Awful MPG, but drives good?

[essenz]

Curious where to start the investigation. I've heard these 83-85 Spiders can avg 25mpg highway (and the PO log books show this).

I've had the car for 3 weeks. Since filling the tank, I've driven about 25 miles - the tank is now on 1/2. So thats like 5 gallaons for 25 miles.

Granted I have 5-10mins of idle time to warm up, and its all stop and go driving - not highway. But man, the fuel needle is just droping in front of my eyes.

The car is in my garage, there isn't a super strong gas smell, and initial inspection doesn't show any leaks. I'm curious what could be the cause for such awful MPG. In general the car runs very good, I guess it "could" be running rich - but not enough to eat that much gas. Its possible the gauge is bad, so at next fill up I'll have a more exact reading of how many gallons.

[18Fiatsandcounting]

Many Fiat gas gauges are notoriously non-linear, and often sporadic in what they read, so I'd check how much gas you actually used by filling up the tank again as you mentioned. Usually the problem with the gauge is the sending unit in the tank, and it's easy enough to replace.

With a fuel injected spider with no gas leaks, it would be very hard to consume more than 1 or 1.5 gallons to go 25 miles.

-Bryan

[essenz]

non-linear.... thats interesting. So full to 1/2 quickly, but 1/2 to empty may take alot longer. I'll go another 50 miles, then fill up again and calculate the diff.

My fuel gauge needle also flutters sometimes. I also had a hard time believing I used so much gas so quickly....

[TheBender]

Agree, calculate off fill up not the gauge.

[18Fiatsandcounting]

A wavering gauge needle usually means your fuel is sloshing around in the tank, but a flickering needle is another sign of a sending unit that's going bad.

Here's another trick I use to see how accurate the gauge is. The tank is roughly 11 gallons, so drive it until it's almost empty. Add 2.5 gallons at the gas station, turn the ignition on, and the gauge should read 1/4. Add another 2.5 gallons and the gauge should be close to half. Add another 2.5 and it should be close to 3/4, and finally, another 2.5 gallons should give a close to full gauge reading.

-Bryan

[spider2081]

I like to calculate MPG every fill up. Its great head math practice for one thing but also its great for pointing out pending maintenance issues.

With a new drive I suggest calculating MPG with each fill for at least 4 fill ups. Reason being one seldom fills a tank to the same point. averaging after 4 fills is a better indication of MPG. I usually fill my tank between 200 and 220 miles. As a rule I don't trust the Fiat Speedometers or odometers to be that accurate so I cross check mine with a GPS.

[essenz]

Ok.... first numbers from the odometer/fill-up method.

80 miles, 8.8 gallons = 9.09 MPG

Granted this is all driving around town, stop/go, no highway miles.... and some idling while the car warms up.

So my dropping fuel gauge isn't an error, I'm actually using alot of fuel.

I imagine 9 MPG is awful, right?

I mean its not leaking gas anywhere, if it did, my garage would stink of gas. On my short list is to check the throttle position sensor. Anything else? Could leaking injectors cause this bad mpg? The odd thing is it doesn't smell like its running rich, and generally drives well. The cold start idle is a bit rough, but smooths out once warmed up. A slipping clutch maybe? The only other observation (not sure if this is normal) is sometimes when accelerating (not crazy) I noticed the gas pedal is bottomed out to stop, strikes me odd that moderate/level grounf acceleration would so quickly reach the end of play for the gas pedal.

[18Fiatsandcounting]

The only other observation (not sure if this is normal) is sometimes when accelerating (not crazy) I noticed the gas pedal is bottomed out to stop, strikes me odd that moderate/level ground acceleration would so quickly reach the end of play for the gas pedal.

Those are pretty lousy MPG numbers, but your observation caught my eye. Unless you are accelerating hard, you shouldn't need to floor it. Try this test: On a relatively deserted road, get up to 40 mph in 4th gear, then floor it. Count how many seconds it takes to reach 60 mph. One one-thousand, two one-thousand, that sort of thing. A normal (stock) engine shouldn't take more than 7 or 8 seconds to go from 40 to 60 mph in 4th gear.

If your car is taking longer than this, something is wrong with the engine. Could be ignition timing, cam timing, something wrong with the fuel injection, or just a really worn out engine. All of those could contribute to low gas mileage, although I have never heard of it dropping that low. Maybe 20 mpg if you are idling a lot or driving it very short distances while cold, but 9 mpg means something is amiss. If it's not leaking gas, it's either burning it or evaporating it. Or someone is stealing your gas in the middle of the night....

