Bad MPG

[ATL 84]

I am noticing lately I am getting some bad gas mileage. I have changed the plugs, cat, rotor, dist. cap, muffler and o2. I need some help on what else to change to solve this problem. This has started about a month ago. It has been 2 months since I did a tune up. Everything else was done in the last 2 weeks.

 

[SoLow]

Yours is a cross fire right?

 

[ATL 84]

Yes, it is a crossfire. I did change the air and fuel filters also.

 

[nyernga]

What exactly do you mean by bad? What's the number?

Len

 

[vee93]

Well..., are you filling up with "bad" gas--as in the cheapo stuff? Wrong octane? Smell gas? Check your gas tank for leaks.

 

[ATL 84]

Well I was averaging 17 to 18 mpg. All of a sudden I noticed I am getting like 14 to 15 mpg. That to me is a lost and the driving style is still the same.

 

[gmjunkie]

Well I was averaging 17 to 18 mpg. All of a sudden I noticed I am getting like 14 to 15 mpg. That to me is a lost and the driving style is still the same.

Gee I wonder if the Colder weather has something to do with it?? junk!!

 

[ATL 84]

Well it has not been that cold, except a few days. I even thought about the winter gas they sell here, and added an additive.

 

[kevman]

I have an '84 and that was the first symptom I had that my fuel pump was getting weak. After installing the new pump, my MPG is around 20. Up from 13. Check fuel pressure at idle and at WOT, it should be between 9 to 13 psi. Hope this helps.

 

[redvett]

Well it has not been that cold, except a few days. I even thought about the winter gas they sell here, and added an additive.

Remember that F.I. has the ability to add & subtract fuel for a 14.1 A/F and everytime you start up a cold engine it takes more fuel and in cold or cooler weather regardless of the winter fuel you will use more since its adding fuel because of the colder weather to maintain the 14.1 A/F.

 

[John Robinson]

Another Veiw

I have noticed that when I come into a town my milage may drop as much as 2 MPG. As soon as I am back on the road it takes 50 to 75 miles to work back the lost milage. But when I am driving in town on my daily errands I have noticed that waiting on a train will cost me up to .8 of a my MPG avg. the same with drive thrus and backed up traffic. At todays prices I shut the car off when waiting in long lines. And as already stated if the ambient temperature is cold then it takes gas to create heat. Heat is required to make the engine run properly. So if the cold air is removing the heat faster then warm air the engine will use more gas to replace the lost BTU's. And finally a tank of bad gas will drop the MPG significately. The good part of that is you get to buy good gas sooner.:confused

 

[ATL 84]

I will check the pressure. I don't think it is the cold. It really has not been that cold in Atlanta. Just last week it was 77 degrees. I think it maybe the fuel pump. I just need to find a meter to read it and go from there.

 

[redvett]

I will check the pressure. I don't think it is the cold. It really has not been that cold in Atlanta. Just last week it was 77 degrees. I think it maybe the fuel pump. I just need to find a meter to read it and go from there.

You can check pressure but if it was low you would have other issues.

 

[ATL 84]

What other issuses would I have. I am afraid to ask.

 

[redvett]

What other issuses would I have. I am afraid to ask.

Low fuel pressure will cause poor drivability. What i think you should do is replace the O2 sensor. If its old and it might be the original one or lots of miles say over 100000mi its time for a new one. The O2 is what controls the A/F mix in closed loop. Also make sure your engine has a 180 deg thermostat at least and the engine is running at 180 or better.

 

[ATL 84]

I did change the o2 and it is not driving that smooth. The o2 did some good though. I thought at first it was the cat and muffler but now I am not sure if that was the problem.

 

[kevman]

When my pump was bad the only other symptom I had was surging engine at idle. There were no drivability problems. The fuel pressure was low causing poor atomization from the injectors. This in turn causes poor combustion. The computer senses an idle drop and adds more fuel. Thus poor economy. This is only my opinion and my experience. I hope it helps.

 

[airman]

You mite want to check your EGR valve mine was running ruff at idle and I did the same things you did with no luck so I pulled the EGR valve and tested it and it had bad vacume leak in the diaframe.It is running a lot better now.

 

[ATL 84]

I would like to thank everyone for helping get passed this problem. I needed both a fuel pump and a egr valve. I did the fuel pump first and did not lose the rough idle or the vibration while driving. You guys are great. My 84 also thanks you.

 

[I_R_DA_ONE]

I would check the cold start relay to make sure it is working... If it has failed it will give you these symtoms... or the cold start switch on the intake...