Hi guys, trying to help my mother with her '03 Chevy Blazer LS terrible gas mileage and very rough idle (especially at stop lights). I replaced the air filter and cleaned the MAF with MAF cleaner.

I've attached a few pics, the first is a disintegrated vac hose I need help identifying and would like to know if it could be the cause of the rough idle.
If you can't see it and need a description, it looks like it goes from the other end of where the PCV valve connects to and runs over the master cylinder for the brakes and into the fender next to the ABS module.

If anyone could tell me what that hose is for I'd be very grateful!

On the passenger side, there is a vac hose going from the air intake to the valve cover, where it connects to the valve cover it is covered in oil. Is there any fix for this? Is something I should be worried about?

Thank you!

 

The larger hose is the suction hose for the PCV as you stated, and even if you were to completely disconnect it it might make a slight idle change, but nothing I would call a "very rough idle" and "terrible gas mileage". The other smaller hose that runs to the fender goes to a vacuum canister first inside the fender, then into the cab where it controls the HVAC system. That one will be less obvious as far has how it makes the engine run if it's deteriorated, but will definitely make the HVAC system not work right (first sign is usually the vents stop blowing air when accelerating)

Is the Blazer showing a check engine light that you can scan?

Any knowledge on when the last time it had a basic tune up?

Could be as simple as old plugs, plug wires, distributor cap and/or rotor, or fuel filter. Could be as complex as a bad fuel injector spider assembly...

 

1) Yes, engine code are P0171 and P0174 which led me to checking the vac hoses. I think plugging that leak could lead to better idling.

2) Last tune up was probably around 25K miles ago so it is at least due for an inspection. Plugs and plugwire/boots are coming soon along with a manual. Have not ordered a dist. cap or fuel filter.

Thanks for the help !

 

If you're positive those hoses are leaking, start by fixing the leaks, it's never good to have vacuum leaks :P

After that, I think you need to start with a fuel pressure test judging by those codes. Running lean, either means it's getting unmetered air via a LARGE vacuum leak, or it's starving for fuel. A large enough vacuum leak to cause that much issue, you would most likely be able to hear it and it would be very easy to track down. Thus why I would lean towards a fuel delivery issue...

If you want to, and didn't replace the fuel filer on the last tuneup, that would be a good easy place to start before even bothering with the pressure test. It's cheap, and takes all of about 15 minutes to do at home. Can rule that out at that point.

If you end up doing the pressure test after changing the fuel filter and the pressure is fine, then you may be looking at the injector spider assembly, common issue for pre-MPFI 4.3's

 

Do I have to do the fuel filter on an empty tank?

I'm still learning mechanical work on cars but it seems like a no brainer to do the fuel filter especially since it's so cheap and could solve my problem.

Also, how necessary is it to pull the fuse relay when doing the filter?

Thanks