Now, brace yourself as this one might be long if I want to explain the problem in detail.
First, this is 2.2 puma in a peugeot boxer, but that should't change much. I've had the vehicle for 3 years, covered well few thousand miles in it (campervan) and there have always been two weird things.
1. sometimes in the night time I would notice momentary (fraction of a sec) headlight dim
2. sometimes when I came to a stop, I heard a radiator fan running even though I'd just stopped a regular drive, chilly weather, night time for example.
A couple of weeks ago I was going away for a weekend, night time, no traffic, 10 celsius outside. I was driving slightly more agile that usually, meaning around 60mph rather than 50 and noticed the light dimming would intensify. I also came to a junction, had my window cracked open and heard the fan running. The temperature on the dash has always shown in the middle, no overheating problems. That night I connected the dots. It was the radiator fan kicking in in random times that was causing the headlight dim effect! But there was absolutely no reason for the fan to kick in in the first place.
Luckily I had my diagnostic interface with me, so I had a plan to investigate on my way back.
The engine stone cold. I brought up the inlet manifold temperature, engine temperature and both radiator fans status. On the start the inlet temperature was showing the same as the engine (I know the fans in the puma are operated by the ECU based on the engine temperature so wanted to rule out faulty sensor).
Started the engine, off I went and observed how the temperature was raising. It was going up steadily. At some point the dash temp. gauge started going up too and when the engine temp sensor was showing around 88-90, the needle on the dash reached the middle and stayed there.
But the temperature of the engine kept going up. Depending on the speed and whether I was going uphill or not. When crusing at low speeds (urban) the temperature kept at low 90s. When going on flat at 50-60mph it would go to high 90s. When going even slightly uphill or accelerating the temperature would go up to 106-107, the fan kicked in (also showing 'active' status on the diagnostics) and it was effectively dropping the temperature I thing down to 93 when the fan would shut off. Every time there was a slight hill, the whole process would repeat. Remember it was a night time, again around 10 Celsius and driving single carriageway with no traffic.
I have to point out here, when I bought the van the cooling system ran on water.
On my return home I was debating 3 reasons.
1. thermostat
2. blocked radiator
3. water pump
What was interesting, the heating worked perfectly fine. I stopped on the way and the radiator seemed cold. Okay, I thought maybe it's the thermostat not opening, but in that case why as soon as the fan kicked in the temperature quickly dropped. If it was blocked radiator, again, how come the fan was so effective in dropping the temperature? And why was fan more effective in cooling the engine down than driving 50-60mph
Anyway, decided to start with a thermostat. Took it out, it was starting to open somewhere close to boiling point so figured it's probably getting lazy, time for a change. Bought a new one but it was opening similar, maybe slightly earlier. Decided to put the new thermostat, bur being there I though I'd take the radiator out to see how's the flow and whether I need a new one or not. There was a bit of a rusty residue inside but I flushed it out and the flow seemed pretty darn good. Fitted it back and with the new thermostat ran the engine on raised idle (it was a hot day) until the thermostat opened. The radiator got hot on its whole surface, the fan did not kick in at any point then (it was stationary). oh, I also flushed the system with a cooling system flush and then using water some two times and pured fresh cooland.
Decided to go away for another weekend and observed the temperature. In urban area it kept 89-90. AS SOON as I pushed the accelerator to get from 30mph to 50-60mph the engine temperature would go up immediately up to some 102 degrees and when I got to my speed it would go down to some 96. Every time I was slowing down before a roundabout, the temperature would go down to 91-92 and then after clearing the roundabout and getting back to speed, same thing, temperature up to 102. At any point during that trip a fan would kick in. The higher temperatures up steep hills would be 103-104.
At that moment it was obvious for me something was wrong with the water pump and that needed replacing. I was suspecting rusted away impeller. Bought a new pump and got to fitting it today. Took the old bugger out, opened it up...and it seems fine. The only suspicious thing is quite a lot of pitting/pockmarks
Seems like the result of a cavitation. Don't know why would it occur? I haven't checked the expansion tank's cap but the system seems to pressurize okay, well if it wasn't there would be a lot of boiling at 107 degrees, wouldn't it? I fitted the new pump anyway but haven't got any other ideas than maybe the coolant temperature sensor is defective in a weird way that it's way to sensitive over 90 degrees and is showing too high readings, but showing correct values up to 90? But that's a very far fetched idea...
What do you guys reckon? These temperatures the engine is running in are way too high, aren't they? Not only that but also the fluctuation is very puzzling. I mean even regular acceleration from 30 to 50-60 would cause the temperature raising from 93 to 102 in a matter of however long it takes to get to that speed. That's mental.
OK, if anyone managed to get to the end, thanks and hope you can throw me some ideas, please.
Cheers,
Dan
PS. worth mentioning, I've done Alps, Pyrenees and last year Scotland and the van never showed any symptoms of serious overheating (I mean the temp gauge on dash always in the middle. the fan would kick in while climbing up the mountains but everything would act as normal). And I know the problem I'm describing has been there even since I've had the van because I've always noticed that headlight dim meaning the radiator fan was kicking in and that happens when the engine temp sensor reaches 106 degrees.
so the problem was there in the first place, something kept triggering the radiator fan after all. but I think a new thermostat and radiator flush solved the issue. even though I thought it still persisted, the ENGINE TEMPERATURE which I kept monitoring dropped good few degrees and the fan stopped kicking in while driving. could have stopped there but because I'm a muppet I kept digging and the only good thing is at least I've got a new water pump 
