Wobbly temperature reading?

Rich H · Post by **Rich H** » Mon Apr 17, 2006 10:21 pm

Coming back up from Preston today I noticed the temp had fallen from it's usual 89-92 (Depending on what is switched on

) to 85 or so then it wandered from 79-92.

Obviously I pulled over ASAP expecting the worst but there is nothing. No water loss, no mayo, no leaks, no smells, nothing. I let it cool down a bit (Went for a piss and a snack) got back in and started her up. Seems ok so I elect to do a short leg to the next junction and monitor.

It would apear that at low revs, tickover-2.5k, the reading is normal, but over this the reading drops with revs to around 83-85 at 4.5k, fuel gauge seems to be more erratic than normal too (Might just be paranoia though)

Is this:

a. The dreaded HGF?
b. Knackered temp sender (I have a spare already)
c. Knackered alternator or voltage regulator (Part of alternator?)
d. Something else?

Any thoughts?

I'm going to try and change the temp senders (Blue and brown) tomorrow 'on-spec'

Cheers
Rich
(Concerned faction)

Rich H · Post by **Rich H** » Tue Apr 18, 2006 6:17 pm

Changed temp senders, what a PITA. Could only get 1/4 turn on the bottom (Blue) one and it was stuck fast! The OE one seems to be black is this a problem? Brown one was easy (:shock:)

Going for a pootle now to check for leaks and odd temp behaviour, wish me luck....

Rich

Rich H · Post by **Rich H** » Tue Apr 18, 2006 7:01 pm

OK: Been for a little run to get the temp up and have a general poke about. The temp seems much more stable and now reads 91 +/-1 instead of 89 +/-1 (I would be suprised if Rover manufacturing tolerance was any better than this!)

She has definatly not used any oil or coolant (Apart from what is sitting on the car park lost during the sender change!) and definatly no mayo visible.

Anyone suggest anything else to check? :Scratch

Cheers
Rich

mac · Post by **mac** » Tue Apr 18, 2006 7:17 pm

clean the earth terminals?

Rich H · Post by **Rich H** » Tue Apr 18, 2006 7:43 pm

I did that a little while ago, I'll leave it for now and monitor like a hawk!

Stewart · Post by **Stewart** » Tue Apr 18, 2006 8:56 pm

Richard
I had a vaguely similar problem. After changing both sensors, the problem was still there. Finally got rid of it by cleaning the earth straps. It was a couple of years ago and I can't remember exactly where they are located. I seem to recall they go from the engine to the chassis or something like that. A quick look at the manual should tell you.
Anyway this cured the problem.
HTH and that it isn't what you are hoping it isn't

Cheers
Stewart

Rich H · Post by **Rich H** » Tue Apr 18, 2006 9:06 pm

Cheers gents, I'll have a better go at the earth straps. I have been thinking about adding a decent bit of cable down the back end of the car and changing both earth straps altogether.

TTFN
Rich

andysc · Post by **andysc** » Wed Apr 19, 2006 3:33 pm

not sure if this will help you, but I've just taken the numberplate & bracket completely off and over the last two weeks without it, its been 89-idle, 84-85-driving. Have you been moving stuff on the front of the car? missing a number plate???
I wouldn't worry too much, the problem with the system is it can take a while for thermal effects to be felt at the sensors, due to the long piping routes.
On another note, my other car a Nissan GtiR used to run @ 85-90deg, changed the exhaust removed cat, & now sits @ 80deg????

robin · Post by **robin** » Wed Apr 19, 2006 4:59 pm

andysc wrote:not sure if this will help you, but I've just taken the numberplate & bracket completely off and over the last two weeks without it, its been 89-idle, 84-85-driving. Have you been moving stuff on the front of the car? missing a number plate???
I wouldn't worry too much, the problem with the system is it can take a while for thermal effects to be felt at the sensors, due to the long piping routes.

For the standard K setup in an elise, the length of the pipes is not relevant. The coolant temperature sensor is at the water outlet elbow (i.e. it measures the temperature of water leaving the engine).

The thermostat is at the water inlet (i.e. where cold water comes back from the radiator).

Thermostat opening is controlled by a bypass pipe that takes hot water from the outlet and runs it into the thermostat over the temperature sensitive bit - this hose is very short. When the engine outlet is hot, the thermostat opens and cold water comes back from the radiator. Provided your radiator is colder than the engine (which is almost always is), then as soon as the thermostat opens, the indicated temperature will begin to drop.

If you are running steady state (i.e. at the normal running temp, everything warmed up) and suddenly freeze the radiator so that it's output is much colder than it was a minute ago (e.g. accelerate from 30 to 70), then the engine will cool momentarily, but the time taken to react is only the time it takes for the slightly colder coolant to get from the elbow to the thermostat - certainly less than one second.

