Help needed with Lean 4.6 2V. Logs enclosed

RallyCobra

Member
Apr 2, 2002
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Here is a WOT 2nd gear datalog from my 01 mustang 4.6L 2V with return style fuel system that does the 1/4 mile in 15.9 sec at 92mph. Obviously, I am way down on power. At about 4800rpm the MAF flatlines and bank 1 goes lean all the way to redline. Bank 2 a/f seems fine. Any ideas? Fuel pressure never drops below 38psi. I have an idea, but I want to hear what the forum has to say before I skew the responses. No misfire codes. History of blown head gasket, and both head gaskets were replaced at the dealer.


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What are the results of a compression test?

Just to confirm that the exhaust system is in good repair. No clogged cats right?

How old are the O2 sensors? How much oil does the motor burn? If oil/coolant fouled, the O2 sensors will not give accurate results.

Is the MAF clean? Note, a dirty/oil fouled MAF will over state air flow at idle and under state under load. This will cause the motor to go from rich at idle to lean under load. Another source of MAF oil fouling are oiled style air filters.

How much power are you making? What has been done to the tune to prevent the MAF from maxing out? What size fuel injectors? I assume the tune has be adjusted correctly.

Finally, the Short Term Fuel Trim STFT say that the motor is running rich (.844). The numbers between bank 1/2 are balanced. However, I am not positive if this is because during WOT, the PCM enters "open-loop" mode.
 
Just got back from a 3 day event at NJMP with the BMW club. A lot of fun despite missing a few sessions to two cracked brake rotors... There was a Mach 1 there, and we ran south jersey parts stores out of rotors. Next time I'm bringing four spare front rotors!

I used the SCT to remove 2 degrees of base timing, and changed WOT a/f to 10% rich. That seemed to improve my power. I will do a log of this setup before changing anything else.

This week I will perform a compression test, and will swtich the plugs and coils bank to bank to eliminate them as a problem. After that, I may swap injectors bank to bank. The guys out in AZ at Bondurant think that its an intake manifold leak, and I should update to a newer plastic/aluminum intake.

MAF was clean, sprayed with solvent, and graphs did not change. Only the stock paper filter has been used. . Cats are almost new, high flow from jegs or summit.

My understanding is at WOT, computer is in closed loop mode.

I don't hear an exhaust leak. I don't know how an exhaust leak would cause a reduction in MAF readings.

I don't recall if I checked TPS, but I will. TPS would affect both banks I would think.

No oil burned, and I only run FLAT OUT!

Car has 9000 miles. I don't recall if we changed the O2's when we got the car 3000 miles ago. The O2 sensors seem to be working perfectly, since they I have done multiple WOT pulls, and the O2 sensor logs look the same.

My newest thought is that I'm losing spark on one of the bank 1 cylinders around 4000rpm. This is causing the engine to lose power, and sending extra oxygen to the bank 1 O2 sensor. That would explain a lot of things, and the computer would show 'lean' since there is uncombusted air that hasn't had it's oxygen used.
 
A misfire would also pump a lot of unburned hydrocarbon into the exhaust causing a rich reading.

At WOT, the computer goes open loop. That's why I asked about the TPS because "WOT" is obviously derived from the TPS. I was thinking perhaps the PCM isn't entering -- or staying in -- power-enrichment mode which might happen if the TPS is flakey, noisy or maladjusted.
 
FWIIW, for a N/A application, an intake leak would normally present as Lean at idle and the amount of lean decreasing as power goes up. The LTFT/STFT values at idle would add fuel (positive or >1). As RPM's increased, the LTFT values would tend closer to 1.

If injectors problems are suspected, consider an injector cleaning/flow test service such as injectorrx.com. Much cheaper than buying new. Once tested, the injectors are as good as new.

+1 on the PCM goes into "open loop" at WOT. The O2 sensors are NOT used.

Cleaning the MAF with harsh solvents can ruin it. The sensor is easily damaged. The MAF should be cleaned with a gentle product that does not leave a residue. Recommend using only cleaners designed for the job.

What is the Max AD counts that the stock MAF will support <stock untuned 980>? For this run, the peak is 820. This would indicate there is still room left. However, I do wonder about the RPM's going up but the MAF flat lining.

I honestly don't know how a misfire will present to narrow band O2 sensors. My initial thought is it will present as rich. But I could just as easily talk myself into Lean due to the fact all that air still being present.
 
Max AD counts in an untuned vehicle is 980, this can be extended to 1023 with a tune. It is not pegging the MAF. You cannot talk about rich or lean without wideband data or else you are just guessing. I suggest you get a good wideband and check things out.
 
You cannot talk about rich or lean without wideband data or else you are just guessing. I suggest you get a good wideband and check things out.

You can in gross terms even with a narrow-band O2 sensor. During closed loop the PCM does this very thing, using O2 sensor transitions to measure rich/lean and adjusting the trims to achieve a time-average of ~450mV. It's very safe to say that anything less than about 450mV on a NB O2 sensor is "lean" and anything less than 100mV is "very lean."

The B1 O2 sensor voltage is less than 100mV where the B2 sensor is showing 930mV. Even though you can't decipher the AF ratio from these numbers it's clear that one bank is rich (B2) and the other is very lean (B1).
 
I have worked through the engine in the past couple of months. Ignition and spark were fine. Compression was good in all 8 cylinders too. Purchased a revised intake manifold and changed it out this week. That fixed the problem!

There was a chunk missing out of the gasket between upper intake plenum and intake manifold. The two gaskets from the intake manifold to the cylinder head appeared to be leaking as well. I don't have new logs, but the power is back. At the same time, I changed fuel injectors as well, but I'm pretty sure the problem was due to unmetered air, as expected.

Haven't done logs with the SCT, but I will likely take the car to JDM in November for a tune.