Ok I have been searching the net for about 3 weeks and here I am posting now. I have an 02 blazer 4x4 that I have had for 6 years now. It has 87k. It is my wifes car. It never got good milage, 16mpg at best...thought I was going to get 19-21mpg. A few months ago the check light would come and go about 3 different times for about a day. Now it is not on. It is not showing any codes.
A few weeks back I filled it up...by the end of the week I got in it and it was empty...filled it up again....147 miles and 14.2 gallons. My wife doesnt have a heavy foot. I had already replaced the fuel filter and cap and rotor about a month prior. Got a new air cleaner and new plugs and it was time to change the oil, and I put in a bottle of lucas oil injector cleaner.
The blazer ran fine, no smoke or soot before all this, just a touch rough at idle. My wife says now its take more pedle to get it to go, so there may be a power loss. I don't drive it much so its hard for me to tell. I used to have a 94 s10 and I know its not a powerful as that was...I gave that to its being a 4x4?
So now after the new plugs and such its getting 11.8mpg. I was thinking O2 sensors or MAS. But I am not getting any major symptoms of either...and no check light. So I think I will just try and clean the MAS and the throtle body. I dont know if I can clean the O2 sensors or not. Any help would be appriciated. Thanks

 

Definitely clean the throttle body, MAF, and the EGR valve (get a new EGR gasket with the integrated screen and don't forget to clean the supply pipe that runs from the exhaust manifold up to the EGR valve with a long 1" diameter brass pipe cleaner).

As you have likely found, there are a lot of threads discussing this, but there are some things you haven't stated as being tried/replaced that are mentioned in most of them that I found in a quick search. If you haven't already replaced the cap, rotor, & wires, do so now using quality parts.

The upstream O2 sensors (ones closest to the exhaust manifolds on each side) are the only ones that are responsible for fuel management. At 87k miles, I wouldn't expect them to be a problem, but without putting a scan tool or oscilloscope to the truck, you really cannot tell if they are providing timely responses to the PCM. If you do decide to replace them, use AC Delco or NGK sensors only. In my experience (albeit somewhat dated now), these two brands far outperform the competition in terms of activity.

A fuel pressure test could tell you if you have a leak or weak fuel pressure. A gauge can be purchased for ~$30 from most autoparts stores. Your engine requires no less than 55psi with the key on, engine off, pump running and should be ~60psi. The pressure should remain above 50psi for 10 minutes after the pump shuts off.

 

I cleaned the MAS, the hot wires were a little darker in color. there was some build up in the TB. Havent checked the EGR yet. got to wait till the wife gets home. I did look for it before but with no luck... I am guessing it is behind the distributor? Its not in front of the manifold.
More info....about a year ago or so it ran like crap at idle....only about 400rpm. It never died on me or my wife. it was a problem that would come and go...no check engine light either. I some how think it was related to a bad battery. I dont understand why because once its started the power should come off the alt. I have had 2 batteries go bad in it and everytime you jumped it the care would run like crap at idle(it would die then).
Again this blazer has been neglected a bit...the wife doesnt know much about anything....if it runs she drives it, now its time to get it back in shape. I realy don't know how long its been getting bad MPG.
I don't trust any mechanic....most will take you for a ride if your not carful. I would rather do everything I can before going to a Mechinic
Now could the EGR be blocked enough to cause the loss of MPG and not throw a Check light? Same with the MAS and O2 sensors?
I am thinking its a combo of everything causing the loss of MPG. forgot to mention that after the new air filter and plugs and lucas injector cleaner I got 11.8 mpg city. so I realy only have to gain 4 mpg to get it back to normal

What about a fuel leak in the spider? I dont smell any fuel or see that anything has been leaking. Could you exspand on the location and what to look for.

 

I strongly suggest checking the possibility of a leaking fuel pressure regulator. Grab a fuel pressure gauge and work through 96+ Fuel System Diagnosis.

 

Ok I will see what I can do with a regulator check...thanks.
So if I am not getting the 60psi or so const. I have a problem. Can you esplain to me how if the regulator is bad how that gives me poor MPG.
Is there any way to check if the spyder is leaking other than pulling the upper manifoild off?
And if the spyder was leaking in the manifold wouldnt I have symptoms? I would think that would mess with the air/fuel ratio. when I replaced the plugs they looked great. whitish with a touch of golden brown. That is why I am frustrated...I have no major symptoms other than poor MPG.

 

Mid-02 was the transition to the MFI engine. If you have the later model year vehicle, you may not have an EGR valve. So you can cross that off your list if you have nothing between the alternator and the A/C compressor.

As far as checking for a leaking spider, if you have pressure bleed down using the pressure test port on the engine, the next step would be to isolate the pump by adapting the gauge to the line that comes from the tank into the filter. If you still have the low pressure and/or bleed down there, the pump is at fault. If not, the leak is inside the plenum.

 

well the wife came home so I thought I would check the EGR....cant find it? I hear that 02's dont have an egr? A little help here please

plus is the presure regulator under the Plenum

 

Well if I have a leak between the pump and the filter...it will give a low psi but wont cause poor MPG. If the regulor is low that still wount give me bad MPG. If it is high then posibly bad MPG. If I have a leak in the regulator though inside, or the spyder is leaking then I will have poor MPG