omc 5.0 erratic temp reading

[gparadiso]

have an'85 omc 5.0 with stringer drive. This season have replaced heads, intake and exhaust manifolds, and carb(edelbrock 1409). Took it for first time and it ran great except that after about 10 min. and 3/4 throttle the temp started rising and rising; i shut it down butit did not seem to be ovrheating; started it back up and got going and temp rose all the way then gauge came down quickly to @ 160 degrees. Was fishing and drifting and i had no problems but on way back to dock , again after about 10-15 min of running near full throttle , the gauge started rising almost to 240 but engine did not give signs of overheating then i backed off on throttle a little and shortly after gauge dropped quickly to about 160. i replaced impeller last season and thermostat this season. Could it be the gauge or bad thermostat? When i completed the engine , I had it started and idling in slip for about 20 -30 min and temp gauge acted normal

 

[Northern Eclipse]

Re: omc 5.0 erratic temp reading

Just a thought but did you use a sealer on the riser to manifold gaskets??? If so you may have globbed some sealer over a water passage..also you mentioned that you replaced the exhaust manifolds.. but did you replace the riser's, since most high rpm heating is a good sign the risers are about to give in.

 

[gparadiso]

Re: omc 5.0 erratic temp reading

on risers or manifolds , i didn't use any sealer and the risers are also new.

 

[crazy charlie]

Re: omc 5.0 erratic temp reading

Possible defective thermostat or perhaps it may be in upside down?? Did you check the power steering cooler for partial blockage??Any chance you may have cranked the engine dry momentarily when on land???Sounds like a worn impeller which can happen so easily and within seconds.Also sounds like blocked risers or manifolds but you stated they are new also.Did you replace them with the same exact ones that were on there???You can also try undoing the petcocks on the block while running the motor(assuming you are raw water cooled)and seeing if rust or sediment come out.