Fluctuating engine temperature

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PembrokeshirePromise

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Hi,

Elderly Volvo MD11C - basically runs OK - a bit slow to start from cold and a little smoke - but its always been like this.

Finally fitted new temp gauge and new sender (previous owner hadn't bothered when previous gauge failed).

Gauge shows increasing temp as the engine warms up, but then when running the needle keeps showing an increasing temp for a few seconds and then drops down to what would appear to be a more "normal" temperature.

Stuck a meter across the sender (and as far as I could see on a running engine) - it appears thr resistance in the sender is also varying (hence the fulctuating gauge possibly).

Stuck a digital thermometer on the exhaust manifold (metal bit just before exhaust pipe) and this certainly didn't change temp in the same way (what temp would you expect this to be on raw water cooled - suspect it might matter which side of the manifold probe is attached)

Any idea why the temp is flutuating (or appearing to be) - could I have a fauly sender - even though new ? - or is the temp actually changing like that (thermostat opening and closing every minute or so ?? - seems unlikely). Pockets of steam hitting end of sender ? Some sort of electrical fault (sender has common ground with engine block ?)

I wouldn't be surprised if the engine was running a little hot - elderly sea water cooled - probably furred up pipes - but it basically runs for hours without any seeming problems. /forums/images/graemlins/confused.gif

Anyone any ideas what to look at - or should I just take the gauge out again and stop worrying as previous owner did ! I know one day we are going to want a new engine - its just a case of when ...

Many Thanks
James
 
I had the same problem when working for Ford as a mechanic.They had a huge problem in around 1999 with having to recall all there vehicles and have 2ohm resistors fitted across the temperature gauges as customers were complaining about the temperature gauge being through the roof.
The temperature was meant to get to 103 deg celsius,and then the fan kick in and if i remember the drop to 93 degrees and remain there,or there about.

How involved is your system?
test your sending unit!The change in resistance is normal.Should change but uniformly!!!
These are complete gauge and sending unit tests!!!!
Try these tests:
Test the gauge:
1)Disconnect the sensing line which goes to the sending unit at the back.It should go to its lowest reading!
2) connect a jumper from the sensing line terminal on the gauge to the sending unit ground or a good engine ground!Temp gauge should go to its highest reading!
3)If the gauge passes these tests,you know it is good
The gauge that is!


4)Reconect the sensing line from sender to gauge!
5)Temperature gauge should go to the lowest reading
6)short the sensing line to the engine block.Temp guage should go to its highest reading.If not the sensing line is faulty,shorted or open-circuited..
You could also be experiencing wiring connection problems and check the main engine wiring harness at the sockets and connections.

loose connections are often to blame forintermittent problems...
If not
Using an ohmeter,(Rx1 scale on an analog meter),to test the sending line first from the gauge to the relevant terminal on the sending unit,then from the other\half of the plug(wiring) to the sending unit.
Both times u should get a very low resistance.!!!Infinity or a high resistance indicates a high resistance or abreak in the wiring/circuit.

To test the sending unit:
1)Switch off ignition
2)Disconnect all wires
3)Test with ohmeter (Rx1 on analog meter) from sending unit to a good ground.
4)Most sending units vary from around 700 ohms at low temp. to around 200 - 300 ohms at around 40deg celsius, down to almost 0 ohms at 120 deg celsius.
Resistances may change from manufacturer to manufacturer,but the important thing is to get a clear change of resistances with change in temperature....
This test works also on oil prtessure except resistances to readings are generally swapped around.0 ohms at no oil pressure and around 200 ohns at high oil pressure.

Hope this helps,sounds electrical,but may be worth cheching your thermostat in a cup of boiling water test and check what temp it closes again.they are a huge cause of many temperature problems in older vehicles.
 
The engine in one of our previous boats was sea water cooled and from “brand new”, always showed a steady rise and fall of about 5 to 8 degrees C when it was running. This was simply the thermostat opening and closing.

This is the problem with fitting lots of instruments, you are going round without a care in the world until you fit a cluster of instruments. You go from the expression “engines running well to day dear” to, is my oil pressure too low? is the temperature too high? should the ammeter not be reading a higher current? that battery voltage looks low! Lot to be said for a bunch of warning lights.

PS Just fitted Oil Pressure, Water Temp, Ammeter Voltmeter and Rev counter etc to the engine in my own boat so I will soon have the same worries.
 
This is a symptom of a fouled up cooling system!

