BX 16v Thermostat

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krekov
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BX 16v Thermostat

Post by krekov »

Hello,
I've swapped the thermostat on my 16v due to unresolved overheating when standing still (everything else checked many times over). The old one was an 88C one, and I put in one opening at 88C too.

The problem still persists, and I've found this site: http://bx16valve.com/technical-info/coo ... -16-valve/
It claims the thermostat is supposed to open at 82C. Is this true?

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Re: BX 16v Thermostat

Post by xantia_v6 »

6° doesn't really matter. It is still 30°C below the boiling point of the coolant, so the thermostat will be fully open long before overheating, and the thermostat can't do any more than that.

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Re: BX 16v Thermostat

Post by Dave_16v »

When I looked at mine, the thermostat was stamped 88, non cat, 160bhp.

Are you sure the fan is operating, the switch that activates it is just down on the left on the radiator as you look from the front. It could be faulty or the contacts may need a clean. It's twin speed, perhaps the high speed is not working.

Can you feel the stat to rad hose heat up when the thermostat opens? Thermostat fitted the correct way round?

I know you say everything else is checked but you never know. 😉
Last edited by Dave_16v on Fri Oct 29, 2021 8:25 am, edited 1 time in total.
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white exec
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Re: BX 16v Thermostat

Post by white exec »

Interesting to read that, as I've just got stuck in to cooling system behaviour on our 19D — twin fan, with AC, and therefore similar in cooling requirements and equipment to a 16S.

Thermostat
The engine thermostat only functions to guarantee a minimum running temperature for the engine block. The 'stat will not begin to open until the coolant in the block (at the 'stat housing/water outlet) reaches its rated opening temperature. It will open slowly and progressively to maintain that temperature, by gradually allowing cooler radiator coolant to be drawn through. The cabin heater is plumbed in directly across the engine block, so, in icy conditions, the engine block and cabin heater can be toasty, while the radiator is largely shut out of circuit.
The engine thermostat has no role to play in setting the maximum running temperature of the system. That is all down to the radiator and its cooling fan(s).

Cooling fans and radiator thermo-switch
This is where the fun begins.

The point at which the radiator electric cooling fan(s) kick in - at low speed then high speed - is set (on BX) by a "thermo-switch" let in to the radiator. This thermo-switch senses the temperature of the coolant leaving the radiator, and about to be sent round the engine block. If it's too hot, the fan(s) are instructed to kick in.

Details of BX thermo-switches on Citroen's parts pages can be a bit difficult to pin down, and several different switches were fitted to both petrol and diesel XU and XUD engines. The most informative pages actually appear for C15 models which got twin fans for their 1.7 engines. Interestingly, a thermo-switch listed here was also used on BX16S.
.
BX thermo switches.JPG
For the C15, three evolutions of thermo-switch are listed:
1st figures are cut-in and cut-out temperatures for Slow speed; 2nd figures are those for High speed.
1264.18 93°/88°C — 97/92 (for cars up to RP 5776)
1264.17 87/83 — 92/88 (for cars from RP 5777 to 8284, as well as BX16S)
1264.49 92/88 (for cars from RP 8285 onwards)

In fact, there was even an earlier thermo-switch:
1264.14 88/79 For petrols, except the 16S
Now that would provide some really cool running!

So why the change of spec?
It does look as if Citroen had second thoughts about allowing the C15 engines to get up to 97°C before the High speed of the fans cut in. This must have meant that block temperatures would have well exceeded 100/110C (remember that the thermo-switch is located at the coolest (exit) part of the radiator) — nothing wrong with that, except that XUD cylinder heads do not thrive on extemely high temperatures, especially if there are air-pockets in the coolant. (Historically, XUD cylinder heads had a reputation for warping and causing HG failure.) So, from RP 5777 (Nov.1992) until RP8284 (July 1999) - 6½ years - C15s were given extra cooling protection by revised spec thermo-switches.

Well, good stuff, but what about the BX19D (twin fan, with AC) here?
When I acquired the car, the thermo-switch was in bad shape: its spade terminals were badly bent, and so I opted to replace it.
I inadvertantly opted for a 1264.18 equivalent (a Valeo 819773), and fitted it.
The RP number discrepancy was small (ours is RP 5800).

Result, though, has been unacceptably hot running. Not in normal driving, or at speed (when there is a good blast of air through the radiator), but when hill climbing, or in slow-moving traffic. Here, the (newly installed) temperature gauge runs up to an indicated 100°C+ (but thankfully not further) before the High speed fan cuts in. I think this is pushing things a bit for an XUD.

