MerCruiser 170 Undertemperature

[mtp9302]

Hi all, thanks for all of your help thus far and hopefully you can guide me through this one as well.

I have a MerCruiser 170/Alpha 1 with closed cooling, and the previous owner upgraded to a 4" heat exchanger as well. It runs great, but at idle or during low-power use the temperature gauge will read around 110 F (the gauge starts at 100, and the needle just leaves the first mark). I noticed this past weekend after about 20 minutes at 30 MPH that the gauge read about 130 F. I've never seen it higher than this since I bought the boat in April.

Based on what I've read here, this is probably a low reading and has one of three causes. I've got some ideas listed below for troubleshooting; is there anything else I should be thinking about?

1. Engine is actually running cool, gauge and sensor are correct

- Pull thermostat and check in hot water for stuck open wax valve.
- Check and see how how the hoses to/from heat exchanger and engine get (is there any way more scientific than putting a hand on it?).
- Anything else?

2. Engine is running fine but gauge is bad/offset

-Is this gauge a type of device where I can check either voltage or resistance across it for a certain output temperature? If so, any idea what those values should be for a given temp?

3. Engine is running fine and guage is telling the truth but sensor is bad/offset.

- Not even sure where this is on the engine, any ideas on what to look for here?

I know a car wouldn't even be allowed to run too cold and would set a DTC, but I doubt if a boat has that sort of intelligence. I also suspect that my tach is not working properly (I'm well below the specified 4400 - 4800 RPM at WOT on the tach), so perhaps that makes the gauge more likely than the other two? I'll stop typing and let the experts weigh in, thanks for any help you can give.

 

[JoLin]

Re: MerCruiser 170 Undertemperature

I'll bet you one 'web buck' that the thermostat is either missing or stuck open.

 

[Eric 3.7LX Alpha 1]

Re: MerCruiser 170 Undertemperature

Check this out: radiator cap with a built in mechanical temp gauge on it:

http://cgi.ebay.com/ebaymotors/THER...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

Throw that puppy on before you do anything and it will let you know what has to be fixed, either gauge/sender or thermostat.

 

[Eric 3.7LX Alpha 1]

Re: MerCruiser 170 Undertemperature

I'll bet you one 'web buck' that the thermostat is either missing or stuck open.

That's my guess too

 

[mtp9302]

Re: MerCruiser 170 Undertemperature

Thanks for your help JoLin and Eric. I'm mostly looking for help making sure I don't miss anything in the troubleshooting process, so I appreciate your insights.

I like that radiator cap, that's the first time I've ever seen one and I can think of a few times I wish I had one.

The stuck thermostat was my first thought too; I was wondering if there was anything quicker/easier to check, as I like to try to get the easy stuff out of the way first while troubleshooting. I'll look for one of those gauge caps and next time I work on the boat I'll pull the thermostat.

Follow-up question until I get to check the thermostat: I know running that far undertemperature is not good for a car, is the same true for an I/O boat engine?

 

[JoLin]

Re: MerCruiser 170 Undertemperature

The presence and proper operation of the t-stat is the first and easiest thing to check.

As for running cold, it's bad for any motor, auto or marine, to run at higher or lower temperatures than recommended.

 

[mtp9302]

Re: MerCruiser 170 Undertemperature

I didn't get to buy a radiator gauge cap as Eric suggested yet, but I did have a chance to pull the thermostat tonight. There was in fact one there and it was in the closed position when I removed it. I placed it in a gradually heated hot water bath and observed it to start to open at about 140F (the thermometer I was using read in Celsius, so I observed 60C). It looked about halfway open around 176F (80C) and was fully stroked at 194F (90C). I performed this test two times and got the same performance both times.

The owners manual lists the average operating temperature range between 165F (74C) and 200F (93C) at WOT, so it seems like the valve is doing what is supposed to. I'll probably replace it anyways since the gasket tore when I pulled it, and I doubt I can buy only the gasket. Based on the testing described above, I don't expect the new valve to change anything.

The owners manual has a troubleshooting tree for overtemperature but not for undertemperature. Does anyone have ideas as to what is the next most logical thing to try?

