Bad Gas Mileage

[1987BuickGrandN]

Stock car, new plugs, wires, coil pack, 120k on engine/tranny. The gas mileage is very bad. I’ve been getting 220 miles off a 15 gallon tank, highway conditions, cold weather, not ever boosted over 10psi. I’d like to take a road trip somewhere. What can I do to improve my gas mileage?

 

[bruce]

Originally posted by 1987BuickGrandN
Right now I'm driving a completely stock car. The gas mileage is bad. I get 220 miles off a 15 gallon tank of 93.

15 MPG under what conditions?.

Just a couple reality checks.
In non boost mode, you're using a 231 CID engine to haul around about 3,700 lbs.. When in boost you're fueling that motor like it's a 460 CID engine.

Neither are going to win any economy runs.

That said, by doing my own chips and paying attention to detail have broken 30 MPG a couple times while on trips.

Trouble with doing the high MPG stuff is that it drives oil temps high, and piston dome temps high, both are bad for detonation.

While you might do a little better, don't look for any serious gains with out some serious work or money.

A fresh Delco O2 nevers hurts.
Not Bosch, or any other brand, Delco only.

GNs were never intended to be an economy car by any measure.
HTH

 

[Two Lane]

You might start with fresh spark plugs, also

 

[Matt Weiser]

Is your car so stock that it still has the original injectors? Could be one of these is sticking open or leaking.

One thing you can do is switch all your fluids to synthetic: motor oil, trans, rear end. Usually good for 1-2 mpg improvement though the stuff costs more. Other common sense stuff: make sure your tire psi is correct, wheel alignment is right, and none of your brakes are grabbing. You might also check for leaky fuel hoses.

 

[1987BuickGrandN]

Originally posted by Matt Weiser
Is your car so stock that it still has the original injectors? Could be one of these is sticking open or leaking.

The injectors are the original stockers. Car runs great, just has bad gas mileage.

 

[1987BuickGrandN]

How are people getting these 25 mpgs?

 

[John Larkin]

What are your true RPMs at say 65mph? Maybe you aren't getting into lockup. I get 22mpg highway with 55s; got that with stockers too. The lockup is a key factor.

 

[salvageV6]

My TCC lockup solenoid went right at 120K miles too.

Got around to changing it at 140K miles and improved gas mileage about 4mpg. for my driving which is mostly surface roads and 20% highway.

See if you get lockup, or can force lockup in 4th with a switch.

Over 20mpg is gravy or bad calculations.

 

[1987BuickGrandN]

at 70mph rpms are in the 3000-3500 range, id have to take the car out for a drive to get exact rpms, but id say they are in that neighborhood. you could be right, lockup might be one of my problems. what should rpms be at 70mph stock converter and tranny. the car shifts out great, firm shifts

 

[scottyb]

Originally posted by 1987BuickGrandN
at 70mph rpms are in the 3000-3500 range, id have to take the car out for a drive to get exact rpms, but id say they are in that neighborhood. you could be right, lockup might be one of my problems. what should rpms be at 70mph stock converter and tranny. the car shifts out great, firm shifts

I'm not positive, but I think 70 mph in 4th gear with lockup is less than 2500 rpm's.

Along with the lockup and OD, what is your rear gear?

 

[FJM568]

Don't forget about the infamous driver's side header crack...It will affect gas mileage a bunch...

BTW, on a fresh motor with only 800 miles on it, I drove to the GSCA NATS 2 yrs ago and was able to get almost 26 mpg with a TA-51 turbo, 50# injs, and a Joe Lubrant 18* street chip...I've now got a header crack and my mileage is now around 15 mpg city/hwy as well...Before the header crack, it was closer to 18 combined city/hwy...But I also have a heavy right foot, too...

 

[1987BuickGrandN]

Why will an exhaust leak cause bad gas mileage? Not disputing that fact. I do have an exhaust leak, thats adding to my mpg story, and I bet the lockup on my TC is bad.

bryan

 

[FJM568]

When not under boost, the exhaust pulses will suck outside air in through the crack and throw your O2 sensor off...It will think your motor is running lean and the ecm will richen up the mixture to try to get the right AFR...

 

[smooth87]

i've gotten 24-25mpg running interstate speeds around 75-80, and i think my gears are deeper than stock(3.55-3.73?) of course i wasnt using the A/C and there were few stops.

 

[crs]

At first, when i got my 87 limited i was getting about 230 miles per tank. Now 6 months later, i'll get about 180-190 mostly highway. I dont know if my stock injectors are sticking or not, cause it idles pretty good. Good luck!

 

[Roc87]

I used to get over 25mpg on the highway, and get over 250 miles in town. Not sure what happend, but on the way to the Nats in Vegas I only got 20mpg. One factor I think was a sticking caliper that I just found and replaced today.

 

[d0n_3d]

guys... make sure your injectors are clean, can't stress that enough...blocked old dirty injectors will throw off the cars fueling like crazy...also make sure the tcc is locking up at around 55 mph...you should be cruising at around 2000 rpms at 60 mph...give or take 100-200 rpms...tire size, weight of car, tuneup conditions, play a major part in gas mileage...also your right foot is the biggest part make sure the o2 sensor is brand new and is a delco...double check to make sure your maf sensor is functioning properly...uhhhh there's millions of things to check...

 

[John Larkin]

At 65, you should be turning about 2400-2600 or so, erring toward the lower end of that range. Almost guaranteed you aren't locking up. On a flat road, set the cruise in 4th then tap the brake. RPM's should jump up about 300-400 when you do that. If they don't, there it is. Replace the TCC solenoid with a 700-R4 TCC. It's longer and has shims with it to compensate for the extra plunger length. I bought one at Advance Auto Parts, just their house brand (Rodeck or something funky) and it's been great. Still expensive but not as much as genuine GM. Word of caution: make sure the connections are very good or you'll be dropping the pan twice. Check continuity all the way to the brake switch before buttoning everything up. Haste will make waste!

 

[Chuck Leeper]

What about?

87, didn't see a comment on the cat converter in your post. Still on the car??
Other comment.. how are these RPM readings being taken?? Factory tach???
Lastly, are the tires the original size???
Just some thots I hadn't seen mentioned...

 

[dpok69]

Is there an easy way to check for exhaust leak? One way is GM top cleaner. But can you use tranny fluid and have it get sucked through a house near the vacuum pod to reveal hidden leaks not visible to the eyes?