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2000 mercury sable 3.0 dohc too lean

Been getting trouble codes p0171 & p0174 system too lean bank 1 & 2. Here’s what I’ve replaced, fuel pump, spark plugs, fuel filter, a few ignition coils, fuel injectors (all six) intake plenum gaskets (upper & lower), checked for vacuum leaks (none found) fuel rail pressure sensor, all four oxygen sensors. Ok so here’s what else I’ve noticed, when I unplug fuel rail pressure sensor, fuel rail pressure goes up at idle 5psi. From 30 (sensor plugged in) to 35 (sensor unplugged).when car is run with sensor unplugged, no other codes other than high output for that sensor code comes on. No lean codes. I know I need to replace my catalytic converters, would this be a cause of this lean code? Need to address the codes first as my plates are expired and I can’t pass emissions with a check engine light on.

Do you mean by ‘unplugging the fuel rail pressure sensor’, you remove a vacuum hose that goes from the fuel pressure regulator to the intake manifold? If so, what’s happening is normal. The fuel rail pressure should go down when you remove that hose. Think about the physics. The fuel injector sits between the fuel rail and the intake manifold. The amount of fuel delivered-- other things being equal – is therefore proportional to the difference in pressure between the fuel rail and the intake manifold. At idle the intake manifold has a low pressure (i.e. high vacuum), and that tends to suck too much fuel out of the injectors. The fuel pressure regulator compensates by lowering the fuel rail pressure.

Is 30 psi the correct fuel pressure? Is that the within the idle fuel pressure spec range for your car? It seems a little lower than I’d expect. Is there any sign of gasoline in that vacuum hose? A common failure mode is for the fuel pressure regulator diaphragm to spring a leak. In which case it wouldn’t work like it should. The leak would allow a small amount of gasoline to get into the vacuum line, so that’s worth checking. By what you say, it sounds like you are on the right track.

If all else fails, you might consider to take it to a shop for a proper diagnosis. Mixture problems can be very complicated to debug for the DIY’er who doesn’t have all the electronic diagnostic tools that a shop has. Sometimes it is necessary to have the Ford specific scan tool to do a proper diagnosis on something like this. So it might pay to have a shop with the correct scan tool to do a diagnosis for you, then you can fix it yourself if you like.

I doubt this is a cat problem. But if you know there’s a cat problem, if it is plugged up, that can cause weird symptoms, so be sure to have the exhaust pre-cat back pressure measured. That’s usually a pretty simple thing for a shop to do.

Have you already cleaned the mass airflow sensor?

A contaminated sensor can cause fuel trim problems

If I was you, I’d have a shop hook up their evap/smoke machine to look for leaks. Perhaps you missed something

By replacing all four oxygen sensors, you essentially shot the messenger

The oxygen sensors only report oxygen content. They are not the cause of the lean condition

Don’t worry about the cats at this point

Edit: Deleted video…wrong fuel pressure control.

It’s also possible to have a vacuum leak inside the dashboard; even with cars using the EATC climate control system.

When I unplug the rail pressure sensor I leave the vacuum line on, unplug the electric connection that sends the signal to the fuel pump to let the fuel pump know at what capacity it should run at. There is no gas in the pressure sensor. Pressure at the rail should be anywhere from 35 - 50. And yes, I forgot to mention I did clean the maf.


As for those fuel pressure specs . . .

You said the specs are 35-50 . . . is that key on or at idle?

Since your fuel pressure measured at the rail is a little low, you need to move further back

Measure deadhead pressure directly at the pump. You will need a master fuel pressure test kit to do this, as the pump doesn’t have a test port

I have worked on a few vehicles in which low fuel pressure was the cause of P0171 and P0174

The fuel pressure regulator on this vehicle is a part of the pump and the pump is new, within the last six months. And that’s at idle


Thanks for the update

I’m curious . . . why did you replace the fuel module?

No start?

Failed sender?

What brand?

Okay… update. Three years later. Restarted trying to fix this. Still have codes p0171 and p0174 and still have bad tags. Been parked for better part of the past two years. Installed new MAF. Because of an IAT sensor code I got while trying to diagnose. I’ve had a shop smoke the engine. No leaks found. I’ve been trying to find some sort of intake leak… Still nothing. If I had hair I’d be pulling it out. If the car was to have an intake leak… the vehicle would be tryong to overcompensate and throw tons of fuel at it running lean? Correct? I’m getting the best gas mileage I’ve ever had since I’ve owned the car. The shop that did the smoke test gave a guess that the timing chain may have jumped a tooth… wouldn’t the engine run pretty violent? I guess the most frustrating part is the car runs great other than a small occasional hiccup while accelerating at times. And has shut itself off while idling. But randomly. I’m starting to wonder if a bad ecu or pcm would be the route cause? I’ve tested a few sensors around the engine compartment all have good grounds. If anyone has any thoughts on where to check next. If appreciate any help whatsoever. I’m pretty sick of throwing parts at this car.

