CA Smog: 2 ppm over the HC limit

[myf16]

This is the first in over 100 California smog tests that any of my vehicles has failed. It seems to me that a limit of 31 ppm for HC on a 1996 vehicle is too strict, but that's what it is these days. My 4.0L tested at 33 ppm. Other emissions were not close to their limits. HC was 20% under limit at 15 mph and 10% over at 25 mph.

I had driven normally 15 minutes before the test but parked for 10 minutes, which I know is not optimal. The gas tank was just filled with regular gas. I changed the air filter and spark plugs about 6000 miles ago. I changed an oxygen sensor at about the same time to eliminate a code. I changed the oil and filter about 2000 miles ago. The drivetrain has 135k miles on it. Original everything else.

For my retest I'm planning to:
- insist that the test occur immediately after hard driving
- remove the air filter before the test
- possibly change the spark plugs again
- probably use a different shop where they may have a steadier foot on the throttle

What do people here recommend for my retest? Also change the other 3 oxygen sensors? I bought a Bosch 15717 last time. I'd have to check whether the other 3 have the same length harness or if I need the longer 15719 for 2 of them. But that's a bit of work and about $90 (ebay prices).

Any other ideas, besides moving to a different state?

 

[1998Exp]

30 ppm is indeed a strict limit; I remember when it was 200... Common wisdom has it that excessive HC emisssions are related to poor ignition. You changed the plugs, but how about the wires? If they are old, there is a good chance the insulation is cracked and arcing some. Besided the reduced emissions, you may be surprised how powerful the engine has become...

This is the first in over 100 California smog tests that any of my vehicles has failed. It seems to me that a limit of 31 ppm for HC on a 1996 vehicle is too strict, but that's what it is these days. My 4.0L tested at 33 ppm. Other emissions were not close to their limits. HC was 20% under limit at 15 mph and 10% over at 25 mph.



?

 

[Flash]

Take it for a highway run and get it good and hot, this will also help clear out any carbon build up. maybe do a Seafoam treatment first then do a highway run.

I've heard that using a high ethanol fuel helps with smog testing.

 

[lobo411]

I also have a 96 4.0 OHV. I did a bunch of things to my X because I was worried that it wouldn't pass last year (in 2012, I was 1 PPM away from failing NOx). Aside from the usual replacement of spark plugs, plug wires, fuel filter, oil filter, running some Techron through the gas tank, cleaning the MAF, replacing the lower intake manifold gasket, replacing the PCV valve elbow (which really improved my vacuum system), my HC numbers went from:

2012:
15 MPH: 32 PPM (Max 47)
25 MPH: 16 PPM (Max 31)

2014:
15 MPH: 39 PPM
25 MPH: 10 PPM

Also, my CO% was cut in half at 15 MPH and 20% at 25 MPH. The NOx reduction was most dramatic: from 493 PPM to 269 PPM at 15 MPH, and from 365 PPM to 50 PPM (!) at 30 MPH.

If I were you, I'd do the usual tune up stuff plus retorque the Lower Intake Manifold bolts (don't replace the gasket...just retorque b/c they loosen on their own) and replace the PCV elbow hose. They have it at Amazon...Motorcraft p/n KCV98.

 

[myf16]

Thanks, lobo411.

I found three 10mm bolts low on the intake manifold but they were already pretty snug.

That PCV hose was soft so I ordered one. I also ordered the two O2 sensors to accompany the one I recently replaced, so I will have all new O2 sensors.

$70 for all 3 new parts from Amazon. (I got lucky and they had a returned O2 sensor on Amazon warehouse deals.)

I think this plus removing the air filter and using a more skilled tester (the guy who I used until 3 years ago) should do the trick. If not, I guess I'll change wires and plugs again, although the plugs were kind of a pain.

One nasty thought crossed my mind. I've been planning to increase the tire diameter from 28+ inches up to 31 or 32. That will decrease the rpm for the 15 and 25 mph smog test, increasing emissions. I might even need to reinstall the original tires to pass. Ugh. Edit: Actually I would only need to reinstall the old speedometer gear for the smog test!

Update: I dug out my smog test from 2 years ago. The big difference is O2 level. Two years ago it was 0.1 and 0.0%. Now it's 1.76 and 1.71%. That means lean mixture (leftover O2 even though HC is down in the parts per million). I think it also means the catalyst is OK and the ignition system is OK, burning all the fuel that's present.

Lean mixture means vacuum leak or the O2 sensors are giving bad data to the computer. I'm hopeful that replacing the O2 sensors and that PCV hose will do the trick. I will also remove the MAF and spray it with whatever Autozone sells. Then I'll spray the connector contacts with DeoxIt (contact cleaner). I'll report results here within a few weeks.

Update2: I pulled the PCV hose and checked vacuum there. It's rock solid and normal. The MAF sensor has a tamper-resistant bolt so I removed the interior cover plate and sprayed from there. It looked pristine, so I'm skeptical this made any difference. I took a rubber hose and listened for vacuum leaks all around the intake manifold. Nothing. for If the PCV hose isn't leaking, and I don't think it is, the O2 sensors look like like the best bet.

