Rapidly Fluxuating Temp Gauge

[mstng]

Hello Folks,

You guys really helped me out last year and it was appreciated very much. I have another problem I hope you can help me with.

Volvo Penta 3.0 with new riser and elbow runs well but am now seeing a rapidly fluctuating temp gauge. It will sit at 160 and then go to 180 and then back to 160. When it drops to 160 you can actually see it move, iow it goes from 180 to 160 in less than 3 seconds. I know the thermostat opens at 160 but at what temp is it fully opened? Is this rapid fluctuation normal or should I be staying at 160. I don't notice a pattern when accelerating. It happens regardless of idle or speed. Do thermostats adjust that quickly. Should I replace the thermostat? It does sit at the appropriate thermostat temp of 160 frequently which makes me think it is working correctly.

My manifold and elbow is barley warm, the only hot hose is the thermostat to the water pump hose. I can hold my hand on the valve cover for about 3 seconds as that is the only hot area. It has a new thermostat and sender and impeller.

Thanks guys

 

[Don S]

Re: Rapidly Fluxuating Temp Gauge

The valve covers aren't water cooled, so not much sense checking them.
Get one of those IR temp guns and use it to check the temp of the engine right next to the temp sender while the problem is occuring, and see if you are getting the same thing (or close to it) on the gun. I very well be nothing more than a faulty sender, but without testing, you will never know.

 

[mstng]

Re: Rapidly Fluxuating Temp Gauge

Thanks Don....

A question. When a thermostat is rated at 160, does that mean it starts opening at 160 and is fully opened at a higher temp. If the temp is correct and it is truly 180, would that be acceptable to run between 160 to 180 or should the temp stay pretty close to 160 most of the time? IOW, how much fluctuation in temperature should one normally see after it reaches operating temp.

 

[propellanttech]

Re: Rapidly Fluxuating Temp Gauge

First all thermostats open at the rated temperature. That means a 160 deg thermostat will open at 160.

I would definitely do two things. Thermostats when old (even sometimes when new) will "stick". That means they will be reluctant to open, and will "pop" open instead of a nice even slow movement between fully closed and fully open.

Second, I would check the oil level in the engine. If an engine gets low on oil, it will sometimes cause the temperature gauge to jump around like you are describing. This typically happens in cars, with a closed loop system, but I'm sure it could do the same in a boat with a open loop cooling system.

Third......I hate to say it, but a blown head gasket will also make a temperature gauge jump like you are describing. this is because the temp sender can be in a pocket of "air". This is due to the engine compression pushing air in to the cooling passages. I have no idea how to tell if you have a compression leak on a marine engine, without doing a compression test. In a car, you can watch the radiator to see if large amounts of air randomly appear seemingly from nowhere.

If it were me, I would try the oil, compression test, then the thermostat. All in that order. If you find a cylinder a lot lower than the rest ( usually two...not just one) then you probably need to replace the head gasket.

I know that is information you do not want to hear, but I hope you get lucky and it's something simple.

James L

 

[backyard mechanic]

Re: Rapidly Fluxuating Temp Gauge

Just for giggles; check for a good ground on the guage. Boat electrical connections have a tendency to collect corrosion on the contact points especially if they are spade type connections.

If you have a wildly fluctuating reading check to be sure the thermo is in correctly. They get old and stick but orientation to the motor is also sometimes backwards. It happens... Be sure the thermo is facing the block.

Senders do go south as do guages. Hope it is an easy fix for you.

 

[propellanttech]

Re: Rapidly Fluxuating Temp Gauge

Just for giggles; check for a good ground on the guage. Boat electrical connections have a tendency to collect corrosion on the contact points especially if they are spade type connections.

If you have a wildly fluctuating reading check to be sure the thermo is in correctly. They get old and stick but orientation to the motor is also sometimes backwards. It happens... Be sure the thermo is facing the block.

Senders do go south as do guages. Hope it is an easy fix for you.

Yes, that is a good point as well. The connections could be a problem but typically they will cause the meter to drop to zero, or just stop working.

The thermostat being in backwards could also do it.

Keep us informed of what you find.

James L

 

[mstng]

Re: Rapidly Fluxuating Temp Gauge

Thanks fellows,

Lot to think about. Haven't been able to get out there and check it out. I do know the thermostat is in correctly and is new, as is the sender.

One thing I forgot to mention is that I have a on board flush hose that when used it stayed at 160, but when on muffs it jumped around 160-180-160 etc. This led me to believe I may have been getting some air entrained from the flush cap not being tight. I changed the rubber washer and tightened the cap and still had problems. The flush hose bypasses the water inlet and power steering cooler. I will check those clamps and make sure they are tight along with the other things you mentioned.

Thanks again

 

[propellanttech]

Re: Rapidly Fluxuating Temp Gauge

Ah,

That is important information.

When on a flush hose you must remember that a hose provides pressure that the engine wouldn't normally see (lower units suck the water up, not have it pushed in under pressure). This added pressure will sometimes (depending on the delivery method) cause temperature fluctuations.

A direct flush adapter is better for it reduces the aperture of which the water can flow, letting the water impeller do most of the work. This limits how fast the water can circulate through the head and block.

With using muffs, special attention should be made to not turn the water pressure up too much (opening the faucet too much). It usually doesn't hurt anything, but can cause an impeller to inverse in it's cavity (vanes are pushed opposite of their normal position).

The extra pressure (if over done) can cause the engine to cool too quickly causing the thermostat to close, then open again, repeating this cycle over and over.

Every time I use flush water I am careful to not open the faucet too much. I have seen problems even on a flush adapter that is designed for a motor or drive. Water pressure can affect an impeller (break it's vanes), but typically if it does, it was time to replace it.

It sounds as if you don't have a problem. I would stick to the recommended flush adapter. If your temps seem fine with it, you probably do not have a problem. I really shy away from the muffs unless working on an outboard that doesn't have a flush port.

My father which was one of the best Evinrude/Johnson Mechanics in the Southeast 49 years ago will not use a flush hose at all. He will drag out the old metal trashcan and drop (not really drop....figure of speech) the drive or outboard into it for a crank. He will let a hose run in the can to keep the water cool, but he has never connected a hose directly to any motor......ever. He says, "you create more problems than solve forcing cold water through a motor, especially if you are trying to tune the carbs."

I have many memories of him putting outboards in that old trashcan........funny, many people do not have memories like that.

James L

 

[propellanttech]

Re: Rapidly Fluxuating Temp Gauge

As Don said, test with a second device (an IR gun is good). Without a reliable "second opinion" you are just guessing.

In general, if every thing is as it should be the guage won't fluctuate much at all. It should come up to 160 (if that is what the thermostat is) and stay there.

Yea....even a touch temp probe would be a good "second opinion". Too bad there is no way to temp the water directly.....

If you get a temp much over 160, there may be a problem. The best place to test for temp is the thermostat housing itself.....it should be at the same temp (or very close) as the thermostat opens (when warm of course).

James L