Yet another gas mileage post (Page 1/2)
damittron JUN 05, 01:50 PM
I have been searching and reading all day (yes, I am supposed to be working, but whatever! ) My car is an 87 GT, has about 1125 miles on it. I have spent about 7 months bringing it back from 22 years of storage in a garage.

The car runs great! My last issue was an EGR issue that was causing some hunting idle, but that is fixed up nicely now. The problem now, is, the car seems to be getting about 9-10 MPG. There is nothing wrong with the fuel injectors (all been refurbished with good fuel pressure), nothing with the fuel filter or pump/sending unit, etc. Basically, the car is like brand new, but getting the terrible gas mileage. The one thing I haven't replaced is the O2 sensor. The main reason being, my exhaust doesn't smell rich or like gas at all.

I do have a scan tool, but haven't had time to plug it in for a reading, but I have no CEL or anything...and yes, the bulb is good.

I ordered a O2 sensor, it said it was an AC Delco, but when I got it, it looks to actually be a relabeled Bosch. I have read that the Bosch are terrible, so it is still sitting in the box on my desk. Should I go ahead and switch it out and see if it helps? I can't think of anything else that might be causing this. Oh, and the entire exhaust is new also, so not a clogged CAT.

D.
FX JUN 05, 02:18 PM
Probably a dumb question, but I actually have run into this before on a friends car. Did you put a fresh air cleaner in it.? Are you sure there is no blockage with the intake, rat nest, bee hives spiders, bats, and so on...
dremu JUN 05, 02:56 PM
Which motor? If the 2.8, the cold start injector may be leaking (you didn't say if you redid four, six or seven L

When I redid mine, I found the O-rings on the CSI were dried, and it was dribbling gas down the block to mingle in with the oil leaking out the distributor. Was hard to sort out what all was in the blotches on the shop floor.

Anyway, point being that the CSI O-rings are worth looking at. Fiero Store has 'em if you can't source elsewhere.

And replacing the O2 sensor can't hurt. Easy enough to switch back if you don't like it.

-- A

[This message has been edited by dremu (edited 06-05-2019).]

damittron JUN 05, 03:06 PM

quote
Originally posted by FX:

Probably a dumb question, but I actually have run into this before on a friends car. Did you put a fresh air cleaner in it.? Are you sure there is no blockage with the intake, rat nest, bee hives spiders, bats, and so on...



New air filter, all rats nests have been removed. Seriously...I had one in the engine compartment fan a couple of weeks back that was causing the fuse to blow. No blockages.
damittron JUN 05, 03:08 PM

quote
Originally posted by dremu:

Which motor? If the 2.8, the cold start injector may be leaking (you didn't say if you redid four, six or seven L

When I redid mine, I found the O-rings on the CSI were dried, and it was dribbling gas down the block to mingle in with the oil leaking out the distributor. Was hard to sort out what all was in the blotches on the shop floor.

Anyway, point being that the CSI O-rings are worth looking at. Fiero Store has 'em if you can't source elsewhere.

-- A



It's a GT, so it is a 2.8 V6. Cold start injector is disconnected...I am in Florida, so no need for it really. That said, no fuel leaks either.
dremu JUN 05, 03:29 PM

quote
Originally posted by damittron:


It's a GT, so it is a 2.8 V6. Cold start injector is disconnected...I am in Florida, so no need for it really. That said, no fuel leaks either.



Duh, forgot all GT's had the six.

In theory if the CSI is still on the fuel rail it could leak just from the fuel pressure even if the electrical is disconnected, but if it isn't leaking, as you say, leave it alone.

Then, yeah, I'd chase basic stuff like O2, timing, etc.

I've been doing the same thing on mine, similar situation, except I drive so little that it takes me forever to get a good sample. Also, I drive a bunch in town which drops fuel economy more than I thought; I guess I'm used to new cars where that's less of an issue.

-- A
Mickey_Moose JUN 05, 03:48 PM
Does the fuel pressure hold after you turn the car off? It should hold fairly steady with a very slow drop over time, if it drops fairly quickly, you have a leak.
Gall757 JUN 05, 05:03 PM
I think your car has an inop. temperature sensor.....the one that feeds signals to the ECM. If the computer thinks the car is cold, it uses pre-set numbers to manage the engine in what we call 'open loop' operation. One of the side-effects of this is lousy mileage......but not obvious rich mixture black exhaust.
damittron JUN 05, 06:11 PM

quote
Originally posted by Gall757:

I think your car has an inop. temperature sensor.....the one that feeds signals to the ECM. If the computer thinks the car is cold, it uses pre-set numbers to manage the engine in what we call 'open loop' operation. One of the side-effects of this is lousy mileage......but not obvious rich mixture black exhaust.



Well, I have a new one of those as well, so I will replace it and the O2 sensor and see where that gets me. Thanks again for helping!
damittron JUN 05, 06:14 PM

quote
Originally posted by Mickey_Moose:

Does the fuel pressure hold after you turn the car off? It should hold fairly steady with a very slow drop over time, if it drops fairly quickly, you have a leak.



Yah, good fuel pressure. Holds for quite a long time after hitting 44 or so PSI. After about 20 minutes, it's still around 40.