Greetings Aston Experts! For the first time since owning this car, it triggered P0174 code earlier this summer. It had been sitting for a couple weeks without battery charger so I chalked it up to low voltage weirdness that we see with these cars. The car was running perfectly so I cleared the code and went on with life. Then many trips later, the same code triggered again. And again, car was running beautifully with no sign of any problem. At this point, it was time to start investigating so off to YouTube I went to research how to troubleshoot these types of lean codes. First, here's the Freeze Frame data pulled from my car:



In both cases the code triggered, and as validated above, I was on the highway cruising with light throttle input. Notice the Long Term fuel trims for both banks are too high! In this case, Bank2 is higher by just slightly, but high enough to trigger Bank2 code (P0174)--presumably when LTFT hits 25%, it will trigger a lean code. Even though it didn't trigger a P0171 (Bank1 lean), clearly the bank1 LTFT is also too high. Based on all my research, combined (total) short and long term fuel trims for each bank should be +/- 5%. In this case, I'm exceeding 25% on bank2 and well over 20% on bank1. At idle, my short and long term fuel trims are hovering around +/- 5%--which appears to be normal. So that rules out a vacuum leak.

I turned my attention to the MAFs and figured they were under-reporting inlet air volume. I decided to simply replace them. Here's a tip for anyone stumbling across this thread and replacing your V12 MAFs: save yourself ~$150 by buying the OEM Ford part instead of the Aston part; they are the EXACT same part!! Here's the Aston part number: 4G43-08-10933 and the same exact Ford part number: 3L3A-12B579-BA. After replacing, I noticed a slight improvement in Long Term fuel trims but still too high (+20%).

So next, I turned my attention to the primary O2 sensors and simply replaced them all since I found a way to do it much cheaper. Again, don't buy the Aston part! WAY cheaper by buying the equivalent Ford part. Picked up 4 brand new Denso O2 sensors (part#2344045) for $111 TOTAL shipped from RockAuto (referenced from 2000 Ford Crown Victoria 4.6L V8 but used in ALL kinds of Ford cars and trucks). In comparison, the exact same Aston part is: 1R12-39-10174 at $160 each!!! That's about 600% markup...good lord. Anyway, swapping all 4 primary sensors was pretty easy...but unfortunately, didn't fix the high LTFTs.

So now, I turn my attention to fuel supply. Hooking up OBD2 data logger shows rock solid fuel pressure at 54 psi. I believe this is within spec. Can anyone confirm? Also, before I dig any further, I have a few questions on the dual ECU setup of the V12 Aston's as it relates to data logging with an OBD2 scanner:

I'm confused to the "Bank" nomenclature with OBD2 in a dual ECU vehicle. In trying to narrow down my issue, I want to make sure I'm looking at the right PIDs. P0174 is Bank2, so is this the left side (Drivers side in USA), rear 3 bank of cylinders or the entire left bank of 6 cylinders? When choosing PIDs to data log, I assume connecting to ECU2 will show me all data on left (drivers side) bank of cylinders? And if this is true, Bank1-Sensor1 and Bank2-Sensor1 are both primary (pre-cat) O2 sensors on the drivers side manifold? If so, this would mean the secondary (post-cat) O2 sensors would be Bank1-Sensor2 and Bank2-Sensor2...Do I have this right? If I connected the OBD2 scanner to ECU1, the exact same nomenclature above applies for Bank1 (passenger) side of the engine, correct? In this case, perhaps a picture is worth a thousand words:


Is this picture accurate? Before I proceed with my troubleshooting, I want to confirm that I'm looking at the right PIDs to see if i can narrow this down. The dual ECU setup is something new for me and is confusing me. How do I know which ECU P0145 was triggered: ECU2 or ECU1? Does P0175 (bank2 lean) indicate cylinders 10, 11, 12 are running lean or cylinders 4, 5, 6?

Anyway, I'll stop here and see what you guys think. Assuming 54psi is within spec for fuel pressure at the rail, injectors might be partially clogged...or something else. Look forward to feedback! Thanks in advance for the support.

