93 OMC 4.3 Cobra Valve Cover Blown

Apparently, when changing the spark plugs on my 93 4.3 OMC Cobra I noticed one cylinder which had not fired in quite some time due to the resistor on spark plug being broken off. Once new plugs were in, I attempted to start motor, upon the first crank of the starter, the motor made a loud backfire and upon inspection the valve gaskets were blown out from under the valve covers on both sides, also the valve covers were deformed after the backfire. My question, what would have caused this much pressure, and secondly how bad is the damage to the engine? Currently the oil level has not changed since the backfire and blowing of the gaskets. The oil is not cloudy or milky, it does not appear to have any water or fuel in the oil.

Would really like to know if just replacing the valve gaskets and covers will be ok or will this just happen again.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

Any ideas???

Re: 93 OMC 4.3 Cobra Valve Cover Blown

change your oil every once in a while and that wont happen.....

coz there's only 1 reason that happens......

fuel in the oil.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

Sorry sir that motor has to be taken down. You have oil in the bilge?

Re: 93 OMC 4.3 Cobra Valve Cover Blown

Check the firing order, you might have switched plug wires.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

captJason said:

change your oil every once in a while and that wont happen.....

coz there's only 1 reason that happens......

fuel in the oil.

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Aircraft engines used to mix aviation fuel with the crankcase oil to aid in cold weather starting.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

mkast said:

Aircraft engines used to mix aviation fuel with the crankcase

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aircraft engines don't have crankcases.... they're dry sump

Re: 93 OMC 4.3 Cobra Valve Cover Blown

captJason said:

aircraft engines don't have crankcases.... they're dry sump

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Most are wet sump engines.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

Think about it, the broken plug hadn't been firing so raw fuel had to be going somewhere, some blown out the exhaust and some washing down the cylinder. I would change the oil and run it, a backfire that would balloon the valve covers must have been a doozie!

Re: 93 OMC 4.3 Cobra Valve Cover Blown

mkast said:

Most are wet sump engines.

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dunno about that one.... sure some are wet sump... but the majority are dry.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

To answer some of the replies:

There is no oil in bilge area, no oil anywhere except oil pan, the boat never ran this backfire happened when it had first spark with the starter. Oil was changed about 30 hours ago, ran fine after that. The plugs went back in same order they came out. Only had one wire off at any particular time during changing them. Unless the firing order had been changed due to the one cylinder not firing. My boat mech asked me this morning to see if it would crank and if so the motor should be fine. I cranked it and it started first turn. Even after 24 hours the oil level has not changed from where it was before attempting to launch. He thinks there was a fuel build up in one cylinder due to attempting to start a few times before the "boom". Yes it certainly got my attention pretty quickly it was quite a backfire. My mech said he has never seen it blow the gaskets and valve covers from a backfire.

Any other thoughts???

Re: 93 OMC 4.3 Cobra Valve Cover Blown

Compression is good?

Re: 93 OMC 4.3 Cobra Valve Cover Blown

Compression is fine in all cylinders, in fact better now than it was before the backfire for some reason.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

listen... a70eliminator hit the nail on the head.

What does the engine combust?? Gas. its the only thing it combusts. It does not combust oil.

If you blew the valve covers out, blew out/off the oil pan. You obviously had a combustion event happen where it wasn't supposed to.
There's only 4 places in an engine that the gas will light.

1. in the intake manifold (typically known as a lean pop but other reasons can make it happen)
2. in the combustion chamber, ON TOP of the piston. (<-- and that's where its supposed to happen)
3. in the crankcase, BELOW the piston. and since we already addressed the only thing that combusts is gas, obviously gas somehow got below the piston.
4. in the exhaust manifold, and hopefully everyone knows this one as a "backfire"

Is there something that's been said that's not clear to you welch181? Something maybe that's got lost in text? The oil level doesn't necessarily have to increase to have a blow out in the oil pan.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

captJason said:

listen... a70eliminator hit the nail on the head.

What does the engine combust?? Gas. its the only thing it combusts. It does not combust oil.

