Limp mode

Tiny
CA7922303
  • MEMBER
  • 2002 BMW 525I
  • 2.5L
  • 250,000 MILES
Overflow tank busted, oil light, check engine light, temperature gauge went beyond hot all at once. No water in oil. No smoke from tailpipe. Engine compartment had no steam and was not very hot if any above normal. Engine now has a miss. Is this limp mode? Will it go out of limp mode when overflow tank is replaced, and coolant is at correct level? Is there a sensor in overflow tank? Thanks for your help.
Saturday, March 12th, 2022 AT 7:02 PM

3 Replies

Tiny
BORIS K
  • MECHANIC
  • 836 POSTS
Hello,

If the engine ECU goes into limp mode the engine performance will be reduced, it will not misfire. The sensor at the bottom of the expansion tank is a coolant level sensor.

See the below video:
https://youtu.be/ddtVMO9ta2Y

How to replace the expansion tank on E39 BMW:
https://www.pelicanparts.com/BMW/techarticles/BMW-5-Series-E39/60-WATER-Coolant_Expansion_Tank_Removal/60-WATER-Coolant_Expansion_Tank_Removal.htm

How to bleed the cooling system, this is on a V* but will also apply to your vehicle:
https://youtu.be/J4p4PRCpOfA

Once the expansion tank has been replaced and the cooling system bled properly, it would be advisable to have a chemical test for a possible blown head gasket done.

https://www.2carpros.com/articles/head-gasket-blown-test

A misfire can be caused by a blown head gasket, faulty ignition coil or sparkplug.
In extreme cases a lack of compression can also be the cause of a misfire.

Please let us know how you get along.

Cheers, Boris
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Sunday, March 13th, 2022 AT 1:35 AM
Tiny
CA7922303
  • MEMBER
  • 165 POSTS
Are you saying that the sensor in the overflow tank would not cause the limp mode to engage?
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Sunday, March 13th, 2022 AT 9:52 AM
Tiny
BORIS K
  • MECHANIC
  • 836 POSTS
Hello,

The sensor in the overflow tank is only a level sensor. A faulty sensor or low coolant level will not bring on limp mode.

Also, even if the engine control module was operating in limp mode the engine would not misfire.

Cheers, Boris
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Monday, March 14th, 2022 AT 9:27 AM

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