1986 Mercruiser twin 4.3L starboard running hot HELP!!

[trevtt600]

Hello All. We got ourselves a new to us 1986 Wellcraft Monte Carlo with twin 4.3L and alpha 1 gen 1 stearndrives. While it was on the trailer I services both outdrives including all new bellows, gimbal bearings, impellers, shift cables ect... Everything i could replace while it was on the trailer I did.

To make a long story short, starboard engine wants to overheat at anything over 2400 rpm. At 2300 it stays steady at 160 degrees along with the port side. Bring it up to 2400 or 2500, temp starts to rise fast. I removed risers which looked in great shaped, but one manifold was plugged. I no no no... i found a good used one.... i no no no... cleaned risers replaced bad manifold, and still wanting to overheat over 2300 rpm. I have also replaced the thermostat on that engine.
I have check water flow from outdrive, and is great, checked flow to all the hoses through out the system and everything is flowing good.

Why is it that from 2300 to 2400 my temp sky rockets?

Please help... I'm thinking full new exhaust system, but it just seems funny the way it acts with 100 rpm difference.

I know you all like pictures, so here she is. Paid $3500 with a triple axle trailer, and engines have 130 on port and 60 hrs on starboard!!

 

[HT32BSX115]

Re: 1986 Mercruiser twin 4.3L starboard running hot HELP!!

cleaned risers replaced bad manifold, and still wanting to overheat over 2300 rpm. I have also replaced the thermostat on that engine.
I have check water flow from outdrive, and is great, checked flow to all the hoses through out the system and everything is flowing good.

Why is it that from 2300 to 2400 my temp sky rockets?

Please help... I'm thinking full new exhaust system, but it just seems funny the way it acts with 100 rpm difference.

Howdy,


Well, you're on the right track.

And you just cannot reliably clean risers and manifolds and expect them to be perfectly clean.

They're made from cast iron. Cast iron slowly begins to rust immediately. The rust slowly clogs both the risers and manifolds.

engines have 130 on port and 60 hrs on starboard!!

Seriously? This is since New or since overhaul? If it's since overhaul, I would question someones judgement if they put old manifolds and risers on new/rebuilt engines.

If it's since new, then YOU NEED NEW RISERS AND MANIFOLDS!

Also, if this is a salt water boat, and raw water cooled, the engine blocks, heads, and intake manifolds may be as clogged as the risers and exhaust manifolds.

If this is the case, you may end up having more than $3500 in it.

Nice looking boat though!


Rick

 

[trevtt600]

Re: 1986 Mercruiser twin 4.3L starboard running hot HELP!!

Seriously? This is since New or since overhaul? If it's since overhaul, I would question someones judgement if they put old manifolds and risers on new/rebuilt engines.
Rick

Short story. PO used boat as cabin since 2000. He replaced both engines in 2001 but hardly used it. My guess is new exhaust when motors rebuilt.

Should i just replace the exhaust system, or is there anything else i can check first? It is just really weird the way it overheats. Temperature is fine at 2400 but wanting to overheat at 2500. Could the thermostat not be opening and letting water flow as need, and this in just the engines way of telling me this?

 

[HT32BSX115]

Re: 1986 Mercruiser twin 4.3L starboard running hot HELP!!

Short story. PO used boat as cabin since 2000. He replaced both engines in 2001 but hardly used it. My guess is new exhaust when motors rebuilt.

Should i just replace the exhaust system, or is there anything else i can check first? It is just really weird the way it overheats. Temperature is fine at 2400 but wanting to overheat at 2500. Could the thermostat not be opening and letting water flow as need, and this in just the engines way of telling me this?

It's very possible that you have a stuck t-stat. Try switching the port with the starboard one and see if the problem switches too.

Engines produce heat directly related to power setting. More power=more heat. If you do not have enough flow, the engine will get hot.

Check both manifolds and risers. are they too hot to touch? If not, you probably have enough flow (to them........you can also use a cheap Harbor Freight IR temp "gun)

If the manifolds/risers are not hot, you may need a new circulating pump. or the t-stat housing might be clogged with "stuff" (rust, debris, impeller fragments etc.......)

If you have had previous rubber impeller failures, the impeller fragments might be anyplace.

 

[trevtt600]

Re: 1986 Mercruiser twin 4.3L starboard running hot HELP!!

It's very possible that you have a stuck t-stat. Try switching the port with the starboard one and see if the problem switches too.

Engines produce heat directly related to power setting. More power=more heat. If you do not have enough flow, the engine will get hot.

Check both manifolds and risers. are they too hot to touch? If not, you probably have enough flow (to them........you can also use a cheap Harbor Freight IR temp "gun)

If the manifolds/risers are not hot, you may need a new circulating pump. or the t-stat housing might be clogged with "stuff" (rust, debris, impeller fragments etc.......)

If you have had previous rubber impeller failures, the impeller fragments might be anyplace.

I replaced the circulating pump on this motor. Forgot to mention that. The manifold is hot on the side, but about the same as the port engine. Can touch, but not hold hand on them. Risers are warm, but can place hand on risers. I cleaned the t-stat housing when I installed new stat.

I am going to by a IR temp gauge and take some readings. As well as remove the t-stat and test in a pot of water on the stove.

Ill take some readings with IR gun, and then go from there. Thanks for the help. Let you know what i find.

 

[HT32BSX115]

Re: 1986 Mercruiser twin 4.3L starboard running hot HELP!!

It's common to be able to put your hand on the risers and have the manifolds a tad too hot to hold on to..... Sounds like you have adequate flow to them.

You might have impeller fragments or other debris in the T-stat housing or intake manifold housing.

Most of the cooling water bypasses the engine and goes overboard through the exhaust manifolds and risers.

*Some* of the cooling water is metered through the engine by the t-stat. (which is completely closed when the engine is cold)

The circulating pump does just that. It circulates water to prevent hot hot spots and steam pockets.....

The other thing I didn't mention but it appears you might be on to is ensure that your gages are not lying to you! swap them too (just use some hook-up wire and switch one temp sender wire to the other side) and see if they're telling the same story as the IR gun.

 

[trevtt600]

Re: 1986 Mercruiser twin 4.3L starboard running hot HELP!!

OK... Update...

Went and purchased an IR thermostat. Went out for a ride while my wife drove, and the temp gauge is totally lying to me. When the gauge says 220 to 240, IR thermostat is reading 140-170... Don't remember exact reading. The hottest spot on the engine i found was the manifold which was 180-200 or so. Which makes since.

I replaced both the gauge and the sender with other units I had, but still the same situation. I am going to bypass the gauge and hook up a gauge right at the engine and see if there is a short in the wire somewhere. If this doesn't work, i don't know what to do. Nothing on the engine seems to be overhearing per my IR gauge.

Thoughts?