Hi.
We have a 1999 Blazer 4.3 with a very high fuel consumption, about 10 mpg. It has around 200,000 miles.

On the last inspection, it had no faults regarding emissions from the engine, and it neither smokes nor smells anything special.

We have replaced the following.
-Sparks, coil, cap, rotor and ICM
-New a / c delco fuel pump and fuel filter
-Spider injectors
-O2 sensors
-Water temp sensor
-Oil and filter
-Air filter

Right air pressure in all tires and all wheels spins easily.
The car drives and accelerates normally.

Anybody have any suggestions?

 

Just a couple ideas to make sure we are on the same page.

Are you calculating the fuel consumption by the amount of fuel you get at each fill-up against the odometer instead of using the on-board computer?
Are you you using the OEM size tires so that the odometer is correct?
Under what driving conditions are you doing your test? City, highway?

I have a 99 with the 3.73 rear end and it will get worse mileage than those with the the lower ratio gears. I also added a Yakima Rocket Box and that really cut down on my mileage but it was worth it for the added storage which negated the need to get a larger rig. At best my mileage was 18-19 mpg on the highway before I put on the rocket box and if I kept it at about 60-65 mph.

Have you checked if any of your brakes are dragging? Don't forget the parking brake shoes either.

If you have the 4-button controlled transfer case, then according to the owners manual, leaving it in auto-4wd mode will reduce mileage but I don't know by how much.

What did the O2 sensor look like when you replaced them (there are 2 or 3 of them)? If very sooty, you could have a fuel leaking from the fuel injection spider under the plenum.

 

We check how much we fill up against the number of miles driven on the odometer.

The tires that are on are original wheel size and we only drive in the city. All the brakes are good, none of them are dragging, the same goes for the parking brake.

There are 4 button switches in this, but since we only drive in the city we only use 2 wheel drive.

The two O2 sensors we replaced looked really good, but we still replaced as we had bought new ones.

Have checked with boroscope and we can not see that there has been any leakage from the injectors under the plenum.

 

Did it just suddenly start using a lot of fuel, or has it always been that way?

 

We hardly use this car, but it has used a lot of fuel all the time we have had it.

 

Dang, that covers all my ideas.

What was your previous mileage? In the winter here in North Idaho my mileage in town can drop to 10 mpg. I know I spend more time idling to get the heater hot. I then found this: https://www.fueleconomy.gov/feg/coldweather.shtml

 

I would perform a fuel pressure leak down test to check for a vacuum leak or other condition that would cause a rich fuel condition. Below is a link to instructions on this site from Captain Hook that has detailed instructions on running the test. If you don't have the tool to perform the test your local parts store will usually let you rent one for free. I'm in the process of troubleshooting the exact same problem on my 2000 ZR2 with no CEL's and engine runs great. Hope this helps.

Fuel Pressure Leak Down Test

 

We have not checked the consumption before as we have hardly used this car.

 

We will take a pressure fuel leak down test and see. Thanks for the link!

 

I get 13-14 MPG combined city/highway. These little Blazers really suck it down. I have a full size 4WD Avalanche that gets better on the same route. If the brakes aren't dragging, no SES light, and the Fuel Trims are normal - you are getting what you are going to get, unless you change where or how you are driving it. (If O2 sensors are off or bad, then LTFTs will also be off).