-Bryan

[essenz]

So here’s what I know….

I’m definitely not leaking gas while parked.

While running it doesn’t appear to be overly rich, doesn’t smell rich, no black soot buildup on plugs or in exhaust pipes.

I guess it’s possible I could be leaking fuel back through the vapor return while driving, or leaking fuel from the rail while it’s pressurized. The injector hoses look super old/brittle, but if that were leaking wouldn’t I smell it while driving?

I definitely need to do the clutch slippage test on a hill. Maybe a slipping clutch plus some fuel leak/evaporate is adding up to be a big mpg issue.

I do know from previous owners logs that the car used to get 26mpg about 10 years/10k miles ago.

[18Fiatsandcounting]

If the fuel injector hoses were leaking enough to lower your gas mileage, you should see gas dripping from the lines at idle, and I think you would smell it at that point as well.

Do the acceleration test that I mentioned above. It will tell you a lot about the condition of your engine. Look for any pinging, missing, sluggishness, hesitation, etc.

-Bryan

[TX82FIAT]

A couple additional thoughts. The fuel line from the tank to the fuel pump and from the fuel pump to the hard lines is a common failure point for problems as the hoses deteriorate because of the ethanol in the fuel. The fuel pressure regulators have a diaphragm that is know to fail in cars that sat at some point or just over time 40 years later. I'd check your fuel pressure using the cold start valve port at the end of your fuel rail. I get 20 to 28 MPG on my 82. If you have a leak after the fuel pump by the rear wheel you might not ever smell it. If the pressure regulator is not regulating the fuel pump pressure you could be putting 40 to 45 PSI when it should be about 35 psi at start up to 28/30 psi when idling. May not be enough to foul the plugs but enough to move MPG significantly. If not a leak or pressure issue, there is a fuel injection trouble shooting guide and the next step would be to troubleshoot the entire system from the airflow meter/flap all the way through to the timing. One variable out of whack could have a huge impact.

[essenz]

Update....

Clutch slippage tests didnt indicate any major slippage.

I did the 40mph 4th gear floor-it test, took right around 7sec to get from 40-60mph - this is a stock engine with 140k miles. So maybe on high side, but this car is original (minus newly installed Vick headers).

I did however find a previous leak I did not see before. I started up the car, then popped the hood about 10secs later. When I ran my finger under the fuel rail (right where the cold start hose is) is was wet with gas (maybe 1-2 drops). Its unclear if its coming from the cold start hose or the cold start injector itself. The plastic part of the injector had some wetness, so I think its leaking from the injector then draining down. I tightened hose clamps and disconnected the electrical plug on the cold start injector. Disconnecting it appears to be help and it doesn't leak??

My TPS also needed a slight adjustment.

So a few followup questions....

1. The cold start injector is not a cheap part ($220), if I only drive my car when its 50 degrees and above, do I need it? I could just block off the ports, and call it day. I was planning on changing the fuel pressure regulator since its original to the car, and a fairly cheap part. I will drive the car for now without the cold start connected - since it appears to leak when connected, but not leak when disconnected.

2. Fuel injectors - though not leaking, these also appear to be original from 84, hoses are super brittle and cracking. I was going to go ahead and swap out for 4x new injectors (RockAuto has Beck/Arnley injectors at $68 a peice)

[essenz]

Update #2:

All new injectors and cold start valve installed, plus some other misc maintenance (I also did timing belt, and installed new aluminum crank and cam pulleys). Lastly, when I checked ignition timing it was set to 0 not 10 BTDC, so reset it to 10, and WOW.... what a difference - drives like a different car.

Just topped off the tank, will drive for a few days and re-calculate MPG.

[essenz]

22.3mpg - that a mix of 60% highway, 20% stop and go highway/rush hour, and 20% city/lights/etc. Not bad!!

[Nut124]

Good progress there.

Check you ignition timing again. The idle timing spec is based on the assumption that the dizzy timing curve works as specified, providing 26-27 deg of mechanical advance as rpm increases. A long shot at best in an old car.

Rather than at idle, check the timing at 500rpm increments or so if you can from 2000 to 5000. Want 36deg total timing at about 3800+ rpm. Then let the idle timing land where it may. If this puts idle timing at much above 15, without vac advance, then you may need to work on the dizzy or retard timing to prevent low speed ping.