IRREGULAR TEMP FLUCTUATIONS - 350 MAG MPI

[thbouro]

On both engines (350 MAG MPI/V-Drives) occasionally I notice irregular temperature reading fluctuations, at any power setting or load. Thermostats and temperature senders are all replaced new. The type of fluctuations (random, rapid, high magnitude) gives me the impression that it should be an indication problem rather than an actual hi/lo temp issue of the engine/water system. Any idea of what could be the cause of the fluctuations? Thanks in advance

 

[CliffA]

sounds like an electrical short or a bad ground somewhere in the wiring but it is odd that both engines have the same issue....

another thought, when you replaced the t'stats did you use the gasket that has the brass grommet?.....the grommet provides continuity between the engine block and the t'stat housing as the housing mounting bolts are tightened down...if one or both sides of the grommet does not make good contact with the metal of the block or housing you can get bad signals sent to the temp gauge....

cliff

 

[thbouro]

sounds like an electrical short or a bad ground somewhere in the wiring but it is odd that both engines have the same issue....

another thought, when you replaced the t'stats did you use the gasket that has the brass grommet?.....the grommet provides continuity between the engine block and the t'stat housing as the housing mounting bolts are tightened down...if one or both sides of the grommet does not make good contact with the metal of the block or housing you can get bad signals sent to the temp gauge....

cliff

Correct Cliff, they had the brass grommet and I put sealant between the gasket and both the metal surfaces, as per the instructions! Could the occasional interruption of continuity be responsible for the irregular fluctuations? How does it work? By the way, I appreciate your assistance!

 

[CliffA]

you should not put sealant on the grommet and the surface of the engine block and the t'stat housing where the grommet is located need to be clean and have bare metal for the grommet to make good contact....the electrical path afforded by the grommet is just a ground source.....I have read posts before that stated the poor ground circuit from the grommet has caused fluctuations in the temp gauge....I have no personal experience with that...I know the potential of issues so every time I have changed a T'stat I made sure the grommet was making good contact...

cliff

 

[Lazy Daze]

For the reasons mentioned, I always put the gaskets on dry. Unless the surfaces are corroded to the point where they some sealer to help seal... then just be careful. When you say the needle is wildly fluctuating, is going all the way to zero? Or is fluctuating higher than normal. If higher than normal, you might have an air pocket - this can be caused by a piece of impeller caught in the t-stat housing, for example, so look in the housing REAL well. Also, be sure you put that white, plastic t-stat collar in correctly (and the t-stat in the correct way!).

 

[cfd95]

read the temp with a scan tool.
You will find the analog gauges are way off most of the time.
My engines run 175 according to analog gauges and 160 according to the scan tool taking reading from engine coolant temp sensor.

 

[thbouro]

In the link below you can find a short video showing the temp fluctuations. Any assessment on the possible cause is more than welcome!!! Thanks in advance!

 

[Lazy Daze]

OK, that doesn't look to be electrical (although I'd still be sure that the contact point on the gasket is clean, just to be sure). Start with disassembling the housing and looking for something stuck in the t-stat. Also look closely through all the passageways in the housing. Inspect the plastic collar, too. You could also try swapping the temp senders from port to stbd to see if the issue follows the sender.

 

[CliffA]

oh boy Dennis....I was thinking just the opposite....I don't think the actual temp of the water/coolant could fluctuate that quickly...I was thinking the speed of the fluctuations and the wide swings of the needle was indicating an electrical problem....

cliff

 

[Lazy Daze]

oh boy Dennis....I was thinking just the opposite....I don't think the actual temp of the water/coolant could fluctuate that quickly...I was thinking the speed of the fluctuations and the wide swings of the needle was indicating an electrical problem....

cliff

It still could be electrical - but "usually" I would see that needle move even faster. It also is "usually" an all or nothing type thing - like turning a switch on/off. But I agree that I wouldn't rule it out. However, if there's a disturbance in the water flow to/around the temp sender or the t-stat, that could create this issue, as well. Especially something that is keeping the t-stat from closing.

