Poor gas mileage 81 FI Spider

[MrWampus]

I have had my '81 for about 3 years now and it consistently gets only 20-22 MPG no matter how or where I drive. My '79 that I had years ago got 27-30 MPG. It runs great, very smooth and good acceleration. Before I just start throwing money at it I wanted to get some opinions on where to start. I also have some clues. For one, I get a lot of black soot from the exhaust. It will leave a black circle on the garage floor if I let it idle a few minutes. Also, it smells "gassy" if I am idling at a traffic light, especially with the top up. And anything I leave in the trunk also comes out smelling like exhaust. It also tends to run cool, maybe 140-150 degrees unless I'm in stop and go traffic. I'm pretty sure I don't have a fuel leak, I've never seen or smelled raw gasoline. I had the fuel filter replaced when I bought it.

The car had 68K miles when I bought it and has 80K now. I'm sure it probably needs new vacuum hoses etc. where is the best place to start? New thermostat to get the temp right? Fuel injectors? Vacuum hoses? Spark plugs?

[RRoller123]

I am no expert, but my 2Cents: Sounds like it is running way too rich. Could be restricted air, or the AFM is shot, or a whole bunch of things. Possibly a PO fiddled with the spring settings in the AFM. Timing could be too far retarded too. I would start there, and do a complete tuneup, including a new O2 sensor at the end of the tune.

[seabeelt]

Car running too cool will not help either. Car will alway think it is cold and supply more fuel-rich condition. Are you sure your thermostat is operating properly and not stuck open?

R/

[spider2081]

when I bought my car the oxygen sensor was disconnected and 22 mpg is about what I was getting. Now I am getting about 27 but I don't drive this car too easy.

[MrWampus]

I hadn't thought about the O2 sensor. I'll check that. And no, I'm not certain that the thermostat isn't stuck open. I replaced it not long after I got the car because ironically, the car was running cool then too. After I replaced the thermostat and the temp was still about the same I just assumed that's where it wanted to be.

[azruss]

if you are running rich with lots of extra stuff out the exhaust, your O2 sensor is toast. 2 things to check on the fuel side. Your pressure relief valve on the end of the fuel rail and your injectors. Also check your start up injector to make sure it is shutting off after a couple of seconds. If it isnt shutting off, it can lead to all your symptoms.

[narfire]

What do the plugs look like, perhaps consider changing them
A few things I'd start with... Check timing.
As mentioned check the vacume hose to the dist. Perhaps suck on it and see if the plunger on the diaphragm moves.
Change out O2 sensor but if it is pooched the engine defaults to an "average" setting. I noticed perhaps a couple of mpg difference when mine was down.
The cold start injector was a good suggestion..

[RRoller123]

Leaking CSI would definitely do this. Good idea, easy to check, just be careful, will spray fuel. Could also be that the Thermo Time Switch is keeping the CSI on. The same test checks both. Just disconnect the 2 allen screws, pull the CSI straight back and pull out of the intake plenum, and aim it into a good sized container. When the engine is cranked over cold, the CSI should spray for about 1-8 seconds (depending upon ambient temp) and then stop and NOT drip.

[majicwrench]

Unleaded gas will always produce a certain amount of black soot at tailpipe, so that is sorta normal.

Vacuum advance is big player in mileage game.

O2 sensor is too. They can stick low (lean) and computer is forever trying to richen it up. Old computer system does not just default to center.

Get some verification on engine temp. As you know, should run 190ish. Everything will work better when up to temp.

Certainly check timing. Why not take a look at CSI, but I would bet if it was leaking, car would be flooded when you tried to start it.

Good luck

[MrWampus]

Thanks for all the great ideas, I have some homework to do. I did just have the timing belt replaced a few months ago and they set the timing when they did that. The engine ran noticeably smoother after that so I think the timing is probably good. (The mechanic was over 40 and didn't complain about it being a Fiat and he had my shop manual.) So, I'll check the plugs, I'm sure they are sooty, and the O2 sensor, (I wonder if the PO unplugged the light to the Ex Gas Sensor light in the dash because it was on all the time...) and the CSI and let you know what I find.

Oh, and the car always starts immediately when I turn the key. It has never failed to start and has only been slightly slow to light when the outside temp is below about 35 but still fires (a few seconds of cranking vs immediate).

[So Cal Mark]

the warning light is on a mileage timer, it has nothing to do with operation of the O2 sensor. Pull the vacuum hose to the pressure regulator and look for wetness. Also check the resistance of the coolant temp sensor

[bradartigue]

At this stage it is not the O2 sensor. Don't replace it yet. If you are running so rich as to smell gas when idling then you need to figure that out fix. Rich running cars destroy O2 sensors (they just make minor adjustments on cars that already work correctly).

Pull the plugs (one or two), report back on how they look.

[wizard124]

http://www.fiatspider.com/f08/viewtopic.php?f=5&t=27531

go to this post to diagnose your AFM. only need a multimeter. Also inspect the connector at the AFM. Those pins can pull back resulting in a no connection and bad idle performance.

[MrWampus]

OK, I have an update. The plugs look good. They seem to be the color I was expecting, white-ish with a slight tinge of beige. Not much soot. What's the trick to reaching plug 4 though? I can't get in there with my socket wrench with any room to turn it because of the heater hose. One extender doesn't get me clear of the hose and if I put 2 extenders on it then it's too close to the firewall.

The other puzzle I have is how to get an allen wrench in behind the bend of the cold start valve. Everything is so close in there I'm not sure how to get in to loosen the bottom screw holding it in.

Anyway, the car still seems to run pretty well but I filled the tank today and calculated 19mpg. 173 miles on a tank and put 9 gallons back in. That's just way lower than I would expect.

[aj81spider]

I use a swivel fitting on the spark plug socket for #4.