Peugeot 407 SW 2.0 HDi 136 Thermostat Replacement

[BadIdea]

Okay, I'm sat in my car at the moment, bonnet open. The water temp on the dash says 70c, while the top hose measures 16c with my temp gun. Assuming I've got an accurate reading, the hose from the bottom of the radiator reads 50c.

Does this suggest that the thermostat is actually doing its job?

[GiveMeABreak]

For your engine, the these are the thermostat settings:

Engine	Starts to open at (°C)	Complete opening (°C)
DW10BTED4	83	95

Air conditioning cut-off: 115°C .
Warning temperature: 118°C

[ekjdm14]

That suggests to me that the thermostat is certainly not stuck open or leaking enough to cause an issue. Being a diesel it might be tough to get it hot enough to open the 'stat without driving, but seeing as you don't have overheating issues I'd think that side of it is OK. It would be nice to know if it's starting to open at the right temperature if possible, but for the moment at least I'd call it OK.

Can you get a reading from the cylinder head at all (if it's too reflective to be accurate, stick a piece of black tape on close to the 'stat housing & measure that), that would assist in knowing whether the gauge itself is reading reasonably accurately.

[white exec]

Okay, I'm sat in my car at the moment, bonnet open. The water temp on the dash says 70c, while the top hose measures 16c with my temp gun. Assuming I've got an accurate reading, the hose from the bottom of the radiator reads 50c.

Does this suggest that the thermostat is actually doing its job?

That suggests something very odd is going on.

Assuming all those temp readings are right, if the top hose (coming away from the cylinder head, temp sensor and thermostat) is reading 16C, then the top of the main radiator should be the same, and as the water in it cools, it drops to the bottom, and thence along the bottom hose - which ought to be cooler, not 50C. Re-check the readings...

If the engine is still warming up (70C on the gauge) and the 'stat is still closed (it should be at a block temp of 70C), there should be no coolant flow along the main radiator hoses. Your top hose figure confirms that, the bottom hose one doesn't.

[BadIdea]

That suggests something very odd is going on.

Assuming all those temp readings are right, if the top hose (coming away from the cylinder head, temp sensor and thermostat) is reading 16C, then the top of the main radiator should be the same, and as the water in it cools, it drops to the bottom, and thence along the bottom hose - which ought to be cooler, not 50C. Re-check the readings...

If the engine is still warming up (70C on the gauge) and the 'stat is still closed (it should be at a block temp of 70C), there should be no coolant flow along the main radiator hoses. Your top hose figure confirms that, the bottom hose one doesn't.

Maybe I've been an idiot and checked the wrong hose. I thought I'd traced it back from the bottom of the radiator, but perhaps it was a different hose. I've attached a couple of photos I took at the time.

[white exec]

You might have been reading a by-pass hose - maybe.
Useful things, these IR thermometers!

[ekjdm14]

You might have been reading a by-pass hose - maybe.
Useful things, these IR thermometers!

That's my thoughts, looking at the photo although not knowing the setup well. Looks a little small to be a bottom hose & I might expect it to be on the other end of the rad, so the bottom hose can go direct to the water pump as a lot do.

To be fair, depending on the design I'd expect a little heat soak into the bottom hose with 'stat closed simply because of the water pump "agitating" the coolant in that area & it not being blocked off by a 'stat down there either. I've had my IR thermometer maybe 8 years now, first got it to aid diagnosing/tuning motorbikes so see which exhaust downpipes were staying cooler (hence intermittently misfiring) and it's been a great tool to have. Having one might've saved the OP a few quid here too, which is always a bonus

[BadIdea]

That suggests to me that the thermostat is certainly not stuck open or leaking enough to cause an issue. Being a diesel it might be tough to get it hot enough to open the 'stat without driving, but seeing as you don't have overheating issues I'd think that side of it is OK. It would be nice to know if it's starting to open at the right temperature if possible, but for the moment at least I'd call it OK.

Can you get a reading from the cylinder head at all (if it's too reflective to be accurate, stick a piece of black tape on close to the 'stat housing & measure that), that would assist in knowing whether the gauge itself is reading reasonably accurately.

I'm going to give that a go next. I've read that these engines run cooler than one might expect, which could explain the dash readings so far. It takes quite a while to warm up, then tends to sit around 70 to 75c. I have recently experienced it go up to 80 to 85c after a longer drive, and although I didn't have have my IR gun with me, I felt the top hose and it was warm, suggesting that the thermostat is opening when required. After a longer drive still, I did glimpse it reach over 90c while at rest; anything to worry about? Oil temp seems to sit between 100 and 110c.

Having driven petrol cars exclusively for the last decade, perhaps I've had unrealistic expectations of fuel economy from the 407. I haven't managed to hit the claimed figures once though since I bought it. I had an average of 28 mpg according to the trip computer until today when I drove 4.5 miles, parked for an hour, then drove 4.5 miles back. This brought it down to 24 mpg, so the economy for that one trip must have been terrible. Does this model have a DPF?

[BadIdea]

Having one might've saved the OP a few quid here too, which is always a bonus

Absolutely! Here's hoping that fixing the heater system won't be too costly.

[white exec]

85-90degC is a healthy running temp for a diesel, and temp going up to 95-100C on substantial uphill nothing to worry about. The electric radiator fan(s) will look after any excess heat.

If 70-75C is actually the engine (block, head) temp when driving (a reasonable distance), that is unacceptably chilly, and will do fuel economy no good. It will also make decent cabin temperature a bit of a challenge in really cold weather.