Or... could just be a flaky temp sender.

It would helpful to know if...
-- The problem started right after the t-stat replacement.
-- The problem doesn't start until the engine is warmed up, or does the needle fluctuate even before the engine is fully warmed up.

Either way, the first thing to do is inspect the gasket, t-stat/housing and temp sender. Since both gauges are reading the same, I'm leaning towards something amiss with the recent install. But that's just a guess without having more info.

Note... I mentioned above about swapping senders. Forgot that you mentioned it's doing it on both engines. Swapping obviously won't do anything.

 

[blaster]

Mine did this when the thermostat was installed upside down.

 

[thbouro]

OK, that doesn't look to be electrical (although I'd still be sure that the contact point on the gasket is clean, just to be sure). Start with disassembling the housing and looking for something stuck in the t-stat. Also look closely through all the passageways in the housing. Inspect the plastic collar, too. You could also try swapping the temp senders from port to stbd to see if the issue follows the sender.

The strange is that it happens to both engines! Both with brand new thermostat casing, thermostat and temp sender!

 

[CliffA]

it has been mentioned a couple times already but are you sure you installed the new t'stats correctly?....it is easy to install them upside down if you are not careful....

cliff

 

[thbouro]

It still could be electrical - but "usually" I would see that needle move even faster. It also is "usually" an all or nothing type thing - like turning a switch on/off. But I agree that I wouldn't rule it out. However, if there's a disturbance in the water flow to/around the temp sender or the t-stat, that could create this issue, as well. Especially something that is keeping the t-stat from closing.

Or... could just be a flaky temp sender.

It would helpful to know if...
-- The problem started right after the t-stat replacement.
-- The problem doesn't start until the engine is warmed up, or does the needle fluctuate even before the engine is fully warmed up.

Either way, the first thing to do is inspect the gasket, t-stat/housing and temp sender. Since both gauges are reading the same, I'm leaning towards something amiss with the recent install. But that's just a guess without having more info.

Note... I mentioned above about swapping senders. Forgot that you mentioned it's doing it on both engines. Swapping obviously won't do anything.

Thank you all gentlemen for your valuable thoughts and assessments; Lazy Daze, as far as I can recall, it happens as soon as the temp is approaching the normal operating reading (about 170F). I think I had noticed that even before the replacement of the casing, t-stat and sender but I am not sure... If this helps!

 

[thbouro]

it has been mentioned a couple times already but are you sure you installed the new t'stats correctly?....it is easy to install them upside down if you are not careful....

cliff

View attachment 57967

I will double check it! Thanks Cliff!

 

[thbouro]

Help again! I also faced RPM fluctuations in both engines, as you can see in the following link, which were not actual (throttle steady at 1200RPM, no engine noise change, I am sure it is an indication problem). Any thoughts? Thanks in advance!

 

[Lazy Daze]

Is the temperature issue resolved?

 

[thbouro]

Is the temperature issue resolved?

I didn't work on that (unfortunately no time available!); actually, the boat is dry docked for the whole summer season, far away from my location, and, whenever I can, I operate the engines for about half an hour (engine flush procedure), following my mechanic's advice. Last time I visited her was last Sunday, where this video is coming from. Temp fluctuation was apparent occasionally on both engines too...

 

[Lazy Daze]

That's too bad you can't use it. When you get back to it (should only take a few minutes to check t-stat blockage/orientation), post back about that. Keep in mind that hoses don't always supply enough cooling water.

RPM... It could just be loose/dirty connections or even a dirty cylinder-selector switch on the backside of the gauge ("exercise" it a bunch of times to clean it). Check connections for clean/tight statrting at the battery.

 

[bobeast]

On the rear of the engine is an electrical "umbilical" cord that looks a lot like a trailer connector. That cord carries all of your gage signals. I had a problem with an intermittent tach that I solved by unplugging that cord and cleaning all the pins in the connector. worth a shot.