Failed Smog: HC, NO OK - but CO is high

[vincewarde]

I have a 1993 Explorer w/4.0 - and I am having a hard time passing smog. I have changed the Oil, Filter, Air Filter, Plugs and pulled codes - everything passed, no indications of any problem. The plug were platinum and had been in there 50k plus. That said, they looked like the motor had been running rich. The only problem is high CO. Everything else passes.

My thought is that if the problem is the Cat Convertor, we should see high HC, not just high CO. Even though there are no error codes, I'm thinking that the O2 sensors could be bad or clogged. I only remember replacing one of them several years ago. What do all of you think? Thanks!

 

[Josh P]

Is it California or federal emissions engine? Do you have EGR? A failed or failing engine coolant temp sensor could cause the engine to run rich.

 

[1.8TTony]

Low temperature thermostat or one that is not closing all the way will keep the engine running colder resulting in high CO numbers.

Dirty fuel injectors spraying an unacceptable spray pattern. A dirty fuel injector may not atomize the liquid fuel enough so that the fuel is properly compressed into a vapor for ignition. Resulting in high CO numbers.

Oxygen sensors......you say you replaced one several years ago.

When you have one good oxygen sensor and one bad oxygen sensor, you replace the bad one. No problem, all is running good.......but eventually the computer is going to see that the new oxygen sensor is working great while the performance of the old one is degrading. What you end up with is an engine with both cylinder banks running rich due to the computer adjusting fuel trims to compensate for the bad oxygen sensor.

Maybe replace the old sensor.

 

[vincewarde]

Is it California or federal emissions engine? Do you have EGR? A failed or failing engine coolant temp sensor could cause the engine to run rich.

I ***believe*** that it is a federal emissions engine. Originally sold in WA state.

Would the fact that it fail with worse readings after I was sure to warm it up point to the temp sensor?

 

[Nadams01]

I ***believe*** that it is a federal emissions engine. Originally sold in WA state.

Would the fact that it fail with worse readings after I was sure to warm it up point to the temp sensor?

possibly, as tony pointed out excess CO is a result of un-burnt fuel in the exhaust. Hook up a vacuum gauge to the tee off of the left side of the intake manifold and watch for a fairly steady needle. the amount of vacuum will vary dependent upon altitude but look for a nice steady needle. An erratic needle or very low (under 10 inHG I think) vacuum could point us to either bad valves not seating fully or a vacuum leak. Pull some plugs off of both cylinders to see if it has un-burnt fuel or oil on the electrode. When your at 1/4 tank of gas, throw in a can of BG 44k into the gas tank, its pricey 30 bucks a can but has the best results for cleaning fuel injectors. There is a TSB about carbon build up on the pistons ( TSB 01-19-7 MSG if you want the PDF) and running a vacuum based cleaner through the vacuum nipple on the bottom of the intake behind the throttle body could help clean the chamber up. The vacuum hose that runs through that nipple leads back to your purge valve for the EVAP system, if the valve is stuck open or getting stuck upon excess fuel vapors can cause a high CO reading. Remove it, and try to suck or blow through it the solenoid is normally closed. using 12v power the solenoid off and on a bunch of times to make sure it is not stuck open. here is a graph for the ECT and IAT. test both voltage with the sensor plugged in by backprobing the wires to measure voltage and disconnect it to measure resistance to compare both signals. Test cold, then test hot. Tolerances are within 15% of graph.

 

[vincewarde]

possibly, as tony pointed out excess CO is a result of un-burnt fuel in the exhaust. Hook up a vacuum gauge to the tee off of the left side of the intake manifold and watch for a fairly steady needle. the amount of vacuum will vary dependent upon altitude but look for a nice steady needle. An erratic needle or very low (under 10 inHG I think) vacuum could point us to either bad valves not seating fully or a vacuum leak. Pull some plugs off of both cylinders to see if it has un-burnt fuel or oil on the electrode. When your at 1/4 tank of gas, throw in a can of BG 44k into the gas tank, its pricey 30 bucks a can but has the best results for cleaning fuel injectors. There is a TSB about carbon build up on the pistons ( TSB 01-19-7 MSG if you want the PDF) and running a vacuum based cleaner through the vacuum nipple on the bottom of the intake behind the throttle body could help clean the chamber up. The vacuum hose that runs through that nipple leads back to your purge valve for the EVAP system, if the valve is stuck open or getting stuck upon excess fuel vapors can cause a high CO reading. Remove it, and try to suck or blow through it the solenoid is normally closed. using 12v power the solenoid off and on a bunch of times to make sure it is not stuck open. here is a graph for the ECT and IAT. test both voltage with the sensor plugged in by backprobing the wires to measure voltage and disconnect it to measure resistance to compare both signals. Test cold, then test hot. Tolerances are within 15% of graph.

