6.2 mercruiser running hot

[im4duke]

2003 Cobalt 240SD
6.2L Mercruiser, Engine SN: 0M603578, 70 hours

I bought this boat from NJ in March 2011 and brought it home to Dallas, Texas. It had 50 original hours on it. I have since put an additional 20 hours on it. Boat is in the condition you would expect for as few hours as it has. Boat is bottom painted and spent first several summers moored, and on the hard in the winters. Obviously wasn?t used much.

When I got it to TX, I replaced the impeller, fuel filter, oil & filter and spark plugs. The impeller was missing 2 vanes. Found them both.

3000 rpm and under, temp gauge reads a constant 175. Maybe creeps up a little if lots of passengers or pulling a tube aggressively (hard turns, etc.).

Over 3000 rpm, temp creeps up, even when running straight with light load. First time it happened, I didn?t know and audible alarm sounded and rpms were limited to 2400. Limped home slowly keeping temp in check.

Local boat mechanic connected his computer which indicated a sensor fault which he says is the MAP sensor. At idle, MAP reading is ~7psi. No other faults indicated. Note, I can run smoothly up to over 4000 rpm with no problem other that I notice the temp climbs so I back down. Note that the MAP sensor is a combo pressure & temp sensor.

Boat is now at local Mercruiser dealer/shop for diagnostics. Would appreciate any insight this forum can share on things to check, be wary of as we troubleshoot this.

Thanks,
Maxx

 

[captainzac]

Re: 6.2 mercruiser running hot

Find the two vanes from the old impeller
most likely still in the system somewhere
maybe Don will chime in he might know where the best place to look would be

Harry

 

[im4duke]

Re: 6.2 mercruiser running hot

Find the two vanes from the old impeller
most likely still in the system somewhere
maybe Don will chime in he might know where the best place to look would be

Harry

Thanks for the response. I wrote up above that I found them already. One was in the impeller housing and the other further downstream. They were both whole, i.e. no missing smaller pieces.

 

[im4duke]

Re: 6.2 mercruiser running hot

Any other thoughts from the forum? For a boat that didn't see much run time in 8 years, could certain components have degraded over time, like the thermostat? Someone local suggested that exhaust flappers may have deteriorated and are preventing the exhaust from escaping well.

 

[im4duke]

Re: 6.2 mercruiser running hot

Shop says they may or may not get to the boat next week. Thinking I might replace a few parts this weekend myself with hopes to resolve it for the Independence Day weekend. I am thinking the following:
Temp sender $25
MAP/temp sensor $200
Thermostat - $15

I know the MAP sensor is bad, so that's a given. $40 for the others and some of my time seems like a small price to pay for a chance, even if it doesn't work out. Thoughts?

 

[im4duke]

Re: 6.2 mercruiser running hot

Anybody with thoughts or advice?

 

[Tahorover]

Re: 6.2 mercruiser running hot

If your MAP sensor is bad, your air/fuel mixture will be way off. So with running hot your mixture is lean.

 

[Stamey]

Re: 6.2 mercruiser running hot

Could be restriction with the exhaust manifolds and/or exhaust elbows. I found this to be the problem with the Chaparral I bought last year. In my case it heated up and overheated, pretty much straight away, due to restriction in the manifolds and elbows. I put a set of new elbows on it and now it only overheats if sitting at an idle for a long period of time (have to wathc it going through no-wake zones). If the RPMs are higher, 1300 or above, it stays at 160. I plan to replace the manifolds also.

When I removed the elbows I was unable to see the exact cause of the restriction, but I suppose it is rust deep inside.
A key clue I had, to suggest replacing the elbows/manifolds, was that I actually blew a cooling hose that goes to the manifold when running at high speed. I thought I had blown up the engine. Turns out that the restriction was that bad in there, and why it had a blown head gasket when I bought it.

No way to say if this is the problem with yours, but if it was kept in the water for long periods of time, and if it was salt water, you can certainly expect corrosion in there.

Chris

 

[lg260ss]

Re: 6.2 mercruiser running hot

Check your impeller housing for any scoring. A rough surface in the impeller housing can cause the impeller to loose its ability to pump enough water, especially at higher RPMs.

 

[im4duke]

Re: 6.2 mercruiser running hot

If your MAP sensor is bad, your air/fuel mixture will be way off. So with running hot your mixture is lean.

