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1989 OMC Cobra 5.7 I/O running hot

6

67-LS1

Member
The gauge in my boat doesn’t have numbers, just dashes but it’s always run between the 1/4 and 1/2 marks. Lately it’s been running closer to 3/4.
I checked the water pump in the drive and it seems fine. I changed the T-stat for a new one (160*)
The water hose from the drive through the steering fluid cooler to the T-stat housing seems cool. The hoses going to the exhaust manifolds are warm. The big hose from the T-stat housing to the engine water pump is VERY, VERY hot. I neglected to check the outlet “legs” of the engine water pump.
In my mind I’m thinking either new stat is junk or the engine water pump is bad.
I’m going to bring my temp gun with me tomorrow just to check real numbers but was wondering if you guys think I’m on the right track.
Any suggestions would be appreciated. Thanks
 
Fresh or salt water?
Have you had the exhaust elbows off to check the cooling passages? If there’re clogged that will cause both the engine & exhaust to run hotter. I always test stats in a pot of hot water to make sure they open at the rated temp & open far enough. When all is right with the cooling system my 1988 4.3 Cobra runs between 160-170 on the gauge, maybe as high as 175 after coming off plane for a while.
 
Temps I get on mine with an IR temp gun:
Intake manifold right under the stat housing: 150-155*, this spot reads closest to the dash gauge temperature that I have found…
Exhaust elbows: 90-100* @ idle and 123*-140 after coming off plane; after idling for a while they will come back down to about 100*.
 
It’s in fresh water. I going to the boat today so I’ll check with my IR gun and report back. I just thought it was weird that I can touch all of the hoses except the large one going from the T-stat housing to the water pump. That one is way too hot to touch.
Also, what are the little brass inserts for in the T-stat gasket? The gasket that came with the tstat didn’t have them.
 
The one coming from the stat housing to the front water pump should be hot because some of the water in it has just exited the intake. The stat housing functions as a mixing valve; with a cold engine & stat closed the incoming cold water bypasses the engine & exits the exhaust. Then when the stat opens that cold water gets sucked into the engine, picks up the heat in the block & cyl heads and then exits the intake & out the exhaust…
The brass inserts are for a Mercruiser gasket OMC & Volvo used a regular gasket. The brass inserts were to ground the stat housing to the intake because Merc put a temp sending unit in their stat housing. Make sure you actually have an OMC/Volvo stat…
 
It’s a Sierra stat, part # 18-3677, 160*.
If it’s running super hot today, I’ll probably pop the T-stat out and come back without it just to see if the flow through the stat is restricted.
Could the engine mounted pump be the issue? Not circulating enough water? That big hose is HOT.
 
If it's not leaking or making noise, the only thing that could cause overheating vis a vis that pump is if the impeller inside is slipping on the shaft, rare but could happen, or of someone used an automotive pump by mistake then the impeller could have rotted off the shaft.
When you have your boat in the water if you disconnect the raw water intake hose at the stat housing, and have someone start it while you hold the hose up (briefly) do you see a stream of water 2-4" high? If not you have raw water intake/pump issues.
I would do that test before changing anything else. Next time I'm running mine I will see how hot that hose gets on mine, it's pretty hot IIRC.
Any hot running/overheating issues I have had with the Cobra, it's never been that front circulating pump. Impeller, wear plate, exhaust manifold clogging (salt water use) were the typical causes. Oh and yeah, I had barnacles growing in the drive water intake on that plastic screen, I wound up splitting the drive and ditched that screen. Once a month during the season I tilt my drive up and rod out the 4 big water intake holes. Made a special tool out of a coat hanger wire just for that little cleaning job 20+ years of salt water use, 6 months of the year, on a salt water mooring.
 
The temps I measured maxed out at about 180* with my IR gun. The gauge is showing 3/4 to the red but doesn’t have any numbers, just marks. So I thinking the actual water temp is ok. I’m going to change the sender and see what happens.
A few things happened yesterday that may or may not be related…
- The oil pressure gauge is jumping around quite a bit.
- When I trim the drive down, the engine died once and seemed like it wanted to die other times.
- The boat has docking lights and when I accidentally hit the switch for them, the voltage dropped to under 10.
- The engine seemed to struggle with acceleration but just one time. It was fine the rest of the day. Except…
- I tried a full throttle run just to see what would happen and the engine sputtered at around 3500 rpm and would not turn any faster.
All of these things seem electrical to me. I did have the engine out recently to change the drive coupler and wonder if I missed a wire or a ground or???
 
Ok for the engine stumbling or cutting out when you cycle the trim; I had that problem years ago and solved it by tossing the junk factory battery cables and installing a dual battery system with Ancor Marine battery cables properly sized. Also make sure that you have good terminals on your battery cables and NO wingnuts at the battery, use self locking stainless nuts or bolt on marine grade battery clamps (what I did years back). Next make sure the starter solenoid connections are clean & tight as well as the 2 solenoids for the trim system….
Oil pressure gauge, you might have corroded terminals or need a new sending unit.
Lastly your alternator might not be increasing charging amps when a load is put on the system and extra loads are pulling your system voltage so low that the ignition coil is starved for voltage
 
Today I pulled the intake hose off of the T-stat housing and started the engine. The water pumped up about an inch higher than the end of the hose. I’d heard that it should be 3-4”s so even though the pump/impeller look good, i’m going to change them. I also flushed the outdrive both ways with the water pump off. Every thing seemed to flow fine.
I also found a loose wire at the starter so hopefully that will calm the gauges down when I get the pump back on.
 
I installed the new impeller and the water rise out of the T-stat hsg end of the intake hose was about the same. I drove it for about half an hour and used my IR gun and everywhere I checked was under 175*.
I also checked the voltage with the engine idling and on my meter it showed 14.2v. When I turned on docking lights, the gauge on the dash dove down to about 10 volts but my Fluke meter only dropped to 14.0v. Same with the blower.
So now I’m thinking there is a gauge issue somewhere. The water temp seems off and the voltage doesn’t show correctly under load.
I’ll be moving the boat from its berth to my shop in two weeks and I’ll have to dig into the electrical then.
Thanks.
 
On what I had with mine with the voltage issue was similar, normal charging voltage at the alt & batteries but low voltage with the ignition on at the fuse box & ignition switch. There must have been a high resistance splice in the harness or maybe the wires feeding the fuse box were undersized. I checked on both pos & neg sides and I found that the issue was on the positive side. So instead of ripping the whole harness apart I ran a supplemental power lead from the battery switch common terminal (fused of course) 8 gauge wire
To the fuse box postive now have same voltage at the fuse box and ignition switch. I know it’s a hack but it works!
 
PS
As far as checking temps with the IR gun I have found that the area that reads closest to the dash temp gauge is the front of the intake manifold right under the thermostat housing. When the gauge reads 160-165 that area reads 150-155. Check that spot & see what you get.
 
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