470 Overheat or Wild Goose Chase ?

[Rake722]

Dear all,

I have been chasing an overheat on my 1977 470 for two years now and I might have been on a wild goose chase all the while.

Boat - Draco 1800ST 5.6M LOA
Drive 1:84 Pre Alpha
Engine 1977 Merc 470 with Aluminium Manifold Kit / Riser / Thunderbolt ignition / Alternator / 4 inch Heat Exchanger / High Temp / Low pressure alarm fitted.

Temp Gauge / Sender - Faria 217F / http://www.asap-supplies.com/search/ts1079/faria-temperature-sender-switch-ts1079

Symptoms -

After taking the boat out and running at 3000 - 3500 rpm for more than 5 minutes the gauge shows over 200F as in pic below -
and needs to be given 5 mins at idle to cool before proceeding. High Temp Alarm does not sound.



I assumed that the engine was going to overheat so slowed and idled.

When I got back I spoke to some guys on breezeworks one of which suggested that I check the gauge / sender accuracy with an IR thermometer and lo and behold this is what I found.

TEST - IR Readings taken on manifold casting directly above the temp sender location

Boat on the dry with muffs
Ran to idle temp and tested with the IR at various locations on the engine and exhaust manifold - gauge read just under 175 F IR Read 160F

See video - at 22 seconds - http://youtu.be/Cg1-6g17jhY you can see where I took the reading - red laser dot marks the spot.

I then ran her up at 3200 rpm and the temp on the gauge began to climb as it did in the water - I was surprised by this. It reached a simillar temp as on the water under load - as in the image below



I make this about 215F

IR Reading = 181F

Would I be correct in thinking that there is a gauge / sender discrepancy and it is reading high ?
Is the IR reading at this location accurate ?
the high temp alarm did not sound either and is set at 217F according to the sender switch the buzzer works as it sounds until engine starts.



Many thanks in advance.

Ron.

 

[Scott Danforth]

rely on your IR temp readings, then do a comparison - ie 215 on your faria gauge is 160 on motor. the temp gauge is merely a suggestion. gauges are not that accurate considering the gauge sells to OEM's for about $5

 

[Rake722]

Thanks for the reply Scott.
Comparison already made, I just needed to confirm that taking the reading with the IR Pyrometer at that location is going to be accurate and thus determine that it is not infact overheating.

Would the water temp inside the manifold be much higher than the surface temperature of the aluminium manifold ?

 

[Scott Danforth]

reading hoses is not as recommended as reading the castings, however you have taken readings off various locations on the manifold.

I did not watch the vid long enough to see if you shot the block or head.. I normally hit the block , head, thermostat housing, then move on to the exhaust mani and risers.

 

[Rake722]

Thanks Scott Reading was taken on thethermostat housing and the casting above the temp sender and they set the highest water jacketed readings on the manifold. Hoses were some 10 degrees lower.

 

[stonyloam]

Usually on a gauge accuracy is about 1/2 of the smallest graduation. For example a thermometer, graduated in single degrees should be accurate +- 1/2 degree. Your gauge shows marks 75 degrees apart, so the best you can hope for is +- 35 degrees. I would suggest you get a new gauge that can give you a little better indication of what is going on. A thermometer radiator cap might be helpful. The fact they the engine cools at idle indicates that there is something going on. Do you still have the 3" exchanger? Maybe it has lost some efficiency over the years.

 

[Rake722]

Hi Terry, thanks for the reply.

as mentioned in my first post it does have the 4 inch HX now.

I am buying a thermo couple and LED display that is accurate to .5C at 100C but it will be a few weeks before its here.

I have been advised and seen advice about use of an IR thermometer here so hence my question about the surface temp at the sensor location in the manifold.

Kind regards

Ron.

 

[Eric 3.7LX Alpha 1]

Read this article from Auto Gauge it explains the readings well:

http://www.autometer.com/tech_faq_answer.aspx?sid=1&qid=57

 

[ewpvic]

Read this article from Auto Gauge it explains the readings well:


YUP!

 

[stonyloam]

Sorry slimed right over the 4 incher. Anyway, once up to operating temp, the coolant temperature should be pretty constant regardless of engine rpm. If when you get the new gauge in you still see a significant drop at idle vs high rpm, there is something not quite right. You might want to check the flow from the exchanger out the exhaust elbow to see if that is at all restricted. Garden hose turned on full through the aft a exchanger hose should flow with little backpressure.

 

[Rake722]

Thanks guys I found the article already and read it yesterday thank you. Terry, the manifold and riser only have 15 hours on them from new and the flow from the exhaust on muffs is both hot and steady but I will fire the hose up it tomorrow and report back. If I am to trust my pyrometer value then the max temp is within operating spec of 165-200F but it does rise from idle to 3200 rpm by some 20F. If this rise is symptomatic of a problem the I must continue trouble shooting The raw water side followed by the closed. One problem is that the boat lives on a trailer and in my highly tidal location floating her is a major effort so muffs are my only option for supply of raw water at present. I could pull the exchanger and have it tested. I can run a clear view pipe from the transom inlet pipe to the HX in to check for bubbles. I replaced the pickup pipe between the gimbal housing and bell with OEM 3/4 pipe and am sure it's not kinking. All closed side pipes are spring reinforced and not clamping under load. Weep hole is clear. Coolant level constant and not losing coolant. I could check the thermostat opening temp and type. All this assuming that there is really an issue even though there has not yet been a high temp alarm. One other thing I noticed is that with muffs on the side with the hose on sprays and the other side all the water is sucked. Blocked pickup or more water pressure hose side of the muffs ?

 

[stonyloam]

You might pull the front cover off of the xchanger to see if there is any junk in there. That is where it would accumulate. Could be that your thermostat is not CLOSING properly, so that your high speed temp is just fine but when you slow down it over cools a bit. Just speculating there. Your new gauge will tell you a lot.

 

[bfjou812]

Just a thought , what thermostat do you have in the boat? I had a similar problem of overheating. Changed to the 4" exchanger, made sure the pump in the outdrive was good, and also ran a 50/50 mix of antifreeze in the closed system with the addition of a product called "Water Wetter" It alters the surface tension of the water to make it flow better ,there are other products that do the same thing this is the one I used.. I also put in a 160º thermostat in it and I could run at 3500 rpm all day long and the temp would'nt get over 170º. Hope this helps.

 

[Rake722]

Video of view inside riser at normal running temp - http://www.youtube.com/watch?v=i-HyOAwi5Cs