Overheating LT1

[don937]

Hey guys,
Here's the problem: I have a '95 Prostar 190 LT1 with 590 hours. I'm having a reoccuring heating problem that I can not seem to figure out. I replaced the impeller in the raw water pump. The old one was gone completely. This needed to be done but did not solve the problem. I removed and inspected the main circulation pump. The impeller was shot in this pump also. I replaced this pump with a complete new unit and the 2 thermostats as well. At idle the boat runs around 180. On plane it hits 240 and I kill it and let it cool. I removed the water pick up hose and inspected the trans cooler for debris. No trash in there. I removed the hose from the trans cooler to the raw water pump and found parts of the water pump impeller. I thought for sure that I had found the problem, but it still runs hot. I took it out yesterday and now it idles at 150 and still shuts down at 240 on plane. I noticed that the 160 thermostat on top needs to be "drilled". The one the dealer sold is not, so I've ordered a drilled one from Miami Ski Nautiques parts department. What am I overlooking ? I've considered that the sending unit could be bad and am planning on checking that. I'm also going to do a compression test to make sure the engine has not gotten too hot. Any suggestions??

[east tx skier]

You might have some impeller bits lodged elsewhere. I seem to remember someone finding some impeller pieces at the joint between the manifold and the riser.

[corey]

Ok, I actually saw this once so its worth checking out. Look on the bottom of your boat where the water intake is. The grills should be pointing towards the bow. if they are not you are unable to pull in water while moving.

[JimN]

Don't even run it if you have the wrong thermostat. How many times has it overheated? Don't do it repeatedly- the heads are really expensive and they don't survive overheats the way cast iron heads do.

Did you recently replace the circulating pump? If it's a regular car or truck pump, it's teh wrong one. The Corvette pump is reverse flow- the heads are cooled before the block and manifolds.

Also. make sure the oil cooler isn't clogged and check it frequently.

[don937]

As far as overheating, I've been out in it about 4 or 5 times since the initial day it overheated. Each time it starts out normal around 120 after 5 or 10 minutes in neutral. Then when in gear, as I get on plane it increases to about 180 and holds there for a minute then it jumps to 200 to 240 and I'll kill it. The warning light has come on a couple of times and it will cut out to prevent from overheating. After letting it cool it runs about 180 at idle in gear back to the ramp. After cleaning the impeller debris yesterday, it idled in gear at 150, but still got up past 200 after a short run on plane. The circulation pump is the correct indmar part speciffically for the LT1 5.7. The pick up fins are in the right direction as well. I haven't taken the riser off the exhaust manifold and checked in there yet. I was thinking the manifold / riser could be corroded in there and not letting water pass, but all the other parts were not in that bad of shape. Will the 160 degree thermostat that is not drilled be that much of a difference? I've thought about taking out the thermostat and seeing what happens. Any thoughts?

[firesmith]

My LT1 had a small bit of rubber from a hose or impeller fin stuck in the 160 thermostat preventing it from closing. With the 160 effectively "stuck open", it would run at about the temp of the 143 thermostat or less, even down to 120. I theorize that if you removed the 160, you would run cool as I describe.

By the way, when I removed the rubber bit, I now run right at 160 once fully warmed, either at idle or at speed.

Good luck finding the obstruction or faulty thermostat. I have certainly heard that you must use the correct marine thermostat for reliable operation.

[JimN]

The thermostat has to have a hole or it'll overheat. If you run it without a thermostat, it'll run too cool, you'll waste gas and possibly dilute the oil with the excess fuel, especially if the cylinders are scored at all. I would wait to get the impeller scraps out and the right thermostat in it before running it on the water again. You should also remove the bleeder lines from the heads (if it has them) and make sure they're not clogged.

[don937]

Thanks for the replys. I will recheck the 143 degree thermostat and wait for the correct drilled 160 to get here sometime this week. Does anyone know the reason why the LT1 requires a hole in the top 160 thermostat? Where should I look on the heads for bleeder lines?

[JimN]

First, the bleeders- they're on the outside have brass right angle fittings and 1/4" hose. Can't miss 'em. Actually, exhaust manifolds, not heads. Maybe someone here can post a photo of theirs.

The thermostat has the hole so the water can continously move, but not in high volume. Keeps the heads cool, keeps air pockets from being an issue and keeps it from overheating. Normal motors have coolant going to the block first, then the heads. The LT-1 heads are aluminum, so they need to be cooled first.

[don937]

It does have bleeders, so I'll check them as well. Hopefully the correct thermostat will solve the problem.
Any idea of what the compression should read on this engine? After the first time it ran hot I checked it and all cylinders were 80 & 81 lbs. Going to recheck tomorrow. What temperature and how much time at that temperature until damage occurs ?

Thanks again.