Limp Home / Fall Back Mode
Discussion
My chariot seems to use this facility when the mood takes it , lots of nasty sooty gack out of the exhausts etc . But not all the time , so , what causes this to happen and how come it can(or seems to) reset itself ? Also I've noticed my temp is now just under 90 degrees instead of the pre-rebuild 80-85 ish . I reckn the rad's a wee bit old and claggy so will be replacing that but what I want to know is , can the high temp produce rough running/ idle symptoms cos it seems to go a treat(when its not got a moody on) from start but a bit crappo when heated up . Anyone got any ideas ?
Nick.............
Nick.............
You could have an air leak, the ECU is trying to compensate by adding more fuel and the leaner mixture will also cause a rise in temperature, check all of your hoses, even the small bore ones, one could be off or have a split. I found one of mine which had perished and nearly snapped in half. Caused rough running as you describe.
A misfire will also cause the ECU to overfuel and soot the plugs, but it wont make the engine hot, more the precats'. I dont think an over temp as such will not cause the ECU to drop into fall back mode, it will just lean the mixture off to keep the exhast composition correct. As for the reset, it might well drop into fall back mode, to get you home, but next time the engine is cold the fault is not so severe to make it fail straight away. The ECU will store the fault code after the ignition is turned off, so you just need to plug a fault code reader in to get the error, and it can store more than one code, but it puts the worse one in the first priority. As already said air leaks around the plenum base, and hoses can really mess the mixture control up, but these tend to cause erratic tickovers as well.
Mark
Mark
P100 said:
Your car doea not have a limp home mode...
If the performance is reduced and sooty emissions are evident,I recon you need to be replacing the Lambda sensors..
They have a direct effect on the mixture and will limit performance in the way you describe.
Thats not totally correct. There is a specific map (one of the 5) that is reserved for a fall back situation, and basically works by ignoring the failed sensor input and using preset values. On TVR mapped cars the ECU warning light is not enabled, so frequently you wont know its in fall back, as the performance loss is not always that bad (The car simply goes "flat" on you.) On USA spec' ECU's (as in Range Rover)the MIL light is enabled, and a fault code reader fitted as standard for easy diagnosis. If you loose a lambda probe, it simply uses the fixed part of the fuel map without lambda correction, that should not be that far out. (assuming the origional mapping is half reasonable). It can trim the mixture one bank at a time, and Lambda correction will still work on the other bank, so if the plugs on one side are a different colour, then this could be a failed lambda sensor. These can be easily tested with a voltmeter across the black and white wires from the probe, and at tick over with the engine hot, you should see a constantly switching signal of about .2-.8 volts at about 1/2 second intervals. Again a fault code reader makes things so much easier if the fault is intermittent.If the performance is reduced and sooty emissions are evident,I recon you need to be replacing the Lambda sensors..
They have a direct effect on the mixture and will limit performance in the way you describe.
Mark
Edited by blitzracing on Tuesday 25th November 12:42
Edited by blitzracing on Tuesday 25th November 12:46
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