My 1990 XJ failed smog for the first time. The numbers were

HC: ------Max Measured ---------CO: Max Measured ----------- NO: Max Measured

15mph: ------123 169 ---------------- .73 1.69 ------------------------ 711 230
25mph: ------103 178 ---------------- .93 1.74 ------------------------ 613 673

A few days before I took it in I noticed some oily black spots on the rocks under the tail pipe. I assumed that it was just carbon from the exhaust and condensation as it has rained a ton lately and I don't drive it much.

When I got home from the failed smog I pulled the plugs and all looked good except for the last one.The crush washer was cattywampus and keep the plug from seating all the way. There was a bunch of carbon blow by on the valve cover by this plug.

My thinking is the plug was in enough to to burn enough of the fuel to avoid a miss but, not enough to get a full burn.

The plugs, wires, cap and rotor 2-3 months old. I drive about 600 miles a month. I fixed the plug ran some seafoam through it replacing the vacuum lines(getting oil in the air filter), putting in a new air filter and am going to change the oil.

Am I right in thinking that that plug was the cause for the high numbers?

Also I am running M/T MTZ's inflated to 35 psi. Should I pump them up to 50psi? This is what it says they should be for towing.

 

That plug was sure not helping.

What brand plugs are you using?

 

Tire pressure won't affect smog readings. 50PSI is max for max load at max speed. I'd avoid it unless you're doing exactly that, and you probably shouldn't be, ha ha.

 

You've done a lot of the right things by doing the oil change etc. One more tip for smog that I've had from informed people on another forum is to make absolutely sure that the car is COMPLETELY warmed up before you take it in for a test: go for a proper drive, if possible with a hill or a stretch of freeway. My guess is that part of this is because an engine not completely warmed up can be getting a richer mixture which will lead to false numbers.

 

I'm using champion plugs.

Its a 20 mile drive to the smog shop so it was definitely warmed up.

I have read that over inflating your tires helps pass smog. I have know idea how. But that's why I was thing of pumping them up to 50psi as they are rated for 50 psi on the side of the tire(while towing).