Poor Mileage!
 

So before I ask anything I'll state a few things.

1998 Chevy Blazer
4x4
4.3L Vortec
Automatic

I've recently installed fresh spark plugs and wires with a new cap and rotor. The air filter is clean. There are no check engine lights. I do not have a cat but I'm running a spacer on the O2 so it won't throw a code. There are no fuel leaks or odors.

Now my problem. Despite what my on board computer says, I'm getting poor mileage. I'm at somewhere between 12-13 mpg city/hwy. My computer says 19.2, but that's a bit off. So is this mileage typical for this vehicle? I understand I will get worse mileage than a truck because the Blazer weighs a bit more, and the 4x4 probably drags it down.

BUT REALLY? I don't romp on it or anything like that. I've seen others get similar mileage. Can I assume my motor is just getting tired- or is there some secret to it all?

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No, that mileage sucks bad bro. I'm getting between 15 and 17 combined and I have a lift and 31s. I'm probably pushing very close to 6,000 pounds with everything on it as well. You have a problem somewhere.

I know a lot of people who have gotten worse gas mileage after they went catless. Despite what you've done to the 02 sensor, I would say the 02 sensor is your problem at first guess. It sounds like more fuel is being dumped in causing worse gas mileage even though the CEL isn't on for the 02 sensor. Also what kind of spark plugs are you running? That could be a mpg problem as well.

Here's a thread you might want to read: THREAD

I'm running 235/75/15 tires. Not very big at all, tire pressure good and such. I'm running NGK plugs. Gapped accordingly and such.. I couldn't tell you specifics on them.

I assumed since it was not giving a reading to the O2 that the cat wasn't working, it wouldn't dump fuel. There are a lot of mixed answers on the topic, and I've never heard of anyone else using a spacer to fool the computer. My buddy have me one that they were using when the cat went bad, but I'm not sure if it makes a difference for mpg. I suppose I could throw a new cat on because its really the only thing that's not stock/factory spec anymore.

did you gap the plugs at .035?? G.M. sent an up date to the line mechanics about 4 years ago about ign. module, to extend it, and improve mpg, set plugs @ .035, if you set them at .043 or.045, resetting at .035 may help.

CSFI gap is supposed to be .060. (CSFI is 96'+)

I would be looking more at leaking injectors or fuel regulator. The downstream o2 sensor only measures that the cat is working. Is has no effect on the fuel mixture.

I took it to a friends shop and he put his scan tool on it. There was nothing out of the ordinary according to the tool.

From what I've read and what I was told- the cat seems unlikely without a code.

The map sensor was in question from my research. I don't think that's the problem, but it's super easy to test- so I will give it a shot when I'm free.

My fuel pressure primes to 60psi and holds at 55psi or so when running. I was under the impression that a leaking injector/ regulator would create fuel odors. Am I wrong here?

Have you cleaned your EGR?

There are no published fuel pressure numbers for the engine running. Way too many variables. Run the test with the key on, engine off. Prime should be 60 to 65 and not go below 55 for 10 minutes.

I have not cleaned the EGR but we checked for proper operation with the scan tool. It's functioning how it should.

Fuel pressure holds fine.