Stinky Exhaust when cold, poor mpg, passed e-test "smog"

[1979lee]

I just passed my states "smog test" passed great. and my car runs great, but the mpg's aren't very good, and Stinky Exhaust when cold, or after the car sits 30 minutes or longer. smells like i have no cat.(but i do)!. after the cat is warm no stinky.

 

 no codes, no rotten egg smell from exhaust no visible fuel leaks. 1997 lumina 3100 V-6 100k. i had the throttle body off last week, checked the egr passage, nice and clear, i didnt remove the egr. but this car i tell you very little/no blowby. very clean inside. in 2014 i did swap in a 2005 impala 3400 tb, so in 3 years and 20k ive had not a problem.  sorry if i rambled...

 

any ideas?

[Nas Escobar]

If it smells like unburnt fuel, you should start checking the ignition stuff. My Cutlass was getting 10mpg and was sluggish but had no noticeable smell (although the cat was bad when I cut it off). Turned out my coils were bad. It would be a good idea to check em first and see what happens.

[1979lee]

thanks for the reply!

 

coil packs.?ok but its not missing.runs great.not sluggish and it has almost new plugs (plaitum delco's) and wireset (40$). 

 

thx

[Nas Escobar]

thanks for the reply!

 

coil packs.?ok but its not missing.runs great.not sluggish and it has almost new plugs (plaitum delco's) and wireset (40$). 

 

thx

 

Mine didn't miss either. Mine was sluggish when cold but ran fine when warm. Granted my coils were pretty bad as well, but bad coil packs don't have to misfire to be bad. They can simply have a weak spark causing the engine to compensate with more fuel. 

 

If you have another GM V6 with the coil packs, it wouldn't hurt to swap in the other car's coils and see if there's a difference. I was the same way as you when I had my issue until I said "doesn't hurt to try". My engine ran super smooth after that.

[Galaxie500XL]

Hmmm....I'd expect, though, with a misfire, there would be oxygen in the exhaust stream, because of incomplete combustion.  I find it odd their test shows 0% oxygen in the exhaust stream...I'd expect at least SOME residual oxygen, even if things were 100% good.  Additionally, HC emissions are very low, I'd expect them to be pretty high if there was a misfire.

[rich_e777]

Clogged up O2 sensor?

[Galaxie500XL]

Definitely a possibility.  Typically, when O2 sensors are approaching the end of their lives, they falsely report a lean mixture, which causes fuel mileage to drop, because the ECM is enriching the mixture beyond what's necessary.

 

I've found that they typically begin doing that a very long time before a code gets set.  On my fuel injected cars over the years, I've typically watched fuel mileage like a hawk.  If I've seen the mileage drop significantly, no other cause could be found, and the O2 sensor was over 30,000 miles old, I make a habit of replacing the O2 sensor.  I've never failed to see fuel mileage and performance improve. 

 

I only will use the OEM O2 sensor in my cars...the AC/Delco O2 sensor is generally around $30.00 on RockAuto.  I've not had great success with store brand, or Bosch sensors, with the exception being the LeBaron, which seemed to like the Bosch sensor just fine.

[1979lee]

thanks you guy's! awsome ideas.  as my o2 sensor is TWENTY years old and 100k, guess i'm changeing it!, how bad a job is it?

if memory serves me, its in the rear head pipe? so its another unbolt tge doggy bones and wrachet strap the engine fwd deals? meh not that diff.

 

how does one test a coilpack. i know it can be done, i did it before on my old 98 lumina(sold in 2013, and its now many beer-cans aka scraped).

 

thx

[Galaxie500XL]

3.1's are pretty easy.  You're correct, it's behind the engine, driver's side, on the downpipe.  With the proper O2 sensor wrench, the only really difficult thing is figuring out what extensions you need to reach back there.

 

Just be thankful you're not replacing one on an LQ1.  The engine is so wide, the only way I was able to replace mine was to remove the plenum...so, both times I had the plenum off to do a timing belt replacement, I replaced the O2 sensor at the same time.

[1979lee]

well the car has been running good. maybe it was just my nose, no really stinky exhaust, well no more then any 20 yr-old car with a cold cat-convert....  it never has ever smoked gray/black or any color!.

would having put on the 3400 tb cause worse mpg's? besides enjoying the power boost lol!

 

thank you

[1979lee]

ok update time.  car has been running good,but the gas mileage is really crap. i went 120 miles in town and i had to add 8.4 us gallons to fill up my car. = 13 or 14 mpg....WTF?   

 

 

i will be cleaning my egr tommorrow. and ohm checking my coil packs....5000k-7000k ohms ive read is the range.

[1979lee]

UPDATE,  fuel pressure regulator vacuume line smells like gasoline?

All coil-packs check good.6.4-6.8 ohms

[Imp558]

Check the IAT and CTS resistance against truth tables and make sure they're okay, or better yet replace them if they're oldsince they aren't expensive. Let's put it this way, the PCM sees an intake charge of -20F and sprays more gas because it's cold, same with the CTS, the PCM bases a lot of the fueling vs temp so if those senders are erroneous but not open or shorted the PCM plays along and doesn't rationalize that the numbers are crazy.

[RobertISaar]

UPDATE,  fuel pressure regulator vacuume line smells like gasoline?

All coil-packs check good.6.4-6.8 ohms

 

the FPR failing and sending large amounts of fuel into the intake isn't unheard of. you should be able to leave the line unplugged, roll the key to run position and see/hear it spray if that is the case.

[1979lee]

Hey guy's,

 

well the fpr just pissed gas out the vacumme line hole, so i put on a brand new fpr.

no more putrid fumes, and the idle has really smoothed out.

cleaned the egr, it did not need it, the egr passages were very clear/clean.

 

so its fixed!(crosses fingers)

 

thanks everyone, y'all are  badasses! i'd buy you a   if i could!

 

update, just drove 20 miles and this car ran great, smoother then it has run in 2 years!

more power too! if a 3100 v6 has such a thing!

no fuel leaks either, starts better, the exhaust fumes have really cleared up, it was just awful before.

 

thanks

[1979lee]

Update 4-30-17

 

car has been running well. mpg's are up, in mostly city driving 17.32 mpg. i do sit idleing with the a/c on for lunch most days. so some loss in mpg's expected. i also ran a good chevron fuel cleaner. 

https://www.walmart.com/ip/Chevron-Techron-Concentrate-Plus-10-oz/16647989

really smoothed out the lil 3100 six. now all I need to do is fix the oil-pump drive o-ring (F**K) changing

that!

 

thanks again.