1997 4.3GL PLKD running cold even at idle (doesn't warm up)

[rkilpa]

Noticed a change in operating temperature the last two outings. Normal operating temp was around 150 and time before last it was running around 125. Last outing the gauge stayed at 100. Assumed I had a thermostat stuck open and would replace it with year end maintenance.

When I replaced the impeller I noticed that one vane was 1/2 missing. Surprising since it had less than 40 hours on it and all the other vanes were like new, but I was sure I would find the pieces in the TS housing and my problem would be solved.

I replaced the TS and to my surprise there were no pieces anywhere in the housing including the bypass hole. I cleaned every passage with a wire brush and could see the brush exiting the bypass hole so I'm sure there are no pieces of impeller stuck in there.

The old TS did look like a few grains of sand or rock were preventing it from completely closing, so I was hoping the new TS would be the solution.

Nope.

Tested gauge by grounding sending wire. Gauge pegs hot. Took actual temp readings with laser and the engine is definitely running cold. Highest reading at the base of the sending unit was 120 after idling for a very long time.

I've removed impeller pieces from the TS housing in the past to cure an overheating issue, but I've never had a overcooling problem before.

I know that the missing piece(s) of impeller must be blocking the path somewhere and need to be found, but if they're not inside the TS housing, where is the most likely place to begin to look so I don't needlessly start taking everything apart?

 

[jerryjerry05]

Re: 1997 4.3GL PLKD running cold even at idle (doesn't warm up)

If??? It went through the housing it probably went on through to the block??
If it's done that then I'd not worry about it as you'll probably not gonna find it.
Is the o-ring under the thermo in right?
Try a different thermo, test it.
Wrong temp thermo?
Any #s stammped on the thermo?

 

[rkilpa]

Re: 1997 4.3GL PLKD running cold even at idle (doesn't warm up)

Are you saying there is no other location the impeller piece could block AFTTER the TS that would cause this situation?

Since I didn't find the piece in the TS housing, I am thinking the 1/2 vane might have come apart in one piece and might have stuck to the inside of the inlet hose instead of washing out when I disconnected the inlet hose from the pump when changing the impeller. When the pump pushes water into the TS housing, maybe the loose piece blocks the bypass but doesn't lodge inside of it and then falls back into the hose when the pressure is removed.

Does this theory sound possible?

 

[skydiveD30571]

Re: 1997 4.3GL PLKD running cold even at idle (doesn't warm up)

Correct, there is no where after the tstat housing that will cause a cool running engine when blocked. Of course there's many places a blockage will cause overheat problems though. Your theory is possible, and wouldn't be the weirdest thing to ever happen. Something is causing cold water to pass the tstat housing and enter the block instead of exiting thru the exhaust...the tstat should be basically closed at 120 deg.

 

[rkilpa]

Re: 1997 4.3GL PLKD running cold even at idle (doesn't warm up)

I just had a chance to work on my boat since the October post. I took off all of the cooling hoses and flushed them thoroughly and no impeller pieces flushed out, so my theory of a loose piece lodging and dislodging in the bypass hole didn't pan out. I replaced the new Sierra after-market thermostat with a new OEM Volvo Penta thermostat (3856961) thinking the cheaper version might have been the problem, but the motor will still not warm up even at idle. When on muffs, the water exiting at the stern was warm, but nowhere near as hot as I've felt it in the past.

I took it out on the water and the gauge took a very long time just to reach 135 at idle. I felt the hoses and the only hose that felt warm was the large hose from the thermostat housing to the circulating pump. The risers were barely warm. As soon as I increased the throttle, the gauge dropped to cold.

The motor idles smooth and runs great, but I don't feel comfortable running it at high RPM's if it's not at operating temperature. I've been keeping it under 3000 RPM's on plane.

I can only wait until spring and see if the warmer ambient temperature will make a difference, but what else could it be if the thermostat housing bypass hole is clear?

 

[rkilpa]

Re: 1997 4.3GL PLKD running cold even at idle (doesn't warm up)

OK...this is probably going to sound really stupid. Since it is still running cold and all I've read says it has to be a blocked bypass hole, I'm not sure that I've cleared the actual "bypass" passage. On the thermostat housing, there is a little passage just inside the inlet nipple that you can't see directly that I've been focusing on. If memory serves, I'm pretty sure this passage connects the inlet nipple with the large nipple that goes to the circulation pump. However, according to the water flow diagram in the manual, the passage I should be concerned about is located between the inlet nipple and the riser outlet nipples, right? What is the best method to locate this passage? Can it be seen directly or is this a by-feel method? If I have the thermostat housing off and I am looking down the inlet nipple as if it were still mounted to the engine, what direction does the bypass passage take: left or up or something else? With the thermostat removed, is the other end of the passage visible where it exits near the riser outlets? I've removed the housing so many times now looking for blockage, I'm thinking I might just purchase a new thermostat housing to fix the problem. I'm guessing that It's got to be blocked so much that I can't even see it. If someone could give me a little guidance as to how to locate the correct passage, it would be greatly appreciated....

 

[BRICH1260]

Re: 1997 4.3GL PLKD running cold even at idle (doesn't warm up)

I had the same underheating issue with my 5.0. After replacing thermostat and temp sending unit, I finally found that the thermostat housing bypass was clogged with several small pieces of impeller. The bypass hole was not easy to see just by looking into the housing as its tucked up deep inside there. I would suggest taking the housing off and feeling up inside with your fingers and or a small dental mirror. Chances are thats where you will find the pieces of you impeller vane.

 

[1994almeida]

Re: 1997 4.3GL PLKD running cold even at idle (doesn't warm up)

I have a similar problem (low temperature) and the cause was the thermostat housing, with 12 years of operation.
With a new thermostat housing, I solve the problem.

Almeida

 

[Lou C]

Re: 1997 4.3GL PLKD running cold even at idle (doesn't warm up)

Are you in salt water? If so those passages can clog up and if its really corroded, replacing the housing and installing an OE Stat should cure your problem. BTW, I have tested many 'stats for that OMC/Volvo style housing and they all start to open at about 155-160 and should be fully open by about 165 or so. You might want to test it in a pot of hot water with an old fashioned radiator thermometer.
I've had this same problem periodically but its always been solved (usually for a bout 1.5-2 seasons anyway) with a new 'stat. Raw water cooling they get crudded up and sticky. And there is always sand and grit that can get under the seat. When I repower its a new long block and closed cooling for me....

 

[rkilpa]

Problem solved

Problem solved

Problem solved. It was a blocked "bypass passage" just like everyone said. I took the thermostat housing off for the third time and used a new "aftermarket" TS housing as a guide. I was able to locate the proper passage and with a stiff copper wire I was able to push out the missing 1/2 vane pieces from my 2 year old impeller. I was focusing on the wrong passage. Just in case anyone else makes the same mistake I made, I have attached photos to guide you in the right direction. I have a good understanding of how a thermostat housing works now.