Couple of theories on the diesel cool running problem?
 

I am starting a new thread as I didnt want to hijack Daznkel's thread any further.

I have been having problems (let's call it an irritant now) since I got the car nearly two years ago (next monday actually :}) .

Started reading through the forum that thermostat was a common issue, so it was one of the first things I checked out, sure enough, it would rarely pass 70 degrees.

Installed the R5 top hose 'stat three times in total. I think I damaged the copper bulbs on the first two, as the temperature would barely reach 72 degrees. But the running temperature did improve. the thrid 'stat, allowed the temperature rise to 80 - 82 degrees under normal running (commuting and to and from the shops).

The temperature would take about 5 miles at 40 - 50 mph to reach 75 degrees, and then to the motorway driving between 70-80 mph for four miles, it would rise to 85 under acceleration and settle to 80 when cruising. This pattern sounds fine and natural to me. It is the numbers that I believe are the issue.

The accepted wisdom, is the original thermostat is faulty, with slow opening. That I accept fully and dont doubt. The other contributing factor is that the engine is very thermally efficient. Again I can accept that fully. I have a replacement OEM (3rd gen) thermostat in preparation for fitting when the engine mount needs to be replaced, may as well, when the engine needs moved about. But until then, I dont want to disturb it.

What I cant get my head around is why the temperature doesnt rise enough to open the R5 stat fully. I havent seen the temperature rise more than 90 ever! Even driving up a steep section of motorway to 90 mph! It dropped to 78 and maintained that on the level for 15 miles at 70-75 mph. I havent also seen the temperature drop suddenly suggesting the stat was opened.

Theory one, is it the radiator?

I can appreciate as you drive along at a steady pace, the temperature can fall with the airflow passing through. But it does stays pretty constant. Just lower than I would like it to be. I am wondering, is it possible that the radiator is blocked somehow as in the picture below. Perhaps corrosion building up and blocking the flow in the section tinted gold. Therefore the stat doesnt open fully and the coolant doesnt fill the radiator quickly enough to fill it. That flow passes quickly through, cooling quickly. The radiator if not filled completely, would not have enough ambient coolant heat to keep 'warm' the flowing coolant. Or perhaps, if the radiator contained 2 litres of coolant, but with the blockage I suggest, maybe only 1 litre is passing through. My thinking is that the 1 litre of coolant would cool quicker than 2 litres, therefore colder going into the engine. (latent heat capacity?)

http://www.the75andztclub.co.uk/foru...1&d=1452045111

I know it is unlikely, but I am trying to think of other potential issues that can cause it to run at lowish temperatures, even with its thermal efficiency. I know that the radiators are the same throughout the range of engines, so therefore were likely designed to keep the petrol engines cooler. But the standard 'stat was designed to open fully at 88 degrees in the diesel which would suggest to me that the coolant should be capable of reaching 90-95 degrees, and maintain 82-88 even with that radiator.

I have read a few others with the same issue as myself, after fitting the top hose 'stat. Some people say this solves their issue. I have read a couple of times, whilst it increases the temperature, it still doesnt increase the temperature to the magic 88 degrees.

Theory two, is the temperature sender faulty?

well, as the it says, could it be the coolant temperature sender? If the temperature sender becomes faulty, and under reads, the ECU would then overfuel? This would cause the reduction in fuel economy, and possibly even a reduction in power. I think this is a more realistic issue, and only affecting a small number of us ....... so far. The coolant could actually be running to the magic 88 degree region, but the sender is reading 10% low.

I am sorry it is such a long winded post, but wanted to include the background too, to eliminate or suggest alternative issues. I do not drive hard, and with it being an auto, have little control of the revs. But even driving hard, the temperature rises to around 80-82, and then falls to 75 when cruising.

Any thoughts would be appreciated, because it is driving me nuts. I understand how the system works. Coolant effectively starts off cold in the engine, water pump pushing it about, over the sender unit, until it reaches the opening temperature of the 'stat, which gently opens as the temperature rises, softening the wax. Coolant flows into the top hose of the radiator, pump and gravity pushing the coolant to the bottom of the radiator via the tubing cooled by the fins. Out the bottom hose, into the engine and back around, with the stat staying partially opened under normal use, allowing the flow to maintain a steady temperature of coolant. So with the radiator issue I suggested above, being unlikely, that only leaves the sender as the other component in the system that could cause an issue like this.

Where is the sender located, and is it easily accessible? It only occurred to me today, but wont get the chance to look for it 'til the weekend (EDIT: just looked it up on Rimmers, seems to be below the inlet manifold, will this need to be removed to access it?)