Regards,
Dan

 

Thanks Matt--the docs are a big help! Plenty of info here to attack next troubleshooting steps. I'll post back results...

 

Recently change the oil? Have you checked the vacuum supply hose to the left airbox that feeds from the manifold by the throttle?

 

I had this same code in my DBS. Reset it a few times but kept coming back. Ended up being a Vacuum leak. A hose had popped loose. Make sure you go over your entire intake track with a flashlight meticulously!!

 

So thanks to Matt and the manuals, I was able to confirm PCM1 indeed controls the right side (cylinders 1-6) and PCM2 controls the left side (cylinders 7-12). I then did some data logging this morning, measuring fuel trims on both sides of the engine and comparing the two in two situations: IDLE and Under Load (while driving).

Fuel trims at idle for both sides of the engine look normal (pic below). Based on my research, this indicates I DO NOT have a vacuum leak. To confirm, I introduced a vacuum leak on the right side of the engine (PCM1) and immediately watched the short term fuel trims add fuel and quickly move near +20%. Plus, you could hear the engine stumble and then run smooth once I removed the vacuum leak.



I took the car out for a drive to measure fuel trims on both PCMs with the engine under the same load conditions indicated by the freeze frame data (light throttle, cruising at ~2800 RPM). Here you can clearly see the long term fuel trims against PCM1 (cylinders 1-6) are much higher than the normal running PCM2 (cylinders 7-12):



The job of Long Term fuel trims is to keep the short term trims around zero. Looking at the short term trims, it appears the optimal air-fuel ratio is able to be preserved by the closed loop system since the values are hovering around zero. If the short term trims were maxed out in addition to the long term trims, then you start to worry about the engine really running lean. Recall the code was P0174, which is bank2 = cylinders 4-6. Long term trims for Bank1 (cylinders 1-3) is running high too, but not quite as high as Bank2 (and not high enough to trigger P0171 code for Bank1)--perhaps this is a clue...

I then logged fuel pressure data, under load, for both PCMs. I overlaid PCM1 and PCM2 'fuel pressure relative to vacuum' in this chart. (I recorded values in two separate runs then overlaid one line over the other, which is why the lines don't match up with on-throttle/shift points [where the fuel pressure climbs with RPM].) But net/net, I believe this proves fuel pressure is behaving normally, under load, at the fuel rail on both sides of the engine:



So net/net I think I proved so far:

1.) My fuel trim problem is under engine load, not idle.
2.) I don't have a vacuum leak given fuel trims look nearly identical on both sides of the engine at idle.
3.) Possible MAF under reporting on PCM1 side; which given the symptoms above would be a STRONG candidate as the root cause. But I replaced both MAFs and data logged air flow rate (at idle and under load) from both MAFs and they look pretty much the same.
4.) Fuel pressure at the fuel rail, under load, is fine and behaving normally.

Next thing would be to check injectors; the passenger side injectors could be partially clogged. I guess if one injector per bank was lazy or partially clogged, it would be enough to effect trims. I'm open for any other ideas as well! In the meantime, I'm going to continue troubleshooting...it's a bummer but I see this as a good learning opportunity.

-Dan

 

Dan

thanks for posting this detail - it's above my skill levels but really enjoy reading about it and learning this stuff.

what OBD reader do you have? And where did u buy it

Graeme

 

Thanks for the tips Matt! I will focus on the injectors next. I’m hoping with a mechanics stethascope I can hear and quickly isolate any faulty injectors. If not, I will then play the swap game...

The data logger software is called “OBD Auto Doctor” (https://obdautodoctor.com). It’s one of the few (or only) OBD scanner software that supports Mac (in adddition to Windows and Linux). It works as advertised and aside from a couple minor quirks, it does exactly what I need as a shade tree mechanic. Super easy to use as well and supports all kinds of Wifi and Bluetooth OBD2 adapter interfaces. I already own Carista and this software supports it.

I’m on the road this week so I won’t be able to post any updates until next weekend. Stay tuned!

