• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Finally Passed Smog

Saudade

NAXJA Member
NAXJA Member
Location
SoCal
1988, 4.0L, Auto, 2wd, 175K.

1st Test:
15MPH: HC:18ppm, CO: 0.10% NO: 1218ppm (fail)
25MPH: HC:15ppm, CO: 0.07% NO: 1030ppm (fail)

Replaced Exh manifold gasket and resealed TB, and EGR gaskets
Tested EGR solenoid and valve (OK), cleaned
Replaced TPS (it was shifting a bit funny)
Replaced CAT (got direct fit) and muffler (Cherry Bomb Turbo). Muffler had a bunch of holes and cat rattled a little.
Added some octane booster to tank

2nd Test:
15MPH: HC:34ppm, CO: 0.05% NO: 68ppm
25MPH: HC:18ppm, CO: 0.01% NO: 53ppm

The NOx improvement was pretty dramatic. The items above needed to be done anyways. I decide to try the first smog test to see if it could pass so I'd be able to wait a bit on the cat and muffler. Since I failed, I figured I might as well get 'er done.

Now I'm good for 2 more years.
 
Well, at least in CA, it's *ONLY* a smog test. ;)
 
Just for comparison, here is the test data I got from my last test a few months back:

87, 4.0, Wagoneer, 248,000 miles emission inspected in one of the tough Texas counties (Houston) earlier this week.

Here are the results:

High Speed: 2441 RPM
HC (ppm) was 8 and the limit was 220 to pass the test
CO % 0.04 and the pass limit was 1.2 %
CO2 % was 14.9
NOx was 0.00 ppm WOW!!!!! :eyes:
O2 % was 5.1


at 698 RPM (Idle) low speed results were:

HC (ppm) 28 with the pass limit being 220
CO % was 0.13 % with the pass limit being 1.20 %
CO2 was 14.7 %
O2 was 5.4 %
NOx once again was 0.00 PPM :eyes:

Not bad for a 20 year old jeep with 248,000 miles on it.

I also found out that since it is a 4x4 they do not run the stress dyno tests, seems the 4 x 4's are exempt for some reason, says they can not test them like they do 2 wheel drive cars.

Note that I have all OEM equipment and IIRC AC Delco brand, single electrode platinum spark plugs.

I am guessing that either your EGR was causing problems, or it was, or even is running lean for some reason based on those high NOx readings. Leaking exhaust manifold should make it run rich, reducing the NOx, not increasing the NOx.

Does the Cat converter also reduce NOx numbers as wel as HCs???

Your test data is most curious!!!!!!

I had heard about 30 years ago that the Cat converters were originally being apposed by some Chem Es becuase they said the Cat converters would increase NOx emissions.
 
For all I know it was the octane booster. I had tested the EGR valve and solenoid prior to my first test and it tested fine. Banging on my cat, I could hear some rattling and my muffler was mostly patches. A few weeks ago, I started getting the funny shift points and OD "drifting" so I knew the TPS was suspect.

So I ordered an OE fit CAT online (like $80), and got the cherry bomb from a local pep boys and some new clamps. Spent one afternoon doing the gaskets while I waited for the cat. Spent a second swapping out the cat and muffler. I should have ordered a tailpipe though. That would have save me a lot of time and energy cutting the old muffler off of it.
 
High NOX is almost always caused by high temperature, which is usually because a bad O2 sensor is forcing a lean condition. Since you did not replace the O2, I'm going to guess that cleaning the EGR was the fix. It could have been the cat though, since it is responsible for separating the NOX into nitrogen and oxygen (the latter is combined with the carbon monoxide to produce carbon dioxide).
 
A bad O2 should cause a rich condition since it's not providing input. At my last smog 2 years ago, my HC was "off the chart" (like 4x the limit) but my NOx was under. I swapped out the O2 and my HCs dropped since it was now leaning out properly but my NOx increased (still under the max but just barely). My mpg got better too.

I swapped out my cracked exh manifold a few months ago and cleaned the EGR valve back then. It tested OK then as well as the solenoid. I had a strange idle problem when cold that lead me to believe that my int/exh gasket may have been leaking.

When I got my cat off I took a look inside. It actually looked OK. The mesh was intact and I could see through it. Didn't appear clogged at all. I had expected it to look worse since I had no idea how long I was driving with the dead O2 sensor. I couldn't believe all the crud that dropped out of my muffler.

Well, no matter. As I said above, I needed these things done anyway.
 
ehall said:
High NOX is almost always caused by high temperature, which is usually because a bad O2 sensor is forcing a lean condition. Since you did not replace the O2, I'm going to guess that cleaning the EGR was the fix. It could have been the cat though, since it is responsible for separating the NOX into nitrogen and oxygen (the latter is combined with the carbon monoxide to produce carbon dioxide).

I missed that. Good catch!

Got hung up thinking about the manifold gaskets he replaced, possible manifold leak making the O2 run it rich, which not increase the NOx, but reduce it, unless the excess HCs from running rich got the cat too hot, thus increasing the NOx?

But agreed, it sounds like the O2 sensor was or is running it lean, or the wiring to the O2 sensor has a poor connection adding resistance to the circuit. Either way he should pull out an analog ohm meter and test the live O2 sensor readings. See "RenX Files" thread for testing details.

THe Renix O2 sensor is 5 volt resistor bridge.
 
Back
Top