Gas Cap/Vapor Canister and Gas Mileage

**Banshee1967** · 01-03-2008, 01:51 PM

Recently my 98 T/A had a significant reduction in fuel mileage. This was not a gradual occurance, rather it occured and was noticed immediately. I went from 315-330 on a tank of gas and now I am down to 200-210. Two weeks after I noticed the drop in mileage, I finally pulled codes 410/412 and 418 (Secondary AIR) After reading alot in these forums and from reading comments from Orion and SunsetOrangeSS, the mileage loss was not due to the loss of the air pump. Although I have replaced both air check valves, PCV, new irridiums, plug wires, cleaned the maf, throttle body, I still have the same (SAME) fuel mileage...no better, no worse. I did change the differential fluid around the same time as the codes went off, but rechecked weight of lube and level and it is 80/90 and level is correct. The codes have corrected themselves and the relays work..and still I have crappy fuel mileage.

The only things I can think of was possibly a vacuum leak. I checked the vacuum and fuel pressure at the rail. Vacuum was 21 inches, went to 15 on acceleration and bounced back up to 21 without any hesitation. No signs or symptoms of burnt valves or unusual fluctuation on the gauge. Fuel pressure was 59 running and 48 key off.

I did notice a vibration/grinding at the fuel pump in October about the same time this fuel mileage drop occured. It lasted a day perhaps and has not occured since.

I have a propensity to overfill the tank to the point where fuel will spill out. I would say likely every time I fill up, I overfill. Now I realize this could have screwed up the charcoal cannister. The fuel cap is original and replaced that mid December but have not driven the car but 30 miles since. The guy at the auto parts store emphatically states that the gas cap is the key...but I highly doubt 8 mpg

I have had this car since new...Is it possible that a gas cap or combination of a bad gas cap and saturated charcoal canister could cause an 8 mpg loss? I am thinking of buying a new control valve, purge solenoid and vapor canister if the gas cap does not work..

Thoughts????

Thanks!

**Orcus79** · 01-03-2008, 03:44 PM

Yes very much so, it is a very bad idea to over fill the tank, it ain't 1960 any more. Over filling can cause fuel to flow back into the cannister and ovther venting for the tank,It could also inpead the return flow of fuel and put undo presure on the rest of system, especially when the fuel warms and expands after a fill. I wouls say replace the canister and cap and see where you end up. Also kep track of how much fuel you put in vs level on the gage, whent he nozel shuts off thats it.

I do that on both my truck and WS6.

**Banshee1967** · 01-04-2008, 01:42 PM

Thanks for the reply!

Used to my 67 and 68 Firebirds..fill up to the top. Will be better regarded in the future as this has been an expensive, yet enlightening problem.

Just ordered a new charcoal canister, purge solenoid and fuel tank evap control valve from the site sponsor. I'll post results as soon as I confirm difference.

If the car is running rich (obviously) Would I need to remove/clean the O2 sensors or should I not unless I recieve a code from the PCM?

**JL8Jeff** · 01-04-2008, 03:02 PM

Did you ever replace the O2 sensors? They won't keep working correctly forever, even if you don't get a code. That is the key item the computer uses to calculate fuel needed so it's probably worth putting in new O2's. I don't think it's worth trying to clean them. You'll probably damage them just removing them. When I did my headers last month, 3 of 4 O2 sensors ended up with some form of damage just from getting them out. Try some new O2 sensors and see if there is still a serious drop in mpg. I'm down about 2-3 mpg from my summer driving but cold weather will do that as well as longer warmups and more short drives instead of long ones.

**Banshee1967** · 01-04-2008, 03:08 PM

I have 126 k on the odometer and the O2's are original. I was told that if an O2 went bad, the PCM would throw a code. If that is not the case, I believe there are 4 of them. Replace all 4? The forward two and the two behind the cats? I have no codes on the car and the fuel mileage dropped 35% in a week. I wouldn't think one bad O2 would result in an 8 mpg combined difference. Multiple O2's maybe..maybe they are all bad? But for 125k, I guess it may be time.

**Orcus79** · 01-04-2008, 07:14 PM

Yea, i forgot about those a bit, I stil think in old school terms, the O2s will wear and get lazy, probably wouldn't be a bad i dea to reaplce them any ways with that many miles, a little preventative work never hurt.

**rbob93** · 01-04-2008, 09:17 PM

Originally Posted by Banshee1967

I believe there are 4 of them. Replace all 4?

Two after the cats.

**Banshee1967** · 01-04-2008, 10:30 PM

Originally Posted by rbob93

Two after the cats.

You guys are killin' me!

So replace the two to the rear of the cats only and see how she fares? I picked up two Bosch 13444 O2's tonight. Guy says three are the same and one is different...but duh, couldn't tell me what one.

So..replace two rear of cats with 15732 Bosch and go from there? I get Bosch 15732 after catalytic convertor, 13444 before cats.

