P0402, did tests, now what?

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?

 

How does it measure the flow? I'd be looking at whatever sensor there may be, and the valve itself. EGR flow goes from the exhaust, back into the intake. If you have a crack or leak, it would need to be downstream to cause excessive flow. Other than that, maybe the valve's not closing all the way (clean the seat?)

How does it idle? EGR on just a little bit at idle can cause rough running. That would point towards the valve seat being dirty.

Not the same problem, but I had an issue with insufficient EGR flow (the engine pinged), and although the engine would stall when the EGR was applied at idle, it turned out there was only a pencil lead diameter of flow left due to carbon buildup in the passages in the intake manifold. After 175K there can be quite a bit of carbon buildup in those passages. Clean anything you can reach in that system.

I suspect the CAT replacement with an aftermarket unit just created a slightly different pressure in the exhaust and pushed something over it's threshold to set the light. Definitely check for exhaust leaks though. Usually you can see black soot around a crack or leaking gasket.

 

It idles fine, it's just a little sluggish, like it was prior to changing the cat. It's my dad's old truck, and I remember how it used to drive. It's only a 2.4l, but it was never sluggish. It would get up and move.

I didn't really want to tear things apart unless I needed to, because in a couple of reports where they cleaned or changed the EGR they broke a stud or two. But if I have to, I have to. Just have to soak it in Corrosion X.

I've read that the tube leading from the exhaust manifold to the EGR can get clogged up with carbon, but it's a PITA to remove. Looks like it would be.

 

I don't see how that tube being clogged would create "excessive flow". Seems it would be the opposite.

You might consider contacting a Moderator to see if you can repost this to the 1st gen forum. That would give you more exposure. I know there is at least one guy on here who owns a 1st gen and also runs a repair shop (BamaToy1997). He's on here frequently giving advice.

 

Sent the request. Figured I'd also post all of the repairs I've made in case it helps.

Plugs (Denso)
Belts (Gates)
Rebuilt starter (copper contact points)
Battery
Catalytic converter (Magnaflow)
Fuel pump
Fuel filter
Carrier bearing
Front wheel bearings
Rear brakes
Rewired aftermarket cruise (they had it plugged into the blinker fuse, moved it to the unused 4wd fuse, then replaced some exposed wires in the switch)
Shocks
Flushed and bled brake lines