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AZDave
03-30-2010, 10:46 AM
My friend's 89 Prostar 351 has developed a sputtering noise from the no 5 spark plug/area. Putting your hand right by the plug, you feel the hot air escaping in sequence either out of the plug, or from a crack in the head, or gasket failure. Boat has about 1200 hours, and mostly in some brakish salt. Noise speeds up and slows down with rpm change. I am guessing head gasket, or exhaust gasket, but wanted some expert advice.

thatsmrmastercraft
03-30-2010, 10:53 AM
Is the spark plus tight? What do the threads look like? Too much corrosion could create a situation where the the plug doesn't seat completely. I always like to start with the simple fix first.

Next step, assuming the plugs OK, is to get as close to the source, cut some sewing thread and tape to the end of a pencil with a couple inches hanging out. Use this to find the source of escaping gas - most likely the exhaust manifold.

JimN
03-30-2010, 11:26 AM
Do a compression and a vacuum test.

TRBenj
03-30-2010, 02:43 PM
Sounds like an exhaust leak. Tighten up the bolts and/or pull the manifold and replace the gasket. While its off, check its condition.

AZDave
03-30-2010, 03:12 PM
We pulled the plug, and it was intact, and seated correctly. I will have him pull the manifold, and check gaskets, followed by compression test. Thanks. I will update when we get a root cause.

Kyle
03-30-2010, 03:41 PM
Does the boat run like you have lost cylinder #5?

I personally would start the boat and use a squirt bottle and spray the area with soapy water to try to find where the air is comming from. It is a lot faster to locate where your problem is.

If the boat is running with full 8 cylinders I bet you have an exhaust leak. Indmar did install my exhaust manifiolds without a gasket but they used silicone instead of a gasket. I have since replaced manifold silicone with new gaskets and everything is great.

I agree with a compression test but some guys may not have a compression guage or tool. This is a good way to find out where your problem is ie. cracks, gasket, or out of the plug area.

Also if you pull the plug and you see black around the top of the plug where the glass part is to where the plug wire goes you may have a leak in the cylinder.

Personally I bet you have an exhaust leak.

JimN
03-30-2010, 03:55 PM
Does the boat run like you have lost cylinder #5?

I personally would start the boat and use a squirt bottle and spray the area with soapy water to try to find where the air is comming from. It is a lot faster to locate where your problem is.

If the boat is running with full 8 cylinders I bet you have an exhaust leak. Indmar did install my exhaust manifiolds without a gasket but they used silicone instead of a gasket. I have since replaced manifold silicone with new gaskets and everything is great.

I agree with a compression test but some guys may not have a compression guage or tool. This is a good way to find out where your problem is ie. cracks, gasket, or out of the plug area.

Also if you pull the plug and you see black around the top of the plug where the glass part is to where the plug wire goes you may have a leak in the cylinder.

Personally I bet you have an exhaust leak.

Autozone and O'Reilly's have loaner tools. You pay for it and get a refund when it's returned.

Kyle
03-30-2010, 04:12 PM
Autozone and O'Reilly's have loaner tools. You pay for it and get a refund when it's returned.

I know. I just dont like dealing with having to get my money back and waiting for 30 minutes in line to rent it and to bring it back.

I am very lucky that I have some friends that have tools. I used a compression test tool and the one I used was from a friend. I am just one of those guys that thinks if you need to borrow tools and you come borrow my snap on tools and you have some tool that I need I am going to ask to borrow it.

If I could not borrow it I probably would just buy a new one and add it to the tool chest. I really cant stand lines at a check out counter. They absolutly make my blood boil and my skin turn inside out. Either the person infront of me has issues or the cashier cant run the machine. Its a beat down that I dont want to deal with twice.