5.0Gi overheats at idle, but fine at 2000 rpm

dypcdiver

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5.0 Gi PWTR Engine No.4110178758
Salt water used for 6 or 7 months a year from new (1999)

Overheating at idle,
I have installed clear hoses T'stat to -> manifold and have no supply at idle, but stuttering supply at 2000 rpm (increasing with revs). Engine pulls really well and WOT is 4900 rpm.

Tests
Clear hoses on suction and discharge of raw water pump, no sign of air (but difficult to see clearly)
Removed outdrive plastic nipple (not cracked) and pressure tested hose from there through transom to outlet of power steering pump oil cooler, it held 30 psi for 10 mins.
Pressure tested raw water pump to 30 psi and vacuum tested it to 25 inHg held both (rotated pump to make sure).

Replaced
Raw water pump impeller (VP)
Raw water pump body (VP)
Raw water pump new seal and Bearing last season.
New T'stat housing and T'stat last season 160 F. Boat will run at anything over 2000 rpm all day at 160 F.
Manifold and Risers are 4 years old.

I'm starting to think I might need to do a compression test, although with 33 C (92 F) and 80% humidity here in Spain I am not looking forward to that or the consequences.

I have run out ideas, anybody else got ant ideas??
 

alldodge

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Are you sure its over heating (used an IR temp gun on thermostat)?

Is the thermostat installed, and has it been tested?

What do the exhaust manifolds and elbows look like?
 

dypcdiver

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Thermostat is a year old and seems to be working ie. only overheats under 2000 rpm over that and it is steady at 160.
My IR temp gun confirms temperatures. The elbows are all OK as I rev the engine over 2000rpm to cool them whilst returning to my mooring.
 

dypcdiver

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[No message]
 

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alldodge

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The in coming hose connects to a fitting from the drive which can get clogged up, have you looked at it?
 

dypcdiver

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I have had the outdrive off and from the plastic nipple to the power steering oil cooler is clear and was pressure tested.

I have .002" wear on the pump bearing body, but the main body is new. I cannot imagine 2 thousandths of wear would stop the pump working!
 

alldodge

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Seems to have ruled out everything but if there is a head gasket problem.

Have you tried to put clear tubing on exhaust hoses?
 

Lou C

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Well you have an overheating at low speed issue. Now I’m not familiar with the VP raw water pump but I recall once some years back I got stuck on a sand bar and sucked up sand... mild hot running at low speed...the impeller looked fine but the wear plate was scored. At low speeds a scored wear plate housing won’t allow the impeller to pump well whereas it may pump well at higher speeds.

other thing to check is when you test the stat make sure that it opens at the right temp but also opens far enough; there is a spec in the manual for this...
yes you might check for leaky head gaskets which can cause overheating or not. When my HGs went it put some water in a cyl but no real overheating.
Last thing if it was used all this time in salt water raw water cooled the block & heads could be full of rust flakes and sand. I might try to flush out the block and heads, some will fill with a liquid that will dissolve rust but that might be risky (head and intake gaskets) so flushing it with the block drains open might help.
 
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dypcdiver

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Thanks for your ideas much appreciated,

I'm about to try a clear hose in the pump output at the thermostat, easier to view there. I'll also pressure and vacuum test the PS oil cooler to pump hose. Then I shall take a sniff at one of the clear hoses off the manifold to see if I can detect any burnt gases.

Lou C. I have .002" wear on the bearing part of the pump and a new main body, I would not have thought that would cause it to not work, if the new pumps were a sensible price I would go and buy one but at Euros 513 that is a bit of a gamble but maybe I'll have to once I have eliminated everything else. If only I had a lathe!!
 
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Lou C

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Agreed that is a crazy price for a raw water pump. I can change the whole thing on my Cobra for about $65 US including the housing, normal rebuild kit is only $40.
I have heard of people replacing the OEM VP pump with a Johnson impeller that fits into the crank pulley. Not sure if it’s any cheaper though. My thought about corrosion is that there is a layer of internal rust inhibiting heat transfer at low speeds but at higher speeds you have enough water flow to carry the heat away.
ps when my head gaskets leaked 4 years ago I did a top end overhaul on my 88 4.3. I had the cyl heads looked at by a machine shop and they found cracks from a previous overheat and cooling passages were eroded by salt water use. So they sold me a pair of remanufactured heads and I installed them and a new VP style center riser exhaust system. Since then it’s consistently run at 155-160 and I think the new heads and better flowing center riser exhaust helped. When I repower this boat sure I will install a half closed cooling system (VP manifolds can’t be closed cooled) that will eliminate the chronic issues of raw water cooling in salt water.

