View Full Version : HELP - Overheating '94 problem

11-04-2004, 11:44 AM
Well, my new (to me) '94 ProStar is giving me some trouble. When I ran it the first time, it seemed fine, then jumped to 200 degrees. I shut it down and took it to the dealer.

They checked the thermostat, the water pump, changed the impeller (even though it looked ok), and ended up finding a clump of weeds at the transmission cooler. They ran it, and said it was fine.

So, I got it home yesterday, and I ran it to change the oil. My water source was good and consistent - no problem there. I ran it for a about fifteen minutes until it got up to 160. It stayed there for a while, then jumped to 200 again. I shut it down. Aargh.

Here's the weird thing - I went ahead and changed the oil, and neither the oil or the engine were that hot. I would have thought that both the oil and the exhaust manifolds would have been quite hot if the engine got up to 200 degrees.

What do you think? What else should I check for? Is it possible that the temperature sending unit or gauge aren't working? Anything else that would cause the temp to spike? The dealer says bring it back in, and I will, but I wanted to get all your suggestions first.


11-04-2004, 11:58 AM

1. Bad sender
2. Poor ground to dash.

11-04-2004, 01:44 PM
It could be the sending unit, or maybe the wire is bad...does it have a short somewhere?

Maybe someone else knows for sure. But I think the way these gauges work is:

The Gauge is hooked up to positive on one side and the sending unit on the other side. The sending unit acts as a resister, breaking the circuit between ground. The hotter the engine gets, the less resistance the sending unit provides making the gauge get higher (the gauge acts just like a light bulb getting brighter). If this is the case, then if the wire grounds out between the sending unit and the gauge, you'll get full power, peging the gauge.

east tx skier
11-04-2004, 03:18 PM
Had it really gone to 200 degrees, you'd have noticed when working around the engine for some time afterwards. Sounds like you're getting a false reading.

11-04-2004, 03:50 PM
The engine really didn't seem that hot. I wondered if I might be getting a false reading. I'm going to check the sending unit and the gauge today.

I fired it up this morning, and it never got above 160. I did notice that the starboard side riser above the exhaust manifold was quite a bit hotter than the port side riser.

I talked to Richard at Discount Inboard Marine this morning (he's incredibly helpful, by the way), and he said I've checked all the issues that could lead to overheating. He suggested that the upper thermostat might be the problem. My mechanic had checked it. I wonder if it's sticking occasionally.

Any other ideas?

11-04-2004, 04:14 PM
You mention an upper thermostat, unless it is a LT-1 there is only one thermostat. 94 TBI should have a 160 degree themostat. Keep us posted

11-04-2004, 04:21 PM
Yes, it is the LT-1. Richard at DIM said I have a 143 degree thermostat right above the impeller, and the 160 above that on the intake manifold.

11-04-2004, 05:02 PM
Keep dealing with Richard he know the the details pretty well. Don't just get any thermostat. I think one of the two has a hole drilled in it for use on this engine. Jimn can clairify I'm sure.

11-04-2004, 07:05 PM
The top one (160) should have a hole drilled in it. You can buy them this way. If one manifold was hotter than the other, it may be the bottom tstat sticking, or maybe just imperfect water flow if you have it hooked to a hose.
I'd start by replacing both t'stats. They take a beating from the crappy water and it's cheap insurance. LT-1's are Al head engines and it doesn't take near as much to overheat them and F up a head, as iron heads. I think the sender is not a marine specific part. May be able to get one reasonably at the parts store.
The boat does have an rpm reduction mode if the engine gets too hot. Did this kick in?
Jimn, does that run off of the temp guage sender, or is it controlled elsewhere?

11-04-2004, 09:14 PM
If the motor was idling for a long time, there's a good chance that the bleeder lines had air in them and was part of the problem. Next time you run it, if the temperature goes up again, rev it for a few seconds to see if it goes down to normal.

How is the water getting into the motor when you run it on the trailer? If you use a Fake-a lake, that probably has a lot to do with it.

The injected motors have 2 temperature senders, one with one wire for the gauge and a 2 wire one that goes to the ECM. The ECM sees the changes in the resistance (by way of the voltage input) and if it goes too high, yes it goes into RPM reduction. DO NOT drive the boat when this is happening.