Overheated GT40

[Sbt3]

Ok due to a bad impeller pump I overheated my GT40 the first time out this year. Since there was no water in the engine due to winterization the temp gauge never read hot. I ran it in idle for about 10-12 minutes. I was getting ready to fill my ballast when I noticed it was hot. I immediately shut it down. I used my ballast pump to fill the engine with water which cooled it down a bit and allowed me to get back to my lift.

I fixed the pump and notice a shake in the boat when accelerating which lead me to believe I had an engine problem due to the overheat. I ran a compression test and they were all 130-135 except cylinder number 7 which was like 90-92. I figure I have a bad head gasket.

So far I have torn the motor down to the block and have the heads off. I didn't notice a tear or anything in the head gasket but I am assuming there must have been a slight leak and since I have only ran it about 10 minutes after installing the new pump that I caught the problem early.

I have ordered a new valve grind gasket set from Skidim which has head, intake, exhaust, ect gaskets. I also order the upper intake gasket. The kit has valve stem seals which I also plan to replace along with the exhaust hoses (didn't look bad but got a little hard) and riser gaskets. New thermostat, dist cap and rotor.

I have two questions-

1- anything else I should look to replace while I have it apart?The engine only has about 300 hours on it.

2- does it seem reasonable that it was the head gasket causing low compression or should I look for some other cause. The cylinder looks good no signs of scoring the head seemed to look ok too. My hope is that it was just the gasket, but again I couldn't see any real tear, but the gasket didn't come off clean either the material was kind of broken down and sticking to the block and the head. The stainless rings were all in place throughout the gasket.

Thanks
Stan

[SquamInboards]

RE: Overheated GT40

All I have to offer is that I bought a GT-40 out of a '97 196 which had overheated due to a bad impeller and had dropped a valve so it required a rebuild. It should be pretty apparent if something like this has happened though so it's not likely in your case. In the case of bad compression:
1. Check your valves and cylinder rings
2. If the head gasket was bad you would probably notice poor compression in adjacent cylinders I believe.
3. A mechanic mentioned putting a small amount of motor oil in the plug hole and repeating the compression test can tell you if it is your rings or the cylinder. If the compression test is better with the oil they probably need replacement/rebuild. If not, start checking your valves.

Just some thoughts. Please someone step in if I'm way off.

[Sbt3]

RE: Overheated GT40

How would I tell if I dropped a valve? Thanks for the suggestions.
I can't put oil in the cylinders now though cause I have it all tore down and the heads off.

[pj]

RE: Overheated GT40

Sometimes those gaskets look ok and there are little cracks or spaces that you just can't see. I'd say you should take your heads down to the shop and have them fluxed and decked just to make sure there aren't any hairline cracks in the head. This happened to me and the guy at the machine shop couldn't detect the cracks until after he fluxed it. Total cost for getting both heads fluxed and machined (including a valve job) was about $125-150.

[tiqueman97]

RE: Overheated GT40

Im with PJ. I had a local (non Nautique dealer) work on my boat a couple years ago. They were however a tournament boat dealer, Centurion and Moomba. Anyway, they put my raw water pump on upside down and never sea trialed the boat before I picked it up (An hour and a half away). Bottom line was, I overheated it 1st time out and ended up cracking/ warping a head. The crack was in between the valves, very small and almost un-noticable until the machine shop pointed it out to me. After 9 months of battling w/ the marina that had worked on it, court papers got me a spankin new engine. The marina has since filed bankruptcy and closed...

Anyway, Take the heads to a machine shop, get them inspected, fluxed and decked. Its worth the money.

[mike-d]

head gasket

did you pull your plug on the # 7 cylinder? was it sparkling clean? if so, you gasket was leaking water into the cylinder, if not and it looked normal, you probably didnt blow a gasket. You would know if the head gasket blew, white smoke all the time.

just my two cents.

Mike
orlando

[Sbt3]

yeah the plug on 7 seemed to look ok. The only time I noticed the vibration in the engine was when it was under load. Basically if I accelerated smoothly it would be very low but if I dropped the throttle it would shake between 2000-3000 rpms and run smooth again once the boat was on plane. Running at 3500-4500 rpm it seemed smooth on plane so I was thinking it was just the beginning of a leak which was minor.

[Sbt3]

ok taking the heads in today to get fluxed, decked and seal tested. I sure hope they are ok. I'll report back when I get the engine back together with what the problem turned out to be.
Stan

[Sbt3]

looks like I warped a valve. I was loosing seal on the intake valve on cylinder 7. I got the heads back and am going to put the boat back together. Thanks for the advice about taking them in. I am glad I didn't get the thing all back together and have the same problem thinking it was just the head gasket. Mine cost about $230 total for the valve job and machining of the heads.

Stan