In the first instance the MD11c is susceptible to blockage of the small channels in the exhaust manifold that convey cooling water from the inlet point half way along its underside to the cylinder heads. One of thes passages is failry short but the other is longer and blocks more readily it is also, of course, more difficult to unblock. You should check the temperature of the individual heads to make sure that one is not getting hotter than the other. An imbalance here will indicate that one of the aforementioned water channels is getting blocked.

These channels can be unblocked once the manifold is removed.

The temperature guage unfortunately gives no indication that one cylinder could be overheating it will only show a high reading when both are, be that due to poor pump output, blockage of the water passages to both or other general restriction of water flow.

I am afraid however that the real cause of the fluctuating temperature is blockage of the water ways in the cylinder blocks themselves and particularly the small passages at the top linking them with the water passages in the cylinder heads.

Clearing this lot I am afraid is a major job. If it is just the passages at the top then they can be cleared once the heads are removed but if the main water ways in the blocks are choked up then they have to come off as well, a fairly major job, and probably the liners pressed out.

You could try chemical cleaning of course but it will need a pretty potent acid cleaning solution to be really effective. I dislike the use of hydrochloric acid in these situations for fear of other damage that may be done. Maybe sulphamic acid (NB I said sulphamic not sulphuric) would be effective but I'm afraid I don't know where you would obtain it, although I suspect that some boiler descaling products may be based on it.

The problem with acid cleaning is getting the solution to where it is needed. If a small passage is blocked then you don't get any cleaning solution through it so it stays blocked.

I am definitely not a fan of acid cleaning, at least not in this instance, but you may be able to make some improvement.

I'm sorry this has not done anything to cheer you up but I have personal experience of this one, fortunately not my own boat so the costs incurred were not out of my pocket even if I did do a lot of the work involved.
 
Many thanks to all for the comprehensive answers...

I already have Nigel Calders book - and agree its excellent - hence got this far with testing (in fact its his fault I bothered to replace the old broken gauge /forums/images/graemlins/confused.gif. Hence I have carried out many (but not all) of the tests suggested by footit - I think the VDO gauge actualy works the other way round (increasing temp drops the resistance in the sender).

As VicS suggests - this is what I suspect is the case - but was obviously hoping that there was an alternative simpler suggestion. I actually have another engine for spares - and so have a cleaned up exhaust manifold - so can try swapping that over as its a fairly easy job.

I have read of using sulphumic acid - but I guess thats a winter job when ashore - or should I try it sooner rather than later whilst there is still some flow through the head ?

I was just surprised at the fluctuating temperature - fairly regularly over a minute or so - would increase over say 10 seconds, then almost immediately drop back to "normal" for a minute before going through the cycle again. I had assumed if the water channels in the head were blocked you would just get it generally running hot.

If I take the gauge back out and ignore /forums/images/graemlins/tongue.gif for the moment presumably it just gradually clogs and gets hotter - until it reaches the point where it seizes ? Previous experience of these engines suggests that even if that has happened if you let it cool you can then run it again for a bit - i.e. the seizing is often not catostrophc - but likely to be the beginning of the end...

Its the balancing act of spending money on a rusty old engine vs investing in a new one. My inclination is if its a £200 or so then OK - if much more you start to reach the point where its becoming a noticeable portion of a replacement.
 
PembrokeshirePromise hasn't mentioned overheating, simply a fluctuating reading on the temperature gauge which is confirmed by voltage measurements. I've observed such fluctuations on a very good, ether-type car temperature gauge and I believe that they're showing the constant opening and closing of the thermostat. The thermostat doesn't open just enough to give the desired temperature and stay in that position, it's constantly opening and closing. In other words... nothing to worry about.
 
I rather tend to agreee with VicS - cooling passages getting clogged, and the thermostat is trying to balance an imbalanced cooling system.

That assumes that you have repalced the thermostat, which may be a bit sticky? Also, check the elctrical connections to your new meter and sender, and any connectors in between, as that would cause the meter to vary.

Does the meter climb into the red before it drops back? If it is within the 'safe' zone at its highest, and never reaches dangerous heights, then ignore it. It is an old engine with limited life expectancy anyway, and start saving for a new one in a few years time.

After all, until you fixed the gauge, you presumably thought it was running OK!

If it is really worrying you, get an independent overheat alarm which will sound if things do get dangerously hot.
 
I would check the basics before considering any dismantling work>

I had a similar problem with the temp gauge on my Bukh but the problem was (is still) caused by the charging system voltage fluctuating. Do you have an "electric" oil pressure guage? - If so does that fluctuate along with the temp guage?

