Originally Posted by brages
So... I'm starting to think that the old MAF is closer to reality than the new one is.
I needed a third car for today, so I decided to get the truck put back together and out of the garage. So first, I did a bit of tightening on the exhaust flange ahead of the O2 sensor. No change.
Then, I took off the upper intake manifold and replaced the intake side spark plugs (dual spark). Wow, they were worn! While I was there, I popped out the fuel injectors and took a peek at them and replaced the o-rings on them. Everything looked good on the injectors. Put it all back together with a new gasket. Cleaned the IACV while everything was off.
Then, I decided to replace the exhaust side plugs too, so I did that. They were just as bad. All Motorcraft, were they original at 265k miles? Who knows?
Put it all back together with the old MAF and when I started it up, I had +20 STFT and -20 LTFT and with time, they converged close to zero with STFT zigzagging back and forth. Looked pretty good to my inexperienced eye.
My old MAF shows about 3.2 g/sec at a 800 rpm idle. Not sure if that's high or what, but I took the truck to work (20+ miles) and it ran OK with no check engine lights. On the drive in, LTFT is running around -10. At idle, it tends to be closer to -5 and driving around, it's a bit up and down. Idle is OK initially at 1000 rpm, but after idling a couple of seconds, it settles down into the 750-800 range and you can feel quite a bit of vibration.
... so here's a little more back story...
I've had this thing about a year. The truck always had some vibrations at idle. A few months ago, the CEL came on with a P0172 (rich) code. I saw what I assumed were higher than correct air flow at idle and replaced the MAF at that time. The CEL went away for a while, then it started running rough and gave a P0303 (cylinder 3 misfire) code. I found that STFT was getting really high (+32) and causing the car to go into open loop, then in open loop it was misfiring. Revving the engine brought down the STFT and put it back into closed loop. I assumed the new MAF was the good one and started hunting for a vacuum leak...
With the new MAF, at idle, you can watch the O2 voltage and it just stays low (0.2V or whatever). It only spikes up if you give it gas.
With the old MAF, O2 voltage is swinging back and forth regularly. It takes about 3 seconds for a whole cycle. (seems slow to me)
I think I'm going to drive it as-is on the old MAF for a while and see if the P0172 pops up again...
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I had +20 STFT and -20 LTFT and with time, they converged close to zero with STFT zigzagging back and forth. Looked pretty good to my inexperienced eye
. .............. agreed
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On the drive in, LTFT is running around -10.
Dont think you can comment on that so soon at all - there are more than one LTFT figure as a function of throttle - you need to drive it quite a lot of hot and cold cycles for it to stabilize - I would say 10 is not necessarily the end of the world or even bad for the maintenance issues you described with plugs etc - its eventually an absolute function of the mechanical condition of the engine and historical maintenance schedules/quality
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O2 voltage is swinging back and forth regularly. It takes about 3 seconds for a whole cycle. (seems slow to me)
IMO its normal - engine speed increases sampling rate and also compensation rate..........
Chasing fuel trim issues with funky plugs is a waste of time .....................gonna guess the code is history by what you describe now - its obvious the ECU is in control of the animal if you have the animal well defined