2003 S-Type 3.0 V6 Base model with 120,000 miles. Slightly detailed post. Repair plans and advice at the bottom!

==ISSUE==
Have worked my way through a dozen minor issues on the vehicle, this is the last worrisome one. Can't find a comparable issue on the forums, but have some ideas for next steps in my repairs.

Issue: After car sits for more than 6-8 hours, after less than 5 minutes of driving, on either city streets or at highway speed, in no particular gear, when accelerating under normal vehicle load, the engine seems to hesitate, rpms waver ~250rpms, for 5 to 15 seconds. Then? Everything is normal and purring along. No error codes, no strange smells, no harsh shifting, clunking, rattling, whistling, shaking, whooping. Zilch.

This varies based on my starting behavior.

Variant 1: Wait 6-8 hours, cold start, at idle STFT varies between -8 to +4, LTFT is between -2 and +2 in closed loop. O2 sensor data on my Torque Android App with a BAFX OBDII adapter is jumping around .2 to .8.

Shift vehicle into "D"rive and usually around 3 minutes later:

*engine feels like it is losing power
*vehicle does not feel like it is downshifting or switching between gears
*no codes
*STFT jumps negative, both banks, but STFT1 2-4 points more than STFT2, varies now between -8 and -14. LTFT about -4 to -8. O2 sensors still varying.

Then?

10 seconds later, LTFT and STFT on both banks within 10% of 0, and O2 sensors reporting a steady 0.6. Occasional STFT "hiccups" for up to 2 minutes, no more than -10 to -14. Then everything evens out for the rest of the day.

Variant 2: Wait 6-8 hours, cold start. At idle STFT varies between -8 to +4, LTFT is between -2 and +2 in closed loop. 02 sensors jumping between .2 and .8. Rev engine to 2500rpms and hold for 1 minute. At 2500rpms STFT varies between -4 and +2, LTFT is still between -2 and +2, but usually closer to 0. Variation on both banks between STFT and LTFT is not more than 10 percentage points. O2 sensors are still jumping around .2 to .8. Ease off acceleration back to 750rpms. Rev engine once more to 2500, hold a minute, ease off.

Shift vehicle into "D"rive and usually around 3 minutes later:

*same symptoms as Variant 1, but noticeably less hesitation
*STFT jumps negative, both banks, but STFT1 2-4 points more than STFT2, varies now between 3 and -10. LTFT about 2 to -8. O2 sensors still varying.

Then?

5 seconds later, EVERYTHING evens out. STFT and LTFT are within 10% of 0, LTFT between -1 and 2. STFT on both banks less than 10, usually between -4 and 4. 02 sensors at .6. No subsequent STFT hiccups in the next two minutes, and random checks on the highway for the next half hour show nothing abnormal.

==REPAIRS SO FAR==
Of many recent repairs, some related to the engine have been either I or a couple of local shops have repaired/replaced: PCV hose, #5 ignition coil, spark plugs, intake manifold gaskets, O-rings, transmission pan/gasket/fluid.

Related to this issue I have cleaned the MAF sensor with MAF cleaner, cleaned the electrical harnesses to the MAF sensor and the TPS using electronic contact cleaner, and I replaced the fuel filter (was pretty disgusting, brown goop evidence of watery fuel). Since replacing the fuel filter, I have run three full tanks of 92/93 octane gas with Techron fuel system cleaner. The only noticeable difference was made after cleaning the MAF and TPS. I let my wife drive the car after that and she couldn't notice the engine hesitation -- I did, but she's right that it is much less noticeable.

==ADVICE ON FUTURE REPAIRS?==

--I am going to check on the EGR valve and clean it if necessary.

--I plan to replace the remaining 5 older ignition coils after the semester ends this month.

--Could this be a clogged injector? A cracked leaking fuel injector?

--Those oxygen sensors. Isn't their life expectancy 80k miles? Due for changing?

--MAF sensor, could it be malfunctioning without throwing codes?

--Torque converter in its early death throes? Since I can leave the car at idle for 10 minutes and still not avoid this issue until I'm accelerating...

 

Yes, maybe the TC. Does it go open loop during the hiccup, though?