I have a 1991 Z28 with a 305 TPI and auto transmission. Back in January I made a thread about it having low gas mileage (12 MPG in town which is where I mostly drive since it is a daily driver) and didn't really ever figure out what was wrong with it. My grandpa and dad always plays it off as everyone on the internet lying about their MPG and that 12 MPG is good in town for my car. When I made a thread about brakes a few weeks ago, a guy with a much larger motor and gears than me said he got even better mileage than me, so it kind of reignited the drive to get this figured out.

There is no trouble codes in the computer. Weird thing that happened. Before we adjusted the timing on the car (it was at 4 degrees before) my gas gauge would go down to 3/4 a tank around 45 miles. Once we increased the timing to 6 degrees as the sticker on the hood said, I can get to around 70-75 miles before the gauge goes down to 3/4 a tank, yet I still end up almost being out of fuel around 120 miles. Anyways, what should I do? My grandpa and dad says nothing is wrong because there is no trouble code, but I'm sure there must be something wrong.

 

It's very difficult to guage mileage vs. fuel needle - the bottom half of the tank always empties faster than the top half. The fuel senders are just not real precise, and the shape of the tank may contribute some.

Car smell rich?

Might try basic tune up if haven't yet - fresh plugs gapped correctly, fresh plug wires, dist cap and rotor, air filter and fuel filter.

Is the torque converter locking? If you go out on the highway, and start from a stop, just normal driving - you should see the tach change at each gear change. After you see 4th, you should see one more little dip in the tach, about 300 rpms drop - this is where the TC locks. Granted you said most of your driving is in town - but the TC can lock as low as 35mph - it's just that you can't see/feel it on the tach unless your accellerating from a stop up to about 60mph or so - that's why I suggest the highway test. If the TC is not locking/unlocking properly, could drop mileage a bit.

Could check coil maybe as well. This is a guess ... but I'm assuming that if the coil wasn't producing enough voltage, that the plugs might not fire 'enough' to get a complete burn, thus a waste of fuel and a rich condition. Sounds good in my head, but like I said, it's a guess, I could be way off base with that one.

Only other thing that comes to my feeble brain is - are you sure it's a 305? Just because the VIN states 305 doesn't mean it is (unless you are the original owner and know for a fact it's never been swapped). I could see a 350 only getting 12 intown maybe. Might see if you can find the block ID on the driver side behind the head to double check - usually has "5.0" or "5.7", but if not should be a 5-7 digit number there - you'll have to spray cleaner there and get the gunk off to read it most likely.

Good Luck!

 

We have replaced plugs once since we got the car and that didn't seem to help. Also with A/C on or off it doesn't seem to make a difference.

I know for sure it's 305. The people we bought it from were original owners and they had idea about how cars worked. They had a local repair shop fix the EGR valve on it, and when we got it apart we noticed they had the wrong valve on (it was too small) and the vacuum hose that was suppose to be connected to it was too big for the port.

Someone once said maybe O2 sensor was bad and was causing the engine to run too rich. Wouldn't a bad O2 sensor create a trouble code though?

 

Not necessarily. The ECM's self diagnostic ability is intended to catch failures in the system but will only set a code if a sensor either goes full high or full low on its voltage scale. "In range" failures are cases where a sensor continues to function within its operating range, thus not setting a code, but is no longer accurate. This can cause huge loss of drivability, performance and fuel efficiency, without ever setting a code.

The easiest way to diagnose this type of failure is with a scan tool. Look first at BLM(block learn module). BLM is long term fuel correction and will point to any issue thats causing a lean or rich condition. Neutral BLM is 128.
Look at coolant temp on ECM data. The ECM makes huge fuel mixture and igntion timing adjustments according to coolant temp. Coolant temp sensor failures are common. Does ECM coolant temp data match actual coolant temp? This can be tricky to determine. You may want to have an infared thermometer on hand to check temp of the radiator and hoses, thermostat housing. The temp gauge is often inaccurate but compare it anyway.
Look at O2 sensor data. run the engine at 2,500rpm for one minute and watch O2 voltage. It should toggle rapidly between .200 and .800v. Then release the throttle and continue to watch O2 voltage. It should drop low when the throttle is released and stay low for a moment (this is decel fuel cutoff). Then O2 voltage should return to toggling between .200 and .800 for several minutes. Watch for O2 voltage to stick low or high or hang in the middle. This may be a sign of a lazy O2 sensor.

 

Hmm interesting indeed. Back when we thought the car was overheating at around 220 degrees we were testing things out. When the gauge on the dash reads between 220-225 the actual temp in the radiator and in the hoses is about 207. Also, when you shut the car off the temp gauge increases a little bit to an even higher number. So maybe the temp gauge is the problem?

If the O2 sensor is cheap enough, I may just buy a replacement one and install it. How much does a new O2 sensor go for?

 

they are cheap for these cars less than 20 i believe.

 

I would definitely start with the O2 sensor and go from there. check to make sure all your vacuum hoses and lines are connected also. Wish I had more to add.

 

Oh wow that's cheap I'll definitely inquire about an O2 sensor here tomorrow and see what it does.