Just to add, interior temperature is fine, and the coolant temperature drops slightly when using the the heater.

edit the above after much discussion, is not relevant to my issue.

In short!

the original theories in the first post are not valid in my current issue.

An inline stat is the quick solution to replace an awkward OEM out of spec stat.

An inline stat that is not watertight will likely cause the engine to warm up slowly. (this is the point I am at now)

Test the stat by filling the cup of it with water and observe its water tightness.

If no water passes through, then fit, and enjoy full operating temps within 5 miles (apparently).

RESULT HERE ;)

I think a semi blocked rad would increase engine temp. Hence the reason we used to block some of the car rads off when I was young. (that did increase the engine temp).

Maybe we should try lagging some of the external piping to aid engine temp. :Snow:

If it was just the sensor, then the car interior would be toasty, but it rarely if ever is... :icon_rolleyes:
...

Thanks for reading through it all :}

I see what you mean about the blocking of the radiator, have done that myself in the past myself. But my thinking is the less coolant, flows through it, the less heat maintained in the coolant. 2 litres of coolant would retain the coolant temp longer. It is clutching at straws I know, and an odd theory.

My interior is toasty, I meant to state that in the above post. I will add that.

I agree with the diesel being very thermally efficient, so there is very little heat being generated. What small amount of heat there is, can be lost through the airflow around the block and head.

However, keep in mind that a considerable amount of what little heat there is, can be dissipated in warming the cars interior via the heater matrix. After getting the car interior and the engine up to full temperature driving - I had to wait around for 30 minutes in close to freezing temperatures. to keep the car warm, I kept the engine running, but the car interior and the engine cooled down quite rapidly.

The radiator itself is far too big for the engine, but that should not make much difference to the warm up cycle of the engine, if the stat is working properly.

The only way for the 75 diesel to stay at temperature in cold weather conditions is for it to be working hard and be supplemented with an FBH. Even opening the windows can dissipate enough heat via the matrix, to bring the engine temperature down.

I'm sure you've considered this, but using the aircon automatically switches on the radiator fan. I believe using the "econ" button instead of the "auto" button disengages the aircon pump and switches the fan off. With the colder weather the fan might make the problem worse.

I'm a little surprised to hear that the heater works well, most reports of poor heater performance seem to be linked to low coolant temperature. Is it possible that the coolant is at the right temperature and it's the temperature sensor that's faulty?

Hi.

Apart from the low temperature reading (disregarding the accuracy of the sender) what other problems are you having? You say the heater is working well, how is fuel consumption?

Alan,

Your variable running temperature related to road speed suggests that your cooling system isn't pressurising correctly. It's perhaps worth looking at the expansion tank cap first.

By the way, your account of the cooling system's operation is basically correct except that in the case of the 4 and 6 cylinder 75/ZT engines, the OEM themostat is located in the return from the radiator, not in the hot flow to it.

Simon

The original thermostat is not "slow to open" it appears to open too early with age or fails to shut properly allowing premature flow of the heated water.
If you are seeing a reading of less than 88C then the Radiator has nothing to do with it as there should be no warm water flowing through it! If you have the air con on Auto then the fan is on and blowing cold air on to the cylinder
block but it is very difficult to prevent that. I have experimented with a higher rated Stat' and my diesel went as high as 98C on a long slog uphill
and fell immediately the stat opened (in summer). The uprated Stat was a
Stant 13352 (205F--96.1C) not suitable for the in-hose fitting as it is a normal sized unit for a housing. The only time you will get your diesel to a high temperature is when it is under a heavy load, just watch the reading drop on the OBD when you go down a hill!

I believe the cool running is the reason that Rover group fitted the FBH, so as the engine reaches temperature quickly, not so YOU are toasty warm.

I have a separate 5L tank for red diesel just for the FBH, I find 20 minutes of FBH before I go out does the trick.

Currently I am in work, so I can't respond properly to all that everyone has said. I will do when I get home for clarity.

However I will say ,
my undertray is fitted and intact.
I don't have an FBH fitted.
I am not using my air con, just econ. heat. Both sides set to 22, if I want it warmer, I dial up the temp to maybe 25, and the temp noticeably gets warmer.
I don't use the heater often, even in this weather, but when I do the coolant temp drops by around 5 degrees. Which is to be expected.
I do have the atc set for feet and demisting when I do use it, which I can feel when it is turned off, when traveling at motorway speeds. (Ram air?)

The coolant temp rises and falls as everyone describes, so I have never concerned myself about this, just the numbers are lower via the obd, than is regarded as normal.