One of my future plans as long as I have the raw water cooled engine is to get some extra Chevy water pump gaskets and make up a couple of fittings to go in place of the circulating pump (thinking of using some PVC plastic & pipe) so I can hook up a garden hose to each block inlet. Then with the thermostat removed but the housing in place, just hook up the top end of the hose to the outlet of the stat housing and then direct the bottom end down in the bilge, this will flush it from the block, up to the head, then out the intake passage where the stat housing is. Might be a pain to have to remove the circ pump to do this though.
Or figure out how to make an adapter to fit on the bottom end of the circulating pump, using some extra radiator hose. leaving the pump in place and flushing it the same way. Have to look at it for a while and come up with some ideas.
 
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dypcdiver

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Bearing side of pump
 

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Lou C

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that lighter colored area, looks to be lower than the rest of he surface...any scoring that will catch a fingernail?
 

dypcdiver

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Thanks for the ideas,

A .002" feeler gauge goes in but a .003" one will not enter at the lighter coloured area.

Tested the pressure at the thermostat 600 rpm 0.5 psi, and at 2000 rpm 5 psi which seemed a little on the low side, new pump has been ordered despite the price.€ 528 ($628) Manifolds and risers are 4 seasons old so if the new pump does not solve it then they will be removed to check for obstructions..

Finger crossed it is not the head gaskets that have leaked, it is too hot to be crawling around in the cramped engine compartment. At the back of my mind I fear that the salt water may have corroded the gaskets.
 

dypcdiver

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Problem still not solved, New VP pump fitted and still no flow at idle. Engine starts and runs perfectly so I have real doubts about head gaskets.
I am about to drain and remove the outdrive lower unit that I fitted last season, just in case I messed up the seals on the water uptake tube and am now getting exhaust gas into the raw water supply.
 

Lou C

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Ok I can share with you a problem common to outdrive boats moored in salt water:
marine life growth inside the water intake area, I had this problem as a chronic issue with my OMC Cobra. They had 4 large water intake holes in the lower unit and an internal water screen. I would get barnacles on this and in the water intake area under the water pocket cover in the lower unit. What I wound up having to do was splitting the drive, removing the water pocket cover removing the screen and cleaning out the barnacles in there. I left that screen out and this allowed me to use a stiff wire to clean out that area with the boat on the mooring with the drive tilted up. Since pulling that screen out I have not had to split the drive since 2013 and I used the tool I made to clean it out periodically. If you’ve never cleaned out that water pocket area that could be your problem. So you’ve done everything else; I’d pull the boat out pull the drive and split the upper & lower. Then remove the water pocket cover clean it all out and paint the inside with Trilux or whatever antifouling paint you use for aluminum. Put it back together with new water tube gaskets if the old ones are at all questionable. Also look for a hole in that copper tube I have heard of that causing overheats too.
Head gaskets usually cause overheating if the way they blow allows exhaust gas into the cooling water. Sometimes they also put water into cyls sometimes not. It all depends on where the breach is.
Tests:
compression test (low in adjacent cyls)
water in one or more cyls
bubbles in cooling water from t stat housing to exhaust manifolds (use clear hose for tests ID 19mm or 3/4”)

when mine blew I had water in cyls and exhaust gas in the cooling water, it did not actually overheat though.
Chevy Marine engines don’t usually blow head gaskets unless they suffer a bad overheat. This happened to me in 2013, it ran fine for 3 more seasons before it blew. However when I took mine apart after 15+ years the machine shop felt that the erosion of the water passages would have prevented them from sealing if they re did them so I replaced them with a set of remanufactured 4.3 heads + Fel Pro marine gaskets and it runs at a cool 160 all the time...
 

Lou C

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running temps...usually 160 except right after coming off plane then it rises to approx 170 then comes back down to 160....

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dypcdiver

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Many thanks Lou C for your time and input,
At last I have found the problem. The Brass or Bronze tube that carries the raw water from the the lower to the upper transmission and sits in the exhaust path has a hole in it due to corrosion.
The exhaust gasses were feeding into the engine water jacket and pressurising it and stopping the water flow at idle.

It is Item Number 12 below.
Of course it was in the upper transmission and the hole was hidden from view on the FWD side so unable to be seen from astern.
 

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Lou C

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Ah great find, just make sure the area below that is clean of marine growth...can you find a replacement, or can it be repaired? Remember raw water systems need a high volume of water in and out....
 

dypcdiver

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New one ordered from local VP dealer 50km away, they said it was in stock and 48 hours delivery,. but they also said the new pump was in stock, but it took 10 days to arrive!
 
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