Engine Still Running Hot

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bsquared

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Joined
Apr 4, 2009
RO Number
31184
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I have a 1999 Mercruiser 7.4L engine. The engine overheated due to the manifolds and risers failing which caused water intrusion into the cylinder heads.

I had an upper engine rebuild done - heads rebuilt, new plugs, rotor and distributor cap.
Replaced the manifolds, elbows, risers, circulating pump, impellor, and thermostat.

Engine now runs good, except the engine temperature is much higher than normal. Normal is around 165-175 and I am running about 195.

The mechanic verified the temperature gauge is working properly.

Any ideas what could be causing the overheating?

Thanks.
 
Could be thermostat.

Get a IR temp gun and take readings all over the engine so you have something to compare to. Same idea goes for compression tests.
 
2nd on the IR temp gun. I picked one up from harbor freight for about $35 and it is invaluable for helping to pinpoint where high temps are coming from. 2 thoughts on the high temps... I know you said you changed the impellor, but did you disconnect the hose running from it (for less than 5 seconds) with the engine running to see what kind of water flow you have comging from it? Did you check the timing as the timing being off can cause it to run hot. Also, remove the thermostat altogether... is it still running hot?

--Rick
 
Sorry, I forgot to mention the mechanic put a temp gun and computer on the motor. He says the temperature readings are normal, but the gauge still read 190-195.

Is it possible the temperature sending unit took a crap when the engine overheated? The temperature went to 220 during the original problem.

I was also thinking of removing the thermostat as well.

Thanks for the quick replies.
 
Gauges are notorious for going whacky. It's common practice to give 'em (and I'm talking about all gauges) a tap in case of a 'sticky' needle (or connection)
 
When that engine overheated probably damaged the sender. If computer and IR shows shows on limit change the sender check connection and wire in case is the insulation damaged.
 
You could also test the sender wire at the gauge end for expected resistance. Faria has a table for that.

http://faria-instruments.com/site_manuals/IS0085E.pdf

But note that while there is an inverse temp-to-ohm relationship , it is not linear, so unless you want to solve for correct ohms at 195* , it would be easier to just check at 100* and 175* on the gauge . Look for 450 ohms and 99 ohms on an American standard seander/gauge set.
 
If the thermostat got truly overheated, its wax pellet (that controls its opening) could have been crushed to the extent that it can no longer fully open the thermostat.
 
quote:
Originally posted by bsquared

Sorry, I forgot to mention the mechanic put a temp gun and computer on the motor. He says the temperature readings are normal, but the gauge still read 190-195.

Is it possible the temperature sending unit took a crap when the engine overheated? The temperature went to 220 during the original problem.

I was also thinking of removing the thermostat as well.

Thanks for the quick replies.

Click to expand...


If I understand correctly, the guage reads 190-195, that should be normal, depending on what temp thermostat you are using.
 
Harlan,

Prior to the engine redo, it usually ran 165-175 or so. In searching several forums, most owners of a 7.4L Mercruiser state the same temperature range as being normal.

Thanks
 
Little more info now.

Raw water/FWC?

Raw water impeller/housing replaced?

Type drive...I/O or inboard. BI/II/III?

Number of raw water pick-ups.

Thermostat housing and springs checked for rusting and interference of free movement.
 
can we assume he is RWC because he said he had an exhaust manifold failure?
 
Raw water system. Raw water impeller and housing was replaced. Circulator pump and housing also replaced.

I/O and thermostat was replaced (160 degree).
 
Raw water cooled systema should use a 145 degreee thermostat. 160 is for closed cooling systems.
 
cmariner32 - Why would the degree of the thermostat cause the motor to run hot once it was open? I was under the impression that at 160 degree thermostat opened up at 160 degrees and would then bring the engine to normal operating temperature. Likewise a 145 degree thermostat would open up at 145. Am I not understanding how a thermostat works? Thanks.
 
quote:
Originally posted by Harlan

quote:
Originally posted by bsquared

Sorry, I forgot to mention the mechanic put a temp gun and computer on the motor. He says the temperature readings are normal, but the gauge still read 190-195.