View attachment 435965

Thanks for the detailed instruction. I'll have to get a vacuum gauge, don't have one at this point. I'll let you know what I find.
BTW, just checked codes again and both KOEO and KOER returned nothing. Nothing in memory either....

 

[Josh P]

What's the CO reading and what's the emissions limit? I'm curious how bad it is failing.

 

[vincewarde]

What's the CO reading and what's the emissions limit? I'm curious how bad it is failing.

The limit here in Idaho (I fled CA) is 1.2, it's failing at 1.53.

 

[Josh P]

The limit here in Idaho (I fled CA) is 1.2, it's failing at 1.53.

Not too far outside the limit. I'm not familiar with the testing process where you are at, is it just an idle test or is the car load tested on rollers too?

 

[1.8TTony]

Is it failing CO levels at low RPM's or is it failing CO levels at high RPM's? ......or at both RPM's?

OBD1 smog test requires emissions to be measured below 1,200rpm and again above 2,500rpm.

If it's failing at idle, you can raise the idle as high as 1,200 rpm. The higher the engine speed, the cleaner the engine burns.

I remember I had a car years ago that would fail the low idle emissions.........the smog tech adjusted the throttle cable so that the low rpm (engine idle) speed was right around 1,100rpm......it passed.

 

[vincewarde]

Not too far outside the limit. I'm not familiar with the testing process where you are at, is it just an idle test or is the car load tested on rollers too?

As noted above by 1.8TTony, it's a low high idle test. It's failing at high RPM.

 

[vincewarde]

Is it failing CO levels at low RPM's or is it failing CO levels at high RPM's? ......or at both RPM's?

OBD1 smog test requires emissions to be measured below 1,200rpm and again above 2,500rpm.

If it's failing at idle, you can raise the idle as high as 1,200 rpm. The higher the engine speed, the cleaner the engine burns.

I remember I had a car years ago that would fail the low idle emissions.........the smog tech adjusted the throttle cable so that the low rpm (engine idle) speed was right around 1,100rpm......it passed.

Unfortunately, it's failing at high RPM.

 

[1.8TTony]

Unfortunately, it's failing at high RPM.

That's weird.......and no codes.......I wonder if the problem is timing chain (cam shaft) related.

If I remember correctly, the timing chain tensioner in that year engine is a mechanical tensioner. You install the tensioner, pull the pin. Spring pressure pushes the tensioner guide against the chain.

What if that spring has lost its strength and the high crank shaft RPM's is creating enough force that the chain is pushing back against the tensioner?...and as a result, the valve timing event is slightly out of time, causing the rich burn condition at high rpm.

Crazy idea, I don't even know if this is possible.....but damn, you've tried everything else.

 

[Josh P]

Acetone will reduce the emissions, but it is important to get to the actual cause of the problem or it could reduce the life of your cat. I'd replace the other O2 sensor, clean the maf sensor, test the ECT sensor. How many miles are on the spark plugs and wires?

 

[vincewarde]

PROBLEM SOLVED!

Well, I decided to replace both upstream O2 sensors (figured out that the one I replaced earlier was the single downsteam sensor). Crawled under and found that the one on the left bank had come loose. I could wiggle it! Strange for something that is usually hard to remove. Replaced both sensors, cleaned the MAF and checked the PCV valve. Originally flunked the test with CO of 1.3 - now the CO is .003!

So, hopefully no more running rich and maybe a bit better gas mileage. Thanks to everyone for your help!

 

[1.8TTony]

PROBLEM SOLVED!

Well, I decided to replace both upstream O2 sensors (figured out that the one I replaced earlier was the single downsteam sensor). Crawled under and found that the one on the left bank had come loose. I could wiggle it! Strange for something that is usually hard to remove. Replaced both sensors, cleaned the MAF and checked the PCV valve. Originally flunked the test with CO of 1.3 - now the CO is .003!

So, hopefully no more running rich and maybe a bit better gas mileage. Thanks to everyone for your help!

Geezus H. Effin' Christ........what an ordeal. Glad you got the rig sorted out.

I'm gonna pop open an ice cold beverage and celebrate for ya.

And yeah, that's a new one.......the sensor backing out of the threads on its own. In all my MANY years of doing vehicle repairs at work (and at home) I've NEVER came across an oxygen sensor which became loose on its own. Usually, ya gotta get a wrench on them and use gorilla strength to break them loose.

 

[Fix4Dirt]

Geezus H. Effin' Christ........what an ordeal. Glad you got the rig sorted out.

I'm gonna pop open an ice cold beverage and celebrate for ya.

And yeah, that's a new one.......the sensor backing out of the threads on its own. In all my MANY years of doing vehicle repairs at work (and at home) I've NEVER came across an oxygen sensor which became loose on its own. Usually, ya gotta get a wrench on them and use gorilla strength to break them loose.