Failed CA smog test high NOx at 15mph
 

Hi all,
1994 Ford F150 w/5.8L E4OD transmission. 239,000 miles. 2wd

Failed CA emissions test. High NOx at 15mph:

Max allowed 690, measured 909

But at 25 Mph passed with flying colors:
Max allowed 592, measured 115

Did a full tuneup (plugs, wires, cap and rotor, oil change, air filter, also replaced ignition coil and a weak alternator) prior to the test. Checked EGR. Engine dies when I apply vacuum to it.

Fuel pressure is 34psi at idle.

For reference, here are the HC and CO numbers:

HC allowed:

15mph: 80, measured 21
25mph: 64, measured 19

CO (%) allowed:
15mph: 0.41, measured 0.13
25mph: 0.71, measured 0.23

O2 measured: 0 at 15 and 25MPH

I did not change the O2 sensor. Also has original cat converters. No codes.
Where do I begin?

Thanks,
John

I might be wrong, but it could be a poor performing catalytic converter. Pretty sure that kills NOx with the high temperature. If you have a cheap laser thermometer you can measure the converter inlet temp, and outlet temp. I think the outlet should be significantly higher in temperature than the inlet temp. Don't know what is a good # though

I thought that as well but discounted it because it was under the limit at 25mph. I'll pick up a laser thermometer and check.

https://www.2carpros.com/articles/ho...ytic-converter. This article is the one I read a little while ago. May be the cause of your failed NOx readings. Next time you test they recommend you run your car/truck for an hour before going to the test center for highest possible temps. I live in Florida, so I have zero experience with emissions testing.

It sounds like maybe your cat might be getting a little weak, but there are some things you can check for. Verify that your AIR injection components are functioning, they help to light the catalyst off. The EGR is also critical to reducing NOx, but you've already verified that it at least mechanically functions. Your HC numbers are pretty low, which suggests your engine might be a little on the lean side, which would cause high NOx. O2 sensors get weak with age, and respond slower and slower until they eventually die. If the O2 has more than 100,000 miles on it, I would replace it.

I think I have a lean condition. The truck is all stock, no exhaust leaks. The fuel pressure drops off idle and the plugs are more white than tan. The o2 sensor is also white. I noticed that the fuel pumps no longer hold pressure in the line. The pressure is 34psi at idle and drops sharply when I rev it. It also has to crank a few times before it fires. It did this two years ago when I smogged it last but it passed then. I really don't want to drop a tank but I guess it's time.

Pretty sure the cat does not deal with nox, but can make more for you. High temps cause nox, and a lean condition at that lower speed is a pretty good bet. Lean enough to burn hotter but not lean enough to misfire.

34psi at idle is fine with vacuum line attached to the regulator, but dropping as rpm rises is not OK: pressure should increase. Does it do that on both tanks? Try a fuel filter first, but may be due for pumps.

Yep changed the filter...Time finally caught up with the pumps... :( It drops on both tanks and they recently started to backfill each other. When I started this thread I didn't have a chance to check all this until today.

Took the truck to a mechanic nearby. They checked the fuel pressure with the return lines plugged and got 60psi on the rear pump but only 40psi on the front. They also said the fuel pressure regulator wasn't holding pressure in the rail. The cats checked out good and the a/f ratio was 14.6 to 1. I will replace the pressure regulator tomorrow and make sure I am running on the rear tank when I smog it again.

Your mechanic probably checked but make sure the HVAC vacuum lines are not leaking. That could cause (or add to) a lean condition too.

I repaired that some time ago when the max ac would stop blowing cold air under load. All is good there.

...Failed again. After replacing the fuel pressure regulator, O2 sensor and running off of the rear tank, the NOx readings for both 15 and 25 mph went through the roof. The CO and HC readings were basically the same as before. I'm just parking it for now...