View Full Version : Overheating LT1

08-01-2006, 05:26 PM
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
08-01-2006, 05:29 PM
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.

08-01-2006, 05:40 PM
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.

08-01-2006, 06:08 PM
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.

08-01-2006, 06:59 PM
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?

08-01-2006, 09:16 PM
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.

08-01-2006, 09:18 PM
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.

08-01-2006, 09:43 PM
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?

08-01-2006, 10:04 PM
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.

08-01-2006, 10:29 PM
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.

08-01-2006, 10:52 PM
don't worry about the auto cooling pump having the wrong direction cooling. the corvette lt1 also had reverse cooling so you'll be fine there. you'll be very hard pressed to find a lt1 cooling pump that is not reverse cooling.

what type of impeller are you using? i've found that the blue globes don't work at all, so avoid those.

you may also want to pull apart your risers and see if there's any rust or corrosion build up in there thats restricting flow


08-01-2006, 11:26 PM
Edited/repeated reply due to slow response.

08-01-2006, 11:29 PM
When I said 'regular' circulating pump, I meant other than LT-1 pumps. The regular car or truck pump will not work.

80 & 81 is way low. Run it to normal operating temp after replacing the thermostat and pull the fuel injection system fuse or the fuel pump relay so you can check the compression dry. It should be more like 150 +/- 10%. Your motor is also higher compression that the regular ones so you need to use higher octane gas. Don't use 87 at all, especially if it's hot and dry.

As far as what temperature and how long- don't overheat it at all. The in-cylinder temperatures while running at high RPM can be around 1200 or more. No water in the motor means no water in the heads where the high temps do the most damage since aluminum melts at about 1220F.

08-01-2006, 11:29 PM
Edited repeat due to slow reponse.

Farmer Ted
08-01-2006, 11:33 PM
180 sounds high, mine runs about two needle widths below 160

08-02-2006, 09:48 AM
SOunds like there's still impeller parts lodged somewhere restricting water flow. You need to take each hose and passage way apart separately and blow out or run water thru to get any impeller pieces out.
That is if the engine is actually overheating. Does it feel noticably hotter?
When running properly (160 deg) w/ good water flow, the exhaust manifolds will feel warm/hot to the touch....you'll still be able to hold your hand on them when the engine is running (they get hotter right after shut down due to no water flow). If the maniflods are too hot to touch with the engine running, it's overheating.
Sending unit may be bad, lower right side of block.
I may be wrong, JimN correct me if so, the LT-1 has 2 temp sending units and the lower one I described just runs the temp guage, and there is another h20 temp sending unit that sends to the ECM to determine engine operating parameters (fuel, timing, etc). If that one ever got to 240 deg, it would shut down the engine I believe.

08-02-2006, 09:58 AM
All of the injected motors have two senders- a one wire (tan/black) and a two wire (yellow and black). If the sender for the engine control sees overheat, it goes into RPM reduction and switches off alternating injector drivers, so on a TBI, it shuts off one injector and on the multi-port motors, it shuts off one bank since they're batch-fed. At this time, the motor runs pretty lumpy and won't go over about 2000 RPM.

If the motor does overheat, the ECM will store that event for 300 key-on cycles so it will need to be cleared from the memory. Codes can be cleared easily, overheats can't and there's a reason for this.

By the way, the bleeder lines are there to bleed off air in the cooling system, not water. If air is allowed to collect in the high areas of the system, it can boil and displace water, minimizing the cooling ability of the raw water and leads to hot spots/hot-cold cycling.

08-02-2006, 12:10 PM
I'm not so sure it has overheated, maybe run hot more accurately describes it. I have been able to put my hand on the riser each time and it did not seem overly hot. I'm not sure how hot the hoses should be going into and out of the circulation pump, but they are very hot to the touch when squeezed checking for pressure. The sending unit could be giving a false reading. It does shut down the rpm's when its supposed to at a high temp. The wierd thing is, I can let it sit for a couple of minutes or so after the temp hits the 220-240 mark and restart the engine and it will jump back down to 180 and has gone back as low as 120. Then I can idle in gear back to the ramp at arond 600 rpm's and be around 150 degrees.
Today I'm going to check all hoses and check the risers for any mone impeller parts and corrosion. When the correct thermostat comes in I'll install and see what happens. As Jim suggested, I will recheck the compression while the engine is at operating temp for a more accurate compression number. Before, I was checking it dead cold and coming up with 80 to 81 lbs of compression.

08-03-2006, 09:16 AM
If the boat's going into rpm reduction then it's seeing high water temps and your temp guage reading(lower sending unit) is in line w/ the sender for the engine control. It does seem strange thought that if the engine is hot enough to go into limp mode, that it would flow enough water to keep the manifold riser temps normal (not too hot to touch).
The impeller that got ate up is still partly lost in you r cooling system I think.

08-03-2006, 06:37 PM
95 PS190 with LT-1 also.

I have written about this a number of times before.
Take out the thermostats and fill the upper housing with a garden hose. Then remove both drains on the side of the block. The starboard side has the knock sensor, and the port side has the drain. Remove both and let the water drain out. Use a bent coat hangere and try to scrape around inside. I flushed out several pieces of impeller doing this.

I have finally concluded that I have gotten air blocks. I cleared them most easily by removing both thermostats and running at idle for a few minutes. Then I replaced the lower and ran at idle again for a few minutes. Then I replaced the upper and things were OK. You must have the drilled 160 Deg. thermostat. Try this and let us know.


08-03-2006, 08:45 PM
I have checked all lines and hoses and everything looks good. Tried to take the risers off and look inside, but could not get the allen wrench in on one of the inside rear bolts. I will have to cut the wrench down a little to clear it.

I will do what you suggest Jim and post later. When going back in with the thermostats, does the one on top lay with the button down and pointed end up, or reversed with the button on top and the point facing down? This may be part of the problem. I have the correct drilled 160 degree one now and want to be certain when going back togather.

08-03-2006, 09:10 PM
If you want to split the manifold/riser, get an Allen wrench with the ball on the ends or it's a total PITA.

08-09-2006, 09:34 AM
I don't know. I will have to open it up and look. I just always make sure I put it back in the way it came out.
You can try calling Indmar, or Skidim.

I am now running just a little cool, and have to inspect the thermostats. I suspect a piece of weed caught, holding it open. Then, since it never warms up enough, it retains it there. I had that happen with a piece of impeller once. So, I will be opening it up anyway, and will draw or take a picture of how it looks.