I have no clue why my 305 is getting this kind of gas mileage. the engine light codes that it has are 15 and 21, which I don't think have to do with the gas mileage. any help?

 

gas leak? any tranny problems, carb or fuel injected

 

No there is no gas leak anywhere that I can see, it's carbureted, and there are no transmission problems as far as I know, it does shift really late though untill it warms up, shifts around 2700, when warm its around 2000

 

does the car flood out thats kinda crazy u only get 8 mpg

 

It has been flooded before but not recently

 

thats crazy but ima try to look in to any weird problem but whats the 13 and 21 code

 

sorry I meant 15*, its the coolant temperature sensor and 21 is the throttle position sensor.

 

ok i honestly dk but i will lookin to it for u

 

alright thanks

 

The coolant temp sensor is very important. If it is bad and reads a low temperature, it will add more fuel to "choke" the engine for what it thinks is cold weather. Eventually this will probably foul out your oxygen sensor and then give you that code too. It might even go so rich as to foul your plugs to the point it might not start someday.

Have you had it scanned in operation mode?

 

I don't know what operation mode is, and yes the plugs seem to always be fouled, but I have a coolant temp gauge I put in thats reading a good 160 degrees

 

And why exactly would these not have anything to do with mileage? I mean really - THROTTLE POSITION SENSOR doesn't ring a bell when talking about Fuel Mileage?

CTS and TPS both tell ECM about fuel - and thus ECM changes timing accordingly - thus bad timing from bad ECM readings off bad CTS and TPS sensors means unburnt fuel and bad mileage.

Your ECM is smarter than you think - fix what it's telling you is wrong before you jump the gun and start grabbing at straws!

 

Those 2 sensors are cheap. Change them both, clear your codes, and see what happens.

On a TBI when the coolant temperature sensor goes out it reads -40* or something ridiculous like that and causes the TBI to DUMP fuel down the bores, so much that it almost kills the engine. Might have a lesser, but still existent, effect on the carb system.

Change those items!

 

The coolant temperature sending unit and the coolant temp sensor are two different things.
Your carb is still computer controlled, there are wires going into it from the ecm.

A scanner, not just a code reader, can read your codes when in code reading mode, as well as tell you what different sensors are reading while your engine runs when the scanner is in operation mode. This will include temperature from the sensor, rpm, throttle position (from the tps), oxygen sensor readings, whether you are in open or closed loop and other things too.
You might be able to get your car scanned at Auto Zone or some other part store.

As Darkshot says, a -40 tps reading will play havoc on your mixture. And you probably have other issues as well.

 

I forgot, 160 degrees is not hot enough, you probably need 195 degrees. Check and replace your thermostat. If the hot water isn't circulating the tps can't get a good reading, and, +++++++ once again +++++++, the ecm thinks the engine is colder than it really is and responds by ? sending ++++++ MORE FUEL ++++++ through the carb.

 

I would find out why they aren't working before replacing them. A screwed up wire is a lot cheaper to fix than replacing sensors. And, spend the bucks to replace the sensors and find out they still don't work because the wiring is screwed up is an exercise in unnecessary spending.

 

Thanks for everyones input, I didn't realize those sensors did so much, I don't have much money at the moment but when I get some I'll replace those sensors and get back to tell you if the problem is fixed, and Basiccamaro can the engine really not be hot enough?, I mean I have a 160 thermostat in there, I should replace it with a 192?

 

160 is low for a carbed 305, I would get atleast a 180

 

Having a 160 degree thermostat in a feedback carb system isn't going to cause the mileage to take a huge dump. It goes closed loop at 110 degrees - but, needs to see that via the CTS in order to go into closed loop, of course.

 

What do you mean closed loop?

 

Ok so I replaced the coolant temperature sensor and the code went away and its running a bit better. I bought the Throttle position sensor and I have no clue how to replace it, I don't want to ruin the carburetor by doing something wrong. Could someone please help me through replacing the tps. thanks

 

The TPS can be replaced without removing the carburetor from the motor but you'll have to be extra careful not to drop anything in the wrong place. You'll need to remove the airhorn after disconnecting the booster lever and choke lever. Make sure all of the screws are out (and the cleaner stud and two forward mounting bolts) and do not force the air horn off.

