Gas Mileage 02 Sensors and Spark Plugs - Once Again

Can these o2 sensors be checked or cleaned or something or should they just be replaced?

The 02 can be checked with a scanner, you should see quick movement between rich-lean when they are doing their thing. No cleaning advised, just replace it!

I am down to 17 mpg now. Changed coils and sparkplugs, ignition wires still have a normal resistance.
What should I do next ?

Do you have any shuddering (misfire) in 4th gear going up hills? Who suggested you change the coils and sparkplugs? Have you scanned to look at @ fuel trims, 02 and EGR performance? Did you install the fuel filter and 02 sensor? BTW, it is not a good idea to disconnect the 02 while driving.

You should look to into buying a scan tool. You've probably have spent enough money to buy a scan tool. A weak coil won't cause the gas mileage to drop that badly and an O2 sensor usually gradually gets weak. And even when it trips the check engine light the gas mileage doesn't suffer that bad. When Ihad an experience with both the bad coil and O2 sensor gas mileage was still 20-21mpg city driving. Its winter now and I can seem to get over 19mpg but I can live with it.

The sparkplugs seemed old, the coils were replaced because they went bad.
The engine runs fine under all circumstances and never hesitates or shutters. It still gets the same top speed as specified in the booklet.
I will top off the tank to see if the fuel gauge may be incorrect.

We had a confusing thread going on here with two members (TheDoughboy & ZiggyPA) discussing similar problems in the same thread. This particular thread was created to separate the two and eliminate the confusion. It is better to start a new thread, rather than hijack another members thread.

The gauge is correct, it was about 17 MPG. This was highway 90-95 mph.
I checked again with highway 80 mph, this is about 21 MPG.
Airfilter was also replaced, but didn't make a difference.

Is this (http://www.ngksparkplugs.com/part_finder/car_truck_suv/partcloseup3.asp?stocknumber=21002) the right o2 sensor (NGK/NTK part number 21002 , pre catalyst) ?
GMparts lists 3 different oxygen for the 91 PA ?

The gauge is correct, it was about 17 MPG. This was highway 90-95 mph.
I checked again with highway 80 mph, this is about 21 MPG. I am down to 17 mpg now. Changed coils and sparkplugs, ignition wires still have a normal resistance.
What should I do next ?Slow down :confused: :rofl:

Can anyone please confirm the sizes of the oxygen sensor, thread size 18mm spanner size 22mm, 'thick' or 'thimble' type sensor?

NTK 21002 is an exact fit.

I replaced the oxygen sensor, and am now up to 22mpg highway.
However I noticed the milage seems to be connected to the cruise control problem? I drove 90 miles highway, for some reason the CC worked perfectly. Milage was 22-23. The way back, I got only 18-19 mpg. There was hardly any wind and I didn't drive faster. But the CC was acting up again, when I got it to work it was going slowly in a up and down motion, slowing and then adding speed and so on.
Does the CC work on vacuum?

The last time I had a cruise control problem an EGR valve code was set soon afterward. When I had it on cruise I would surge up and down like you described on the same trip that I alway take. Which was pretty flat. It would surge to like 3000 rpm even though the cruise was set on 60mph and it wouldn't stay at 60mph either. hope this helps.

22mpg highway is still not that great, not even if you have the performance gear ratio...

Also this being a 91 Park( first year for this C body design ) The mileage should be better than 22 Mpg..

This engine is also EGR'less... One of the 3 years GM made the 3800 without the EGR..

For the O2 on these cars I usually recommend the AC Delco AFS 20... Stay away from Bosch O2's with the 3800's

I have a Bosch O2 sensor that I bought early this year and it has set a code one time. Changed my old O2 sensor which was getting lazy but didn't experience a significant increase in gas mileage.

Why wouldn't you reccomend Bosch O2 sensors for the 3800 and how much do the Ac delco or NGK O2 sensors cost.

I paid something like $35 for the NGK sensor including shipping, but that was in Europe and US car parts are hard to get.

The speed increase/decrease is only small, about 2 miles but noticable. But on the long term I believe this affects fuel efficency.
Overhere mpg is advertised as 24 to 25 highway, is this correct? My PA has the 3.08 or 3.09 ratio I think.

I installed the MPG+ ignition wires, which have 0 ohm resistance and am now up to 23 mpg highway (80-85). The best result so far, and a MPG increase of about 7-10%. Still there would be another factor of the poor milage, with these wires I should get 26 mpg highway.

Also what is considered as an average "highway speed" in the US?

You should probably check out http://www.fueleconomy.gov/
I believe it would tell you what is considered average if you knew where to look. I believe it would also tell you what mileage you should expect to get on your car with the new system (which has lower expected numbers), unless it doesn't go back far enough for your model year. I think basically if there was a rating originally there is a new one now, but I'm not sure. However, up until a year or two ago, I always averaged the highway mileage from the old system. Now I'm lucky to average the highway mileage on the new system. That being said, I don't know if it is age getting to the engine or whether something needs fixed, but just because you are getting what it implies you should get doesn't mean you never got better.

Thanks for the link Dustin.
It lists the 91 PA 16 MPG city, 24 highway and 19 combined under the new system, and 18 city, 27 highway and 21 combined under the old system.

Curiously enough, the milage of the "new system" is about what I am getting. Maybe I should get 25 mpg highway with these supersonic ignition wires, but it seems the milage isn't as bad as I thought.