WalshG
New Member
- Joined
- January 8, 2017
- Messages
- 6
- Reaction score
- 3
- City, State
- Nashville, TN
- Year, Model & Trim Level
- 2004, AWD 4.0L Flex Fuel
2004 Mercury Mountaineer AWD 4.0L Flex Fuel. Started to idle crap and stall when warm. It's my daughters car and she got stranded. When I arrived it started fine and I drove it home to work on it. IF you are driving it seems to be okay, when you slow down close to idle it becomes a problem. I have chased my tail for nearly 2 weeks. I have changed EGR, Throttle Body, Injectors, PCV Valve, MAF, Intake Manifold Seals, Fuel Pressure Sensor, Plugs, Coil Pack, Plug Wires, Cam Sensor, Crankshaft Sensor, fuel filter, changed oil and oil filter. I have done a compression tests (while cold) and all cylinders are at 190 psi. There are never any DTC 's.
When it is cold it idles fine with near zero short term fuel trims. As it warms up, takes about 10 minutes in these cold temperatures, the short term fuel trims start to climb progressively up to 50-60% and then it starts missing and stalls. I have a ballenger wideband in the exhaust to check what AFR I really have. It shows close to stoichimetric and then as the STFT start to get worse and worse it eventually looses control and goes very lean. I have all vacuum/air sources to the intake blanked off (PCV, EVAP, CRANKCASE VENTIALLATION, Accessories). I even have a blanking gasket plate to stop EGR exhaust flow. I have sprayed starting fluid and propane all around the intake, no effect. After it goes into this mode it is difficult to get started after that, it always stalls, even if I disconnect the battery and do the PCM reset procedure (postive to negative jumper for a few minutes) before starting. Fuel pressure always looks good so I have not considered changing the fuel pump.
When the fuel trims get bad, before it gets to the stall point you can see form the log I raise the rpm and the fuel trims adjust down somewhat, but never close to zero. As soon as you go back to idle they quickly go back to very high.
One thing in the log that caught my eye is the commanded fuel pump DC value. It is mostly at 18.5% and roughly when the STFT's starts to increase ( I assume to compensate for a lean O2 condition) then the fuel pump is being commanded to progressively smaller values. The seems odd to me. I assume that the fuel pump is being commanded to smaller values becuase the fuel pressure (PSI) is staying at 39psi and therefore it thinks it can slow it down. It's interesting that slowing the fuel pump correlates to the progressivley leaner condition, The confusing part is that the fuel pressure stays high. Is the fuel % DC command purely related to fuel pressure and do I have a bad pressure fuel sensor (which is new)?
Can't find the insert file option on this forum to add the scan log??
Update: I disconnected the fuel pressure sensor and the problem went away. The PCM set's the fuel pump DC command to 17.5% and does not lower the fuel pump speed signal since it does not know what the fuel pressure is (sensor disconnected). As a result no more lean idle and no stalling, STFT's close to zero. Problem is identified. This Fuel pressure sensor is brand new as of 3 weeks ago. There must be something wrong with the signal (too high). I need to figure out why the new sensor is giving me a high reading, or if there is something else wrong with the fuel pressure/delivery. I replaced this sensor 3 weeks ago as there was (over the last 1-2 months) a few instances of fuel rail pressure signal low DTC. So something is still not right, either the new sensor is not accurate or something else is wrong with fuel pumping. Looking back through my emails this was a generic ebay part, I have a feeling it is junk and has cost me days of trouble and chasing my tail, in part due to my learning of this ford/mercury pcm. Thanks to HPtuners I found the correlation. I will replace it with a genuine ford part and post an update.!!! I may have learned a painfull lesson about generic ebay parts.
When it is cold it idles fine with near zero short term fuel trims. As it warms up, takes about 10 minutes in these cold temperatures, the short term fuel trims start to climb progressively up to 50-60% and then it starts missing and stalls. I have a ballenger wideband in the exhaust to check what AFR I really have. It shows close to stoichimetric and then as the STFT start to get worse and worse it eventually looses control and goes very lean. I have all vacuum/air sources to the intake blanked off (PCV, EVAP, CRANKCASE VENTIALLATION, Accessories). I even have a blanking gasket plate to stop EGR exhaust flow. I have sprayed starting fluid and propane all around the intake, no effect. After it goes into this mode it is difficult to get started after that, it always stalls, even if I disconnect the battery and do the PCM reset procedure (postive to negative jumper for a few minutes) before starting. Fuel pressure always looks good so I have not considered changing the fuel pump.
When the fuel trims get bad, before it gets to the stall point you can see form the log I raise the rpm and the fuel trims adjust down somewhat, but never close to zero. As soon as you go back to idle they quickly go back to very high.
One thing in the log that caught my eye is the commanded fuel pump DC value. It is mostly at 18.5% and roughly when the STFT's starts to increase ( I assume to compensate for a lean O2 condition) then the fuel pump is being commanded to progressively smaller values. The seems odd to me. I assume that the fuel pump is being commanded to smaller values becuase the fuel pressure (PSI) is staying at 39psi and therefore it thinks it can slow it down. It's interesting that slowing the fuel pump correlates to the progressivley leaner condition, The confusing part is that the fuel pressure stays high. Is the fuel % DC command purely related to fuel pressure and do I have a bad pressure fuel sensor (which is new)?
Can't find the insert file option on this forum to add the scan log??
Update: I disconnected the fuel pressure sensor and the problem went away. The PCM set's the fuel pump DC command to 17.5% and does not lower the fuel pump speed signal since it does not know what the fuel pressure is (sensor disconnected). As a result no more lean idle and no stalling, STFT's close to zero. Problem is identified. This Fuel pressure sensor is brand new as of 3 weeks ago. There must be something wrong with the signal (too high). I need to figure out why the new sensor is giving me a high reading, or if there is something else wrong with the fuel pressure/delivery. I replaced this sensor 3 weeks ago as there was (over the last 1-2 months) a few instances of fuel rail pressure signal low DTC. So something is still not right, either the new sensor is not accurate or something else is wrong with fuel pumping. Looking back through my emails this was a generic ebay part, I have a feeling it is junk and has cost me days of trouble and chasing my tail, in part due to my learning of this ford/mercury pcm. Thanks to HPtuners I found the correlation. I will replace it with a genuine ford part and post an update.!!! I may have learned a painfull lesson about generic ebay parts.