 

UPDATE! I decided to swap injectors 1-3 and 7-9 given those are at the front of the engine and much easier to access. Here's my logic: the high long term fuel trims on Bank1 PCM1 would now move to Bank1 PCM2, if in fact one of the injectors was at fault for causing the lean condition. Result: NO CHANGE to Bank1 PCM1 or Bank1 PCM2 fuel trims! Therefore, I've ruled out fuel supply as being the root cause of P0171/P0174. (Fuel pressure at the fuel rails is fine, which means fuel pumps and fuel filters are fine and now confirmed fuel injectors are fine.)

Given I've ruled out fuel supply as the root cause, this is good progress in the troubleshoot! I then re-focused on my initial hypothesis that I have an "air" issue. Again, given that the issue is seen on all 6 cylinders on PCM1 and the data suggests I don't have a vacuum leak, I re-focused back to my initial fix which was the MAF sensor. Recall, I replaced both MAFs as the first thing I did at the top of this thread. But is it possible that I received a defective MAF sensor? And as luck may have it, did I happen to replace the original MAF sensor on PCM1 with a brand new defective one? Back to the OBDII data log: I closely compared AirFlow Rates at idle and part throttle for both MAF sensors. Here's the smoking gun:


(PCM1 = light blue line, PCM2 = dark blue line)

At idle, the grams per second air flow rate is nearly identical...which corroborates with fuel trims being normal at idle. But look at the difference in air flow rates once I give it some throttle and hold around ~2k RPM. Not only is PCM1 air flow rate erratic (as compared to PCM2 MAF), PCM1 MAF is under reporting incoming air flow by ~25%...which is exactly the same amount the fuel trims are trying to compensate!

I tested my theory by replacing the new MAF sensor on PCM1 with the old/original MAF sensor I pulled from PCM2 (that was working correctly). Once I fired up the car, I immediately noticed PCM1 pulling/reducing fuel trim at part throttle! BINGO!! After a short drive to let PCM1 re-learn the long term fuel trims, here's the result:



Bank1 and Bank2 long term fuel trims on PCM1 are now much closer to mirroring PCM2, which means NO MORE P0171/P0174 codes!!! The lines/data don't match up exactly but that could be due to the fact that I need to drive around more to complete the re-learning process of the long term fuel trims on PCM1. (In fact, you can see the short term fuel trims for Bank2-PCM1 [pink line in chart above] still dipping down to -5% which means PCM1 still pulling/reducing fuel from the long term trim. Compare this chart to the "Fuel Trims at Under Load" I originally posted above and I classify this as a FIX. :-)

So net/net, the moral of the story: you can't always guarantee a new part is 100% functional out of the box--especially a highly sensitive electronic gadget like a MAF sensor.

Best,
Dan

 

Dan

thanks again for posting this

all very interesting and well done on sorting it out

Graeme

 

Thanks for the good job !

so which MAF sensor should we buy?

as this :
https://www.ebay.com/itm/FORD-LINCOL...ion=3#shpCntId

 

That’s the one! Exact same Aston part at 350% savings. 😎 Here’s a pic of OEM one I pulled from my car:

 

I also have P0171 and P0174 on my V8 Vantage. As I changed the Intake system to the GT4 like Version and both banks showed a fault, a vacuum leak was suspected. Even with meticulous inspection and a smoke test, a vacuum leak was not found. Today I changed the MAF sensors but the fault came back. Live data logging at idle showed:

Fuel System Status Bank 1 = 2
Fuel System Status Bank 2 = 0
Intake Manifold Pressure 34 Psi
Short term fuel trim bank 1 = -4 to +6
Long term fuel trim bank 1 = 3
Short term fuel trim bank 2 = -12 to -2
Long term fuel trim bank 2 = 13
Mass air flow 5.83 g/s
Throttle position 12,941176 %
Location of o2 Sensor 51

any ideas on what to do next?

 

Check a similar V8V post that one of the forum members just submitted: https://www.6speedonline.com/forums/...74-15-v8v.html . Are the two codes being triggered while you drive at highway speed? Long story short, your MAF is probably picking up a turbulent air stream, causing it to read incorrectly and causing your long term fuel trims to go wonky.