'84 C-10 can't pass Cal smog Nox

[freppun]

On June 27 posted about EGR aperture size for 350ci .
Finally got correct part but it didn't reduce NO enough on one of the measurement categories (15mph @ 50% load, 1363rpm).
My figure was about 23% over the max before new EGR valve. Now is down to about 16% over, but anything over is a fail
Any suggestions?
Part of the problem is that there is no way to know what the problem is or if a fix has brought down the figure enough, without going back to the smog guy for another $45 test.

[VileZambonie]

The only thing you are failing for is NOx? Are the EGR passages clear of carbon? Did you verify the function of the EGR circuit? If you pull up on the EGR valve diaphragm does it stall the engine at idle? Is there an area you can go where you can do a top engine cleaning and make tons of smoke?

[freppun]

All other measurements are very good - below the average.
Old EGR valve, that held a vacuum, but was very gunky, didn't cause a stall at idle when I used my hand vacuum pump. New one has vacuum bleed-off, so hand pump doesn't work. It will be hard to get to the diaphragm to test, manually.
Not sure what you are referring to with a top engine cleaning and making tons of smoke.
Would that be for carbon build up in the cylinders?

[bd]

Reach through the vents underneath the diaphragm cover from below and lift upward on the diaphragm.  Does it kill or nearly kill the engine?

[freppun]

Vents underneath are small round holes, too small to fit a finger through.
Will have to find some kind of tool to fit in there to try that.

[bd]

Don't use anything with a sharp point.

[JohnnyPopper]

Curious about how this turns out.

While your truck is a little newer, for '78, Cal's allowance is huge. Gross polluter huge:'(

Happy I'm not in that upper range 

[MIKE S]

How is the timing? To much advance can increase Nox. To much carbon in cylinders can lead to to much heat and high Nox. Running to lean can raise Nox and lower CO. There are tests that you can do at the tailpipe with the proper equipment but frankly the average person cant afford. How old is the cat conv. If it is the original pancake replace it. If it is an aftermarket replacement more than a few years old it is probably weak or dead. Those are not made to last. A cat is not that expensive. Make sure it is calif certified.   

[JohnnyPopper]

Usually the smog station checks the timing as part of the inspection, so it's probably in range.

The carbon build up is a possibility, might be what VZ as referring to. Makin Smoke...

Seriously though, does the cat burn nox? Can't say that I know...

About 45 years ago, I had a diesel mechanic looking at a 396 I was having issues with.

He had me get it up to OT (operating temp) and then ever so slowly, to my horror, pour a coffee cup full of WATER down the carb, goosing the throttle to keep her going.

As he revved it to about 3500 RPM, gobs of carbon came blowing out of the tailpipe.

He explained the process and my heart started to beat again.

About 15 years later, I bought probably the last aftermarket "Water Injector" system on the shelf. If you're not familiar, It comprised of a micro processor that measured RPM and vacuum, and would pump a stream of water into the carb via a metered fitting on your air cleaner. The theory was it did the same as raising fuel octane, it slowed the burn rate. It probably does other things scientifically that someone else can explain.

I used it on a C20 w/350 4BBL, that really used to struggle to get up the local grades with the camper, kids, and their toys. After installation, it was like magic. What used to be dropping into 2nd, and sometimes 1st, became a strong 2nd all the way up.

I think they closed the doors in the late 80's, I later put it on my 3+3, and there is a story there too. 'hydro lock' is an ugly thing.

[freppun]

I don't drive the truck much, just use it for hauling, though I just totaled my usual driver so the truck has moved up in the ranks of a needed vehicle.
Cat is only about two years old, the engine is not the original, but a 350ci crate motor, from Chevy in place of the OEM 305, with all the 305 smog gear.
Made the change more than 15 years, or so, ago and it has passed with not too many problems until now.
Shouldn't have much carbon build-up, since there is probably less than 30,000 miles on it.
I've had it to the smog check guy twice and can't keep spending the $45 each time I think I've brought down the NO.
On the dynamometer it is only the 15mph @ 50% load that fails, the 25MPH @ 25% load passes OK.
I think I'm going to have to bite the bullet, go to a CARB referee and get approval of the engine change, so that I can take it to a smog certified repair station.
If I just take it in, without going to the referee, the shop probably won't work on an unauthorized altered vehicle.

[MIKE S]

Be very careful with this engine change with a referee. If you make an engine upgrade in California and it was not available in that model they may not allow it. In 1984 in California on a 2 wheel drive 1/2 ton you could not get a 350. Only a 305. Now if it is a 49 state emissions vehicle in California now you can have a 350. Is this a computer controlled carburetor? If it is they are very difficult to adjust and may be part of the problem.

[JohnnyPopper]

I totally agree with Mike, avoid making the engine change an issue. That's not what the problem is. You may be inviting more drama than you need to.

[freppun]

The problem is that I can't know if anything I do to correct the problem has done the job, without going back to the test place and paying another $45.
If I take it to a certified smog repair station, they'll have the equipment to see if the fixes have brought the NO levels down, but may discover that the engine is not the original.
I suspect that Mike S is right about the computer controlled carburetor, as the smog guy has mentioned both times about it running lean, based on the measured CO%.
But the jets are the right size for a 305ci, while the engine is a 350. Will adjusting the idle mixture make enough of a difference? Getting to those screws is a chore as they are at the bottom of the Qudarajet.

[JohnnyPopper]

First, there's no way to tell a 305 vs 350 from any visual aspect. Chevy made the same block since 1955.

It does raise the question about the carb: if it is jetted for a 305 it may running lean.

I can't say I know enough about emissions to be helpful, there are others here that are more knowledgeable, hopefully they will weigh in.

Remember: "the main thing is not to panic"

[JohnnyPopper]

Sorry to run on....

The questions about your EGR valve are still out there: does manually opening the valve stall the engine?

In these types of things you need to eliminate the variables one at a time.

A 23% increase in displacement (cubic Inches) is way low on the list.