View Full Version : Bad gas mileage....HELP

07-23-2006, 05:39 PM
Hey guys, been a while since I posted to the list (I used to post the old one).
My 87 GN has just started to give really bad mpg. Set up is stock block, TA49, green stripes (30#), Thrasher 92, stretch intercooler, cone filter, adjustable regulator, heated O2, 3" d/pipe, hooker duals, no cat. Usual stuff.

Used to get low 20's around town, 30 mpg on the motorway - Thats here in the UK, our gallons being 4.5 litres.

Took the car on a 180 mile round trip, mostly on the motorway at about 70-80 mph. I could literally watch the gauge drop. Got home and filled up, the car gave about 15-16 mpg!!! The car has ben stored for about 7 months and runs fine except for the crappy mileage. Idles great - open loop thrasher idle - shows no codes. Spoolo up is a little lazy from a stop, then takes off like hell. Spools fast at cruise.

Since this I have repaired my cracked d/side header which was pretty bad. I had a new O2 sensor in there, which I switched back to my heated one, and a new thrasher chip which I also switched back to my known good old one. Cleaned and gapped plugs, good coil pack and wires. All the pin out voltages from the ecm are what they should be. My Tlink readings all seemed ok, but I now have problems with it talking to the car. Another story which I will start in the appropriate place after this mail.

The problem remains and I can't think were else to look. As I said, the car seems to run fine all except for the possessed fuel gauge. The car is really drinking gas at an alarming rate and at almost $8.00 gallon just aint funny.

Any help you guys can give is much appreciated. Hi to Scott Keller by the way, glad to see your name still up here.

Ian Reeves, UK.

07-23-2006, 06:29 PM
Hey Ian,

Nice to see a familiar name from the old list, other than Scott and Ken I don't see too many that I recognise. :dunno: I do not post very often either (still like the old list format better than the forum but thats just me).

Dunno about your mileage,... maybe one of the rubber fuel lines is dry rotted or cracked????

Mark Flick

07-23-2006, 07:05 PM
Hey Mark,

is the old style list still available?

Rotted pipes? Can't smell gas but....... I did change the fuel filter after storage. I'll get under the car tomorrow and check the fittings and pipes.



07-23-2006, 07:59 PM
have you checked the fuel mileage since the exahust leak repair?

07-24-2006, 01:45 AM
Yeah, I thought was the problem. The header had been cracked for a long time but not bad. Never enough to really effect anything. When I took it off to repair it was REALLY BAD. I put it back and expected things to return to normal. But alas.

Ian, UK.

08-19-2006, 07:59 PM
Well, finally found the problem. Out driving and the MAF failed. Car stumbled like crazy as soon as you touched the gas pedal. Tapped the sensor.....car stumbled.

Managed to get another sensor thanks to one of the few turbo Buick owners over here and praise be, MPG has doubled.

Just thought I'd let you all know.

08-19-2006, 08:17 PM
Classic diagnosis method. Glad you were able to find it without too much trouble. Did you see any bad numbers on a scan tool prior to this?

Unplugging the MAF should have made it run better in 'limp home' mode as well while it was estimating MAF numbers.

Glad you got if figured out.

08-19-2006, 08:56 PM
Had to drive home in limp mode. Didn't have any scan tool data which was a pain in the rear - the cable to my old T link fried, I'm in the process of updating to the windows version.
MAF failed two days before my annual pilgrimage to the strip. My freind shipped me the MAF from his GNX so I could race for three days - what a guy.

Two morals to this - things don't always code and can't say how important a scan tool is. My head would have ached less.