VERY LOW MPG help

[bierry]

To continue my saga to pinpoint why I've been getting 9.5 MPG lately (both the DIC and 600 miles of calculation correspond), I ruled out sticking brakes using an infrared thermometer to check the caliper and disc temps and all were fine.

Now I'm thinking it's a fuel leak (xover tube?), bad O2 sensor, leaky injector, or a defective block allowing gas to seep into the crank.

Since I've confirmed the DIC is actually correct, the computer must actually be injecting this much gas into the engine. I can therefore rule out fuel tank leak, defective block/rings, and leaky injector because otherwise the DIC should be on the high side? What's left? Bad O2s? No codes have been set, other than Service A/H.

Car is out of warranty and is completely stock, save zip tie mod.

Any help greatly appreciated.

[wallyman424]

im pretty sure a faulty o2 sensor would trip a code. so have you checked them? good luck

[Dave C. '04 Z06]

With my 1995 I had the same symptoms. But the 1995 would eventually set a code saying the O-sensor was bad.

What had happened there was I had made the cats freer flowing (IE: Gutted) but since the passenger O-sensor was right near the cat, since the cat was now gutted it didn't keep the o-sensor hot enough so the thing would read slightly lean, the computer would richen it up, then it would start clogging over time and a dominoe effect would occur where the car would keep richening, making the o-sensor clogging worse until it eventually wouldn't read anything.

So I would think that two things I would look for on your car is:
1. Faulty O-sensor (oxygen sensor)
2. A leak in your exhaust somewhere between the engine and the oxygen sensors that might be allowing fresh air to get in there, which would cause a false lean condition and make the car richen up.

I would lean toward the latter, because if the o-sensor was bad the PCM would probably report it, but who knows?

-Dave C. '04 Z06

[minivette]

Check the codes. I had an 02 fail on my Z28 and the computer automatically defaults to the maximum fuel tables to make sure the motor is getting enough fuel. I was getting really crappy mileage until I changed it.

[minivette]

Whoops, didn't notice you had already checked the codes. Try hooking up a scanning software and check the 02's.

[bierry]

Dave C,

What you said makes sense, and while I'm not sure whether I have an exhaust leak, my 01 burns alot of oil so that might be fouling the O2s even tho the sensor themselves are fine, causing them to read lean. I'm taking it to CARTEK in the next couple of weeks since I'm only 18 miles from them fortunately. Thanks.

Bill

[bierry]

BTW can the rear O2's be edited out while I'm at it?

[minivette]

Yes, your rear 02's can be edited out. However, if you are burning so much oil that you are messing up the front 02's, you have much bigger problems that need to be addressed.

[bierry]

It's burning a quart every 800 miles or so, haven't gotten the rering and prob won't since it's a lease and out of warranty. After 24K miles I think the O2s have had it...hopefully it's just the O2s.

[Subdriver]

Some suggestions:
- Try putting in new plugs
12571164 (GM Iridium)
- Try cleaning the MAF with electrical cleaner (I bought some at radio shack). Be very careful not to break the wires.
- I replaced my front O2 sensors last weekend. They are relatively inexpensive from one of the forum parts vendors.
Oxygen Sensor Front 25312184
Oxygen Sensor Rear 25317697
There is one front/rear each side (2 each total).
It was somewhat of a pain to get them out, but doable. Unplug them first so you can rotate them out of the header without twisting the cable. Install them before plugging them in for the same reason.
- replace your air filter
My car runs much better with the above changes. You may have bigger trouble, but the above is easy and relatively cheap.