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93 NOx Too High


Well-Known Member
August 18, 2004
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City, State
Year, Model & Trim Level
'93 XLT
My '93 4.0 won't pass inpection. Too much NOx, over 1100 where 905 is the spec.

It's had some overheating troubles this year, at idle with the AC on, so I can see where too-high combustion temp could be the cause.

However, I don't see an EGR -- the likely source -- on this vehicle. In combing the archives here at the forum, I found some opinions that EGR wasn't installed on non-CA vehicles until midway through the 93 year model -- same time as the switch from R-12 to R-134a. This was definitely an R-12 vehicle.

With no EGR and no OBD codes flashing, what should I look for?

The generic form from the inspection lists:
  • 02 sensor
  • EGR
  • ignition timing
  • feedback sensor failure
  • vacuum lines
  • catalytic converter

Again, no codes. I did have a bout of acceleration ping last summer, which I traced to an intake leak (near the MAF sensor, which I cleaned) and fixed.

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I had the same problem this year. I cleaned the MAF and other pre smog things and forgot to reset the computer. Here in California, I was afraid of falling in the "GROSS POLUTER" catigory so I put it in the shop where they did 1 hour of magic and then smogged it without problem.

What "magic" did they do??


yeah im interested.....i already have to pay to get stock exhaust put on to pass i have no cats or anything.

I sprayed half a can of quick-start (ether) all around the hoses, intake manifold, and intake hose. No change in RPM. So it's looking less like a vacuum leak, though that's not completely ruled out.

I also took the grill off to see if the radiator was obstructed or not sealed around the edges. I discovered a foam seal that prevents loss of suction across the condenser and radiator, which is cool -- supposedly this year model had cooling problems that the seal was supposed to fix. I did straighten some bent condenser fins, but there weren't a lot.

This truck has a new radiator, fan, and fan clutch. I'm still puzzled as to why its combustion temperatures would be too high.

check your oxygen sensors

msmith65 - if you look under your hood there is an emissions sticker that will list the types of emissions controls that the Ex has. I would be VERY surprised if it didn't have EGR. As far as emissions controls go, EGR controls only NOx. NOx are formed under high heat and high pressure, so advanced timing can cause them, but that is unlikely in a distributorless ignition system. A lean condition can cause high combustion temps, so look for a vacuum leak. If no vac. leak and the EGR functions, suspect the catalytic converter. Those are really the only common reasons for excessive NOx.

I would be VERY surprised if it didn't have EGR.

So was I. Here are the stickers. Sorry for the blurriness.



Studying them closely, I find no evidence of an EGR at all. I had read here in some other posts that EGRs were added partway through the 1993 model year.

Sorry about being away, tons of stuff in the way. What the shop did was reclean the MAF and reset the computer.