OK...I've followed the old posts on this problem and don't see much resolution so here is a new thread...

Recently began getting about 10mpg around town when I was always getting 16-17mpg for the same ride.
Background: ... this is a new engine (Jasper) that has run great for 6k miles...all still seems good, runs great, no codes, smooth etc.
Checked: Checked the vacuum lines as best I could, all are new with the engine swap..., fuel injection system is new with engine swap, distributor is new, air cleaner is fine, pcv is fine, egr seems fine, can't find any leaks. Just changed both oxygen sensors before the cat and still only get 10mpg!!!

What else can I check? Gas cap seems good...hear a release of pressure when opening the cap, engine runs fine, cat seems fine (exhaust is not pukey smelling), no codes...but suddenly over the last two weeks we can just about watch the gas guage drop as we drive!!! ...

Oh...one thing I'm suspicious of...the 4 wheel drive seems to drift into engaging kind of...we occasionaly start to hear a hum with driving speed and have to toggle the 4x4 and 2x4 buttons to get it to stop...but it doesn't seem as though the 4x4 actually engages unless we push the button nor does it feel like it is inadvertantly engaging over the last few weeks causing the low mileage...but I figure it is worth mentioning...

Any new ideas or followup answers that don't appear in the old threads about this problem???

This is a '97 Blazer 4x4 with the 4.3l, w/ A/C

 

You would see somewhat of a decrease in fuel mileage if the front axle were attempting to engage. I'm guessing you have a 3-button 4wd switch on the dash, but since you haven't listed what you are driving, I'm not going to speculate further.

Bad gas? Crappy aftermarket O2 sensors?

 

Suggest checking all vacuum lines and connections.

Just a thought.

 

Vacuum lines are all new with the engine swap and seem OK...

The ox sensors are both Bosch...is there any field test for them? I don't know of any.

 

How long have you had the truck? What sort of mpg did you get in cold weather last year?

Also where is your temp gauge sitting? If it's around the demark half between 210 and 100, you have the same problem several of us are seeing

https://blazerforum.com/forum/2nd-ge...en-pics-67856/

I checked my temp gauge with a live ECT scan yesterday and it is accurate. Thermostat should keep coolant at or above 175 for closed loop.

Today, it's even lower, though it's no colder

 

Here's the clue....


As Kyle mentioned, the 4WD vacuum actuator switch on the transfer case is sticking. Vacuum is slowly leaking past the seal in the switch and partially engaging the front axle actuator, that's the "hum" sound you're hearing. When it finally fully engages the front axle, your fuel mileage goes right out the window! The ball is spring loaded and should push in relatively easy and spring back out on its own.

The vacuum switch



is located on the driver side of the transfer case, inboard and slightly above where the front drive shaft connects to the transfer case. Dorman part #600-500 available at most auto parts stores for under $20 and takes less than 15 minutes to replace it. Pull the vacuum connector off*, 15/16" 6 point deep socket, 3/8" drive, and a ratchet. For 2 door, reach forward over the crossmember from the rear. 4 door you can get right at it. Piece of cake!

* If fluid is present in the lines, they must be cleaned out, (big nasty job)..... Remember to check your transmission and transfer case fluid levels.

 

I just popped mine off and there was fluid leaking out from the hoses. What does this mean? Im still sorta new to this. Is there an easy way of cleaning this out? or do the hoses all need replaced? Why is there fluid in it to begin with? Thanks for any more info. And thanks for all your help so far. Very grateful.

 

It means that A) the o-rings inside the switch have failed & B) the transfer case input seal has failed allowing transmission fluid into the transfer case causing an over full condition. Please be courteous to the author of this post and stay on the topic it was originally created to cover. If you have further questions that are not covered already in past threads on the forum, please create a new thread to discuss them.

 

The switch seals are bad and anytime you find fluid in vacuum lines, do not start the engine again until you isolate the other ends of the lines. The real danger is that the fluid migrates into the heater controls section.
Once isolated, both ends open you can spray wd 40 in then blow out with compressed air. But if the lines are the least bit stiff/brittle you should just replace them. Bulk line is available wherever pro shops buy their parts.

 

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch