In trying to diagnose an acceleration issue in my 94 Loyale, I've noticed a temperature drop across the 2nd cat.  The 1st cat temp climbed 100-200 degrees, but the 2nd cat temp dropped about 20 degrees. 

I've also compared tailpipe exhaust temp against my known working 93 Loyale, and its much cooler.

What is the temp difference between the inlet and outlets of the cats? Are you sure the 2nd cat is not just a resonator? Do you have access to a vacuum gauge? Thats the best way to test for plugged exhaust.

I'm 99% sure its a cat.  Its not round and has the same type of heat shield as the front one. Cat 1 (front) was about 100 deg hotter at the outlet, cat 2 was 20 deg cooler at the outlet. 

I'll try a vacuum test tomorrow.  Any vacuum line on the intake will work? ...or is there a specific one you would use?

Edited September 3, 201510 yr by jetpilotdc10

Loyales have 2 cats.   Yes, the temp should be higher after each one.  Your second one may or may not be partially blocked, but it is not working.

I really hope you're right DaveT!  Do you think it may be the cause of my sluggish acceleration?  I don't know the history of the car, but I do know it had been parked and inoperative for the last 2 years.

The cat /s not working has no effect on the engine - except in the case of the core getting blocked or breaking loose and blocking the flow of exhaust. I had one break loose, suddenly had very low power. Idle or very light load seemed normal.

Simple test. Unbolt the exhaust from the engine, so there is a half inch or so gap . Make sure something will hold it there. Rig some wire or whatever. Take a short drive. It will be loud. If you have power, the exhaust os blocked.

Simple test. Unbolt the exhaust from the engine, so there is a half inch or so gap . Make sure something will hold it there. Rig some wire or whatever. Take a short drive. It will be loud. If you have power, the exhaust os blocked.

Removing the offending cat/exhaust would work. But you run the risk of stripping studs and needing to replace gaskets etc. its simple enough and may work flawlessly, or it may not.

If its plugged enough to cause severe driveability issues then you will see the behavior with a vacuum gauge. Drive the car and warm it up well. You should use a manifold vac port, get your idle reading 15-22hg or whatever. Rev the engine to 2500 - 3000 rpms. The gauge needle will dip for a second and should settle higher or the same as the idle reeding. Hold it at the increased rpms for a couple mins, if the gauge starts dropping, your exhaust is plugged.

Keep in mind if your car is missing and running poorly you are going to see odd converter temps.

Ha, I wish removing 20 year old exhaust bolts was simple . I might start with the vacuum test.

The car idles perfect and runs fine in cruise, but under any load she bogs down.....the more load, the worse it is. And its worse at lower RPMs. At 3500 rpm it almost comes back to life....almost.

Edited September 3, 201510 yr by jetpilotdc10

Yeah, I forget they can be a pain because I have always ended up having to remove them for one reason or another before they got that bad. And once I've removed them, they have anti seize and stainless nuts. If one stripped, it needed a Helicoil anyway..

Vacuum test complete.  Was about 19hg at idle.....revved 'er up and it dropped to about 15hg and then quickly back to 20hg.  I guess the cat is ok (not plugged), so the search for my problem continues.  Thanks everyone!!