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My m5 started slightly overheating, so I took it in to the dealer, fully expecting a bad thermostat. They let me know it was the fan clutch, replaced it, and told me to come pick it up. They also told me that one of my cats was clogged, and that it would cost $2,800 to replace. Apparently a code was stored. 3 miles down the road, the car overheated again. I took it back, and the service advisor told me that the clogged cat was causing the overheating issue.

3 questions:

Does the BMW ECU store codes for a bad cat that my OBD2 code puller would not show (pending or otherwise)? I'd like to know this before I replace the cat(s).

Is there any possibility that a clogged cat could cause overheating? Doesn't seem feasible to me, and the car is not down on power.

Should a <30 day old fan clutch be showing significant rust on the nut where it attaches to the crank pulley? From everything I've seen, this nut is integral to the fan clutch, and as such should be new.

Thanks,

Chris
 

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I can't comment on the diagnosis, but I would certainly not spend $2600 on an EOM cat. First, there are probably guys here on the Board who've upgraded to a performance cat, and probably have their oem's. Second, for that kind of money (less really) you could pick up a set of Supersprint race cats. Even less for the Evo's.
 

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My m5 started slightly overheating, so I took it in to the dealer, fully expecting a bad thermostat. They let me know it was the fan clutch, replaced it, and told me to come pick it up. They also told me that one of my cats was clogged, and that it would cost $2,800 to replace. Apparently a code was stored. 3 miles down the road, the car overheated again. I took it back, and the service advisor told me that the clogged cat was causing the overheating issue.
Idiots

3 questions:

Does the BMW ECU store codes for a bad cat that my OBD2 code puller would not show (pending or otherwise)? I'd like to know this before I replace the cat(s).
No. The Cat is part of the emissions system which is what OBD2 covers.

Is there any possibility that a clogged cat could cause overheating? Doesn't seem feasible to me, and the car is not down on power.
Uhhhh........NO. A clogged cat will cause the car to stall, run poorly, and a whole host of other issues, but overheating will not be the first symptom.

Should a <30 day old fan clutch be showing significant rust on the nut where it attaches to the crank pulley? From everything I've seen, this nut is integral to the fan clutch, and as such should be new.

Thanks,

Chris
Depends on the nut they used - if it is stainless, then no. Otherwise, it might. Should it be new? Well, I can tell you I'd be mad if they re-used a $0.99 nut on a major repair...

d-
 

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Find another dealer, not only are they stupid, but they are dumb.

I agree with D, they reused the parts on the flywheel. Also, if you have less then 75k miles they should replace the cat foc. I believe that is the situation that Irish found hiimself in.

-Kirk
 

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The problem I think with dealers are they don't service M5's a lot so don't know a lot about them and also if you tell them something is happening to your car they think back to when they last had that type of problem with a different car and just tell you that part needs changing.
They also never really do a proper diagnostic and as for fixing the problem forget it they just replace parts.
 

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+1 Douglas

A bad cat would throw a OBDII code for the rear O2 or Cat efficency. I have already replaced the water pump on my M5 as it was making a slight whine. I have replaced many water pumps on BMW as they are common failure. It is difficult to diagnose a car with words and not actually seeing the vehicle. If nothing is leaking out, it could be several things from a stuck thermostat, a water pump or a head gasket. You need to find a good indy or another dealer.
 

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I've seen overheating caused by thermostat not correctly lined up, air in the system after service, even leaves and stuff covering the radiator. Check those first.
 

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overheating

My car had an overheating problem. I changed the fan clutch same problem. I replaced the thermostat spent about 120 and no more heating problem. Id try the thermostat before replacing the cat. Good luck.:M5launch:
cj
 

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that big "nut" is integral to the fan clutch, its not replaceable by itself.

Just because the part is new on your car, doesn't mean it was just manufactured, it could have been sitting at the dealer, warehouse, etc for weeks/months/years.

What are the outside temps, and how much is "slightly overheating"? just to the right of the middle? just inside the red on the gauge?
 
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