Ford 4.6L - P0174

[Falcon_LS]

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?

 

[Colt45ws]

You can unplug the MAF; the PCM will resort to the Failed MAF tables for load information. If the MAF is the issue, it will run better. Not great, but closer to what it should be.
The fact its adding so much fuel and your wideband is showing it running rich, but it STILL thinks its lean does kinda point to an O2 sensor problem. Since its limited to one bank, that should eliminate the MAF as an issue.
Oh, you might check voltage right at the O2 sensor and/or ohm out the harness from the sensor to the PCM to ensure the signal is actually getting back. On a 16 year old car, anything is possible including a faulty connection or a broken wire.

 

[Falcon_LS]

Colt, I did unplug the MAF with the engine running. It got worse and ran like poop.

The engine and transmission harnesses are both brand new - with Ford's practice of discontinuing parts every 10 years, I was lucky to find a brand new one made in 2011 right from my local dealer. I made it my business to replace everything that could give me trouble down the road, when the car was merely a rolling chassis for three years. The heat here plays havoc with plastics and wiring, so it was actually a fast selling item. Still, I will double check all connections to make sure nothing is off. I got rid of the SGA2 PCM and switched to a BXT4, because the old PCM wasn't configured for oxygen sensors (or emissions for that matter).

 

[Colt45ws]

If you unplug it while its running, it will get very confused and run worse. I always shut off the car and unplug. I did that when I drove with no spark plug and it ran 100% better than with the MAF plugged in.

Oh very nice! I didnt know you had gone though such an extensive rebuild. Im thinking of doing something similar and building my own panther with the car I recently bought.

 

[yvon_la]

Sound like you got electrical induction issue .. Start from fram verify that all ground wore don't touch anything else or are close .(double the wire size in distance .then from the alternator follow the big wire .and make sure they aren't closer then double the wire size in distance .lastly same so startor wire .once you are positive things are properly airgapped. You ll likely need to unplug batter wire for 5 minute then replug and test thing. Make sure no small wire aren't closer then double the biggest wire size (likely the startor wire) here is whats likely going on .you alternator likely send and receive electricity via frame .if you got a wire close to the frame and your 200 amp alternator send electricity most of the time the small wire gizmo are mode for less then 5 volt less then 5 amp so yes 200 amp would trigger something .eldctricity might be fast but but its also very lazy . Like why should I pass by the moon .there is a path right here .you can't see the path but electricity can. Anyway you wouldn't believe how many time induction problem was my issue on big rig . Want to keep it simple ? Go to hardware store get hot water foam protector .nust slide a small length when things touch problem solved. Yes sadly it mean you ll have to go to dealer and erase fault code active passive and historic passive (or whatever for call those tree officially.after the various wire are air gapped

 

[The_Eric]

Unplug the O2- the computer should default to 450mv. If the trims come down and the engine stabilizes then you know you've hit pay dirt.

It really does sound like a stuck O2.

 

[Linctex]

It sounds like "unmetered fuel" is getting in somewhere.
I have seen this be caused by 2 things:
1) injectors that are not seating "closed" when there is no elec signal triggering them.
2) fuel leaking past a ruptured diaphragm in the fuel pressure regulator and entering the vacuum line

There is also this, but it is VERY rare:
3) a shorted trace on the ECU circuit board electrically opens injectors at time when they aren't supposed to

 

[MichiganMadMan]

Originally Posted By: Linctex
It sounds like "unmetered fuel" is getting in somewhere.
fuel leaking past a ruptured diaphragm in the fuel pressure regulator and entering the vacuum line


^This sounds like it to me as well. When the diaphragm gives up, you get a leak that is made worse by higher vacuum in the engine and lower engine speed, which matches the symptoms you describe. It is also getting "old enough" where the elastomer bits are giving up from the heat cycles, age, and ozone damage. It can also drown one side of the engine in fuel, while leaving the other more or less untouched.

I'd put a clear section of hose on this and watch it at idle for gasoline running by. I'd also have a fire extinguisher right next to me in case it does go wrong.

 

[Colt45ws]

I would agree but his O2 sensor readings from that bank show it still thinks the car is running very lean, even though it has added +40 to the fuel trim and his wideband is showing very rich..
I think I would try unplugging the O2 as well.

 

[punisher]

Remember you have a problem that is effecting one side much much more than the other. Common stuff like MAF problems, vac leaks, internal fuel leaks, hit both banks.

If you have the time, swap sensors left to right and see if the problem follows the O2 sensor. That being said.....MAF sensors are tricky sometimes.. On that model when the MAF is beginning to be a problem child, idle trims go rich, part throttle goes lean, WOT don't matter. The thing is it would still effect both banks equally. Many times you have one O2 weaker/slower than the other. When you start to have lean problems from a MAF, or low fuel pressure that side codes first. Good luck with it.

EDIT: Ck both bank's O2s. Ck trims, cross counts etc and get an actual comparison between both banks.

 

[Trav]

Suspect a cracked plastic intake as its only effecting one bank with no O2 or MAF codes, propane torch with a hose is your friend for this one, run the engine and go over the suspect area, when the fuel trims change that's the spot.

 

[MichiganMadMan]

So Falcon_LS, any update on what was actually wrong? Looks like quite a few ideas were offered up, I'd be interested in seeing who got it right.

 

[Falcon_LS]

Intake manifold runner on the driver's side had a crack in it, probably due to torque stress. The manifold was removed and reinstalled twice - once when the original all-plastic manifold was replaced with the updated aluminum/plastic composite and the second time when the engine was pulled for upgrades. Since the manifold only had 11,248 km (7,030 mi) at the time of upgrades, I thought buying a new one would be a waste and reused it. Extreme heat - both generated from the engine and high ambient temperatures - took its toll sooner than later. It probably would have been fine, if it wasn't for the second removal and installation.

It did take a good while to find it - no amount of carburetor cleaner or propane helped. In the end, I had to block off the air filter housing and use the EVAP smoke machine hooked to the vacuum lines.

I was looking into using the all-aluminum Edelbrock Victor Jr. intake manifold, but the fuel rails need to be replaced and new AN fittings are required. Not that I have a problem with it, but the total came up to $780 with no gains over the OE aluminum/plastic composite. The other option I considered was Professional Products' Typhoon, which is a three piece design. Reading up on several reviews, however, it turns out the bolts do not align that well when putting the three pieces together and that is not cheap either - $900 for a hand polished manifold or $550 for a satin finish. The OE/Ford Racing manifold was the last standing option and I went with that.

 

[MichiganMadMan]

Thanks for posting that. It sounds like it was quite a struggle to find it.