Detective needed Overheating one side only?
 

2000 7.4L MPI



Overheating on left side



Right side is cool as can be



210 on water temp gauge



Hoses on left at bottom of manifold and exhaust appeared close to melting. Could not hold hand on manifold or elbow for long at all.



Hoses entering and exiting the T-stat housing were filled with boiling hot water when I drained them to check for a blockage. Right side hoses cool.



1. Impeller- perfect and installed correctly

2. Thermostat- replaced with Merc 160 degree

3. Elbow- looks clear

4. Manifold- looks clear seals were replaced 7/6/7



My thinking on the one side hot one side cool is that the inflow of water doesn't split until the water circulator pump.



In my motor (I believe) seawater follows this flow:



-Enters thru the bravo

-Passes thru the seawater pick-up pump

-Passes thru the power steering cooler, oil and fuel coolers.

-Pases thru the T-stat

-If below 160 degrees it passes thru to exhaust manifold

-If over 160 is diverted to the circulating pump and then splits and enters the block

-Exits motor passes back thru T-stat housing exits to manifolds



I would not think it would be any of the coolers clogged, or both sides should be hot....the flow of water doesn't split until the circ pump.



Is there any secret to removing the hose from the oil cooler? It looks like removing it to check for blockage would be difficult, due to obstruction of oil cooler etceteras.



The boat is being looked at by the pros this afternoon.



I showed them the impeller, so they could skip that step.



They are going to:



-Backflush

-Check the internal exhaust flapper

-Check circulator pump



Then what?



Hoping to get back on the after for Sat or Sun....although there's a 60% chance of rain again. It has actually rained in the Austin area 33 of the last 37 days! haven't had the boat running properly since end of May!



What is the function of the bypass hose that comes off of the top of the block and re-enters the circulating pump?

What style of exhaust manifolds? If aluminum like a Gil, look for PC that has delaminated and is blocking the flow.

Suspect a blockage on that side exhaust. Water will flow in then stop, with no flow, it overheats.

Pull the hose off the t-stat housing that goes to the hot side manifold/riser. Stick the garden hose in there and see if it will run water through. Chances are it's blocked and you'll get water coming back at you :D

Manifolds are OEM original.

They have the standard light rusting on the inside, but this doesn't limit water flow.

Won't the internal exhaust flapper cause the water to backflow even if there is no blockage?

Where would I see the blockage?

I have pulled it apart at the elbow and the manifold and I cannot see any blockage?

It could be anywhere in the hose, manifold, riser...
I doubt an exhaust flapper would be water tight at garden hose pressures. If I have flappers, they're way worn out because when I run water through, it comes out no problem :D
Do you have thru-hull or thru-hub, or switchable exhaust? I'm not familiar with thru-hub or switchable, but I don't think they have flappers?
BTW, the bypass helps prevent pressure build up in the block/heads when the t-stat hasn't opened up...at least that's my understanding.

handfulz28

I have thru-hull exhaust

Thanks for the insight on the bypass

This is the diagram of my cooling system:

You must have a clogged exhaust elbow . Did you flow check it like handfulz28 suggested? I did lot read your answer. You will not see the blockage it must be flow tested. Elbows don,t last forever and the catch all the debris that goes thru the motor in addition to any scaling the motor releases.
Jim

mobilmercman

Thanks for the input.

I will ask the mechanic to check that first.

My bet is in the elbow, possibly right near the end.

When running, is any water coming out on the side that is overheating??????

If not, its blocked somewhere on that side.

Well the mechanic tried everything and they were never able to recreate the extreme overheat condition.

They were still seeing a 20-30 degree difference at the manifold, when measured with a laser temp gauge.

They ran it on muffs and for an extended period on the lake. WOT, stop-n-go, stop-n-heat soak.

They reversed the output from the thermostat housing and the temperature increase switched sides as well. They then ran a test using clear tubing and saw very few bubbles and the temp increase still followed the left side output at the T-stat.

Waterflow out of the thru-hull exhaust is equal on both sides.

They backflushed

They re-checked the impeller, t-stat housing, elbows, risers, gaskets.

Changed out the 160 degree t-stat for a 140 degree....that helped a little on both sides.

They even called Mercury Marine; who indicated that it was not uncommon for some motors to run up to 30 degrees hotter on one side or the other.

Also, the temp was returning to the normal range on both sides rapidly when returning to low speed and idle from WOT runs.

Thanks for the update, sounds like your boating again.

Yeah, but I hate that nervous, "Looking at the gauges all the time" feeling.

It's like when you think you heard your outdrive make "A Noise"!

Ran it myself this morning. Ran strong right up to the rev-limiter. No strange sounds or smells.

Air temp was 85 degrees.

Water temp was 85.

At idle in the slip it ran up to 170 degrees and was hotter on the left than the right.

At 3,000 RPM it comes up to 175 degrees on the gauge

At WOT 4,680 RPM for 15 minutes...it came up to 190 degrees on the gauge.

When I slowed back to 3,000 RPM it came back to 180 degrees immediately.

The left side is much hotter than the right side in all conditions.

I ran the RPM's up to 2,500 while out of gear; looked under the swimplatform and the water flow appeared equal from both exhausts.

I am considering adding an oil temp gauge as an added layer of protection. Where should the sender be placed? is it 1/8", 1/4" or 3/8"? I would have to replace the stock dash panel to add the gauge or I could just remove the non-functional depth gauge....never use it anyway; I just mind the markers & stay on the deep side.

I was looking at the gauges so much that everytime I looked up I had such bad persistence of vision that I could see 7 huge circles floating in the sky.

Decided to go ahead and add an oil temp gauge as a back-up

I don't like that the water temp gauge is at 185 @ WOT!

I need some reassurance that I am not boilin' oil

I don't remember if anyone ever confirmed this for me, but I "heard" that Merc indicates engine water temp should be approximately raw water temp + 100 degrees. So if lake temp = 85, engine = 185 sounds dead on and perfectly fine, especially at WOT. If you're in fresh water, warm is good.
I would expect oil temp around 250 +/- 25. Look forward to reading your results.