Hello, I just purchased an 87 Volvo 740 GLE wagon for a road trip to Canada with my son. I was surprised to have it fail smog with high NOX, but not by a lot. The rest of the readings were well within range.

High NOX indicates a lean/hot condition so I have fixed a low fuel pressure issue (bad regulator) which upped fuel pressure at idle from 30 psi to 37 psi. I also found a leaking vacuum hose upstream of the throttle. I replaced the oxygen sensor with a new Bosch unit, replaced the platinum plugs with regular NGKs, did the Seafoam decarbonizing treatment, locked the airbox door in the cold position, replaced fuel filter, cleaned my fuel injectors in my ultrasonic cleaner (didn't test them though) and tried another mass air flow sensor to see if it would up my AFR readings, or put out more voltage at idle than the unit I have (it was the same, 2.2 volts at idle - manual transmission idle speed.)

Despite all that work, and it does run better, I am hesitant to get it tested again because the oxygen sensor still has the same readings on my digital voltmeter that it had before I did any of the work. It goes back and forth between 0v to .8v. I have read that it should be more like .1v to .9v-1v, which is richer.

Should I still be worried about a lean condition here? Or should I just go ahead and have it smogged again?
Also, the temp gauge in the dash has the needle about an 1/8 inch to the right of center. Is that normal?

Your thoughts are appreciated.

 

It is wrong to say that elevated NOx is a result of lean conditions. Elevated combustion chamber temps are what cause high NOx. The rule of thumb is either the EGR valve or a bad catalytic converter. Your O2 is fine; when they fail, the ecu defaults to rich, not lean. Platinum plugs suck in redblocks so you did good there. When you say the airbox flapper is in cold mode, I hope you mean it is closed and not allowing preheated air into the air box. An open flap is for cold starts and should close via the thermostat in the air box quite soon. Allowing the heated air in for any length of time is a surefire way to toast the AMM. Check to see if the car has an o.e. Volvo branded cat. If it does, it's probably no longer effective.
To simply get the car to pass smog, all you need to do is retard the timing roughly 4-5 degrees. After the test, put it back.

 

Thanks for the response. Unfortunately here in CA the car must have stock timing settings to pass smog. It's no matter though because I want the engine to run correctly. And if it is running lean I want to fix that. I believe that leaner mixtures do cause higher combustion chamber temperatures.

I locked the airbox flapper so no heated air comes through.

I did a quick test on the cat with an (infrared?) temp gauge and the exit was about 75 degrees hotter than the intake side (300 degrees or so) so I assumed it was working. Maybe its not.

Would an 87 740 red block have an egr valve? I don't see one anywhere.

And where is my temp gauge needle supposed to be?

Perhaps if the test is done with the car cold that would help, it did when I had high NOX on my 88 BMW 325i.

 

Yes, running lean elevates combustion chamber temps. All I'm saying is it is folly to make the leap of logic and say high NOx is caused by running lean. It is caused by higher temps...which rarely, is a result of lean conditions. Federal 87's did not have egr's but I haven't a clue about Cali cars. If you don't see it, probably not. You might try looking around the intake and exhaust manis w/ propane and check for leaks. When they smog in CA do they actually put a timing light on the car? How else would they determine proper timing?

 

Yes, they do put a timing light on it here in CA. I've seen them do it on several of my cars.

I did try the propane trick on the intake side and found no vacuum leaks. Never thought of doing it on the exhaust side, but if that works I have no idea how. I do think i hear a small exhaust leak so I'll have to check that. I just read that it can keep the cat from working right if it is upstream of it.

I think the engine may be running hot as well. I'm going to check it with the infrared gun and maybe change the thermostat to a colder one.

 

Wow...and I thought Dallas inspectors were nosey! I was tired last night; I meant to say use the propane around the intake and injectors...exhaust leaks will indeed make the O2 read wrong. Remember, it's the O2 that tells the ecu what to do.

 

I'm a little surprised that the sensor voltage gets all the way down to 0 VDC. Are you taking the measurement with the wires connected like they normally would be? If so maybe try a different meter, but the important thing is that it is constantly cycling between rich and lean.

 

Yes, I just pulled back the rubber boot and left the O2 sensor connected. I have an analog voltage meter I can try.

I took the engine's temperature after my 20 minute drive to work in stop and go traffic. With engine idling, the plastic ends of the radiator were 188 degrees, while the thermostat housing was 210 degrees and climbing when I stopped to keep my hand from getting cooked.

 

With O2 sensors, if in doubt, throw it out. They live in a very hostile environment and really have a useful life of 80-100k miles at the very most. Even before that, they start to slow down in their response time and adversely affect data into the ECU.
I've never owned an older car that I regretted replacing an o2 sensor on, always a decent gain in mileage and midrange running.
I'm 100% behind Swiftjustice on the high NOX thing too. Too many times, people chase electronics problems when diagnosing EFI systems and forget to think that the engine is still an engine, fundamentlaly the same as it was before it was computer controlled.
With any fault, you could consider ALL the factors and symptoms, not just chase one factor (suspected lean).

 

The car has a brand new Bosch 02 sensor, which reads exactly the same as the old sensor. if you have suggestions beyond what I have tried, to get it to read correctly, I'm all ears. My hunch is that it has a tiny exhaust leak.

 

Turns out I have a slight exhaust leak at the exhaust manifold/down pipe gasket. I had to suck some ATF into the engine and partially plug the tailpipe with a rag to see smoke coming from it. It certainly couldn't be felt by hand.

Let's hope this fixes my problem!

 

Fixed the exhaust leak with new gasket. Engine runs much better at low rpm. Clutch engages smoothly now, whereas it seemed to chatter before.

Unfortunately the 02 sensor still reads the same.

 

This sucks. Car failed smog again with similar readings. Nox was still too high at 25 mph and just barely passed at 15 mph.

CO was slightly above average (.10 percent) and HC was slightly below average (16 ppm) at 25 mph.

NOX did decrease a bit. From 689 ppm from last test to 606 ppm at 25 mph. Max NOX allowed at 25 mph is 511.

 

Found that my flame trap hoses were quite loose, definitely causing a vacuum leak. Wish i had fixed that before the smog test this morning. Fixing it did not change the oxygen sensor readings.

Tried spraying starting fluid around the intake manifold gaskets, vacuum hoses, TB, etc, no signs of more leaks. Going to try smoke testing to be sure.

I really hope this isn't going to take much longer or its going to do me in for sure.