Maxthe222

Hey again guys,

Finally got my GTA back out on the road, it had such a litany of issues from the mistreatment of the previous owners,
-the spark plugs were just sitting lose in the chambers,
-fuel filter blocked
-oil filter blocked
-heater core leaking (nightmare job on RHD cars, no option but for dash to come out and HVAC is flipped upside down on other side of the car)
-transmission mount was destroyed
-radiator leak

Gave the car a super through and through once over, most of the general service items have been replaced, but one thing still remains;

Why is the fuel economy so bad????
I will drive this car down the road, up to the shops, not going anywhere more than 60kmph, maybe a couple of revs while idle here and there but nothing over 3000rpm for a second, and the tank has already depleted by 15-25%??? We are talking about less than 30mins of driving here, it's been like that since I got the car? It is impossible this is the standard for a 5.7 350 V8 chev motor with both EFI and overdrive!
For contrast, my 1979 Y84 WS6 with a carb'd 403 Oldsmobile 6.6L motor backed behind a TH350 3-speed auto (no overdrive!) drinks significantly less fuel!! A 30 min drive in that car barely moves the needle, so I know it's not just a "oh that's what V8's are like" thing, does anyone have any suggestions?

There are definitely no fuel leaks, the injectors seeeeeeeeeeem ok at the moment, the gauges are in good condition (albeit butchered to RHD), why would I be getting such bad fuel economy? It is a bone stock motor, the emissions equipment hasn't even be removed, the motor has never been disassembled. Maybe it previously had bad fuel? But I put a fuel tank in the car with 98 unleaded premium fuel (98 is kind of the good standard fuel here in Australia, 91 is the economy, and 94 is the standard). Previous owner may have used E10 which is like that 10% ethanol fuel, I know that isn't good for the injectors but really don't seem bad at the moment (or mechanic doesn't think they are the problem). I feel like I would not be able to go for a drive for more than 3 hours without needing to refuel which to me seems insane, my '79 Y84 403 Olds can go a whole day of consistent driving with one tank.

Are there common problems like this?

Thanks!
-Max

Komet

Is it sluggish, does it smell rich? Check your base timing, verify egr is functioning. What do the spark plugs look like? Is it throwing any codes?

Vader

"Bad" is a relative term. Kliks per liter is an absolute measurement, which could be more useful.

Also, and for what it's worth, my '86 has used E-10 for probably most of its life. All my vehicles use E-10, an dthe only injector problems I've had were in the '86 TA, after it sat for a long period with aging fuel (at 8 years old it had just under 18,000 miles).

I'd be more suspicious of a lazy oxygen sensor indication (or exhaust leak causing a false lean) than the injectors.

Maxthe222

Ok, tank is 14 gallons = about 50 litres. I drove one kilometre, yes just one kilometre, and that appears to have consumed 20% of the fuel gauge, let's assume the gauge is accurate to 40L in the tank, so that would be 20% of 40 litres, that would mean the car needed 4 litres for one KM!!!!! That is definitely bad! Even if it was an overestimate at 20%, the gauge moved that kind of distance consistently on other drives too, 10% is still 4L for 1km of driving! That can't be good!

You mentioned exhaust, actually I think the exhaust is toasted on the car, but all the way at the back by the muffler, i'll have to look into that...

Maxthe222

Really, really sluggish. I will be foot to the floor and feel like i'm not getting anywhere, it's like a 8-9 second 0-60.

The plugs were bad when I got the car, and probably the wrong size. On one side of the engine, they were barely even screwed into the socket, and the other side was really black. Threw those out, got new NGK equivalent GM Delco replacements.

How do I check if the EGR is functioning? I can tell the catalytic converter has been replaced, that is definitely new. It still has all the EGR stuff ontop of the motor. Timing I will check, but how do I check if it is throwing codes? The check engine soon light turns on for a split second when starting, but that is normal.

ironwill

The fuel gauge in these cars (and most other cars, for that matter) is not accurate enough to use to figure mileage. And based on frequent questions concerning fuel gauge operation on this site almost by the day, you're fortunate that your fuel gauge moves at all.

Fill the tank, drive a couple hundred miles, and then figure your mileage the traditional way.

Maxthe222

I suppose that is a good point, but I am paranoid of being stuck out in the middle of nowhere with no fuel!

ironwill

Once you have figured the car's approximate mileage, simply reset your trip odometer to zero every time you fill the tank. You'll then know how far you've driven on that tankful and how much further you can go before running out. No problem.

GeneralDisorder

iTrader: (1)

Fill it. Drive it 50 miles. Fill again. Divide 50 by the gallons it took to fill the tank back up. That will approximate the mileage. Do this 10x and average the results to make it more accurate.

GD

Maxthe222

I'm going to completely empty the tank to find where zero fuel really is on the gauges. One thing I just discovered is:

the car still had the original 1987 AC Delco fuel filter on it, and ho-boy it was completely blocked.

Full of dirty and grime. Replaced it, now operates just a little better... just to the point that it is noticeable. That handy manual roof odometer may come in handy for testing the mileage!

Rolling Thunder

My money's on the fuel pressure regulator. It's controlled by engine vacuum and when the diaphragm ruptures it starts dumping gas into the intake manifold. This causes the engine to run poorly, black spark plugs particularly on cylinders near where the vacuum line hooks up, and of course bad fuel economy. Take off the vacuum hose and see if you see or smell gas. Be mindful of the fact that depending on the severity of the leak it may or may not leak fuel without vacuum being applied.