gas milliage has gone from 30mpg to 9mpg

hi ! iam trying to figure out what has caused my 1986 olds delta88 royalle it has 3.8 ltr engine with fuel injection to drop from 30mpg down to 9mpg in less then 6yrs! i have rplace the cataletic (sp) converter and done tuneup on it changed fuel filters numerous times and a still getting bad millage from it! help? i hope you can help me thanks tom

 

I would take you car out and run it at high speeds for about five or ten minutes - then shut it off quickly ( no idling) and check the plugs for light brown color.

If your plugs are OK do a compression check to see how you sit with the engine hardware. You should have readings (around 120 to 140 per cylinder with even run throughout the cylinders).

Check the fuel pressure at the rail before and after shut off. A loss of pressure after shutoff can indicate leaky injectors = poor fuel economy.

Check the EGR valve. Pull it and clean it. Those usually cause some other driveability issues when they act up, but they can hurt your mileage. You can also clean your MAF sensor with a solvent from an auto parts store.

You have the full range of sensors on that car. You should have a scan done on it for codes. The O2 sensor can kill fuel mileage, but I would scan it before replacing anything.

Hope this helps,

Bob

 

That was highway speeds, sorry.

 

Oh, oh; by now he's probably on one of those helicopter videos of high speed chases.

 

 

Ok, Ok, so it's all my fault.

 

And this vehicle still runs good? (Since you never said it no longer does)

And you still remember how to do mileage checks?

And you aren't parked all the time in sand or gravel and do not realize you have a gas leak?

 

Or have a neighbor with a syphon hose?

And I second the question on how it's running. A car capable of 30 mpg should be running like crap if it's sucking gas at 9 mpg.

 

don't clean the MAF sensor...it's not a hot wire sensor.

I also have to believe that it runs badly...forget about driving it like a wild man and examining the plugs. it's got computer running it. Take it to someone who it knowledgeable to determine if the computer is in charge of what's going on or if it's being lied to by a faulty sensor. a failing coolant sensor would cause this and not turn on the MIL on a OBD I vehicle.

likewise, don't bother with the EGR at this point...it's not going to cause a 60% drop in fuel mileage unless all you do is idle