bad millage


Asked by Jun 11, 2015 at 10:46 PM about the 1999 Mercury Grand Marquis 4 Dr LS Sedan

Question type: Maintenance & Repair

How to reset the ECU to factory settings

6 Answers


Undo the battery for a few minutes,

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But if there is no engine light on in car,it is set.

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If you are getting bad gas mileage ,change plugs,wire,s ,fuel filter,and air filter.

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Disconnect one of the battery cables, turn on the headlight switch, wait at least 1 minute, turn off the headlight switch, reconnect the battery cable. This will erase all memory in the system, so any codes that were stored will be no more. After that drive the vehicle as you normally would, it should relearn the vehicle and your driving style in about 20 to 30 minutes driving time. If during that time the check engine light should turn on, then you have a hard fault and it will have to be repaired before the system will operate properly.

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Best Answer

Replaced the intake manifold(recall)Thermostat,wires, coils, plugs,alt, battery, cleaned MAF and throttle body.The car runs and start.Just choke @99 mph.Used to go 110mph.I think my son reprogramed the ECU so I don't get killed,lol,.It botthers me, because my 99 marquis LS used to glide @70-75 mph, without pushing it.Now drops the rpm and have to step on the gas. I think that's the reason for the bad millages.Otherwise have 144,600m and looks, runs like new.Thanks.


If he did a reprogramm with a chip replacement then doing a reset just won't work as the changes are part of the chips main operating program. But if he just did a change with like a bully dog unit then a reset will put it back to factory.

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