jccog

My 2000 Grand Marquis has a rough idle. I am getting about 19-20 mpg highway, which is below average for this car. The car has 90k miles on it. I just replaced all the coils with no improvement. I then unplugged each coil one by one as it was running, and the idle dropped with each coil, so I know it is firing on all 8 cyls. I have no check engine light nor do I have any codes stored in the ECU. The rough idle is especially noticable with the car in gear. I have also tried new plugs, fuel filter, fuel injector cleaner in the tank. Does anyone have any ideas???

Thanks

amo4387622

I would check to see what you voltage is at the TPS sensor. Also I would think about cleaning you MAF sensor. And check and see if your IAC is working proply.

88grandmarq

Agreed, clean or replace the MAF sensor. Ford has recently had a butt load of TSB's regarding dirty/faulty MAF sensors. the most common symptom is a rough or surging idle.

there is a new product on the market at the parts stores that is specifically for cleaning MAF sensors. DO NOT use carb cleaner as it leaves an oily film. Electrical contact cleaner is the next best alternative if you can't find specific MAF cleaner.

make sure you disconnect the negative battery cable to clear the PCM memory after you clean the sensor.

jccog

I tried cleaning the MAF with napa electronic parts cleaner with no change. How do I check the IAC as someone suggested? I haven't checked the TPS voltage yet either? Can anyone tell me which wires to probe on it to check it? What about injectors? A buddy told me that someone else ended up having a dirty injector with a similar problem. Still getting 19 mpg after cleaning the MAF.

Thanks

Johnny Mullet

PM me your VIN# or post it here. I will check for service bulletins.

Johnny Mullet

The only thing I found was an update for the PCM which can be reprogrammed at your Dealer for a charge.

You do have an open recall...................

05N04 - Some of the all-composite intake manifolds used on 4.6L SOHC engines may develop fatigue cracks at the coolant crossover duct which could result in external coolant leakage. Drivers may be alerted of this potential leakage by the coolant level and/or coolant temperature warning systems. The recall will only be performed if a coolant leak is detected at the intake manifold crossover.