Bumping an old thread, forgive me.
My 94 4.0 HO is currently failing the AZ NoX test (required every 2 years). 2 years ago, it failed the NoX, at about 228K miles on the clock. First time it had failed in the 10 years we've had it. At the time, I suspected old injectors not flowing well, and since I didn't trust them, I decided to replace them all. It took care of the failure...that time.
Sometime back near that time, I had noticed the temp gauge showing uncharacteristically high after 20-30 minutes of driving, not coming down with movement. One day, I caught a lean-limit code on the freeway, and ended up putting a MAP sensor in it, which made it seem to idle a bit better too. Since all the temp sensors were old as dirt, I replaced them all. But never got rid of the temperature creep. Had the 3-row Modine rodded out (radiator guy said it was clean, but had a crack at a hose), did not change anything. O2 sensor is fairly new, at 15K miles today.
This year, with 240K on the clock, I've really struggled with it.
First failure was 3.01 (just over!), and to try and get it past, I dropped the thermostat to 180, and checked it out for vacuum leaks, with a small one found at the MAP sensor fitting. Idle vacuum is right around 16-17" creeping to 18 when it warms up, which seems right.
No luck, went to 4.00 this time (!). Notice that when he's done with a pass on the tester, it's reading hot (not a big surprise, since it's a 4-5 minute road-speed test with just a fan in front). So started looking at everything with a fine tooth comb. All sensors buzz out to the right values cold or hot. Not sure I hear the fan clutch kicking in when I think it should, so I put a fresh one in it. Also found that the ground stud near the rear right side of the block seems a bit loose, so I snug it. Road test seems nice and smooth, and I don't notice any temp creep for a couple of days.
Retest. Fails again at 3.30. Now I'm digging deep...
One thing I note is a bit of a ratty idle that comes and goes. Plugs/wires/cap/rotor are all fairly new (5K miles), plugs are clean with good color. Suspecting that a misfire is likely to throw off the ECU in closed-loop, I try a new coil. That does seem to clean up the idle some, but that temperature creep still shows up here and there.
So, I picked up an IR thermometer to look at the radiator and catalyst. The catalyst shows about a 100F rise, which is what I think it should do. Radiator shows about 180 at the inlet, with about a 10-15 degree drop. That seems right with the 180 thermostat. But the dash gauge (which is out of calibration) shows temps at 190-200 or so I think. Since the sender is right next to the #6 cylinder exhaust port, I still suspect a lean condition due to the heat.
Today, had a little time to look at the O2 sensor with an o-scope, right at the ECU input. Never tried this before. From what I've read, an early MPI setup like this should switch rich/lean/rich at about 5/7 Hz or so (150/200mS). But that's not what I see at all; what I really see is 1 to 1.2 seconds at idle (!), picking up to about 2-300mS when kicked up to 2500RPM or so.
When I first kick it on, it will float around 400mV, then start slowly switching from 200-700mV. After running it awhile, I'll see it go 100-900mV (which should be full range). It seems to spend a bit more time rich than lean, but the lean time could be long enough to get the combustion temps up. Or so I think. I can even "hear" the change in the exhaust to match the switching.
Not having anything to compare it to, this seems like it's working, but the slow rate may be a real problem. Anyone have any data I can compare to?
(sorry that got so long winded...)