard side cylinders so Mike was right too. So heads are going to the machine shop and I'm going to have to get new manifolds....wooooh haha
Click to expand...
Good at least you know exactly what wrong . Hopefully you find a crack in the head not block…
 
Kola16

Kola16

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I have not had the time to test the block yet, but I got a quote from the machine shop and it was $1000 to repair my stock, plain Jane cast iron heads that came off an old SBC 400. That seems like I might as well buy new heads for that price? Can anyone recommend some good heads for a 355 with flat-tappet lifters? I want to eventually get a 383 with roller rockers so maybe heads I could eventually put on that? Would Vortec heads be an easy conversion or would there be too much involved in that switch. I know I would need a new intake and self aligning rockers, but would the valves have the proper lift? I have dished pistons.
 
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