Failed smog high NOx at 15 mph

1995 Tacoma 2.4 L manual stock 380K miles. Failed smog because of high NO 990 ppm (386 allowed) @ 15 mph. Smog test results were:

15 MPH - 990 ppm NO | 0.02% CO | 10 ppm HC | 0.37% O2 | 14.69% CO2
25 MPH - 155 ppm NO | 0.13% CO | 5 ppm HC | 0.09% O2 | 14.82% CO2

Just prior to testing replaced both the vacuum modulator and electrical solenoid. Both were original. Solenoid failed with an electrical open and vacuum modulate was questionable but likely working.

If I apply vacuum direct to EGR valve the car does stall. Did a road test with a vacuum gage teed into the vacuum line at the EGR valve. No vacuum at 15 mph. Vacuum starts to show up around 18 mph and is there at 25 mph.

I am leaning towards believing that the exhaust back pressure on the bottom side of the vacuum modulator is inadequate. I think the small metal tube on the bottom of the EGR valve that should put exhaust pressure on the bottom side of the diaphragm in the vacuum modulator is mostly plugged. I would think this tube would be opened to the exhaust pressure and therefore should not hold any vacuum. However, if I apply a vacuum to the exhaust backpressure tube it drops slowly versus instantly as I was expecting. Is this an indication this tube is mostly plugged?

Am I on the right path? If not what are the suggestions to identify the very high NO at 15 mph?

Thanks in advance for any suggestions.

 

Solved.

The issue and solution. - The bottom side of the vacuum modulator has exhaust back pressure applied through a small metal tube on the EGR valve and then a rubber hose. With the Tacoma not running I applied vacuum to the small metal exhaust back pressure tube. Although it would not hold a vacuum it would bleed down relatively quickly. This suggested a partial plug in this tube. I purchased a new EGR valve and applied vacuum and it bleed down instantly as expected.

When I applied vacuum to the EGR valve the engine would stall. I always thought that meant the valve was working. With new vacuum lines, vacuum modulator, electric solenoid I thought I would pass, but was wrong. I had high NO at 15 MPH, numbers above.

IF you have high NO, I would strongly recommend two test of the EGR valve:

1) Applying vacuum to the vacuum port of the EGR valve to stall the truck

2) Applying vacuum to the tiny exhaust pressure tube on the EGR valve to verify it is wide open. If there is even a rapid bleed down of vacuum clean the tiny tube of obstruction(s) or replace the EGR valve. Checking local stores, on-line suppliers, I found the EGR valve on Amazon for $56 and passed smog on the second test.

15 MPH - 164 ppm NO | 0.11% CO | 39 ppm HC | 0.22% O2 | 15.07% CO2
25 MPH - 053 ppm NO | 0.05% CO | 13 ppm HC | 0.07% O2 | 15.18% CO2

Not bad as I have owned the truck is Dec 1997 and put 340K miles on it (367K total) and this is the first smog parts replaced. The truck is stock and drives like a dream. Doesn't drop or burn a drop of oil. Never had the valve cover gasket, intake or exhaust manifolds off. Had to rebuild the starter, replace the radiator, and clutch once and one of four fuel injectors. Tires, brakes, batteries, and oil changes multiple times.

 

Thank you for coming back with such a clear and concise solution to your problem! this'll help anyone searching in the future

 

I'm so glad 2000+ vehicles don't have to do the dyno/NOx test in CA. Seems like such a pain in the ass

 

Every 2 years in Colorado it's needed

 

Not needed here.

 

Wait what. My 99 was put on the dyno for smog a few months ago

 

Just an OBDII scan and visual check for 2000+ here. Having no codes but still failing due to emissions gases would be a pain to solve IMO. Not to mention getting flagged in the system for a failed test

 

Thats why I said 2000+

1999 and older have to do the dyno/NOx test in CA. 2000 and up, do not. Just a visual inspection and OBDII scan.

 

Did you ever have a code for EGR flow insufficient? Maybe it doesn't trigger code at 15mph, but that's the speed of the smog test.

 

No current codes. I has surprised that the Tacoma failed. There was no hint of any issues. The Tacoma runs fantastic.

I did have an EGR flow insufficient code once about 2 years ago. I read, then erased the code. It never came back. The code was triggered going up a very steep driveway. Because the code did not come back, and the Tacoma had recently past smog back then also, I decided it was likely a combination of engine temperature (had been hot but stilling off for about 1 hour), engine rpm, load, and speed.

 

Did you replace the EGR valve or the EGR modulator valve, thanks