Hope it all goes smoothly for you. Fingers crossed!

 

For diagnosis, my mechanic measures pressures in the exhaust system. He diagnosed an old Ford this way, but the concept applies. I don't know the values other than to say upstream high pressure is bad, low pressure is good. I think it should be in the very low psi range or inches water column. Maybe pull the O2 sensors and measure upstream and downstream pressures. He says the honeycomb material can actually melt if engine is running super rich. They can also get contaminated with coolant (ouch) or too many fuel additives.

I found out some time after buying it, the PO for my 94 D1 had cut holes in the cats and removed the innards, then welded the holes shut again. The 94 does not have downstream sensors. I have since swapped in a wye pipe from my 96, that seemed to reduce some annoying resonance in the cabin. Good luck with the diag!

 

Thanks for sharing that experience/advice.

I know as well from experience that the honeycomb can clog. Had it happen to a friend's '76 Grand Prix on a trip from LA to Mammoth for skiing, car had no power by the time we got to Bishop, found a mechanic that did emergency repair: punched out the cat converter with a long rod. (this was back when gas stations still had mechanic/pump jockeys and bays and lifts).

Do you think running it for 6-8 months with a P0102 code (bad MAF) could create the rich condition that could clog the cats?

Regarding measuring pressure/flow, are you saying that the flow in a cat is unidirectional, i.e. free flowing from the exhaust manifold to the tailpipe, but not the other way around?

If not unidirectional, then testing for pressure/flow at the O2 sensor ports seems like a good idea before taking the Y-pipe off entirely, right? If you agree then I'll try that first with the shop vac or leaf blower (if I can rig up a reducer) at the tailpipe to create the pressure/flow.

Also, I picked up another IACV while at the junkyard picking clean that D2 that Best4x4 turned me onto, cleaned it and shook it and the valve moves very easily, which it doesn't do on the other two IACVs I have (on one the valve moves only reluctantly when shaken, on the other valve doesn't move at all, and both IACVs "hum" like they're stuck when then are plugged in with the key in position II), so swapping that out (again).

 

I just watched a couple You Tube videos on the topic. First one checked O2 sensor readings while getting on it up a slight incline. If rich, then getting enough fuel, if lean there is a fueling problem. Then they checked upstream pressure with a 15 psi gauge. Scotty Kilmer checked using an 8 psi gauge. Consensus was pressure should be below about 1.5 psi even revving the engine. The badly plugged cat went over 6 psi. If pressure high at the upstream port, then check downstream to verify condition of the muffler (and resonator I suppose). If downstream is high then the muffler is plugged (e.g. from bits of broken cats).

Cats use a ceramic honeycomb structure to maximize the surface area of exhaust gas exposed to the catalyst (e.g. platinum). Flow can go either direction. Of course when the engine is running flow goes from engine to tailpipe. I hope this is helpful.

 

That's really, really helpful. Great way to test the y-pipe without removing it first.

Can you send the links to the youtube videos so I can see the setup they used for pressure testing, as I am trying to visualize exactly what they used to adapt a pressure gauge to the O2 sensor port (hole).

BTW, I have a parallel thread going on wrt this issue in case you want to see what else I recently did to try and fix the issue:

https://landroverforums.com/forum/di...-109436/page6/

 

AustinLRBill, AutoZone and Amazon (and others) sell exhaust back pressure test kits with threaded adapters with o-rings for checking exhaust back pressure at O2 sensor ports for less the $70. An adapter for checking cylinder pressure might also fit. I suppose we could drill out and tap a bolt to make our own adapter.

There are many videos on line to check exhaust back pressure. Search You Tube for Scotty Kilmer "How to tell if you need a new catalytic converter" for a good video. It really is a very fast and simple test. A bit more complicated because we have two to deal with. A tapered rubber plug is used as an adapter held in by hand with threaded-in adapters shown for reference. An older Kilmer video drills 1/4 inch holes that are sealed with metal plugs after testing. Search "Exhaust Back Pressure Testing - 2 Methods" if you want to see a way to check using a cylinder pressure transducer and an oscilloscope compared to mechanical gauges. I thought it was cool to see the 4 strokes in real time.

I did not find the video that checked fuel trim first, but the essence was flooring the gas pedal up a slight hill only got to 25 mph (lack of power), and the O2 sensor showed rich to indicate enough fuel was getting to the engine. If the O2 sensor leans out, then you're not getting enough fuel and need to fix that issue first.

I have been reading that other thread. Seems you are getting plenty of help from all the right folks. Good luck, and looking forward to your test results.

 

Thanks, JohnZo. Lots of good direction. Will be crazy if a clogged cat (maybe only bank 1 as that's where all the backfire action seems to be emanating from) has been my problem all along.

But not losing sleep (too much) over all the other stuff I've replaced during my "throwing parts at it" phase as I am looking at it as preventative maintenance on a 200K mile D2.

I am busy this week, but will get to it this weekend.

Any idea as to the thread size and pitch on the O2 sensor (e.g., M14 1.0) as I don't have any spares lying around?

My brother has a nice setup with stick and wire welders, a Bridgeport Mill and a South Bend Lathe (and naturally a nice floor drill press), so fabricating an adapter if needed is not a big deal, but I am going to check out the kits first.

 

I looked at NTK 25623 spec said 18.0mm thread diameter, did not show pitch. Best to measure. Guess 1.5mm.

There are so many videos about clogged cats, it makes me think it is not uncommon, but probably more common where size was compromised to save space (think low riding economy cars, maybe even seven Porches

 

6 of 7 of the Teutonic cars are cat-free! (only the 2000 Boxster is in any way modern, and that currently has a balky top that needs attention -- no rest for the wicked it seems). What was I thinking, only 2 of the 7 are cat-free: 911 longhood and 914; the 924S, 944 x2 (one is getting an LS motor someday), 928 and 986 Boxster are all catted -- but since they are all over 25 years old, I never even think about emissions issues as no inspection is required -- if I ever have an issue with any of those cats, I'll just take the sawz-all to them). Heck, only the Boxster even has an ODBII port.

Too bad a clogged cat doesn't show up on the spark plugs. I replaced mine as part of this troubleshooting process (50K miles on them) and they were perfect in terms color, electrode degradation, etc.. showed no signs of anything unusual going on in any cylinder.

BTW, on a totally unrelated matter, if you saw my post on Lucas, Prince of Darkness and general opining on the English engineering philosophy, as an owner of one English vehicle (I desperately want an overdrive spitfire and a TR8 -- what can I can say, I'm a masochist) and several German vehicles, I think that the mating of an English engineer -- clever and inventive but not terribly focused at all on RMQA (reliability, maintainability, quality assurance), for them really just an afterthought, if even that -- and a German engineer -- focused almost totally on RMQA but not terribly innovative, more followers than leaders -- would result in one heck of a great automotive engineer. Of course, that's unlikely unlikely to happen as, like oil and water, they don't mix very well. Now I have both the English and German members of the forum mad at me (though I have never seen a German on here).

 

My grandfather was German.