 

[lobo411]

Ya I forgot to mention that I also changed the O2 sensors. Big PITA because they were welded in by years of heating and cooling cycles. I don't think I bothered to do a freeway run before the 2014 test either, so that probably hurt my low end HC results.

 

[myf16]

The good news is I passed the retest. The bad news is that I only passed by 1 ppm because the problem persists. Oxygen is even higher than before, at 1.8 to 1.9%. All my emission levels are way above the averages for passing vehicles.

I'm wondering if one of the injectors isn't putting enough fuel in its cylinder. I don't know how to diagnose something like that.

California sure is throwing a lot of its residents' money into reducing air pollution by trivial amounts.

 

[lobo411]

Congrats on passing! Did you ever change the o2 sensors? It's definitely annoying to have to deal with the smog stuff, but I have to admit that my explorer runs a lot better since I started trying to keep it in "smog-ready" condition.

 

[2000StreetRod]

weak cats?

Maybe after almost 20 years your catalytic converters are weak.

 

[SoNic67]

In California they have double the number of cats... they might be the culprit.

I would just... move to a sane state.

 

[2000StreetRod]

new cats

When I upgraded my exhaust system (prior to an M90 blower) I installed two high flow cats (one for each bank). Each one was rated 49 state legal for up to a 6.7L engine.

They were inexpensive but the custom fabrication to incorporate them wasn't cheap.

I grew up in California (Chico) long before pollution controls. Back then in urban areas the only time the sky was clear was after a weather front passed thru. Now California vehicles probably generate less pollution than lawn mowers.

 

[myf16]

Congrats on passing! Did you ever change the o2 sensors? It's definitely annoying to have to deal with the smog stuff, but I have to admit that my explorer runs a lot better since I started trying to keep it in "smog-ready" condition.

Yes, I now have 3 new O2 sensors, nearly new plugs, cleaned MAF sensor, and replaced PCV hose. There's no vacuum leak.

The O2 level was zero 2 years ago and it's not zero now, so the engine is getting too little fuel or too much air. That would have to be MAF or injectors (or computer) I think. Since it was working fine 2 years ago I suspect a bad injector. That needs dealer diagnostic gear which is $$$. Unless you folks have a better option.

Incidentally, this vehicle has only one cat on the 4.0L non-SOHC V6. I don't know that it's a California vehicle but it probably is.

 

[myf16]

OP back again. This 96 4.0L 4WD Explorer barely passed (by 1 ppm) on the re-test in 2015 and it's up for another test soon.

One interesting fact is that last summer it had a check engine light related to oxygen sensors. Since the sensors were new, I didn't pursue the problem. The error has not appeared for months now. I suspect that winter gasoline (less oxygenation?) is better matched to this engine's control unit.

My smog test can be done any time now through August. That gives me the choice of winter gas or summer gas. Which is more likely to nudge the hydrocarbon level lower, winter or summer gas?

Second, I have seen conflicting recommendations on premium vs. regular gasoline. The engine is rated for regular, but if there is carbon in the combustion chambers raising the compression ratio then premium might make a difference. I hear no knocking with regular gas. My smog guy says buy the 91 octane brand name. What do you folks think?

 

[Josh P]

http://www.sciencedirect.com/science/article/pii/S2215098615302469
Some fuel additives or engine cleaners are acetone packaged in a nice bottle. XtraMPG is a good example.

 

[scrivyscriv]

How is your fuel mileage?

 

[myf16]

It has always been poor. Maybe 14 mpg. It's a big, heavy 4WD vehicle. But the engine and transmission are running perfectly.

 

[rb142]

You are probably not running lean. Your oxygen numbers are up, which is telling you it is not being consumed in the combustion/aftertreatment process (O2 sensors measure the oxygen left, not strictly rich/lean -- you have too much fuel as well, as shown by the high HC numbers). Most likely this is because the catalytic converters aren't doing much anymore, and aren't consuming the leftover combustion gasses (using up both the O2 and the HC content). Verify the behavior of all the O2 sensors, and check what is happening with the fuel trims. I suspect you will find low catalyst efficiency

 

[1998Exp]

It has always been poor. Maybe 14 mpg. It's a big, heavy 4WD vehicle. But the engine and transmission are running perfectly.

Have you ever replaced the ignition wires? If not, you may be surprised.

 

[myf16]

Have you ever replaced the ignition wires? If not, you may be surprised.

OK, you convinced me. Should I spend the extra $15 or so for Motorcraft wires rather than Denso? If the Denso fit is not exact, that $15 would be cheap avoidance of frustration.

I guess I'll find some acetone too. That link was great.

 

[96eb96]

What are your fuel trims at idle and 1500 rpm. I also have a 96 and replaced the EGR valve because it leaks air thru the pintle.