Is it possible the temperature sending unit took a crap when the engine overheated? The temperature went to 220 during the original problem.

I was also thinking of removing the thermostat as well.

Thanks for the quick replies.

Click to expand...


If I understand correctly, the guage reads 190-195, that should be normal, depending on what temp thermostat you are using.

Click to expand...


I don't know of any common marine gas engine RWC or FWC for which 190-195* is considered normal , though that is not yet critical temp.. A gauge reading like that is an indication of flow , gauge or sender issue that should be addressed by a responsible boater to avoid dealing with "critical" at an importune time. 190-195* leaves very small margin to "critical".

bsquared- How warm/hot do the riser/elbows feel to your hand at idle and cruise speed in gear?
Hopefully you can keep your hand on them constantly without burning.

If the mechanic says the gauge is OK and the IR gun says the engine is at proper RWC temp of perhaps 145- 165*? or even a little higher as you mention, then it will be worthwhile to test that sender.

What readings was the IR actually showing at the thermostat housing area? Do you know what temp thermostat is actually in there so you will know what correct gauge range should be? If it's not on the thermostat you can test it in a pot of warming water with a candy thermometer to see when it starts to open & is full open.

Even if a higher temp FWC-type ( 160 to 170*) thermostat is in there, IMO it should not allow 190-195* accurate gauge readings if flow is OK .

If, on the other hand, the IR gun(& sender resistance ohms check )verified the 190-195* gauge readings , it would be time to check RW hull intake, RW strainers, sea cocks , hoses for crimping, air intrusion , suction-collapse or delamination, as well as correct attachment at the RW pump inlet/outlet, and confirm air is purged . And confirm gaskets with proper # holes were used installing the new exhaust parts.

If your Merc has check balls & springs in the RW circulation system system those ecould be an issue as well .

If everything checks out with all that, there could even be a little exhaust flow restriction or lean running condition.

I don't believe cmariner32 was indicating any open thermostat would be causing your overtemp reading,... only advising what he felt the RW system noramlly would use, though I think that may vary a bit. In general , RWC cooled systems have lower thermostats to minimize salt "precipitation" in the engine cooling passages that occurs at higher temps for somewhat improved combustion efficiency normally seen in FWC engines that have no SW in them.
 
Good drscrption Sandy. Cmariner, Newer Merc RWC engines are coming with 160 degree thermostats, so the old dictum of "143 for RWC and 160 for FWC doesn't hold true anymore. Also many fresh water boaters changed to 160 deg stats to allow their engines to run at the higher temp for more efficiency.
 
I suspect many of the newer RWC engines are MPI and need higher degree t-stats to help the sensors perform correctly and allow the ECM to optimize efficiency. It's a tradeoff with the salt-precipitation issue.
 
The original over heat was probably not from the riser leak but some as yet undiscovered cause. Does it cool way down when you return to idle? If so then the gauge and thermostat are working but the engine is not getting enough water. Check the through hull and remove all the input and exhaust hoses to look for internal failures or blockages.
 
I had what I believed was an overheating problem last year. The boat is an 89 Formula with twin big blocks. my gauges were reading 190-200 all the time. We replaced manifolds, risers,impeller and housing, cleaned put the oil coolers. My mechanic suggested i check for water volume. turned out I was only flowing 1.5 GPM. we found an old TSB from merc suggesting that the addition of a thru hull in addition to the thru drive cooling would increase volume and keep the engines cool. We added 1.25" thru hull fittings with valves prior to the oil cooler. It worked amazing. The water flow increased to 6.5+GPM. I run 143 TSTATS. and digital gauges. My temp NEVER goes over 143. It was a lot of work but the boat runs cool as a cucumber. Hope this helps. I know how annoying temp issues can be.
Steve
 
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