Having run like this for a few days, I decided to look again at thermo-switch specs, and have now plumped for, and fitted, the 1264.17 (an FAE 37800), which cuts in fan Slow speed at 87 rather than 93, and High speed at 92 as opposed to 97°C.

Some prolonged hill climbing today has brought the temp gauge into a much more comfortable region, so am pleased with that.
_______

Although C15 and BX16S are poles apart, with 19D sitting somewhere in between, the twin-fan cooling system is shared by all of them. Citroen obviously rethought cooling requirements for these engines, and so, if high running temperatures are an issue, it looks sensible to consider how the cooling fans are being asked to operate, which is all down to the thermo-switch.
Chris

krekov
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Re: BX 16v Thermostat

Post by krekov »

Dave_16v wrote:
Thu Oct 28, 2021 9:23 pm
Thermostat fitted the correct way round?
Actually, what is the correct way to fit it? Doesn't it just fit one way in?

Exec, I have fitted a lower temp thermo switch to my 16v. Full fan speed from 92 to 88 and half speed from 87 to 83. The issue is, by the time the fan kicks in at half speed, the gauge is already up to a 100. And full speed kicks in only intermittently at 110.

The hose where the thermo switch is just doesn't get hot enough, while my head is cooking. I've flushed the system twice, the coolant is clear and free of any debris. I checked the temps with an infrared thermometer and the head and thermostat area is indeed up to ~105 when the gauge is at 110.

I tried to bleed the system many times with the bleed valve on the thermostat housing and on the hose stuck to the rad. There's no air coming out, only fluid.

I'm just at a loss with this situation, can't figure it out.

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Re: BX 16v Thermostat

Post by Jaba »

Have you done the feel the rad test? When the engine is fully up to temperature run your hand down the inside of the rad to see if the lower part of the rad might be blocked. The rad will feel cold towards the bottom if this has happened. No amount of temperature tweaking will overcome the lack of cooling capacity of a partially blocked radiator.
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Re: BX 16v Thermostat

Post by Dave_16v »

krekov wrote:
Fri Oct 29, 2021 10:56 am

Actually, what is the correct way to fit it? Doesn't it just fit one way in?
Short end to the hose, long end to the engine. I cannot remember if it is possible to fit it wrongly on our cars but there are plenty about which have space on the hose side to get it wrong.

Having fitted a switch with a lower trigger point, it would be wise to fit a stat with a lower operating temp to work with it. That hardly gets mentioned if at all with that particular mod.

Your fan could be active with stat still closed a lot, you have two different ranges in operation. The original set is a closer match, better to replicate it. That will get the coolant moving better through the engine so the fan can cool it lowering the temperature as a result.

Another thing to consider is the water pump. It might be one of the after market versions with smaller orifices, they restrict flow which would impact mostly when stationary because revs are low.

I am not saying the stat will fix it but it is worth swapping and taking it from there if problem persists.
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Re: BX 16v Thermostat

Post by krekov »

The thermostat is new, opens at 82C.
I've felt the rad and it's hot in all places.
I do not get the "stop" light, if that means anything. The temp sender is new.

But:

Something I've noticed is: turning the heater blower up to max makes it stay at 90C forever, and the radiator fan turns on at 90C on the gauge too.

But without the heater on, it will not turn the fan on until around 100C on the gauge, and it will go to 110C with the fan regardless.

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Re: BX 16v Thermostat

Post by white exec »

Krek, you do have twin fans on the car? Just wondering if both are operating...
Your 82deg 'stat and thermoswitch combination should be ok.

If you do open up the 'stat housing (to check it's in the right way round!), just as an experiment you could try leaving the thermostat out. That will make warm-up (of engine and cabin heater) a much slower process, but should reveal whether the radiator fans are able to contain top-end temperatures properly.

The most worrying thing you say is that by the time the fans kick in at half speed, the gauge is reading 100. That can't be right.

Dave's point about an unhelpful thermostat (or even the pump) is spot on: the radiator and its fans won't be able to get cooled water to the engine block if there is a constriction (stat, pump...) somewhere. The 'stat must be fully/substantially open before the fans start to kick in. Despite newness, I'm thinking a 'stat problem here. If you take it out, do a saucepan test on it. It should start to open at the marked temperature (82), open progressively and smoothly, and certainly be fully open by 88-90.
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Re: BX 16v Thermostat

Post by white exec »

Just done some more prolonged mountain road climbing in the 19D this morning. With the 1274.17 switch in place, the gauge went to just over 90C today, instead of just over 100C with the previous switch. That seems like a more comfortable margin. It's also allowed low fan speed cooling to happen sooner, and to hang in for longer, which has improved the rate at which temperature reduces when climbing eases off.
Chris

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Re: BX 16v Thermostat

Post by Dave_16v »

krekov wrote:
Fri Oct 29, 2021 1:42 pm
The thermostat is new, opens at 82C.


