czerta - Thanks! That's good info to have on the Davico replacement.

MichaelW - Attached is the info the emissions agency provided on replacement converters. They should have provided this info during your appointment. My engine family is TLR4.658GFEK which is found on a decal located to the left of where the hood latches.

All the catalytic converter sites I've researched only have CA/CO approved TLR4.6H8G1EK which CO would probably accept, but you'd need their guidance on that. After I was quoted $2.5k, I gave up.

On the final test prior to the in-depth inspection my NOx GPM was 3.0275 which is just over the 2.50 limit.

 

@jvarnadoe thanks - yea, I did receive that same report after my in-depth test. Just thought it was strange that Boulder Muffler's system was showing that there aren't any cats available for that engine group and had to check directly with the emissions office. Trying to get a better understanding of this before dropping another thousand bucks on parts...

Are you just driving yours on expired tags or is it sitting on your driveway?

 

At this point im tempted to suggest you go with a CARB universal that meets the requirements. The details on universals can be found https://discoweb.org/oldsite/cats.htm just make sure the P/N is CARB and take it to a quality shop. Weld in should be at most like $300.

 

Old Indy just sits in the garage for now. Luckily I have other means of transportation. I do drive it around the neighborhood about once a week to keep things a movin' and a flowin'.

 

Maybe a dumb question, but can anyone answer this? The previous owner may be willing to ship me the full y-pipe from his other D1, so I'm wondering whether this is likely to get the NOx levels within legal limits or if a pair of brand new carb compliant cats is still going to be necessary.

 

It's hard to say since there are so many variables: 4.6 vs. 4.0, performance cam, unknown condition of the original cats, etc.

I always passed emissions testing in CO with the original cats until the attempted theft and damage to my driver's side cat.

 

Yea, I actually looked up the engine group for his other D1 and it's different for the '97 4.0L, so probably not worth trying that option. the good news though is that the '99 4.0L and 4.6L are of the same engine group so the swapped engine theoretically shouldn't impact which cat it needs.

Re: the performance cam, I've pretty much ruled that out as an issue since all of the results of the 5 gas test indicate that my air-fuel ratio is within spec. Or am I misunderstanding something there?

 

Update: I had new carb-compliant universal cats welded in, which (after a ~40 minute highway drive) brought my NOx values down from 8.9 to 5.5GPM - so clearly the cats weren't doing their job, but there's still an upstream issue causing excess NOx to be created. (Note: the 4.0L D1 and 4.6L RR engine are from the same engine group, so the cat should work for either).

I took a couple of drives and monitored my short- and long-term fuel trims and MAF values. Here's what I found:

• LTFT: sitting at -3.9 for both banks, indicating a rich condition. This also seems higher than what others on the forum have reported.
• STFT: fluctuating in what feels like a normal range, mainly between -5 to +5 ish, with occasional spikes as high at +10. Notably though, they hang out more in the positive range than negative (like 75% of the time they're positive). Does it seem odd that ST and LT FT values would be opposite like that?
• MAF values: I'm seeing 0.06-0.08 at idle, and then values up to 0.4-0.5 at light revving and as high as 0.8 for hard revving (pulling onto highway). Note that the ultraguage apparently scales down by a factor of 1000 for D1s, hence this being decimals. I'm not finding many threads on this subject with people reporting "normal" values, but 0.8 seems to be higher than what others report. Does this seem like a red flag?
• O2 sensor values: Upstream sensors are fluctuating within the normal range, though I noticed the downstream bank 2 sensor will sometimes hang out at 1.020. The upstream O2 sensors and cats are new so I'm guessing this is just a sensor that's close to dying. Am I correct that the downstream O2 sensors don't impact ECU management?

Some ideas, roughly in order of cheapest next steps...

• Seafoam treatment: again, new engine only has 3k miles on it so my first thought wouldn't be that there's buildup getting burned off, but I guess if the NOx values have been this high for those 3k miles then maybe this could move the needle. Thoughts?
• Colder spark plugs: some have suggested this. Current plugs are nearly new, but I can see how colder ones would help reduce NOx since it gets created at 2300 degrees. Y'all see any downsides to this approach?
• MAF replacement: I foolishly cleaned the MAF without first checking the readings, so unsure if that made any difference - but it didn't look dirty to begin with. However it's the original SAGEM MAF with a manufacture date of '98. Does anything about the above indicate that the MAF could be bad? I did the trick where you unplug it while the engine is running and the STFT values tanked to -25, so clearly it's "working", but I wonder whether its responsiveness has degraded. I've seen folks on other forums comment that MAFs are only designed to last for 10 years and 100k - mine has 25 years and 215k...
• Timing advance: Roadside Werx in Denver suggested checking this next, and quoted $1000. The Colorado emissions tech (who seemed super knowledgeable and has no financial incentive here) said he doesn't see any indication of a timing issue, so I'm holding on this for now. Commented that for these NOx levels it'd have to be so far advanced that it's pinging, which it's not. That performance cam is really hard to ignore though as a possible source of the problem...

Thoughts? Anything else? Appreciate answers to any of the above questions

 

Regarding LT and ST fuel trims, your numbers look very good. Not unusual for LT and ST to move in opposite directions. LT finds a mid operating point (based on elevation, barometric pressure, temperature, etc.) then ST moves O2 around the stoichiometric point, while allowing acceleration and deceleration. Monitor both at idle and at constant highway speeds. Maybe try a different elevation.

Maybe try a couple simple adjustments to see what happens. Try some old style fat copper plugs. Try changing the gaps. Try different gas, pure gas vs ethanol.

I understand the theory that the sensor 2 O2s do not impact ECM, but I have my doubts. Try unplugging both downstream sensors. Even try unplugging the upstream sensors and see what the default mapping gives you. This assumes you have easy access to the 5 gas monitor equipment, and time to experiment.

 

Thanks - probably a dumb question, but I should be looking at plugs for a 4.6, not a 4.0 as it was originally constructed, right? Or is the ecu going to freak out if I run the "wrong" plugs? And you're suggesting going with a shorter gap, right?

Ohhh the things you need to learn when your vehicle isn't stock...

Also noticed today that I'm missing the O-ring that sits between the airbox and the MAF sensor. Doubt that's contributing here since it's on the airbox side of the MAF sensor, meaning it shouldn't throw off the air flow measurements, but I guess I'll add that to the list of cheap things to fix...