Gas Mileage gone somewhere

My 97 blazer 4x4 was getting about 300 miles to the tank. now after a tune up from our local mechanic it is getting about 245-260 at best. $ wheel drive is not stuck on and it is running the same as it was before the tune up. The engine is idiling the same and not backfiring or sputtering at all.

Any ideas on what it could be? Or any ideas of where to start, beside driving the truck into my mechanic?

Thanks for your help in advance.

Sounds as if it is running pretty rich, wouldn't you say?

Just kidding. Do you have the SES light on? Was it ever on? Need a little more input.

did he use AC/Deloc parts of HO-Chi-Min flashing blue lite specials

You might want to find out if he adjusted the ignition timing. If not done correctly, it could be wrong and affect milage. Other possibilities are change of driving habits:
In the winter people sometimes warm up their vehicles (more running, less driving).
Starting a fuel leak, injector or regulator, enough to affect milage but not enough to affect performance.

Sounds as if it is running pretty rich, wouldn't you say?

Just kidding. Do you have the SES light on? Was it ever on? Need a little more input.

The SES light comes on and off. It was on and the mechanic said that it needed a tune up for it to go away...Well that was wrong. It still lights up every once in a while. The winter is here in NY so we do tend to let it idle for a bit, but only for 5 min at most before going out to go.

When the light is on it doesn't seem to run any different from when it is off. I haven't had another mechanic look at it, but I am getting to that point. Just sank almost $500 into it last week before christmas...Hurt a little bit.

Thanks.

First, take it to autozone and get the EXACT codes stored in the computer. Once you have that we can help a bit more.

In most instances, as you described, the O2 sensor may be the culprit. However, the mechanic that did the tune-up should have checked for the codes prior to the tune-up. As per "rlith"'s suggestion, go get the codes, it removes some of the guess work.

Any time the check engine lite comes on it goes into a pre set computer mode for fuel and spark control and the fuel milage WILL go way down. That is what it is designed to do.

Got the code from AutoZone. It is a PO300 and they are saying that there is a Misfire on one of the cylinders. The truck is running and idiling fine and a mechanic cleared the code this morning. Said it was misfiring on cylinder 3...And other thoughts for me to keep in the back of my mind.

Thanks!

Typically that denotes a bad injector... It's either clogged or not opening all the way. Simply replace the #3 injector and you should be fine.

Typically that denotes a bad injector... It's either clogged or not opening all the way. Simply replace the #3 injector and you should be fine.


Thanks for the help on this!

Have a good New Year. Nice looking Blazer. Checked out the site that you have.

Thanks for the help on this!

Have a good New Year. Nice looking Blazer. Checked out the site that you have.

Thanx.... I'll hopefully have the v8 put in at springtime...:)