OP
krekov wrote:
Thu Oct 28, 2021 6:35 pm
Hello,
I've swapped the thermostat on my 16v due to unresolved overheating when standing still (everything else checked many times over). The old one was an 88C one, and I put in one opening at 88C too.
Maybe I have misunderstood but in the OP, it reads both the old and new were 88c, ie.like for like opening as you stated at 88, not 82. Isn't that why you questioned the bx16valve site info where it mentions 82c below?


The problem still persists, and I've found this site: http://bx16valve.com/technical-info/coo ... -16-valve/
It claims the thermostat is supposed to open at 82C. Is this true?


If you have reduced the fan switching criteria, the stat opening point should be reviewed. In other words,if you have put the same value back in as you originally stated, one stamped lower should be considered.

Also, if your radiator is old, it may well be restricting flow.
It's been mentioned before that the valvers like a clean radiator because if the flow is slow, the engine temp can become considerably higher than the radiator exit temp (sensor area) and a slow flow makes it worse.
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Re: BX 16v Thermostat

Post by white exec »

Looks like listings for both 82 and 88 thermostats:
Haynes: 79-82°C
Citroen Parts (ServiceBox): 1338.03 listed for the 16S
Mister Auto (PSA/Stellantis): lists both as 1338.03 equivalents, and as the item for the 16S

However, Citroen's mechanic's reference booklet
Private Cars 1992 050920 for AX ZX BX XM
has these pages:
.
Thermostat and pressure cap
Thermostat and pressure cap
.
Cooling fan(s) and thermo-switch
Cooling fan(s) and thermo-switch
.
Water temp warnings
Water temp warnings
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Re: BX 16v Thermostat

Post by krekov »

Dave_16v wrote:
Fri Oct 29, 2021 4:57 pm

Maybe I have misunderstood but in the OP, it reads both the old and new were 88c, ie.like for like opening as you stated at 88, not 82. Isn't that why you questioned the bx16valve site info where it mentions 82c below?


The problem still persists, and I've found this site: http://bx16valve.com/technical-info/coo ... -16-valve/
It claims the thermostat is supposed to open at 82C. Is this true?


If you have reduced the fan switching criteria, the stat opening point should be reviewed. In other words,if you have put the same value back in as you originally stated, one stamped lower should be considered.

Also, if your radiator is old, it may well be restricting flow.
It's been mentioned before that the valvers like a clean radiator because if the flow is slow, the engine temp can become considerably higher than the radiator exit temp (sensor area) and a slow flow makes it worse.
Sorry, I've changed the thermostat to an 82 today, that's what I meant. I had changed it previously to an 88. Now the car has an 82 in it, to fit with the lower temp thermo-switch for the rad fan.

I'm wondering about the restricted flow, I can squeeze the hose coming out the bottom of the rad (where the thermo-switch screws in) almost closed, there's not much fluid in there.

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Re: BX 16v Thermostat

Post by white exec »

The bottom hose should always be full of coolant, even if upper reaches are struggling!

Before thinking about renewing the radiator — maybe no bad idea if it is the original, the history of using good coolant might be uncertain, and there's the possibility that a leak-stopper might have been used — it would do no harm to clean it out (and the whole of the cooling system) with a good system cleaner. I can recommend Liqui-Moly Radiator Cleaner (Kuhlreiniger) #2506. 1 x 300ml can will be sufficient; just follow the instructions on the can or the website. It's good stuff, and will clean out oil, grease, deposits and calc.

If that doesn't improve cooling, replace the radiator.

I've had a couple of cars that no amount of flushing would restore decent cooling, with the radiators in both cases having no 'cold spots'. Replacing them with new transformed cooling. Cross-flow radiators with fine tubes can be prone to restricted flow, if coolant quality/changes is neglected.

Final question: Do you have one radiator fan on the car or two?
Chris

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Re: BX 16v Thermostat

Post by krekov »

I have just one at the moment, but I will be fitting a second one using the schematic I believe you gave me.

The radiator is still the original. So just to confirm, does the half-empty, squeezable hose coming out of the rad mean the radiator is at fault?