Bosch, Denso..what ones are you guys preferring?

**rbob93** · 01-04-2008, 10:55 PM

One is different because of the wire length.....other than that they are the exact same.
You asked if you should replace all four.....O2's don't always throw codes when bad so it's a guess, at $75 each that's an expensive "give it a try" so do so only if you feel ok with it.
The front two are used in a closed loop I believe, that's why IF you change them, do the rears.
I hear Bosch are crap sensors.....I don't know for sure.
And yes, the canister could be saturated.....That's a very real possibility as is a vacuum leak.

**Banshee1967** · 01-04-2008, 11:32 PM

Well, considering $3.29 a gallon for premium and a reduction of 35% of fuel economy, and given the 126k on the odometer, I feel real "ok" with replacing the O2 sensors. Like you guys said about the O2's "preventive maintenance".

I have the charcoal canister coming from GMparts direct and will only replace the O2's and canister seperately after a few tanks of fuel in order to alleviate any error in pinpointing the source.

**rbob93** · 01-05-2008, 05:44 AM

^Smart troubleshooting.....let us know your outcome

**Frost** · 01-05-2008, 06:24 PM

your mileage issue has got to be nearly fully caused by O2s.

There is no reason that mileage should suffer over your evap system, especially to that extent. It's just to handle tank-vapors in an environmentally friendly format.

Some mentioned changing the rear O2s. Don't, they are used solely to monitor the cats and in no-way contribute to any PCM fueling schemes. The front O2s control basically ALL fueling outside of WOT. Over time, even if they do not fail outright, they drift. Even if they throw no codes they should still be changed out at least every 50k miles for mileage sake.

**rbob93** · 01-05-2008, 10:10 PM

Well I'd sure like to get the straight dope on what sensors do what.....someone help us out here please

**180ls1** · 01-05-2008, 10:40 PM

Originally Posted by Frost

your mileage issue has got to be nearly fully caused by O2s.

There is no reason that mileage should suffer over your evap system, especially to that extent. It's just to handle tank-vapors in an environmentally friendly format.

Some mentioned changing the rear O2s. Don't, they are used solely to monitor the cats and in no-way contribute to any PCM fueling schemes. The front O2s control basically ALL fueling outside of WOT. Over time, even if they do not fail outright, they drift. Even if they throw no codes they should still be changed out at least every 50k miles for mileage sake.

so you are saying change the front ones and not the back ones? even with no codes.

**Frost** · 01-06-2008, 03:29 AM

Originally Posted by 180ls1

so you are saying change the front ones and not the back ones? even with no codes.

With over 100K on them, heck yes! The problem with narrow band O2s, and even the Bosch lsu widebands is that they don't fail outright, they drift over time. Stoichiometric AFR (~14.62:1) is based on a middle of the road voltage for the front sensors. Fueling at cruise, in closed loop includes a scheme whereby the fuel trimming system is constantly swing things slightly rich then slightly lean. The middle point for this back and for is stoich. Over time, it takes a richer mixture overall to maintain the swing, as the sensor begins to drift from it's manufactured calibration. With tuning software and a wideband, you can stretch this out a bit by lowering the switchpoint voltage until the middle of the road matches 14.6-14.7, though most people jus tneed new ones. Once they drift like that, they usually have become kinda slow responding anyway. Like I said, almost all fueling outside of WOT and immeadiately at startup comes from feedback from these sensors.

**Frost** · 01-06-2008, 03:32 AM

Originally Posted by rbob93

You asked if you should replace all four.....O2's don't always throw codes when bad so it's a guess, at $75 each that's an expensive "give it a try" so do so only if you feel ok with it.
The front two are used in a closed loop I believe, that's why IF you change them, do the rears.
.

You had it right, then you went against yourself

The front two are used for closed loop fuel control. They are the feedback for fuel trimming. The rear two just monitor the cats, nothing else whatsoever.

**rbob93** · 01-06-2008, 04:49 AM

**Bobby97SS** · 01-06-2008, 06:09 AM

The two after the cats will not affect your mixture in any way. Dont waste your money on them. Maybe you could use it at Barnes and Nobles for a book called Fuel Injection and Electronic Engine Management by Charles Probst.

**Banshee1967** · 01-06-2008, 06:10 AM

I replaced the front two about an hour ago..stay tuned sports fans....

**Too Fast** · 01-06-2008, 07:07 AM

Yep, ASE Master Tech here, front O2s are the only ones to be concerned about. Another possibility nobody has mentioned is partially clogged catalytic convertors. They can do that sometimes, causing drastic decrease in MPG. Rear O2s only for catalytic convertor efficiency monitor.
Another thing: thermostat. If it is opening too soon and staying open too much, the engine runs cooler and never goes into closed loop, thus running richer. Fixed a car like that last summer, not an LS1 car tho.
Keep us posted.