Fuel economy loss

[ralley]

I have run the numbers, and the verdict is in. Zeppelin is losing fuel economy over time. From a high last Xmas of about 12 MPG, the last 3 tanks ran 7.8, 6.7, and 7.2 MPG.

1991 350, 78K miles. Running an MSD ignition, gearvendors, and a very clean air filter--less than 2K on the last tune-up.

I get an occasional whiff of fuel when driving.

Any ideas??

Best to all

Rob

[SteveH]

That is a significant change in mileage. It shouldn't have changed, but the thing that has the biggest effect on fuel mileage is timing, so check that. If that's good, I'd try running some injector cleaner thru it.

[john hd]

ethanol blend?

john

[slowmover]

Also look to collapsing exhaust system (internal) and fuel supply lines, pump, filters. Carb may need overhaul. On a vehicle that sits too much, there are far too many gaskets that deteriorate. Check alignment, brakes, bearings too.

[purman]

have you change the plugs and wires?
Is it fuel injected. are the injectors leaking gas. They might be if you smell it.

[ralley]

Thanks to all so far. I think, as the path of least resistance, I'll go with a hefty dose of injector cleaner first, and see what happens. Vehicle sat for the first 6 months of 2008. My notes do not indicate if I used a fuel stabilizer (did not expect to be down that long).
Best, Thanks--Rob

[Silvertwinkie]

Malfunctioning EGR valve can do this. PCV also may be an issue. Leaky injectors could also do this, type of gas, plugs, wires, etc.

91s have OBD1 which does have some check engine status if it were serious enough to trip. From the sound of it, you have no check engine light on. Are you burning any oil? Burning oil could mean the piston rings are not doing that well, and a simple compression test could rule this in or out. If even one piston is not up to spec it can throw the whole shabang out of wack.

[RBolton]

Most of the suggestions so far would normally be accompanied with a loss of power or performance. Have you noticed any? Do you smell raw gas? This would indicate a leak -tank, lines, injectors etc. Rich conditions resulting in poor fuel economy could be caused by a bad oxygen sensor or vacuum leaks.

[ROBERT CROSS]

Six months in storage, gas smell. Ck lines for leaks, also the fuel lines (module) at the tank. Look for pecker tracks down the side of the tank, or seepage at fuel filter, better yet change it, be careful not to twist off the lines.

Run the engine at idle, spray carb cleaner around intake manifold gasket and throttle body, (or carb) base gasket, egr valve, any change in rpm= vacuum leak.

[SteveH]

One more thing...you didn't mention performace, that I remember, but a choked, or partially choked catalytic converter will really mess them up performance wise and economy wise.

[Poprivet]

seafoam man, seaforms good.

Are you sure all eight cylinders are firing?

[thecatsandi]

Originally Posted by ralley

I have run the numbers, and the verdict is in. Zeppelin is losing fuel economy over time. From a high last Xmas of about 12 MPG, the last 3 tanks ran 7.8, 6.7, and 7.2 MPG.

1991 350, 78K miles. Running an MSD ignition, gearvendors, and a very clean air filter--less than 2K on the last tune-up.

I get an occasional whiff of fuel when driving.

Any ideas??

Best to all

Rob

Ethanol in your fuel will do that. They do that in the Winter to keep polution down. I tried a tank of ethanol in my old truck and the milage dropped like a rock. 50%. I never went back. You do not have a choice in Maryland in the winter.

[samb]

I have to agree with ethanol ( I call it that &%$* corn syrup) being the problem.
I have kept track of my mileage for the last 14 years. Every time, without fail, I have noticed a 35% to 55% drop in mileage with a tank of ethanol. In addition, if you do not use an ethanol rated fuel stabilizer the fuel will separate in just a few months, adding to poor (or no) performance. And now, thanks to my governor, I cannot get gas without 10% ethanol at any time. He has nicely made it possible for me to purchase straight gasoline for my boat. The only problem with that is that it is a 180 mile round trip just to buy boat gas at an outrageous price from the nearest dealer.
#@%** corn syrup!!!!!!!
Sam