LIMP MODE: the way prom changes timing to componsate for age of a motor? True? - Third Generation F-Body Message Boards

TBI89Formula · 05-24-2003, 07:23 PM

Hi, I have an '89 Firebird Formula with TBI 305. When I first bought my car with 96,000 miles on it it ran 16.2's consistantly. Now at 107,000 miles it runs 16.6's consistantly! My one friend says that after a certain amount of miles (say 100,000) some GM cars's computers enter this mode called "limp mode" which makes the timing weaker to componsate for the motars age. (Theoretically, before it was zero degrees to 37 degrees, but now its 0 to 22). Is this so called limp mode real. If so could a new chip cure this, by tricking my car into thinking its new again? mY friend says that the prom in my car is simmilar to a memory card in a game system as it remembers things like mileage. Is this true? Thanks, Ben

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Chuck! · 05-24-2003, 07:52 PM

No. Tune your car up and you'll probably find the times you were pulling stock.

Slade1 · 05-24-2003, 08:13 PM

Limp mode is real, just not how you have it described. Limp mode is a state in which the ecm can't make heads or tails of the sensor readings meaning something is completely fuxored in your engine. What it does is richen the fuel and disables advance timing altogether in a bid that you may be able to get the car to a garage. In this state it I think it ignores as many sensors as it can, just maybe working with TPS and defaulting to rich fuel tables.

badxs · 05-26-2003, 09:35 AM

yes he is right it only goes into ( limp home mode) when something is to far out of range on the motor in hopes of gettint to a shop

Dewey316 · 05-26-2003, 12:08 PM

yes, limp home mode is just like the describe, it is a very conservite fuel/spark table that is built into the computer so that if you were to have a serious failer of 1 or more critical sensors, the engine will enter limp mode, and will disregard most sensor readings and run completely off pre determaned SAFE tables , so that you can still drive your car home (thus the name 'Limp Home') without causing serious damage to your engine.

get a good tune up , and i bet your car goes 16.flat.

the ECMs does not really learn (it does to a limited extent, but nothing like your friend is describing) your ecm doesn't care how old the engine is, or how many miles it has, it just takes in sensor readings, and tells the injectors how long to fire, and tell the ingition how much timing to add or take away, that is really all that it does.

badxs · 05-26-2003, 03:26 PM

hes right the ecm are quite "stupid" they dont do nearlly as much as people think

Grumpy · 05-27-2003, 08:14 AM

Actually limp home mode is when the processor fails and the ecm lets you get home on fixed timing, and the redundant fuel devise.

But, it's become related to any Check Engine Light nowadays.

At one point the ecm had a break in mode, but that was abandonded many years ago. Way too many customer complaints about doggie performance.

There is no high mileage cap on anything, they all have to pass the EPA test cycles and that includes high mileage testing.

Engines do wear out, and passing oil thru the PCV system will help to prematurely trigger the knock sensor, since oil detonates easier then gas, so performance drops. Just as one example.