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Just reading my new Haynes, and thought that this was cool.

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The cooling system is fitted with a fail-safe mode, this comes into operation in stages when the temperature of the engine is too high:

STAGE 1
The cylinder head temperature sensor transmits a signal to the engine ECU, which then moves the gauge into the red zone.
If the engine is not switched off and the temperature continues to rise the multi-function warning light will come on.

STAGE 2
The engine ECU will contol the engine by starting to cut out two cylinders and restricting the engine to below 3000rpm. When this occurs the engine waring light will also illuminate. If the temperature should drop back to normal, the ignition will have to be switched off and then on again, to revert back to four cylinders.

STAGE 3
If the engine temperature continues to rise, the engine will be totally disabled before engine damage occurs. The engine warning light will begin to flash, indicating to the driver that the engine will be switched off after 30 seconds.
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I knew about the two cylinder bit, but not about the shutting the engine down in 30 seconds, a pretty good feature if you ask me, could possibly save you a new engine, if you are silly enough to keep it running through the first two stages.
 

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Interestingly, my 2004 Focus ZX3 2.3L Duratec PZEV has been known to go into fail-safe cooling mode without ever throwing an overheat code or moving the needle on the temp gauge on the instrument cluster. It does throw P0300, P0301, and L0304, though, as a result of the misfires inherent in the failsafe mode, and I get a flashing check-engine light. Nowadays, if I just turn the key off and then back on again, I can continue driving with no problem. Freeze frame data shows a normal temperature when the misfire codes were thrown, consistent with the fail-safe mode actually cooling the engine before the misfire counter counts enough misfires to throw a code.

The first time this happened to me, several years ago, I had several undiagnosed cooling system issues and didn't know they were causing the engine to overheat from time to time. The check engine light would flash as I crested a slight hill, and if I didn't get off the freeway quickly enough, the engine would shut off completely. If I turned the engine off and waited a few minutes, I could restart the car and get back on the road.

With no overheat code or abnormal temp on the freeze frame and the intermittent nature of the problem, it took forever to get this diagnosed as fail-safe cooling due to a nonfunctional low speed fan switch and cracked cylinder head temperature sensor insulation. My recollection is that the Ford service manual for this model/year doesn't even mention fail-safe cooling mode, and everything I had read online mentioned it only for ZETEC engines and other cars or other model years. But a Ford dealership mechanic figured it out, armed with an independent shop's discovery of how to reproduce the misfires (engine speed about 3300 RPM) and no ECU signal to fuel injectors 1 and 4 when the misfires were happening.

Now, several years later, I'm having the problem again, but at least this time I know where to look, and sure enough my coolant level was low today, so next stop is for a cooling system pressure test to find the leak.

Rebeccah




Just reading my new Haynes, and thought that this was cool.

-------------------------------------------------
The cooling system is fitted with a fail-safe mode, this comes into operation in stages when the temperature of the engine is too high:

STAGE 1
The cylinder head temperature sensor transmits a signal to the engine ECU, which then moves the gauge into the red zone.
If the engine is not switched off and the temperature continues to rise the multi-function warning light will come on.

STAGE 2
The engine ECU will contol the engine by starting to cut out two cylinders and restricting the engine to below 3000rpm. When this occurs the engine waring light will also illuminate. If the temperature should drop back to normal, the ignition will have to be switched off and then on again, to revert back to four cylinders.

STAGE 3
If the engine temperature continues to rise, the engine will be totally disabled before engine damage occurs. The engine warning light will begin to flash, indicating to the driver that the engine will be switched off after 30 seconds.
-------------------------------------------------

I knew about the two cylinder bit, but not about the shutting the engine down in 30 seconds, a pretty good feature if you ask me, could possibly save you a new engine, if you are silly enough to keep it running through the first two stages.
 
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