As for the vacuum hoses, when we did the EGR valve we found the hose on the back of the TPI planum was almost falling off along with other loose hoses. We have fixed those problems since then.

 

Be sure to buy a good quality O2 sensor. Cheap O2 sensors can cause more trouble than they solve. Bosch is good. Denso is better.

The O2 sensor is the ECMs eye into how well the engine is using the fuel its given. The ECM uses information from the O2 sensor to adjust fuel trim. Basically, there are two seperate sides to fuel management.

The MAF, coolant temp, TPS sensors make adjustments to fuel injector pulsewidth based on engine load, throttle opening, and engine temperature. In the case of the TPS, asyncronous injector pulses, or additional injector pulses are managed to compensate for sudden throttle openning. These adjustments are based on preprogrammed "maps" which are written in the PROM and are intended to match a particular engine. This is the primary fuel management system.

The second fuel management system consists of the O2 sensor and the ECM's "fuel trims". The ECM has both a short term and a long term fuel trims built into its programming. In the case of our 3rd gens, Integrator is short term fuel trim and BLM is long term. These fuel trims allow the ECM to make fine adjustments to injector pulsewidth based on oxygen content in the engine's exhaust stream. By monitioring exhaust oxygen content, the ECM can determine whether the engine is receiving too much or too little fuel. It then adjusts first the short term fuel trim to compensate for the lean or rich condition. When the short term fuel trim reaches its adaptation limit, the ECM adjusts its long term fuel trim so that short term fuel trim is able to properly compensate for under/over fueling. Should the long term fuel trim reach its adaptation limit, either rich or lean, the ECM stores a trouble code for fuel system lean or rich, whichever the case may be.

Should the O2 sensor become inaccurate, the fuel trims will not be correct and the engine may run rich or lean without setting a trouble code.

 

Thank you, very informative. I'm definitely going to pick up a new O2 sensor now.

 

I would say 12 local is not that uncommon. Mine is not too far from that. It could be your driving style.

 

Your's is a 355 and, if your avatar is a photo of your engine, looks to be modified. It atleast has SLP runners. This thread concerns a stock 305. It should be closer to 16 or in town. Could do even better with mild driving and in proper working order.

 

Sure 12 is on the low side but its not out of line for a 305 and local driving. No one asked what his in town driving consisted of.

 

This is true. I still would like to see him connect a scanner and look at BLM and Integrator. Then hed know for sure if the engine and fuel management are running correctly.

 

I picked up a Bosch O2 sensor today for $20. Were going to install it this weekend and I'm going to go fill up my tank that same day. We'll see how the MPG goes.

Sorry that took so long to reply, I just had all 4 of my wisdom teeth removed last wednesday so I've been recovering

 

How do you figure? I've had the wideband on cars that were damn near 128BLM, some were 15:1, others were closer to 12:1. Not a good indication as far as I'm concerned.

-- Joe

 

Are you sure 120 miles is actually 120 miles? Have a friend follow you in another car with both cars trip odometers reset to 0. drive 10 miles and ask him if he has the same mileage as you do. Another culprit may be the rear end. If someone has switched out 3.23s for 3.73 ot 4.11s it will definitely hurt your mileage. My 350/auto race car gets 17 and i drive "spirited" alot of the time.

 

So how is your gas mileage doing now? I know for my 350 the buildsheet says 16 mpg city and mine is still all stock. However I only get 100 miles out of about 12- 13 gallons of gas. I have not yet checked the timing (dont know how to and my "car savvy" neighbor doesnt know how to do it on my "newer car") but I plan to. I also plan on getting a new O2 sensor so that hopefully it will help out the with the performance and the rich smell and the poor gas mileage. However I wont know until March or until we have a break in snow to test it out.

 

I have a 1984 z28 camaro 305 with the quadrajet carb. The car run very rich when its cold and take s long time to warm up. When the car warms up it does lean out out and run smooth. It is going to be getting a full tune up in the next week or so. ( plugs, wires, cap rotor). If that doesnt help what else can i do to help it run better when the engine is cold.

 

It's a carbed car and the nature of the beast. Carbs do not like the cold. The electric choke helps, but only so much till they warm up. It will run poorly when cold no matter how tuned up it is compared to fuel injected cars. Now, if you go out, start the car, let it run for 2 minutes minimum at high idle to warm up, when you kick it down to normal idle speeds it should run pretty well. I know mine likes to stall/bog alot until it's pretty warmed up.

You just won 't ever be able to go out to a cold carbed car in the winter, start it, throw it in drive or in gear and go...and have it run smoothly. It'll always lug around till it warms up. Just the nature of the beast (In my experience...others may have a different story).

 

Ya i just have to get the choke on the car working again. Its has to run for atleast 10 mins with being on the gas for the car to warm up and idle. I am going to be putting a new thermostat in because the on in the car now is always open. Once i do the and fix the choke its should be better.

 

Camaros generation 1-4 have all been cold-natured beasts. Gen 5? I dunno yet, haven't driven one in the cold yet.
Why didn't you start a new thread?

 

The ECM coolant temperature sensor isn't connected to an instrument dash gauge.