CA Emissions Test Fail - Hi NOx at Low Speed

I’ve got an '87 Jeep Wrangler with a 4.2l straight six that just failed it’s emissions test here in CA and I was hoping somebody might be able to point me to the problem.

The engine has 250k miles on it - I rebuilt it a couple of summers ago when it had about 225k so it’s got about 25,000 on it. The motor is tight, and runs well - standard mileage etc.

The problme is the that NO emissions at the 15mph part of the test are high, but everything else is good:

15mph 25mph

CO2 15.0% 15.3%

O2 0.7% 0.0%

HC 18 ppm 15 ppm

CO 0.02% 0.20%

NO 1275 ppm 157 ppm

Does anybody have an idea as to whats up with it?

Thanks in advance.

High NOX readings can be due to high cylinder head temps. Check the EGR valve and make sure is is clean as it is a common source of trouble for this kind of problem.

I’ll try checking that this weekend - I was kinda thinking it might be the case, but since it was only at low speed, I wasn’t sure.


could also be the catalytic converter,especially with that mileage. make sure you have vacuum to the EGR valve as well

Check the timing. Timing advanced a little too much can increase NOx. Do the timing check at idle, and with a load on the engine equal to 15 mph (If an automatic transmission, in gear, foot on brake and gas). If a manual transmission, you could raise the drive wheels, and applied brakes and gas to simulate 15 mph. (It would, probably require two people.)

Make sure the plumbing to the EGR valve is clear as well as the EGR passages in the intake manifold, so all cylinders are getting exhaust gas.

The O2 and CO #'s suggest it’s running lean at 15mph. Could be a carburetor issue.

In addition to what circuitsmith mentioned, it might be a vacuum leak of one of the intake manifold gaskets or hoses that is not bad enough to affect the 25 mph readings.

Hi everybody -

Thanks for all the responses - I apologize for not getting back sooner, but I got called out of town for work and then got lazy re: car work.

I pulled out the shop manual and checked out the EGR system as suggested, and couldn’t really find anything. The valve was clear, and moves in/out freely. The CTO switch seems to be working (vacuum when car is warmed up, none when cold) and there’s vacuum through the system, and when you pull the valve open manually at idle, the engine starts to run like crap.

The only thing I can see that might be off is that the vacuum seems a little weak when I plug the hose with my finger, but that’s subjective and I don’t have a
vacuum gauge to test with.

My current thought is to clean the carb out (it’s pretty clean now anyway) and give it a good long drive before heading to the re-test this coming week.

I will report back afterwards.

Thanks again


Testimony time. I had a 1989 Mazda 626. It failed emissions for NOx. Checks of the EGR system were inconclusive. A thought struck me right between the eyes (Maybe, you can see the mark it left). Timing was lowered, in those years to reduce NOx (That’s when I got struck). I checked timing. It was on the high side; so, I loosened the distributor hold-down bolt and retarded the timing a tad (My memory can only handle, “a tad”). Short story, long: it passed the emissions test with flying colors. Can’t hurt to try that, can it?

I’ll give that a shot as well. Shouldn’t take but a few minutes (plus another 30 or so to find my timing light… :slight_smile:

I just tweaked the timing - the shop manual says 9-deg @ 1600rpm with the vac advance disconnected. I had it at between 9 and 10, reduced it to between 8 and 9.

My car failed the NOx at low speed. So I want to try to retard the timing. How many degrees should I retard the timing from the factory specification?

Don’t go radical. Just a degree or two lower, and check out the EGR system. Timing re-set won’t cure a defective EGR system.

Now you see one of the reasons the Fed removed the abilty to change timing. People were altering timing instead of fixing the real problem. Same thing with idle mixture and idle speed. They wanted everything that could be tweaked taken away from peoples hands.

When I worked in a TV shop during high school we used to joke about designing a TV with only one knob: the channel changer.
Volume not loud enough? Sit closer
Picture too dim? Turn the room lights off!
Shut the thing off? Pull the plug!

Muntz had the right idea…

I believe on that old Rambler Motor you can check the timing with a strobe light. High NOX at low RPM usually means too much spark advance. Check to see that the centrifugal advance (built into the distributor) is working properly and not stuck on full advance.

I retarded the timing to 12 degrees (factory spec is 15±2 degrees) and added a bottle of “Guaranteed to Pass Emissions Test Formula” by CRC to a tank of premium grade gas. After using 3/4 of the whole tank of the premium grade gas, took the car for a re-test. It passed the test without any problem.