97 Mercruiser 5.7 slight misfire

[John_S]

Re: 97 Mercruiser 5.7 slight misfire

Hate to say it but a really clean plug says water intrusion.

While I wouldn't have thought new plugs would have been only slightly tarnished from just running/de-bugging, having one that is pure white is a give-away. And given the recent work, I'd say the manifolds/gaskets, as well.

You may want to get some lubricant in that cylinder as soon as possible. It doesn't take much to get some heavy rust going. The valves and seats would be of the biggest concern.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

While I wouldn't have thought new plugs would have been only slightly tarnished from just running/de-bugging, having one that is pure white is a give-away. And given the recent work, I'd say the manifolds/gaskets, as well.

You may want to get some lubricant in that cylinder as soon as possible. It doesn't take much to get some heavy rust going. The valves and seats would be of the biggest concern.

Thanks John and Bubba,
I pulled the risers this am and there was blow by on the gaskets, I checked the machined surfaces with a straight edge and they looked good. I went over to a marine repair shop and picked up a diff type gasket set. This shop paints the machine surfaces and uses quicksilver perfect seal on the this type gasket they use and torque to about 40lbs. So I cleaned and painted the surface today and will reassemble the risers with new gaskets and sealer in the AM. I will also give cyl#1 a dose of lube. Funny that #1 was the clean plug with a center riser. Both sides had blow by but only one plug was clean looking. I had about given up on an ignition problem. I hope this solves the issue once and for all. Thanks again. My Iboats.com order arrived today, got all new coolant hoses so I will install those tomorrow as well.

One question; Should I let the sealer setup overnight before I fire the engine?

 

[bruceb58]

Re: 97 Mercruiser 5.7 slight misfire

You don't use sealer on the Mercruiser gaskets. You used real Mercruiser gaskets right? You also don't want painted surfaces.

I also really don't think a leaky riser gasket is going to cause a misfire. How do you think water is going to get into the cylinder when the exhaust valve is open while the engine is running? Maybe when you turn it off, you will get water in there.

I think you have 2 problems. A leaky riser gasket and a misfire.

This is also an example of why I use OEM manifolds and risers. You don't have to worry about them.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

You don't use sealer on the Mercruiser gaskets. You used real Mercruiser gaskets right? You also don't want painted surfaces.

I also really don't think a leaky riser gasket is going to cause a misfire. How do you think water is going to get into the cylinder when the exhaust valve is open while the engine is running? Maybe when you turn it off, you will get water in there.

I think you have 2 problems. A leaky riser gasket and a misfire.

This is also an example of why I use OEM manifolds and risers. You don't have to worry about them.

Me and a buddy were debating that very thing, how a small amount of water could affect it while running. Maybe I have two issues, we will see. I used the gaskets that came with the osco manifolds. When looking for new manifolds I never saw any OEM manifolds for sale. I'm sure a dealer would order them, but I did not check on them. I went with made in USA Osco's. Just curious, how much does a set of OEM manifolds and risers cost? As far as painted surfaces, I have read both ways. The shop I went to today, guess it's yesterday now, has been in business for 20 years or more and they paint them so when they sweat they dont get rust around the edges of the joint, he said customers come back and say they are leaking if they don't paint them.

 

[bruceb58]

Re: 97 Mercruiser 5.7 slight misfire

Your mistake was using the OSCO gaskets. If you use the Mercruiser ones you should be fine.

If you want OEM parts, I use http://www.fasteddiesmarineparts.com

You just need to make sure that you get a quote for the shipping on heavy parts. They give a 20% discount depending on the parts. That is where I buy all my Volvo parts.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

Your mistake was using the OSCO gaskets. If you use the Mercruiser ones you should be fine.

If you want OEM parts, I use http://www.fasteddiesmarineparts.com

You just need to make sure that you get a quote for the shipping on heavy parts. They give a 20% discount depending on the parts. That is where I buy all my Volvo parts.

Thanks Bruce, Live and learn, I won't use them anymore. I didn't know they used that many diff types of gasket materials until I went to the shop yesterday. The guy brought out 4 or 5 diff types and explained the why and why nots so to speak.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

I was leaning that direction when you said they were all new. At idle (minimum exahust flow) it can/will cause a misfire. At higher RPMs it was probably such a small amount of water the engine could handle it. (Until it got worse.)

Why cyl. #1? Umm, why not. LOL One lung will always have little better comnpression/vacumm or you may have the bow low enought the water is running to the front. Hard to say beyond it being the luck of the draw.

When it comes to the manifold / riser mating surfaces I like to put a thin film of grease on the surfaces, set the riser in place (no gasket) and take it back off to inspect the grease for any area that was not properly mated. Sometimes you would be amazed at just how poorly they mate up.

FYI: I have never painted these surfaces (thats gong to make them uneven) and cannot imagine any real benefit if they are machiend properly and you use a quality gasket.

I just finished putting the risers back on with new gaskets. I have to get to the job now and don't have time to run the engine and re-torque so it will have to wait til AM. Took a pic of the original Osco (green) and the new gasket that I got from the shop. I sure hope this solves the mystery and I don't have a electrical misfire as well...

 

[hoylegj]

Re: 97 Mercruiser 5.7 slight misfire

I just finished putting the risers back on with new gaskets. I have to get to the job now and don't have time to run the engine and re-torque so it will have to wait til AM. Took a pic of the original Osco (green) and the new gasket that I got from the shop. I sure hope this solves the mystery and I don't have a electrical misfire as well...

Did the gasket fix the problem only I have just had a service with the riasers cleanedand now have a misfire, fitted new dist caprotor arm plus and still have the problem. Thanks Geoff

 

[Don S]

Re: 97 Mercruiser 5.7 slight misfire

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