Engine overheats when A/C is off, error codes P0491 and P0492 - BMW M5 Forum and M6 Forums
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post #1 of 2 Old 8th July 2018, 03:12 AM Thread Starter
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Engine overheats when A/C is off, error codes P0491 and P0492

Hi guys

2001 e39 540i

I replaced front bumper with an m-style one at a shop. After about 30 mins of driving, CEL light came on and the engine started overheating. I turned on the A/C and the engine temp went back to normal right away. With A/C on I also noticed that the cooling fan was spinning at a higher speed, so fan clutch is working well (but only with A/C on)

At home I ran OBD diagnostics and got these codes:

P0491 reported by 17
P0492 reported by 17

So the symptoms are:

1. When A/C is on, the engine temp is normal and the cooling fan is spinning at a higher speed
2. When A/C is off, the engine overheats and the cooling fan won’t speed up.
3. Error codes P0491 and P0492

Please help
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post #2 of 2 Old 4th August 2018, 03:55 PM
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I've done a few swaps/replacements of aux fans and main fans of the on the S62 and M62 engines, and I don't think the mechanical fan clutch was controlled electronically in any way.. however, the aux fan (the electric one on the front of the radiator) definitely is, and is commanded on by the DME based on A/C load, radiator outlet temperature, intake air temperature, and vehicle speed.

Those OBD codes are for failure of secondary air injection, if that was working before, the pump probably came unplugged during the bumper swap. It's behind the fog light on the front passenger side of the car (LHD models).

If the mechanical cooling fan ISN'T spinning with the engine at idle, it sounds like the mechanical fan clutch may have failed.. although, I've only seen them fail to disengage, not fail to engage..but anything is possible. Could also be water pump issues, water pump not being driven properly by it's belt..
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