Hi guys, this is my first post here.

I'm looking to smog pass my dad's Volvo 740 wagon.
It is a 1989 2.3 liter 740.

What's already done:
The engine was recently rebuilt after blowing the head gasket, engine was dipped in acid and cleaned, so were the valves.

Its running great and would have passed CA smog if CA did not create stricter standards for passing smog in this year compared to previous years.

The Catalytic Converts have been fairly recently replaced, with not too many miles on them. I am cautious to replace them again before I get some questions answered.

The smog check sheet is attached.
It is clear on the smog check sheet that the measured emissions-- HC (PPM) and (CO^2) decreases from 15 mph to 25mph as the car speeds up and the heat within the catalytic converts increases, which shows that the catalytic converts are working properly while getting hotter.

The baffling issue is the NO (PPM) is that it increases, as the car speeds up. NO is normally caused by the engine combustion temperatures above 2500 degrees as far as I know... ?

Has anyone had any issues with this or any luck resolving this issue?

My suspicion is that there might be a small air leak in the exhaust system, before the Catalytic converter, causing this.. as the Oxygen sensor works to adjust the engine to decrease these issues, but the extra oxygen entering the system might throw this off.. .just a speculation.

Really just looking for some answers, bring it on guys.

 

You say that the car has a "rebuilt" engine and that opens up a whole new can of worms as far as what they did... Why don't you go back to the shop that rebuilt it and get them to check it out? Volvo's generally pass smog easily. May be just a good tune up, and preheating it will do the trick... But since the recent rebuilt, who knows what they did or did not do, replace, etc.

 

I don't know about that sheet....I thought the following to be the "MAX"

NO (PPM)
MAX AVE MEAS
920 625

 

545 seems freaking low.....My smog tech suggested two things.......O2 sensor or air leak on intake.....check the exhaust manifold gaskets along with the one just before the O2 Sensor....but yes your shop should check it out....maybe did a "hurry up get it out of here" job and screwed the valve adjustment.

 

usually a bad 02 sensor...base idle too high...bad MAF or an exhaust leak that will cause a no pass....

 

I heard the Mass Air Flow sensors are pretty bulletproof in these cars.

The engine was rebuilt by my dad, he took due care himself as he loves the car.

The NO (PPM) does not benefit from increasing catalytic efficiency, as far as I know... so the SMOG guy said at least. The smog guy is great and gave as much advise as possible and did a re-test. But he told me "check the boards if someone has had similar issue".. which even surprised me because he's done it for 30 + years.

 

Bullet Proof?? WRONG! All MAF's have a piece of platinum inside them that takes a temperature and tells the ECU the how much fuel mixture it should be giving at that time. Over time...that platinum breaks down giving inacurate readings...so here's your choice keep asking questions or 1. Check your car for a miss or jumped sparked by checking plugs,wires,etc. 2. Check ur exhaust for leaks 3. Test O2 sensor 4. Test MAF...these are really the only things that would not make your car pass smog. Or try to run your car for an hour or so before you go through the test and see if it passes. Sometimes if the car is really warm...A "rebuilt" motor has NOTHING to do with a car passing smog....this is part of your emissions system. Unless your dad did not do the rings correctly and you are blowing blue smoke...this is not an engine issue but an emmissions system issue or a tune issue...good luck and stop thinking that car parts last forever cause they don't.

 

Appreciate the info. Thanks.

 

my question if I may butt in is, which is the correct O2 sensor to buy? I see an array of them, both Bosch, and others. Mine is a California 940 base model.