The ECU doesn't care what kind of plugs are there. Get whatever is on sale for testing. Autolite plugs are fine. Yes, spec'd for your motor, not the motor that isn't there anymore.

My trucks run better with a wider gap (0.045 - 0.055 inch vs 0.035 spec), but this is an experiment.... Maybe even try regular vs premium gas, or an octane booster additive.

I still think a little tweak is going to get you across the finish line. If you are daring, could even try a gallon of mineral spirits or pure ethanol in a full tank to see how the exhaust gases are changed.

I put a little silicon grease on both o-rings at the MAF to seal just a little bit better. I did the mineral spirit trick when I was trying to revive my cats (it makes them burn a bit hotter). It was only later I realized the guts were busted completely out of one set by the PO. The trucks ran fine with it.

 

make sure the sensor plug into the MAF is clean and well seated

 

The MAF is not complicated. There's a hot wire element and a temperature sensitive element close beside it. More current is sent to the heated wire to produce the same heat at the sensor when airflow increases. There really is nothing to wear out (although weak solders are problems on other Disco printed circuit boards). Maybe use a magnifying glass to look at the hot wire and the little bead next to it to see if a chunk of crud has gotten baked on. If so, carefully pry off the chunk of crud.

Racing fuel has methanol in it. Maybe try some racing fuel to see if you can pass the test.

 

So after feeling pretty good about my fuel trims, they now seem to be jumping around a bit.

Long Term trims were sitting at -3.9 for the first few days I monitored them, then when I took it in for another test, they shot up to -6.8. (They don’t pop the hood for the treadmill test so they wouldn’t have screwed anything up - just a coincidence.) On the 20mi drive home they ended at -9.1. NOW, after changing the air filter and installing on O-ring on my MAF (mine had been missing), it’s crept back down to -7ish.

(It would obviously be a miracle if the missing MAF o-ring was the culprit all along but I don’t know about that.)

what do you guys make of long term trims fluctuating that much over the course of only 50-75 miles? I’m wondering if that points to a vacuum leak, since the amount of air coming in would fluctuate kind of erratically.

going to order some parts to smoke test for a vacuum leak just so I can rule that out, but wondering if that might point to anything else.

 

That degree of LTFT change is not extreme. Doesn't raise any red flag, so long as the O2 trends look good. Remember LTFT adjusts for elevation changes (atmospheric pressure), weather, and other variables.

Since there is higher vacuum at idle (when throttle is closed), compared to highway cruising (when throttle is open), if a vacuum leak is present, the more air gets in during idle vs highway. Typical symptom is adding fuel (STFT) at idle, then adding much less or subtracting fuel (STFT) on the highway. If that is not the case, then probably not a vacuum leak.

By the way, I installed a fresh set of spark plugs in my '96 GEMS yesterday, with extra wide gap 0.054", Champion RN12YC. One range hotter than spec (might help reduce some black sooty shadow marks on the old plugs insulators, NGK BPR6EP 0.039"). Truck has not complained yet. Mostly used for short trips around town, with occasional highway drives. It loves the snow we got last week.

 

Thanks, that's a really helpful explanation. Actually, what you described is pretty much what I've seen the past few days of monitoring trims. At cold start, the short term trims are lingering in the very positive numbers (+10 to 25) until I get out of the parking lot and start really moving. At highway speeds they go back into their usual range of +/- 5 or so. They don't shoot back up when I'm at a stoplight, but my thinking was that there could be a small vacuum leak that's mostly sealing up once things heat up and expand. Guess I better do a smoke test!

Good to know it can handle slightly different plugs without a fuss. Coincidentally I spoke with someone on the Discovery USA facebook group who said he was also failing for high NOx, went to the same rover tech in Denver that I went to, and they found his plugs were over spec at 1.3mm gap (so same as the extra wide ones you just put in). So I guess I'll go the opposite way with Champions that are 1 step cooler and see if that moves the needle for me as much as it did for him.

 

It is normal for extra fuel (rich condition) to be added during cold starts. It may take a few minutes before normal trim levels are reached as the engine warms up. So be sure to compare idle and highway trims when the engine is warmed up.

Of course, smoke testing might be worthwhile anyway, at least for peace of mind. Any tiny leak can make a difference in engine performance.