If your gasket is crappy you may need to purchase a rebuild kit for a replacement. You will also need to decide whether to complete a full rebuild of the carb while you're there at the TPS.

Some pictures:

https://www.thirdgen.org/forums/carb...-ccc-qjet.html

 

As was stated, above, the TPS is located in the carb. If you look closely at the big green accelerator pump arm on the top of the carb/driver's side, you'll see a little metal dowel/pushrod on the underside of it going into the carb. That's what moves the plunger on the TPS inside the carb (a new TPS should come with a replacement dowel/pushrod which you can use if your original goes missing or is chewed up). Gotta take the airhorn off the carb to access it. Yes, there is stuff you can screw up inside there. Most common is not getting that little metal dowel/pushrod back in correctly when you reassemble. It's gotta stay all the way up in it's hole through the airhorn as you put the airhorn back on (I put a dab of thick grease on it so it kinda sticks in it's hole without sliding right back out when you turn the airhorn over to put it back on).

If you're not comfortable with digging around inside a QJet I couldn't fault you for seeking the help of a professional mechanic who is familiar with QJets for this one. Yes, you can do it yourself, but it's kinda hair-pulling the first time if you don't have someone to show you how.

If you lived anywhere near Philly I would do it for you no charge, just to help out a board member.

 

If you lived near me I would do it for you faster.

Coincidentally I just replaced the TPS on my 85 TA this past Monday (holiday). As it failed the ECM began commanding torque converter unlock and sure enough the scanner was showing varying TPS voltage readings at steady highway cruise.

 

Not to hijack, but where is the cts on a 305 tpi? I dont have any codes, but I do have a half melted sensor hanging off the passenger side head...

 

Thanks everyone, I ended up replacing it myself, the car definitely runs better somewhat, but the gas mileage only went up a gallon and a half, It , when the car warms up to operating temperature it runs way worse than in the cold, it stumbles and bogs every once in a while, It also has hesitation from idle to anywhere on the gas, It almost sounds like the engine shuts off for a second then starts to move, and it still shifts really high( I still have no clue why). But the voltage of the tps is set at .63 and it's suppose to be at like .46 or something, would that be the reason for terrible gas mileage and all these problems?, I would adjust it but I cannot find the tool anywhere. thanks

 

In my experience 0.63 is probably close enough, certainly not far off enough to cause any kind of mileage issues. Instead of the special tool, I've always used an appropriately sized roll pin lightly tapped over the set screw to adjust the TPS.

Before delving into the carburetor anymore I'd thoroughly check the ignition system. Have you changed plugs, wires, cap, rotor? Timing set correctly? Timing advancing as it should?

If it's shifting at the wrong rpm check out the the following website for info on adjusting the TV cable: http://www.tvmadeez.com/

Make sure the cable's good, has the proper range of motion and you can feel the light tension of the TV valve when you pull on it. I replaced the one on my 85 when I put the new motor in last Fall. The cover on the old one was broken in places and it was only a matter of time before it failed.

 

I'm not sure what you mean by roll pin lightly tapped, but thanks a lot for that website on how to adjust the tv cable, I didn't get to drive it much tonight but it looks as it is fixed. thanks!

 

Just drove it today, It's shifting perfectly, thanks again, when I fill it up i'll get back if the gas mileage has gone up at all.

 

It's at about 12 Mpg now.

 

Time for fresh plugs, wires, cap, rotor and a new O2 sensor. Now that you got the big stuff fixed it's time for a good, basic tune-up. Especially the plugs- they could be a little whacked from all the bad running before. Everything else because who knows how old it is (unless you replaced them yoruself recently).

That stuff is usually worth another 1-2 MPG if it's been on the car for a while. But like I said, especially the plugs in this case.

 

Alright cool, I'll do that later that week and get back here to say what my gas mileage is.

 

the coolant temp sensor being bad will make your car run in open loop all the time and that means it will run extremely rich. fix it and you should be good unless you have other problems.