Check the system voltage while the engine is running and the temp guage is fluctuating. Is the system voltage (battery voltage) fluctuating "in line" with the temp guage fluctuations?

Thermostats to get "tired" after a while and can become slow to respond to the change in water / coolant temperature. Have you considered replacing the thermostat?.

Having eliminated the above it would sound like a partially choked water jacket - I used Boiler de-scaler (Fernox™) to de-scale my Bukh by arranging a bucket in the cockpit with a feed line to the engine raw water pump and a second bucket under the exhaust through hull fitting to collect the waste coolant and a 12v in-line pump in the second bucket to recirculate the water back to the bucket in the cockpit. Flushed the engine with fresh water (ran the engine under load against the pontoon until temp was up) while allowing the exhaust to discharge to the marina then putting the second bucket below the exhaust and recirculating the discharged coolant. and adding the Fernox™ to the cockpit bucket - took about an hour and a half to do the job and there was a lot of crud discharged. After flushing finally with fresh water I removed the anode and had a look inside the block and was impressed at how clean the inside of the block was, at least the area I could see. Same with the thermostat and stat housing. Worked for me and saved stripping the engine.
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hammer.gif
“Artificial intelligence is no match for natural stupidity”
Skype id:cliffillupo
 
Thanks Cliff,

I had wondered about charging fluctuations - as presumably as the sender is simply a variable resistor the gauge is showing the difference between the charging voltage and the gound. I wasn't quite sure how to check this - is testing across the battery terminals OK - they won't act as some sort of "buffer" reducing the visible fluctuation.

I did try a meter between the sender and the engine block - which appeared to also be varying - thereby eliminating any cabling issues between sender and gauge - but didn't carefully check against fluctuating charging.

No oil pressure gauge unfortunately - just a warning light.

I haven't investigated the thermostat yet - again I guess an easy thing to check / replace - is it possible to choice for "tiredness" - or is it just a case of boilng up and seeing when it opens / closes

All I need now is 8 pairs of arms and two telescopic heads so I can hold all the wires in place and check all the gauges at the same time (or maybe a friend !) /forums/images/graemlins/crazy.gif

If none of the above show up the issue I guess maybe its a bucket in the cockpit job - can't make matters much worse I presume ....

Thanks again.
 
I started all this messing as I wasn't particularly happy at the idea of having no temperature indication (e.g. no way of knowing if impeller failed, inlet blocked etc).

Being naive I thought that replacing the existing defunct temp gauge would be the simplest solution !

I was looking at putting a temperature alarm on the exhaust as you suggest - but wasn't sure what temp max to get (from RS) - hence the messing with the thermometer - and my now confused state /forums/images/graemlins/confused.gif

Pretty sure the engine isn't actually overheating (ran for 3 hours continuously at the w/e without any real sign of problems (apart for me behaving lack a jack in a box checking gauges, meters etc) - but no idea if its running hot really.

Can anyone tell me what temperature I should be measuring ?

I know that ideally water should be about 60C to avoid furring up on se water cooled engine - but how to measure?

What would you expect the temp of the block to be ?
The exhaust manifold (probably differs depending on position as it has a cold water inlet pipe on it)?
The exhaust hose itself ?
The water by the time it made it out the skin fitting ?

I also noted the variation in the temp on the exhaust manifold dropped once we had moored up (with engine still running but idling) suggesting to me that with more water flow it might run cooler. Then when switched off temp rose - presumably as theheat from the engine block warmed it again.

I can't help it - all this measuring - I'm a scientist - unfortunately a biologist - so not much use when it comes to mechanics - and whilst I maintain these engines have a mind of their own - I can't really call it living ! /forums/images/graemlins/tongue.gif
 
If reeac was right (which of course I would love to be the case !) - presumably if I ran it with the thermostat removed I should see a steady gauge ?

Yippee - another test to do - this is what happens when theres a weekend with hardly any wind - too much time to fiddle with other things! /forums/images/graemlins/blush.gif
 
[ QUOTE ]
If reeac was right (which of course I would love to be the case !) - presumably if I ran it with the thermostat removed I should see a steady gauge ?


[/ QUOTE ]

Direct water cooled engines should not go much above 70C, and an optimum is around 60 - 65C Commercially available exhaust alarms usually trigger at around 90 - 100, as they are designed to respond to an exhaust that has started running dry. I have heard of alarms that have sensors on the block as well, but do not know who makes them.

