Passed the smog test...barely

[e21pilot]

My 1992 4.0 2WD automatic Ranger is running really well with just over 250K miles on the original engine (and 4th transmission). I took it to the smog test station today and was surprised to find out that despite having much lower HC and CO readings than the average of vehicles for this year still on the road and even lower NOx readings at 25mph than the average. the NOx at the lower speed of 15mph was well higher than average and almost at the max allowed (711).

This means it just passed today and maybe would not have on a different day. This was despite my usual tricks of fresh oil change, rear tires pumped up with air, and a well warm engine.

Wondering what may be causing the problem with higher than average NOx at the lower RPM? I have read lots about EGR issues but I don't believe the 1992 4.0 OHV engines had an EGR setup. I also read about cat problems but it seems with all the other readings being well under average at high and low rpms, the cat is not the issue. I have changed the thermostat in recent years and the temp sensor but I have always observed the the engine runs cool always just at the very bottom of the normal range and only getting slightly hotter in the summer when towing uphill for instance.

Really wondering what I could start to look at now before the next smog test comes and I may have to scramble to investigate this.

 

[racsan]

Ive heard (but dont know for sure) that having a tank of high grade gas could help. My ‘93 4.0 had no egr setup.

 

[Blmpkn]

Ive heard (but dont know for sure) that having a tank of high grade gas could help. My ‘93 4.0 had no egr setup.

Higher grade gas than what it needs I'd think would make it worse

 

[rubydist]

NOx goes up very quickly as the mixture gets lean, so most likely its running a little lean for one reason or another.

 

[sgtsandman]

How old are your O2 sensors? They might be causing an issue since they are only good for so long. If I remember correctly, 10 years or 100,000 miles is pretty much the life expectancy.

 

[scotts90ranger]

Yeah, oxygen sensors or cleaning the MAF sensor would help richen it up a smidge, both are likely to read more off at lower speeds and loads especially the MAF

 

[dvdswan]

Pass is a pass. Just know next time it probably won't.

 

[lil_Blue_Ford]

Did the capacitors pop in the computer? I’ve heard that can happen and cause issues

 

[RonD]

NOx is a toxic gas that is always present in gasoline engine exhaust
More NOx is generated as cylinder temperatures go up, i.e. engine is under a load so cylinder temps go up accordingly

EGR systems add exhaust gases to the intake "air" when engine is under load, and ONLY when its under a load, and this reduces the cylinder temps because it slows down the "burn"/ignition in a cylinder, and lower cylinder temp = less NOx generated

Cat converters should clean up 90% of CO and NOx gases
So you could have an older Cat that is less efficient now
Does your tail pipe still drip water?
Cat converter changes NOx(Nitrogen Oxide) into Nitrogen and water vapor, drip, drip, drip, lol

+1 on the older O2 sensors, 12 years or 100k miles and they should be changed, better MPG and lower emissions, these are the ONLY sensors that wear out

15MPH test is a fairly high engine load, which is why they do it, especially in an automatic because engine may be lugging because trans is in the wrong gear
Did they shift trans into 2 for the test, they usually don't because they want real world test, and people don't shift if cruising at lower speeds, lol
Lugging an engine generates extra cylinder heat

 

[rubydist]

This is what I am saying - as the ratio goes even a little lean, the NOx goes way up.

 

[Eddo Rogue]

Mine also barely passes, every time.

+1 on cats and O2 sensors as well. its usually those.
Thankfully replacement cats for most gen rangers are available and not astronomically expensive.

 

[RonD]

+1 on Lean, Lean = higher cylinder temp burns, so higher NOx

 

[e21pilot]

First off, thanks for all the posts. Very informative.

To answer the questions, the front O2 sensor was replaced about 5 years ago but the cat is original as is the rear O2 sensor. Many parts of the fuel/ignition/emissions system around the area of the intake were replaced about five years ago as well when I was chasing a high idle condition back then. The computer was also sent out back then and found to be OK.

Since then, it has run really well, always starts right away and idles very smoothly. I run it on 87 most of the time and only switch to 91 when towing. The only issue it has is occasionally when its really hot outside and I get the engine hot and park it, it runs rough for a bit when starting probably because of some fuel vapor condition caused by the heat rising in the area of the fuel line but, this did not occur during the smog test which was on a cool day.

Is a low rpm vacuum leak a possible explanation here? If vacuum is the highest at lower rpm, it might tend to run richer than leading to higher HC than when the rpms increase. I did replace all the vacuum hoses 5 years ago with silicon ones but it is still possible to have a vacuum leak somewhere else. I might get out the propane torch and see if it make a difference anywhere.

 

[rubydist]

I would start by replacing both oxy sensors - they are not that expensive and one for sure needs replaced.

 

[e21pilot]

I could replace the front one again. I have heard the rear one doesn't do anything for the operation of the engine and I wonder if its worth replacing? Now that I am thinking about it, I wonder if the 1992 even has a rear sensor? I have so many vehicles I have forgotten!

Also, it wasn't discussed but I was running on 87 which is the lower octane in my area. I wonder if the smog test results would improve if I were to run 91 instead?