Not sure if you have the same engine, we ended up going to the dealer for bank 1 lean, bad o rings on the plenum. I do not know where they are but it was over $500 for the bill in maybe 2011
Don’t know if it was a line, but they said they had special diagnostics as our regular shop could not fix it.

I’ve replaced those gaskets. More than once. Tried doing the running propane around all the possible openings/leaks. Only time I get an increase in rpms is when I’m near the actual bottom of the airbox

But I may throw my hands up and take it to the dealer for a diagnostic.

If the chain had jumped a tooth, I would expect to see a cam/crank correlation code on an OHC engine of this vintage.

Thank goodness. That is reassuring. I’m currently re-tracing steps. Been working a lot lately. So haven’t had any quality time to really look at the situation. Currently I’m going to focus on the connector to the dpfe sensor. (May have melted before changing the clogged catalytic converter that was blowing the sensor off the egr tube). And take another look at the vacuum line going to the top of the egr itself. Also last night just for giggles I checked the ecu but holding the wire loom on. It was kinda loose. So I tightened. So far I’ve driven approx 60 miles. And all I have is a pending p1131 code. Turned the vehicle off several times in those 60 miles. Still averaging almost 21 mpg’s which is great for this car.

I did not say gasket, I said plenum o rings, like I said I do not know where they are. Glad the bank lean has gone away.

I’m a little unclear what the current situation go got there OP. Do you still have the “lean” codes? If so, how that all works, the engine computer is constantly estimating how much gas it should be injecting (for that driving situation) based on several sensors, the most important of which are the MAF, coolant temp, intake air temp, and engine rpm. It injects that amount, then checks for an overly lean or rich mixture using the pre-cat O2 sensor. If the O2 sensor says it is running lean (too much O2 in the exhaust stream) , then it injects a little more gas until the O2 sensor no longer is showing lean, and sets a lean code. It sets the code b/c the first sensors are supposed to more or less agree w/the O2 sensor. If they don’t agree, something is amiss. But the engine could well continue to run fine when this happens, b/c the computer is correcting the fuel air mixture using the O2 sensor.

The most likely reasons why there might be more O2 in the exhaust streams are

  • Air is leaking into the engine, bypassing the maf. Common causes are the brake booster and the pcv valve. Injector o-ring seals, various gaskets, fuel pressure regulator are a less likely but possible cause. Exhaust leaks can cause this too, especially if upstream or near the o2 sensor. EGR problems are another cause.
  • The maf is not accurate. Common causes are that the sensor is faulty or the sensing wires are dirty, bad electrical connection to the sensor, or air leaks anywhere in the path between the engine air filter and the throttle body. most common are air leaks in rubber boots that connect those parts.
  • The fuel injectors are not injecting as much gas as the computer thinks they are. The computer has no way to know how much gas an injector injects. It assumes based on the specs of the parts the car is designed with that the injectors inject a certain amount of gas for a 1 ms pulse. If the injectors are clogged, or the fuel pressue is low, not as much gas as the computer expects is injected. When not enough fuel is injected, excess O2 goes into the exhaust.
  • Faulty intake air and coolant temp sensor. this items rarely fail, but the electrical connectors can corrode or just fall off.
  • Power supply problems to the computer or the sensor. Bad battery, bad alternator, bad grounds, bad voltage regulator on the ecm.

Okay… So… I broke down and took it to Ford to have them diagnose the problem. 4 hours later and 75 dollars diagnostic fee… they have no idea what the problem is. They metered the fuel pressure and checked for intake leaks, and tested all components related to and tested all wires to and from all components surrounding a system too lean condition. First two hours was checking Everything I had already checked and found to be normal. Which was reassuring that I’m not doing anything out if the ordinary as far as what tools I have at my disposal.they do have the more advanced scan tool that gives them live data which. Everything was found to be within spec… they did say that the bank 2 o2 sensor was rapidly up and down where as the bank 1 o2 sensor was basically flatlining

When I did replace the fuel injectors I used a set of remanufactured injectors off e-bay. I suppose those could be the problem maybe? Do you think it would be worth the money to buy motorcraft injectors to install?

Buy only OEM injectors, would be my (inexpert) advice.

That’s a strong clue; sure not within spec.
I’d try swapping injectors between bank 1 and 2 before buying new parts.
Inspect the spark plugs.