It seems like the only way I could actually be undertemperature is if something is causing too much flow to the heat exchanger, but I think the only way it can get there is through the thermostat, right? That's the way it seems from the manual cooling circuit, which would seem to rule out any passage blockages (that would make me run hotter, not cooler). If the thermostat is working, maybe it is an electrical problem (gauge or sensor is telling me I'm undertemperature when I'm really OK)?

 

[generator12]

Re: MerCruiser 170 Undertemperature

If the temp. gauge is the only indication that the unit is running cold, you need to double check it. Get an infa-red pyrometer (can be had for $30 or less) and check it against some of the temperature measurement devices around the house including the interior of your oven. Compare it to any thermometer in which you have confidence. Once you have ascertained its accuracy, go to the next step.

Start the engine on muffs and run it until the temp. reaches its peak, checking it with the infa-red unit in a spot as close to the temperature sending unit as you can get. As the temp. rises, write down the indications from the infa-red unit and the dash gauge every minute or two in a way that enables you to later correlate the two readings as taken at the same time. This will give you a profile of the engine temperature as she heats up. (Or, do it as she cools down. What you're trying to learn is whether your gauge is accurate.)

You may well find that you're chasing an engine temperature problem when it's actually a gauge problem.

The 170's sometimes have a problem after some years of operation in which the temperature begins to indicate lower than actual. I've seen it in the forum and on my own boat. I still don't know whether it's the gauge (basically an ammeter) or the sending unit (basically a temperature-sensitive resistance) that causes it, but I replaced both on my 170 because it was reading cold all the time.

I replaced then with Sunpro components since the new gauge and sending unit together cost me less than the sending unit alone from Mercruiser suppliers. (And I knew the new stuff would work - I didn't have to guess whether the malfunction was in the old gauge or the old sending unit.) Lo and behold, the engine temp. was normal all the while (165 degrees F) - the gauge was misleading me.

If you look around you'll see this problem mentioned several times in the forum.

Good luck bud...!

 

[Lyle29464]

Re: MerCruiser 170 Undertemperature

Might check the sending unit. Some times if gasket sealer or tape is used it can change the reading.

 

[bigdan1]

Re: MerCruiser 170 Undertemperature

if you read yourself , you alreday know all there is to know


you have already the best tool to verify this : your hand palm !

just put your hand on the upper hose feel it : if at 130 , youll be able to leave it there if at 180 it will feel quite hot !

and it is so easy to verify thermostat , why fool around , do it !

cold engine builds carbon on valves , pollutes and consumes gas like hell .

go for it !

 

[missalot]

Re: MerCruiser 170 Undertemperature

I recently had to replace the T'stat on one of my 3.7ls. It opened like it should but was REAL slow to close. Engine warmed up fine, ran at temp until a good run at cruise speed, when you slowed back down it ran cold (verified with an IR gun). Tested a new T'stat and it closed real quick when it was cooling off. Solved the problem for me.

 

[mtp9302]

Re: MerCruiser 170 Undertemperature

Thanks all, appreciate your insight. I'm going to put a new thermostat in it as soon as I can get to the store. I'll probably drain the coolant loop as long as it's half-empty now anyways (no idea when it was last done). I'm not expecting these things to fix it, but they'll definitively rule out the thermostat.

At this point I'm thinking the next easiest thing to check/replace is the temperature sending unit. I have a few questions about this:

- This device is threaded into the manifold right next to the thermostat housing, correct?

- Is there a way to test this sender? The manual says there is not, apparently if you suspect it is has a problem you just put a new one in?

- I was reading somewhere else on the web that I could get a bad gauge reading if I use a sender that is of a brand that is different from the gauge. Have you found this to be true (ex can I use the Sierra version or do I need a Faria sender)?

Thanks for your patience and your help. I'm starting to get a feel for the reputation these engines have; mine is currently running well and I'd like to keep it that way.

 

[Lyle29464]

Re: MerCruiser 170 Undertemperature

Did you check the sending unit? if it is not grounded you gauge will be wrong. just jump the case to ground and see if your gauge changes.

 

[stonyloam]

Re: MerCruiser 170 Undertemperature

This appears to be a case of chasing ghosts, based on a assumption of what was wrong in the first place. Occam's razor states if there are a number of explanations for observed phenomena, the simplest explanation is preferred. You observed that the temperature gauge was reading low. You made the assumption that the gauge was correct and the temp was in fact low, where you should have chosen the opposite conclusion, that the fault was the gauge. You have done a ton of work and still don't know if you actually had a problem in the first place. Simplest explanation: bad gauge. Check: measure engine coolant temperature, against gauge reading. If temp is actually low work from there. Good luck.
P.S. I never worry that my 470 is under temperature.

 

[Eric 3.7LX Alpha 1]

Re: MerCruiser 170 Undertemperature

Thanks all, appreciate your insight. I'm going to put a new thermostat in it as soon as I can get to the store. I'll probably drain the coolant loop as long as it's half-empty now anyways (no idea when it was last done). I'm not expecting these things to fix it, but they'll definitively rule out the thermostat.

At this point I'm thinking the next easiest thing to check/replace is the temperature sending unit. I have a few questions about this:

- This device is threaded into the manifold right next to the thermostat housing, correct?

- Is there a way to test this sender? The manual says there is not, apparently if you suspect it is has a problem you just put a new one in?

- I was reading somewhere else on the web that I could get a bad gauge reading if I use a sender that is of a brand that is different from the gauge. Have you found this to be true (ex can I use the Sierra version or do I need a Faria sender)?

Thanks for your patience and your help. I'm starting to get a feel for the reputation these engines have; mine is currently running well and I'd like to keep it that way.

Not trying to beat a dead horse, but if you would have gotten that radiator cap with the built in thermometer, you would know in an instant if the thermostat, or temp gauge/sending unit was at fault instead of playing a guessing game of testing this and that and changing parts for nothing.

You could use it to confirm if you dash gauge is accurate or wrong, and even see when the tstat opens by watching the gauge.

It is a mechanical not electrical gauge, which are much more accurate.

That cap also works when you DO NOT have a problem just to confirm temp readings while working on the engine while it's running.
Guess u like doing it the hard way.

 

[NHGuy]

Re: MerCruiser 170 Undertemperature

Get an infrared thermometer, awesome tool.

 

[mtp9302]

Re: MerCruiser 170 Undertemperature

Hi guys, I probably didn't state my objectives very well, so I apologize for the confusion. I'm definitely trying to verify that the temperature is indeed good, which most of you are theorizing that it is and I'm right there with you. However, I also want to fix things such that the gauge reads the correct temperature, which I why I asked the follow-up questions about the sender/gauge. I realize it's more work than is required just to verify that the gauge temperature is wrong. I went in on this boat with my father-in-law, so I'm not the only one operating it and I want the gauge to read correctly when I'm not at the helm.

Eric, I wasn't blowing you off about the gauge cap even though I'm sure it seems that way. I'm not set up on eBay, so I've been doing a search for the best price and I still can't get close to the eBay listing you showed me (the best I found is $27 on Amazon). I'm going to buy it from somewhere, it just hasn't happened yet and I'm trying to do some things with the sender/gauge in parallel because I want to gauge to read correctly as well. I appreciate your help, I assure you your advice is not being wasted.

 

[mtp9302]

Re: MerCruiser 170 Undertemperature

I wanted to give an update on this so that everyone who was nice enough to help me didn't think I was just wasting their time.

Since my last post I performed the troubleshooting prescribed in the Mercruiser manual to verify a good gauge (short the sender terminal to ground and verify that the needle reads 100%). I did this on two gauges, the temperature gauge and another I trusted was reading correctly, and both behaved as the manual said they should.

I also did flush the coolant and reinstall the same thermostat with a new gasket. After letting it run at a fast idle for about 20 minutes, the temperature gauge was reading about 110 F just as before. This time, however, I bought a K-type thermocouple to plug into my multimeter and took readings all over the place, including at the sender and the actual coolant itself (the radiator cap was off, so I stuck the probe in there). The highest value I was able to read was about 136 F, which doesn't match the gauge but indicates that the undercooling is a real condition.

Around the same time I did this, our fourth child was born and I haven't had a chance to work on the boat since. When I get a chance to work on it again, I plan to repeat the above fast idle test, this time with an IR thermometer I have since bought. I have compared the thermocouple to the IR thermometer in several different conditions (freezer, truck engine, room temperature, etc), and they are fairly close. If the test with the IR thermometer repeats the results of the thermocouple pretty well, I'll replace the thermostat and sender and try again.

If that still doesn't work I'll probably ask for some more of your expertise in a new thread, but until I get to work on it again thanks for your help.