Mercury wont idle

Dustin Shaw

Asked by May 20, 2015 at 07:00 PM about the 2004 Mercury Grand Marquis LS Ultimate

Question type: Maintenance & Repair

I have a 04 grand marquis 4.6l and it wont idle. It gave me a the codes for bank 1 and 2 is too lean. So I looked around unplugged mass air flow sensor ran better. So I bought a new one and plugged it in and does the same thing. Runs good unplugged but with a rough idle. With it plugged in it stalls on idle. I'm mechanically inclined so what do i test? I dont see nor hear any vaccum leaks. What do I test where do I start?

7 Answers


If you've ruled out a vacuum leak, check the fuel pressure. It may be low due to a clogged fuel filter, failing fuel pump, or failing fuel pressure regulator. Check the upstream oxygen sensors as well. The downstream oxygen sensors are not involved since they do not control air/fuel mixture adjustments. -HTH. -Jim

Best Answer

Interesting! I had the same problem, ran well with MAF disconnected. I new one fixed it, but I had replaced the upstream O2 sensors prior, even though they didn't give codes except lean. I'd replace them, just because. How many miles on the car?


If those things don't work, think about the TPS. (throttle position sensor) They are mechanical, and can wear out.

Dustin Shaw

Yea looked at tps and o2 sensor prices will be replacing praying not ecu. Car only has 120k. Havnt replace any mechanical stuff. Only had since 97k. Only thing ive done to it is put p71 H pipe with 3 inch exits at rear. Ran great with those. I did put little over 500 miles on it that day when problem hit. Trying to find way to test vacuums and im borrowing 10k car tester today should tell me. Ill update this tonight. Car will be adding another 500-600 miles hopfully.... this weekend

Dustin Shaw

Ended up being crack in vacuum line on throttle body


Thanks for letting me know. Those vacuum leaks will "get you"


Yes, thank you for letting us know. Very glad you got it fixed and that it was an inexpensive, relatively easy fix too. -Jim

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