My car, 88 350tpi didnt pass emissions today. They measure CO only, and it was high....5.1, limit is 3.5. (This is in Sweden).
Im thinking maybe I should change the oxygen senor? There is no ECM code,
but mpg is high... 15mpg, mixed driving mostly highway....
I also got some STP injector cleaner, thinking at least it wont hurt...
Trying this, what would the next step be if it fails?

 

You're right…injector cleaner won't hurt. If your O2 sensor is malfunctioning, it should throw a code. Make sure the engine is fully warmed up before you have it tested.

If it fails again, my next move might be to look at the catalytic convertor. Someone else may want to chime in…good luck!

 

Ok, thanks for the info
I already have the o2-sensor, so will change it and see if it makes a difference....
The cat is probably stock from -88. I checked the last emission check report from 2013. back then the co read 0,1 now its 5,1. It has been driven maybe 400km(640 miles) since then. I hear a common trick here is to put some carburator alcohol in the tank, but would rather like to fix the problem....

 

Forgot to mention that the car runs well, apart from being very thirsty

 

How old are the spark plugs? Mine looked like this, changed to new ones and also wires, distributor cap and rotor and changed the O2 sensor. Radical drop on emissions with new parts, HC (unburnt fuel) dropped from 900+ ppm to about 250, limit is 600. Here in Finland they measure HC, oxygen and bunch of other values too...

 

They give/test no other readings? Hmmm... Simple solution to high CO is raise the overall operating temp (ie, go in for testing after driving around a while) and lean out your mixture. It'll throw off so much else, but it'll get your CO down.

 

A weak O2 sensor can continue to operate without setting a trouble code. But it will cause the ECM to richen the fuel mixture in the process. Which can cause high CO levels.

Replacing the O2 sensor is relatively inexpensive and easy to do. And will likely solve the problem.

 

O2 sensor replaced. Going back to get it tested again....

 

I have had an Edelbrock 3860 intake, asm runners and ported plenum lying around for some time. Now I decided to have it installed by a mechanic. The o2 sensor was replaced a few days ago. The mechanic will tune the engine after the intake swap(no new chip).
I ll ask him to change oil and oil-filter, then hope for a passed emission test and a few hp extra