1999 Tacoma DLX/SR5 (DLX style but has SR5 badges) 2.4l 2wd ext. cab auto, 175k miles, stock.
Truck threw the code for a bad cat, P0420. Idiots at the muffler shop convinced me I had a cat built into the manifold. After searching for a replacement, I tore it apart only to discover there was no cat, it's not a Cali emission truck and only has the one cat. Muffler shop wanted $350 to replace, I got a Magnaflow bolt-in cat for $100 and had it installed.
The CEL immediately went off, and I considered it problem solved. Truck had more power, idled better, all good. Then the other day the CEL came back on and it lost some of it's performance again. I had it checked, code P0402. Excessive flow on EGR system.
Did some research as well as read up in the Haynes manual.
Checked the VSV. Supposed to be around 36ohms resistance, mine was 34.5. Seems close enough. Clicked when 12V applied. Held vacuum and released when 12V applied. All tests good.
Cleaned the vacuum modulator filter. Didn't look that dirty to begin with.
Applied vacuum to the EGR valve with engine running, engine stumbled and nearly stalled. Released vacuum, engine runs smooth. Test good.
All hoses are tight, no cracks or leaks that I can find.
I read that the factory exhaust manifolds are notorious for cracking, and it looked like it might indeed have a hairline crack in it that I noticed when checking for a manifold cat. Could this have anything to do with the P0402 code?
So now what? I'm thinking that if there is something at fault with the EGR system, that could have been what threw out the cat in the first place, and now that that is fixed it's tossing the code for the original problem. (CEL was on when I inherited the truck)
What do I test now, and how do I test it?