Exhaust Temps at Pre & Post Catalytic Converter 1994 80 Series Emissions (1 Viewer)

Aug 12, 2020
A little help, please. I'm trying to test for how tired my cats are. I'm looking to compare my exhaust gas temps to a like vehicle, which is breathing healthy.

1994 model uses 2 side-by-side, separate cats, which vent into a converging Y-pipe at their outlet. Using an IR temperature reader on each of the downpipes at PRE cat, on the 2" pipe, right at the flange, I get about 400F deg. POST cat, after the Y-PIPE, on the 2" pipe, right at the flange, I get about 440F deg.

My understanding is that, if the cats are working in spec, then there is a reaction going on, inside the cat, which should raise the temperature, as it leaves the cat. So, PRE-CAT temps should be AT LEAST 100F deg COOLER than the post-cat readings. Put another way, the back should be MUCH hotter than the front. Obviously, I'm well below that in my application.

I'm asking for this in order to get confirmation. Like everything else on this ---- thing, not much is straightforward as it might be on many other vehicles. There's a lot of corrosion on these cats (these are aftermarket, btw) mounting flanges. So, removal will likely require cutting them out and getting new sections. I'm much less concerned about outright blockage of the cats; the performance of my vehicle is otherwise smooth and even.

Also of note on my vehicle:

- O2 sensors appear newish. Aftermarket. I still plan to test these. But this temp check seems like a good lead.
- Cleaned throttle body butterfly
- New iridium plugs & gapped (old plugs had nice toasty color, but were worn/burned down)
- Recent NGK ignition wires
- New distributor seal, cap and rotor
- Seafoam in tank
- 91 octane of non-ethanal gas
- Fresh oil change
- Codes on OBD1 - I get 71 (EGR). I was getting 24 & 25 (the lean AND rich codes. Disconnecting the ground cleared them, yes. But, these are not flashing after 120 wiles of highway driving. But I wouldn't be so surprised if they may or may not come back after more mileage.

After doing all of the above work, my CO dropped from .71 GPM to .41 GPM.

I'm passing with HC @ 1.05 and NOx @ 2.16

Thanks for your help.
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