If you blew the valve covers out, blew out/off the oil pan. You obviously had a combustion event happen where it wasn't supposed to.
There's only 4 places in an engine that the gas will light.

1. in the intake manifold (typically known as a lean pop but other reasons can make it happen)
2. in the combustion chamber, ON TOP of the piston. (<-- and that's where its supposed to happen)
3. in the crankcase, BELOW the piston. and since we already addressed the only thing that combusts is gas, obviously gas somehow got below the piston.
4. in the exhaust manifold, and hopefully everyone knows this one as a "backfire"

Is there something that's been said that's not clear to you welch181? Something maybe that's got lost in text? The oil level doesn't necessarily have to increase to have a blow out in the oil pan.

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So is a70eliminator correct? Just replace the gaskets and covers, change the oil and run it?

Re: 93 OMC 4.3 Cobra Valve Cover Blown

Ayuh,....

The Only Reason ths could have happened was by Gas in the Oil.....

Maybe it was theplug Not firing,..??
Maybe your Fuel Pump is leaking Internally,..??

But you most Definitely had Gas in the Oil....

Re: 93 OMC 4.3 Cobra Valve Cover Blown

captJason said:

dunno about that one.... sure some are wet sump... but the majority are dry.

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Go to the closest airport, look at the aircraft on the general avaition ramp,
if you find ONE aircraft with a dry sump engine I'd be surprised.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

^^^Same with marine engines not having PCV systems...


In all my decades of being a mechanic both professionally and personally I have never seen a backfire deform a valve cover or oil pan. I've worked on v-8 engines that had over 3 qts of raw fuel in the oil due to stuck injectors, "running" yet no backfire or fuel-in-the-oil ignition.. Ignition takes place in the combustion chamber and even with a bunch of raw fuel sitting on the piston I have yet to see that "combustion" travel beyond the cylinder into the oil pan/crankcase.

Obviously, Welch181 had a bunch of "pressure" instantaneously leave the "combustion chamber" and enter the crankcase. This either went past the rings or head-gasket. I doubt anything ignited in the crankcase but no-one could say for sure..

Welch181,
- Does the oil badly smell like fuel or oil (with a normal level I doubt it)?
- Is the oil pan deformed at all?
- Will the deformed valve covers accept new gaskets (if so, start by replacing them and not the covers to mitigate cost, then continue diagnosis and operation)?

You mentioned it started back up and ran fine. About all you can do is fix any oil leaks regarding the valve covers etc, ensure the oil is OK and then run the beast. My long term worries would be other engine seal damage and possible piston ring or head gasket issues. However, could have been just some fluke issue of combustion that caused no short/long term issues other than the evident.

I'm sure interested to hear your outcome.

Best of luck

Re: 93 OMC 4.3 Cobra Valve Cover Blown

KJSmitty said:

In all my decades of being a mechanic both professionally and personally I have never seen a backfire deform a valve cover or oil pan

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not a backfire... that's fuel ingition out the exhaust.

I've personally seen it happen 3 times. the actual event of it. 2 out of the 3 times the engine contained it. the third time blew the oil pan off the engine. not split the pan, but completely off the engine.

Re: 93 OMC 4.3 Cobra Valve Cover Blown

captJason said:

not a backfire... that's fuel ingition out the exhaust.

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Most all mechanics know that whether the "ignition of fuel" goes out the exhaust or intake manifold is contingent upon either/both ignition and valve timing.

Backfire is a common term reflecting either a burst of combustion "back out of the exhaust" or "back out of the intake". Combustion out of the intake normally does not only imply a "lean pop" as you mentioned (some setups maybe) - rather an ignition/timing issue resulting in combustion while the intake valve is still open.

Regarding Welch 181, to have done what he has described leads one to think that the combustion took place while both valves were closed yet significant enough that normal rotation/engine operation could not expel it fast enough to keep it from bypassing the rings and/or etc, forcing all of the pressure into the crankcase. Or as you mentioned, the ignition literally continued into the crankcase igniting fuel vapors from the oil etc. No dispute there.

Now, to have an oil pan shoot off due to a like issue/event. That would have been quite the sight let alone quite the mess...


Cheers