1999 Mercury Grand Marquis LS, 4.6L V8, 220,000 miles

Originally the check engine light came on showing an error code of P0174 – engine too lean bank 2.

The shop that we took it to said that it could be one of three things, because the engine is getting too much air and not enough fuel.

  1. Air conditioner malfunctioning
  2. Fuel filter needs replaced
  3. Car needs a tune-up

All of those items have since been done. The check engine light went off, for about 2 hours (drive time) then came back on.

We brought it back in and they hooked it up to a diagnostic computer and this time said its the MAP sensor. So we replaced the MAP sensor. Less than 45 minutes of driving, the check engine light came back on.

The engine runs extremely rough. Also lately the speedometer on the digital dashboard has been acting strange. The speeds it displays bounce all over the place, and frequently get stuck at 0.

Although these problems arrose at the same time, the speedometer seems to act up whether the check engine light is on or not.

A video of speedometer problem can be found here: https://www.facebook.com/amy.jo.iannone/videos/10210980608297703/

So I have a few questions now that I seemingly keep poring money into my car but the problem does seem to get fixed:

  1. What could be causing the check engine light to chronically continue to go on, and the car to run rough?
  2. What could cause the odometer to not work correctly?
  3. Is it likely these problems are related?
  4. I love the car, and I have poured a lot of money into it, but it does have a lot miles, is it perhaps time to look for a new car (meaning do these problems sound so serious that its just one of those situations where the car needs retired)
  • 2
    the 174 code may be a tune up issue, but i doubt it. if you haven't had a tune up done for a while pull a plug and measure the gap. i'd suspect the upper or lower intake gasket is leaking or that a o2 sensor is stuck lean/exhaust leak before the sensor. i'd be interested in looking at fuel trims and how the o2 sensor responds and go from there.
    – Ben
    Commented Sep 8, 2016 at 1:00
  • 1
    This is why throwing parts at codes without diagnostics will waste a lot of money... and might fix the problem. What did the tune up include? Commented Sep 8, 2016 at 1:30
  • PCV Valve? Easy to get to and cheap to replace. If it's due for a change replace it even if it seems to be good (by mileage you should be on your 3rd one). Commented May 15, 2017 at 3:00
  • I was getting a P0174 on an '04 Grand Marquis...also had a flashing check engine light . Changed the PCV valve and put a bottle of Techron in the tank and it's happy. Not sure which did it (if not both), but it was cheap and easy. Commented Sep 25, 2017 at 4:48

2 Answers 2


TLDR; Find a new shop to go-to as that one isn't doing anything for you besides taking your money. Also, get a service manual for your car if you want to be able to verify that troubleshooting is being done or get involved in a knowledgeable discussion with a mechanic/technician. (i.e. mechanic tells you the MAP sensor is bad, you ask for him to show you how they know, verify they used the procedure in your service manual or presents a logical argument like "it's physically cracked and circuitry is exposed and dirty/burnt/melted")

Unfortunately that P0174 code isn't very helpful. Everything before the oxygen sensor could be an issue as well as the oxygen sensor.

You would check the following components in this order:

  1. O2 Sensor. With a decent OBD2 reader, you or the new shop can test the oxygen sensor, is it reading the correct voltage? If yes, make your way up the system to the next component. If not, try removing it, cleaning it with brake cleaner, reinstalling and retesting it. If it still fails, replace it.
  2. Exhaust system. Check for leaks, cracks, anything to cause a pressure drop. A lean condition can be caused if the oxygen sensor gets an incorrect reading and the engine tries to over-correct by reducing fuel.
  3. Spark plugs and ignition coils. It is less likely to be the coils but they can be individually tested per the procedure in the service manual. If the spark plugs are in terrible condition, replace them. https://www.ngksparkplugs.com/about-ngk/faqs/spark-plug-faqs/how-do-i-read-a-spark-plug
  4. Fuel pressure. Verify correct fuel pressure at the location shown in the service manual. You can find cheap fuel pressure testers online although a good shop would test this anyways.
  5. If the above fails, come back and let us know what you have found so that we can guide you in a better direction.

Some general advice:

Stop going to that shop, they are not diagnosing or troubleshooting your car, they're throwing darts at it.

  1. Air conditioner malfunctioning, The AC system is in no way connected to your exhaust.
  2. Fuel filter needs replaced. Unless you have been using just terrible gas, fuel filters don't really go bad or clog anymore, either way this could have been checked by verifying the fuel pressure which they obviously didn't do.
  3. Car needs a tune-up, why? If the shop can't show you which components have failed, there's not much reason to replace them.
  4. they hooked it up to a diagnostic computer and this time said its the MAP sensor. So we replaced the MAP sensor, this is another dart throw. If your MAP sensor was bad enough to cause a lean condition the car would run terribly all the time. Often any MAP sensor issues can be solved with cleaning anyways.
  5. The check engine light was only off because they reset it and it took your car the 45 min to 2 hours to complete it's drive cycles to ready the sensors.
  6. You should correct the other issues before worrying about the speedometer as it may be supplying correct input but the input is bad because the engine isn't running correctly. After correcting the engine issues, you can check again and see if the speedometer is indeed running into other issues.

Based on mileage I'd say you are due for your 2nd set of plugs if they haven't been done. Possibly coil packs, too.

What was wrong with the AC?

What did the 'tune up' involve. Hearing that term with modern cars make me suspicious - the powertrain controller tunes itself...

Not sure if the speedo issue is related to anything else, could just be the cluster on the way out...if not maybe all the gremlins are some kind of problem in the powertrain controller? Not sure if the speedo is getting the speed sensor signal straight from the sensor or if it's going through the powertrain controller on this platform.

Anybody scope the O2 sensor? How is the fuel pressure? Dirty injectors?

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .