1987 Force overheating at low speeds

[tinkma]

Hello,
I have a 1987 bayliner with a Force motor. Model# 856x7b Serial# 3746. It has been mostly run in fresh water. (sorry for the following simplistic terminology. I am fairly new to this) The over heat buzzer sounds at at low speeds (including neutral) but it does not at higher ones. (there is no rpm guage on the boat so I don't know that information) I used an ir thermometer to confirm that it is indeed overheating at 175 deg. It has a new impeller. I tested the thermostat in hot water and it does open fully. Water comes out the thermostat opening when I removed the thermostat.
The manual mentions the pressure relief valve but I am having difficulty locating it. I don't know if this could be the problem.
I am clearly missing something. Does anyone have any ideas?
Thank you.

 

[Redbarron%%]

I have no idea about the outboards, but for the Sport Jet engines a low idle will cause them to overheat. I would think that the outboards would run at a lower rpm and cool OK, but if it were very low perhaps they would overheat as well.

 

[foodfisher]

Correct impeller, installed properly with key?

 

[tinkma]

The previous owner put the impeller in and I assume that he did it right. (something I will need to revisit) What about the pressure release valve? From what I understand, if it is stuck open then the engine will have problems cooling at low speeds.

 

[Nordin]

The pressure relief valve is under a cover in the left low part of the exhaust side.
Port side of the engine left in the lower "corner".
The screws (2) are small and they can snap very easy and if it has been running in salt water the cover can be stuck and a PIA to remove.
Be very careful when you trying to remove the screws and the cover.
Force 90Hp from 1990 basically the same engine but with a lager bore 3,375 in instead of 3,312 in did not have this valve.

 

[jerryjerry05]

If the impeller is new? Like the PO said???
The first thing I'd do is drop the lower unit and check the impeller.

When you removed the thermo, was there and corrosion or crud built up on it?
Or in the hole it came from??

Ask the PO if the old impeller came out whole or any of the vanes are missing?
Missing? then the cooling system needs to be back flushed to remove the pieces
of the old impeller.

The pressure relief valve stuck or not shouldn't make any difference in the cooling.
Usually they stick closed from setting and that makes the water flow stronger
at slow speeds.

Like Nordin said, they can be a PITA to remove.
They are NLA from the parts warehouses.

 

[WinnerCougar74]

I had exactly the same problem with a Chrysler 135 hp. It started for me after going through some water plants in late summer shallow water last year. It started early this year after a lot of idling around searching for fish on the finder. Last 2 times I have been out it went away. I idled around for no more than 10 minutes and the alarm never went off. What a relief. For me I think if there was a clog it may be gone now. When I posted my question most said it was my pressure relief valve on the lower exhaust side of the motor. I was running without a thermostat. I was very scared to break off those bolts on the valve. The impeller housing bolts were seized when I put the new impeller in. A 2 hour job turned into 2 weeks and a lot of work. I did a little extra running at cruising speed to try to give it all a good flush plus I am going to have to be really carefully with the Chrysler motor and shallow water in the summer.

 

[tinkma]

thank you all so much for all your input. I changed out the impeller and that did the trick.