Thanks. I had the shop order the MAP sensor and thermostat so I can have those ruled out before the holiday weekend. Does a failing map sensor usually lead to a lean condition? I was thinking the opposite. FWIW, at idle, the MAP was reading ~7psi.

 

[im4duke]

Re: 6.2 mercruiser running hot

Could be restriction with the exhaust manifolds and/or exhaust elbows. I found this to be the problem with the Chaparral I bought last year. In my case it heated up and overheated, pretty much straight away, due to restriction in the manifolds and elbows. I put a set of new elbows on it and now it only overheats if sitting at an idle for a long period of time (have to wathc it going through no-wake zones). If the RPMs are higher, 1300 or above, it stays at 160. I plan to replace the manifolds also.

When I removed the elbows I was unable to see the exact cause of the restriction, but I suppose it is rust deep inside.
A key clue I had, to suggest replacing the elbows/manifolds, was that I actually blew a cooling hose that goes to the manifold when running at high speed. I thought I had blown up the engine. Turns out that the restriction was that bad in there, and why it had a blown head gasket when I bought it.

No way to say if this is the problem with yours, but if it was kept in the water for long periods of time, and if it was salt water, you can certainly expect corrosion in there.

Chris

Wow, that must have been quite a restriction to blow the hose off. Hoping that's not the case here. I think the shop will run it in their pool and use a IR temp gun to check temperatures. Is there a systematic process/order to check components for cooling water restrictions?

 

[im4duke]

Re: 6.2 mercruiser running hot

Check your impeller housing for any scoring. A rough surface in the impeller housing can cause the impeller to loose its ability to pump enough water, especially at higher RPMs.

Makes sense, will check that too.

 

[im4duke]

Re: 6.2 mercruiser running hot

Update, the shop replaced the water pump/impeller housing. It is a hefty brass part and was scored pretty badly. It was believed to be causing cavitation around the impeller and thus not pumping enough water at higher (>3000) rpm. Unfortunately, that did not solve the problem. The shop put almost its entire team on it for a day and believes it has now identified the real cause. Apparently a hose that carries cooling water from the outdrive into the engine mates to a plastic fitting/insert on the back of the engine. This fitting is deformed and collapsed such that it is less than half open. They have to remove the engine to access the fitting for replacement. My guess is that the original impeller came apart after I took posession of the boat, blocked cooling flow which lead to a high temp condition, which melted this part. Who knows...


So...the engine comes out soon. What other work or preventative maintenance should I consider having done while the engine is out? My list so far:

• pressure wash the engine compartment
• lube steering and shift cables
• inspect gimble bearing
• check oil pan for corrosion/pitting
• ...

Thanks for any help and advice.

 

[Jesse Walter]

Re: 6.2 mercruiser running hot

I have the same exact problem with my 6.2 Bravo 3. I rebuilt my pump and still had no flow. Then I figured out that I had a blockage. Called my dealer and he told me that the fitting on the back of the engine probably collapsed. May be a 4 hour job or he might have to pull the engine. I was able to snake myself next to my engine and could tell that there was no way I could do it myself. I consider myself a pretty decent mechanic but I'm not trying to tackle that job. So I bit the bullet and dropped the boat off at the dealer today. :facepalm: This is the first time I've ever had to take any of my boats to a dealer and I've had boats for 15 years.

Jesse

 

[im4duke]

Re: 6.2 mercruiser running hot

Well, several hours of labor and $$$s later, engine is fixed and runs perfectly now. The shop replaced 2 parts: a hose that feeds the cooling water into the engine and a plastic fitting that mates to it. Apparently both were collapsed and restricting the flow of water, presumably due to temperature. Maybe $20 for these parts. They were able to slide the engine forward, so didn't need to remove it entirely. I will look up the part numbers and edit this post later. All in, it was $2400 in parts and labor that could have been as low as $500 I think if the exact problem was known from the beginning....

 

[Jesse Walter]

Re: 6.2 mercruiser running hot

Don't feel too bad. Mine ended up costing about $1700. I had to get a new sea water pump also because the housing what scored, probably from running dry.

 

[im4duke]

Re: 6.2 mercruiser running hot

Couldn't find the hose and fitting pieces in the parts diagrams.

I also replaced the pump for the same reason. They thought it was cavitating and was the culprit. Probably would have worked fine with the original even though it was scored. Maybe I can get some scrap value for the 5 lbs of brass...it was ~$350 for the part.