So I'm tearing out what's left of my hair here. Car's an '01 Mercury Grand Marquis with the old reliable 4.6L Modular V8 - not stock by any means, but I'll not get into that.
This fiasco started a couple of weeks ago, when I started noticing unusual readings on my wideband AFR gauge all of a sudden - well below 14.7, indicating the motor is running rich. My gas mileage certainly took a hit, barely averaging 200 miles to the tank. I'm not noticing any performance issues, but the AFR has been reading rich at idle - anywhere between 9.8 to 11.2 - and it starts stabilizing at higher engine RPMS, namely 2,200+ rpm, although it takes a while before it starts reading 14.5 - 15.0. Light acceleration in gear (no downshifting) will trigger 10.0 - 12.5. Heavy acceleration reads between 10.0 - 11.9.
That being said, STFT on bank 2 is horrendous. I've seen figures anywhere between +11 to +25 and even wilder readings like +32 to +41. Never have I seen fuel trim readings like that before, and what's even weirder is a CEL wasn't triggered - until this morning when I got a P0174. LTFT wasn't affected when I last checked, but given a P0174 is now stored, I'm sure it has now.
There was a minor vacuum leak, it was the vacuum hose that goes from the back of the throttle body plenum to the EGR vacuum solenoid on the passenger side firewall. That did not fix the problem however, and it seems to be getting worse. I went over all vacuum lines with a fine comb, including the HVAC connections and automatic parking brake release - no other vacuum leaks. I've checked for exhaust leaks - before the upstream sensors and post upstream sensors - there is none. There are no misfires either, and no misfire codes. At this point, I'm down to 8.7 MPG. Running Euro V isn't cheap either, 17 gallons at $3.58/gal every 2 days gets expensive very quickly.
I'm not on my computer right now to post a screenshot, but my oxygen sensor graph looks like a disaster. Bank 2 is consistently near the 0 mV mark, whilst Bank 1 constantly fluctuates between 100 mV - 900 mV, albeit on the lazy size. Raising the idle to 1,500 rpm, Bank 2 would go up to 300 mV (won't go any higher) and back to 0 back and forth. The Denso oxygen sensors have 8,415 miles on them, I can't wrap my head around the possibility of them failing, but it's certainly possible.
I have an 80 mm OE Ford/Hitachi MAF sensor, reading 1.0 V at idle. It goes up to 1.5+V as idle speed is raised. If I'm not mistaken, the reading should be between 0.6 V - 0.9 V - perhaps some knowledgeable folks will chime in here. Cleaning it with CRC MAF Cleaner had no impact on the reading. Some people have had success spraying it with brake cleaner followed by MAF cleaner, although I'm hesitant to do that. The MAF is brand new (not rebuilt) as well. This was part of an upgrade to a Mercury Marauder intake. That said, despite regular air filter changes, the dust in this part of the world is so fine, it still goes past the filter. The zip tube, going from the air filter housing to the throttle body, has a tint of dust inside.
The DPFE sensor is new, and also has less than 9,000 miles on it. All sensors were replaced when the motor, transmission and rear axle were rebuilt last year. I was getting a P0422. It was resolved after the shredder valve tube, gas cap and vapor canister vent solenoid were replaced. EGR passages are not blocked, and the EGR valve functions as it should. CHT and ECT both function as they should. Fuel injectors are also new and were flow matched by Trav, no problems there either. Fuel pressure regulator also tested good, no problems there. If this was a fuel pump issue due to low pressure, I'm pretty sure it would affect both banks and not just one.
The only "repair" I've done recently was replacing a faulty brake booster check valve. It takes a bit of effort to push it into place, so I did use WD-40 to help. Not sure if that would negatively affect oxygen sensors.
I don't like throwing parts to fix a problem, but I am considering replacing the oxygen sensors to see if anything changes. Any input or ideas?
This fiasco started a couple of weeks ago, when I started noticing unusual readings on my wideband AFR gauge all of a sudden - well below 14.7, indicating the motor is running rich. My gas mileage certainly took a hit, barely averaging 200 miles to the tank. I'm not noticing any performance issues, but the AFR has been reading rich at idle - anywhere between 9.8 to 11.2 - and it starts stabilizing at higher engine RPMS, namely 2,200+ rpm, although it takes a while before it starts reading 14.5 - 15.0. Light acceleration in gear (no downshifting) will trigger 10.0 - 12.5. Heavy acceleration reads between 10.0 - 11.9.
That being said, STFT on bank 2 is horrendous. I've seen figures anywhere between +11 to +25 and even wilder readings like +32 to +41. Never have I seen fuel trim readings like that before, and what's even weirder is a CEL wasn't triggered - until this morning when I got a P0174. LTFT wasn't affected when I last checked, but given a P0174 is now stored, I'm sure it has now.
There was a minor vacuum leak, it was the vacuum hose that goes from the back of the throttle body plenum to the EGR vacuum solenoid on the passenger side firewall. That did not fix the problem however, and it seems to be getting worse. I went over all vacuum lines with a fine comb, including the HVAC connections and automatic parking brake release - no other vacuum leaks. I've checked for exhaust leaks - before the upstream sensors and post upstream sensors - there is none. There are no misfires either, and no misfire codes. At this point, I'm down to 8.7 MPG. Running Euro V isn't cheap either, 17 gallons at $3.58/gal every 2 days gets expensive very quickly.
I'm not on my computer right now to post a screenshot, but my oxygen sensor graph looks like a disaster. Bank 2 is consistently near the 0 mV mark, whilst Bank 1 constantly fluctuates between 100 mV - 900 mV, albeit on the lazy size. Raising the idle to 1,500 rpm, Bank 2 would go up to 300 mV (won't go any higher) and back to 0 back and forth. The Denso oxygen sensors have 8,415 miles on them, I can't wrap my head around the possibility of them failing, but it's certainly possible.
I have an 80 mm OE Ford/Hitachi MAF sensor, reading 1.0 V at idle. It goes up to 1.5+V as idle speed is raised. If I'm not mistaken, the reading should be between 0.6 V - 0.9 V - perhaps some knowledgeable folks will chime in here. Cleaning it with CRC MAF Cleaner had no impact on the reading. Some people have had success spraying it with brake cleaner followed by MAF cleaner, although I'm hesitant to do that. The MAF is brand new (not rebuilt) as well. This was part of an upgrade to a Mercury Marauder intake. That said, despite regular air filter changes, the dust in this part of the world is so fine, it still goes past the filter. The zip tube, going from the air filter housing to the throttle body, has a tint of dust inside.
The DPFE sensor is new, and also has less than 9,000 miles on it. All sensors were replaced when the motor, transmission and rear axle were rebuilt last year. I was getting a P0422. It was resolved after the shredder valve tube, gas cap and vapor canister vent solenoid were replaced. EGR passages are not blocked, and the EGR valve functions as it should. CHT and ECT both function as they should. Fuel injectors are also new and were flow matched by Trav, no problems there either. Fuel pressure regulator also tested good, no problems there. If this was a fuel pump issue due to low pressure, I'm pretty sure it would affect both banks and not just one.
The only "repair" I've done recently was replacing a faulty brake booster check valve. It takes a bit of effort to push it into place, so I did use WD-40 to help. Not sure if that would negatively affect oxygen sensors.
I don't like throwing parts to fix a problem, but I am considering replacing the oxygen sensors to see if anything changes. Any input or ideas?