1994 Sedan Deville w/ 4.9L didn't pass emissions. They said the HCs were too high. The pass level is 220 PPM and it put out 290 PPM. It passed the CO test without problems 0.91 and the pass level is 1.20. I am very surprised by this as the car runs great. The SES light is not on and no codes come up. I did have Premium fuel in at the time of the test plus you have to idle for a while before the test. Any info on what I should address to solve this problem would be appreciated. I just changed the oil, fuel filter, airfilter, PCV valve. Should I change the plugs and plugs wires? They are fairly new with only 8K on them at most. Any info would be appreciated. Thanks.

Even if the O2 sensor isnt setting a code, it might be getting tired. For as cheap as they are, I'd change it anyway. Can't hurt becuase it'll save you some gas, and might even get you down to the acceptable HC level. Of course check your vacuum hoses too.

Even if the O2 sensor isnt setting a code, it might be getting tired. For as cheap as they are, I'd change it anyway. Can't hurt becuase it'll save you some gas, and might even get you down to the acceptable HC level. Of course check your vacuum hoses too.

shange the egr valve too

clean two egr tubes inside throttle body

I live in Plano, Texas where the emissions requirements are kind of stringent and my '91 failed the emissions test with new plugs, new PCV valve, new O2 sensor, new catalytic converter, new EGR valve. It had over 190,000 miles and was running like a top at that time...as a matter of fact I saw guy with a Chevy van that was smoking like a cigarette and it passed!!!! Well, I had to reluctantly take it to the dealer (Crest Cadillac here in Plano) and they cleaned out the intake passages (and I suspect some other things) but $140 later it passed and it has passed every since. I will admit it was a good $140 because I'm sure doing something major would have cost more!!! so I say take it to a well respected dealer in your area (if your regular mechanic can't figure it out and does not do inspections) and let them take a stab at it.

Take a look at what makes each emissions. don't R&R parts guessing.

HC = Hydrocarbons are unburned raw fuel that has gotten back out the tailpipe. typicaly causes are:
a. lean mixtures, flame front has not been exposed to all the fuel.
b. Poor chamber/poor piston design. raw fuel was trapped in crevices where it was not able to be exposed to the flame.
c. too much cam overlap
d. stuck open PCV
e. vacuum leaks (hoses, throttle body loose, etc)

CO = CO in a way is similar to HC, but very different. This is fuel that was exposed to the flame front fully, but not enough free O2 was present to burn the HC molocules. Common causes are:
a. dirty air filter
b. too rich mixture
c. leaking injector
d. wrong injector timing (miswired)
e. wrong ignition timing
f. wrong ignition wiring
g. thermostat bad, is P.0.4 reading 92 C or higher?
h. check Integrator and Block Learn values (P.1.6 and P.1.8 I think) for Integrator at 128 and Block Learn around 128. If BLM is high >150 at cruise, not accel or decel, then there is a vacuum leak, or something making the mixture lean and the ECM has to compensate. If below, <90 at cruise, not under decel or accel, then it is rich and the ECM is pulling OUT fuel to keep the mixture in line.

NOx = Oxides of Nitrogen
This is an effect of high combustion chamber temps. Caused from chamber temps over 2049F. Common problems are:
a. EGR plugged or not operating
b. Excessive chamber deposits
c. Too high ignition lead
d. Too high compression
e. Camshaft advanced too far
f. Lean mixtures contributes to this problem

Now, using this info, look into what would cause the emissions problem that you have. Do not arbitrarily replace parts not having a clue on what they do. EGR valves last 200K+ miles on most cars. O2 Sensors typically last a lifetime if not abused (poisoned from antifreeze in exhaust - look for white sparkly dots on sensor, wrong RTV used that is not O2 sensor safe, etc). Cat Converters also do not just go bad, only if they are abused like O2 sensors. You can check O2 sensor with diagnostic display (I don't know proceedure on 94, ask on the 4.9L forums)

So check timing and set to 10 degrees with pins a-b on ALDL shorted, do idle learn proceedure and check items like plugs, wires, cap and rotor for wear, deterioriation, damage.

Was engine fully hot when you tested it? Cold engines will fail. On your way to the emissions station, go drive for 20 min on the highway at highway speeds. Then at the drivers testing let car idle and get good and hot. Hotter is better for emissions.

My 91 Bonneville passed with 0.00% CO and 3 ppm HC at idle and 0.01 CO and 5 ppm HC at 2500 rpm.

1994 Sedan Deville w/ 4.9L didn't pass emissions. They said the HCs were too high. The pass level is 220 PPM and it put out 290 PPM. It passed the CO test without problems 0.91 and the pass level is 1.20. I am very surprised by this as the car runs great. The SES light is not on and no codes come up. I did have Premium fuel in at the time of the test plus you have to idle for a while before the test. Any info on what I should address to solve this problem would be appreciated. I just changed the oil, fuel filter, airfilter, PCV valve. Should I change the plugs and plugs wires? They are fairly new with only 8K on them at most. Any info would be appreciated. Thanks.

Get that bottle of that "gauranteed to pass emissions" from your local automotive store and follow the directions and it worked great for me...passed with flying colors even with the flowmaster daul exaust that I have on my 92 deville. For 10 bucks or so it was worth every penny for me anyways

I would go with a new (AC/Delco only) O2 sensor. Make sure the throttle body is clean, and the air filter is new.

Also, replace the PCV valve with a new AC/Delco unit.

Something else that can really affect the test is oil quality. Change the oil with a good quality oil and replace the filter. I'd recommend Rotella T 15W40.

EGR function should not affect HC's directly, but can influence in other ways by causing an overly rich mixture if the exhaust gases are not being metered into the system properly. EGR function is directly calibrated into the fuel maps.

I doubt the convertor is bad, since the CO is good. The convertor mainly controls CO and to some extent the HC.

Another secret weapon I use is to retard the timing about 8 degrees and pour in a jug of methyl hydrate into the tank.

I just went through all this BS to get my '86 Fleetwood to pass.

Mike

Yes, old or bad engine oil WILL make a difference. Oil burning in the chamber messes up emissions. I think mainly NOx, but being the rings aren't sealing, it is likely HC problems too. The fuel can mix with the oil that gets left in the chamber and burns, but burns slowly, so HC is likely a result.

So good fresh oil is always a must before an emissions test. Synthetic is rumored to have better ring sealing properties. But has yet to be proven.

thermostat bad, is P.0.4 reading 92 C or higher?


How do you get into that part of the ecm to read those readings? I have a 87 and I havent been able to find those readings. Please help as I would like to see the temp.