Removing the stat may or may not be a solution, and I have come across both cars and boat engines that actually overheat with the T/stat removed, at tickover! Something to do with the pump not circulating the water correctly at low speed when the stat is missing. My Bukh 20 is one such, but there the stat actually opens a bypass, so that without it, most of the water goes down the bypass, not through the engine!

Worried me no end, till I sussed it!

But it would be well worth checking the stat is not the problem first, by simply replacing it.
 
[ QUOTE ]
Thanks Cliff,

I had wondered about charging fluctuations - as presumably as the sender is simply a variable resistor the gauge is showing the difference between the charging voltage and the gound. I wasn't quite sure how to check this - is testing across the battery terminals OK - they won't act as some sort of "buffer" reducing the visible fluctuation.

Thanks again.

[/ QUOTE ]IIRC the sender has one lead that goes to the gauge. The Gauge has two leads - one to "IGN" and the other to the sender. The voltage you want is between the block and the lead that goes from the gauge to "IGN" depending on the gauge you might be able to pick up the voltage from the terminal on the back of the gauge - do make sure you do not disconnect any leads.

Whilst the Fernox™ treatment worked on my DV24 there is no guarantee it will work for you.
----------
hammer.gif
“Artificial intelligence is no match for natural stupidity”
Skype id:cliffillupo
 
Thanks Cliff,

I think the gauge has 3 connections - sender, Ignition and grnd - but can see the test you are suggesting...

Thinking about it - presumably if I measured the resistance of the sender between the terminal and the block - and I saw fluctuations this would either be genuine temp changes or faulty(new!) sender.

[ QUOTE ]
Whilst the Fernox™ treatment worked on my DV24 there is no guarantee it will work for you.

[/ QUOTE ]
Taken as given thanks. /forums/images/graemlins/wink.gif
 
[ QUOTE ]
PembrokeshirePromise hasn't mentioned overheating

[/ QUOTE ]
Ok I know. I was trying to give as comprehensive an answer as possible to the possible cooling problems with the MD11C based on my own experiences as well as adressing the specific problem asked about. I would have thought it clear if my response was read through carefully.
[ QUOTE ]
The thermostat doesn't open just enough to give the desired temperature and stay in that position, it's constantly opening and closing. In other words... nothing to worry about.


[/ QUOTE ]
I disagree. The thermostat will settle at a steady position under steady operating conditions. Ok the opening will vary as load etc varies but it wont oscillate between open and closed and the result should be a steady temperature reading on the guage varying only a little as the conditions vary. You don't see the temperature guage in your car constantly fluctuating as you drive along at a steady speed, none of mine ever have! (You may of course once stationary and the cooling fan cuts in and out , but that is a different situation).
 
[ QUOTE ]
Removing the stat may or may not be a solution, and I have come across both cars and boat engines that actually overheat with the T/stat removed, at tickover! Something to do with the pump not circulating the water correctly at low speed when the stat is missing. My Bukh 20 is one such, but there the stat actually opens a bypass, so that without it, most of the water goes down the bypass, not through the engine!


[/ QUOTE ]

Thanks for posting that explantion. I have seen the advice before that an engine should not be run with the thermostat removed but I have never understood the reasons (apart from running too cold, of course).

Doesn't apply to the MD11C but running too cool leads to condesation in the rocker covers and that over time can cause surface rusting of the valve springs with their eventual failure. The result is not a pretty sight, I can assure you, and expensive to put right.
 
[ QUOTE ]
I'm a scientist

[/ QUOTE ]

Glad to hear that. At least you'll be thinking about problems.

The difference between an engineer and a scientist is that a scientist thinks about a problem while an engineer hits it with a big hammer and only thinks about it when he's broken it.
 
Vic, Many thanks for all the replies..

If you are correct and the root cause is blocked cooling channels - can you explain why this causes cyclical fluctations of the temperature (from your first post I take it this is what you saw in your friends engine ?).

Just curious really to undertsand the process behind - I would have assumed blocked channels would lead to reduced flow and hence a constantly higher temperature.

So whats going on - - are there little pockets of water that get trapped and then vaporised into steam before escaping to be replaced by more water to repeat the process - or what ?

And your dislike of the acid approach suggested by Cliff is based on what - fear that it might eat gaskets / seals etc - as well as salt crystals etc ? Just trying to weigh up pros and cons ...

Thanks
 
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