Corvettaholic

Hey everyone,

I used to get about 29/30mpg on the freeway at 70mph, now I get about 20. About 3 weeks ago I had some weird fuel issues, so I shotgunned it by replacing fuel pump (definantly dead), O2 sensor (getting codes about it), IAC valve (fixed the idle problem), fuel filter, and 120amp alternator (voltages were off, flickering lights). Also found out that cylinders 3 and 4 are getting crap for compression. Think thats piston rings or a partially blown head gasket? The heads are about 6 months old, a set of those trickflow twisted wedge aluminum ones. But about the gas mileage... at low speeds the problem comes and goes. Driving around at a constant speed my instant-gas-mileage-o-meter fluctuates from 16mpg to 28mpg at 50mph. It used to just sit at 28. The freeway is even worse where it will never go higher than 20mpg. This problem started happening about a week after I got it back from the shop. I used to have a real bad vacuum leak from my PCV system a ways back, fixed now, and I suspected I had another but could never find it. More symptoms: off the line when I stomp on the gas, it almost dies then flares to life. My check engine light only turns on if it takes more than 2 or 3 seconds to start the car, which only happens on a cold start. When driving, if I stomp on it, it will hesistate really really bad, then usually take off like normal. I did a quick lookover and I didn't see any cracked rubber, but maybe its something common I don't know about?

Frizlefrak

What year car? Any codes? How many miles on engine? How many lbs of vacuum at idle? Needle on vacuum guage steady? Is car coming up to temp and going into closed loop? How many pounds of fuel pressure? Low compression on both 3 & 4 )both banks of engine)? How many lbs of compression on other 6 cylinders? I'm assuming you've checked or serviced plugs, wires, cap & rotor, fuel filter.

BTW, welcome to the forum. A little more specific info, and we'll nail the problem down. :cheers:

65Z01

To check those cylinders squirt in a copla drops of motor oil to see if compression comes up; if so it's rings.

Verify base timing is at 6deg BTDC.

Check engine vacuum at idle.

Check that the MAF sensor wires glow for 2sec after you turn off the engine; if not service the MAF bun off relay.

Use the method on my site to verify that the ECM goes closed loop and stays there at steady elevated RPM; if not you will need a scan tool to see if the problem is in the O2 sensor or the coolant temp sensor.

vader86

Usually when your gas mileage goes south its the O2 sensor. If its giving a code its dead.

ittlfly

All the above posts are good things to check. To your question, a vacuum leak can cause gas mileage problems. The O2 sees a lean exhaust and the ECM fattens up the mixture to compensate. Some of the wasted fuel may ignite in the cats causing a popping.