Cheers,
Robin

andysc · Post by **andysc** » Wed Apr 19, 2006 6:01 pm

will the volume of coolant not have an effect? it's an 8L capacity in the system, usual for this engine is 4.5L(in the rover guise). will this not mean the system will be very "start/stop", ie from startup when cold, thermo shut until temp increases, then thermo opens and lets in "cold". This repeats until
everything in the system is flowing due to thermo being open constantly? I've noticed on cold mornings the car takes probably a good 10miles sometimes to fully reach temp.
the thermostat will control the release of the coolant from the radiator, but what error range does it cover? From what is happening to mine, I was thinking that allowing a greater volume of air to cool the radiator was allowing a drop in system wide temperature. I've had a look at the service manual but can find no mention of the operating temp & error. For this kind of item maybe a 6% leeway is ok?

I think I've confused myself now

robin · Post by **robin** » Wed Apr 19, 2006 11:20 pm

andysc wrote:will the volume of coolant not have an effect? it's an 8L capacity in the system, usual for this engine is 4.5L(in the rover guise).

I don't believe it makes any difference - you don't care what the volume of water in the unregulated part of the system is. The more water you have the longer it will take to reach fan-on point in traffic, but apart from that it will make no difference.

andysc wrote: will this not mean the system will be very "start/stop", ie from startup when cold, thermo shut until temp increases, then thermo opens and lets in "cold".

No different from any other car ... even in a wee rover 200 the water in the radiator on a cold morning is going to be ~70-90C cooler than that in the engine. The critical factors are (a) the temperature difference between rad and engine and (b) the rate at which the thermostat can react to changes in temperature of the water coming from the bypass.

andysc wrote: This repeats until everything in the system is flowing due to thermo being open constantly? I've noticed on cold mornings the car takes probably a good 10miles sometimes to fully reach temp.

If the temperature is stable at whatever temp yours happens to end up at, it's because the thermostat is constantly opening and closing (or at least changing the amount it's open by). It will only be constantly (wide) open when the radiator cooling is insufficient to match the waste heat coming from the engine - e.g. idling in traffic - no air flow - or racing up a mountain in second at 7,000 rpm - masses of waste heat, little air flow.

The heater matrix is a mini radiator - if you have it set to "hot" then you will be dissipating heat from the engine that means it will take longer to heat up (this is deliberate - most people prefer their engines to take longer to heat up if it means that they get cosy toes sooner!).

andysc wrote: the thermostat will control the release of the coolant from the radiator, but what error range does it cover? From what is happening to mine, I was thinking that allowing a greater volume of air to cool the radiator was allowing a drop in system wide temperature.

The radiator has mammoth cooling capacity (up to 200BHP engines can run on the standard radiator, depending on climatic conditions). Assuming you are moving at a reasonable speed (20mph+) then the water in the radiator will be close to ambient temp - 70-90C less than that in the engine. Adding airflow might make it 5C cooler (optimistic).

You are right that the larger the difference in temperature between the radiator and engine, the harder the thermostat's job is. Impulse changes in either engine load or radiator temperature may cause it to oscillate and thus you might see +/- a couple of degrees momentarily while it sorts itself out. However, at steady state it will cope with any mix of engine load and radiator coolant temperature.

andysc wrote: I've had a look at the service manual but can find no mention of the operating temp & error. For this kind of item maybe a 6% leeway is ok?

From new I would expect +/- 1C on the thermostat operating temperature. That will widen as the thermostat gets older (actually I think it will only get colder, not hotter). However, if the only tool you have to measure it with is the dash then it has much higher error - the dash display is very sensitive to resistance in the earth path and changes in current draw).

I seem to recall the standard thermostat temperature is in the low 90s, while the stat fitted to the remote stat kits is in the low 80s. Of course the engine is designed to run at close to 100C continuously, so the reduced stat temp is going to cost a little in terms of efficiency, but it gives you a bit of a buffer against things going wrong

On balance, I would prefer to see the remote stat kits run a standard temperature, but the parts aren't available.

If you really want to confuse yourself, go see how the Freelander pressure-sensitive stat conversion operates

Cheers,
Robin

andysc · Post by **andysc** » Thu Apr 20, 2006 9:19 am

cheers Robin,
the confusion has lifted! Have you had experience in engine cooling design?

Andy

Post by **Shug** » Thu Apr 20, 2006 9:51 am

Heh, Robin has experience of most things and if he doesn't, he can usually show you the maths to prove any theoretical point he may have...

Resident SE1 Planet-Sized-Brain.....

robin · Post by **robin** » Thu Apr 20, 2006 1:29 pm

andysc wrote:cheers Robin,
the confusion has lifted! Have you had experience in engine cooling design?

Andy

Nope - I am software engineer!

It's just a feedback loop controlled servo system like any other ...

Robin

Rich H · Post by **Rich H** » Thu Apr 20, 2006 4:18 pm

But either way my temp appears to be back to (near) normal....

Cheers gents, everyday is a school day