[ekjdm14]

Indeed, those running temps if accurate are nothing I'd be concerned about. The fact it gets up to 90 at times suggests everything on the "getting warm" side of things is working, I suppose summer traffic will tell if the fans work (or actuator test in DB) but seems pretty much normal for now I'd say. Really would have expected more from it in terms of economy though, considering those figures are pretty middling even for a petrol. Not sure what I'd consider normal, having no experience of the 2.0HDi (only the little 1.4 and the earlier XUD's) but considering our non-French car ('06 Volvo S80, 2.4 5cyl diesel) manages to average around 38-41mpg I'd want better from the smaller engine in a lighter car, even with the automatic.

White exec, both of our 1.4 HDi engines consistently showed temp around 70-75 in normal driving, good heater within 5mins, & I feel the gauge may be a little pessimistic hence my thoughts of measuring at the head itself when fully warmed up. When I first experienced this my thoughts were it was running too cool as well but from what I read on here I seem to recall finding a lot of others with similar gauge readings. I think I chalked it up to the efficiency of the direct injection common rail design meaning less energy wasted as heat, hence making it tricky to attain a stable running temperature over 80c in normal driving (even with a working 'stat) but it could just be that the gauges read a little on the cool side?

The oil temperature is something I wish the little 1.4HDi made use of though as that seems more reasonable around 100-110c in this case.

[BadIdea]

That suggests to me that the thermostat is certainly not stuck open or leaking enough to cause an issue. Being a diesel it might be tough to get it hot enough to open the 'stat without driving, but seeing as you don't have overheating issues I'd think that side of it is OK. It would be nice to know if it's starting to open at the right temperature if possible, but for the moment at least I'd call it OK.

Can you get a reading from the cylinder head at all (if it's too reflective to be accurate, stick a piece of black tape on close to the 'stat housing & measure that), that would assist in knowing whether the gauge itself is reading reasonably accurately.

Over the weekend I experienced the engine getting to temp, and the heater working! However, it took quite a drive for that to happen, and it does seem to want to drop back cold pretty quickly. My guess is that the thermostat is not closing completely. After this discovery, I waited until the following day to check temps with the IR gun again. I found that the top hose started off around 9c, but rose to 16c before the water temp was anywhere near the 'stat opening temp. After taking it on a fairly short trip, I pulled over and measured the block temp (just anywhere I could get a reading on the metal; thankfully it was dull enough, but I was armed with tape just in case) and found that it seems to sit in line with what the dash says. The highest temp I recorded was 77c. Sometimes when idling after a drive, the temp can momentarily reach 85 or 90c. I guess the thermostat fully opens at that point and rapidly cools the block back down.

[BadIdea]

Indeed, those running temps if accurate are nothing I'd be concerned about. The fact it gets up to 90 at times suggests everything on the "getting warm" side of things is working, I suppose summer traffic will tell if the fans work (or actuator test in DB) but seems pretty much normal for now I'd say. Really would have expected more from it in terms of economy though, considering those figures are pretty middling even for a petrol. Not sure what I'd consider normal, having no experience of the 2.0HDi (only the little 1.4 and the earlier XUD's) but considering our non-French car ('06 Volvo S80, 2.4 5cyl diesel) manages to average around 38-41mpg I'd want better from the smaller engine in a lighter car, even with the automatic.

I was surprised to read in the manual that my particular spec has a whopping kerb weight just shy of 1800 kg! Granted, this is with the usual "75 kg driver, 95% full tank". For some reason, the manual doesn't list the fuel economy for this spec, but the closest match, the 407 SW with 4-speed auto and 2.0 HDi 136 is supposed to have an urban figure of 30 mpg. The 6-speed is very slightly heavier, but I would imagine the extra ratios make up for that. Still, I've got nowhere near that figure. I currently have 22 mpg on the trip computer. By comparison, my 3.2 litre M3 with two more cylinders and 200 more horsepower wouldn't be far off under the same conditions! I average around 18 mpg for similar trips, and have managed 32 mpg on a long journey.

[white exec]

Over the weekend I experienced the engine getting to temp, and the heater working! However, it took quite a drive for that to happen, and it does seem to want to drop back cold pretty quickly. My guess is that the thermostat is not closing completely. After this discovery, I waited until the following day to check temps with the IR gun again. I found that the top hose started off around 9c, but rose to 16c before the water temp was anywhere near the 'stat opening temp. After taking it on a fairly short trip, I pulled over and measured the block temp (just anywhere I could get a reading on the metal; thankfully it was dull enough, but I was armed with tape just in case) and found that it seems to sit in line with what the dash says. The highest temp I recorded was 77c. Sometimes when idling after a drive, the temp can momentarily reach 85 or 90c. I guess the thermostat fully opens at that point and rapidly cools the block back down.

Everything you describe points to the thermostat not closing properly/completely...
- premature warming up of top hose
- long drive needed to get up to proper temperature
- temp dropping back rapidly (when less fuel is being burned)
A good 'stat will only allow gradual changes in the gauge reading, as it opens and closes progressively, and by small amounts.

Good news that the block itself (and heater) is getting decently warm.
Replace that 'stat with a good new one.

[BadIdea]

Everything you describe points to the thermostat not closing properly/completely...
- premature warming up of top hose
- long drive needed to get up to proper temperature
- temp dropping back rapidly (when less fuel is being burned)
A good 'stat will only allow gradual changes in the gauge reading, as it opens and closes progressively, and by small amounts.

Good news that the block itself (and heater) is getting decently warm.
Replace that 'stat with a good new one.

Yeah, I'm relieved it's not the heater system. I didn't fancy having to dig into that. Just have to find a decent thermostat now.