Sudden terrible gas mileage; or, The Disease with No Symptoms

Well...going on your "it must be only something I touched" theory.....since you said you removed the intake manifold....well then....IF you neglected to hook back up the vacuum line from the throttle body to the dizzy.....the car would run okay....but for sure fuel economy would suffer...😓

Also keep in mind that as time goes on....our gasoline gets crummier and diluted to a crappy E10......you fuel economy will not be as good as it was several years ago.....not as bad as you are experiencing now....but....

If....you are convinced the fuel economy is for sure suddenly worse....and suspect the car is running rich....or unsure if the O2 sensor is okay.....you could I suppose install....at least temporarily.... an A/F ratio gauge like this one:
www.ebay.com

2" 52mm Blue LED Digital Air Fuel Ratio Gauge Car Racing Gauge Narrowband Meter | eBay

Fit for Narrowband Oxygen Sensor. 1 x Digital Air/Fuel Ratio Gauge (NOT include Oxygen Sensor）. This gauge will both show the digital and analog LED. Color: Smoked Tint lens when "off",Color LED Display when "on". ).
www.ebay.com

Poke around on Ebay and amazon and AliExpress and lots to choose from. The one above is just a random example...Just make sure you pick a "narrowband" gauge which is what the X1/9's O2 sensor puts out. Some come with a sensor....you dont need that...just the gauge.

It gets its signal from your existing O2 sensor. They are typically not highly accurate...but will tell you if your O2 sensor is working....AND...will show if the car is running way too rich ( or lean ) . And besides....the fancy dancing light display is sure to impress the women.....

You can also get a "wideband O2 gauge". These are way more accurate but require also a wideband O2 sensor ( usually included in the kit ) that you will have to weld in a bung into your exhaust pipe for. And will be more expensive ...but seem to be getting cheaper every day....
 
So, to recap - the facts are:
1. Car performs as usual [including temperatures]
2. Fuel consumption has increased from 30MPG to 20MPG. {same route, same season, same driver and style]
3 No smell of fuel from anywhere, anytime, any condition = no "leaks"
4. Change in consumption was sudden and appears timed with work around the top of the engine [replace short FI hoses]
5. OEM O2 sensor was changed in passing at the same time and has been changed again but high consumption remains. And remains unchanged so there was no effect from the second O2 sensor change at all.

Thoughts:
No smell of fuel means that the stuff that is disappearing must be doing so via the proper channels = inlet, bang, exhaust. No fuel [rich] smell in exhaust means AFR must be such that still getting full [normal - ish] combustion. The car had and has no cat so no impact from that

Systems disturbed were inlet manifold and everything that interfaces to it. Mech, elec, fuel, sensors, gaskets.

Dumb ideas are I have left:
Somebody is literally tapping your tank and stealing fuel.

A rag got left in the inlet ducting - not blocking it off, but making the engine work harder at all speeds, pull more vacuum etc.

You left the caravan hooked up?
 
There is a FI dedicated air temperature sensor in the Air Flow Sensor that enriches if air is cold - maybe it died?

1703655633650.png
 

x1/9 backfire

Have an gold 85 X1/9 that backfires when I decelerate in gear. What do you guys think could be the reason?
xwebforums.com

I know this isn't the same symptoms, but if you read post 4 from this thread, it explains that the throttle position sensor works in 3 ranges. Perhaps you have a different issue where the closed/idle position of the TPS is working correctly, but the part throttle is signaling full throttle for some reason. Anyone with more knowledge know if that's possible?
 
Take it up to John/Gildo and let them check it out. The cool thing is that not only would they hopefully fix the problem but Gildo will find seven other things wrong with your car that you didn't know about.

There are no shops (other than the boys mentioned above) in the DC metro area who have the ability to work on Fiats reliably. If it was a Fiat you take out just a few times a month for fun then I would say just forget about the mileage and enjoy the car but I know you drive your Fiats like daily drivers.
 
The gaskets between the intake plenum and intake manifolds only go on one way, but you can reverse them and have part of the gasket protrude into the intake tract. I doubt this would account for you drastic reduction in mileage, but something to check if you take it apart again.

As has already been stated, it could be debris in an injector. If one (or more) cylinders is not running correctly it could cause your problem. Reading the plugs may not show the problem unless you perform a plug chop, run the engine at highway speed and quickly push in the clutch and kill the ignition. Then pull the plugs and have a look.
 
Steve Thomas said:
So, to recap - the facts are:
1. Car performs as usual [including temperatures]
2. Fuel consumption has increased from 30MPG to 20MPG. {same route, same season, same driver and style]
3 No smell of fuel from anywhere, anytime, any condition = no "leaks"
4. Change in consumption was sudden and appears timed with work around the top of the engine [replace short FI hoses]
5. OEM O2 sensor was changed in passing at the same time and has been changed again but high consumption remains. And remains unchanged so there was no effect from the second O2 sensor change at all.

Thoughts:
No smell of fuel means that the stuff that is disappearing must be doing so via the proper channels = inlet, bang, exhaust. No fuel [rich] smell in exhaust means AFR must be such that still getting full [normal - ish] combustion. The car had and has no cat so no impact from that

Systems disturbed were inlet manifold and everything that interfaces to it. Mech, elec, fuel, sensors, gaskets.

Dumb ideas are I have left:
Somebody is literally tapping your tank and stealing fuel.

A rag got left in the inlet ducting - not blocking it off, but making the engine work harder at all speeds, pull more vacuum etc.

You left the caravan hooked up?
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Outstanding summary, Steve, I appreciate that! Didn't stuff rags anywhere during my work, so that can't be it. Disconnected the 38' boat and the Airstream trailer from the trailer hitch, so it can be that either. I've even considered the gas tank theft idea, but the gas is being used so fast I can basically watch the gas gauge dropping while I'm driving, so probably not theft. Truly a mystery.

Debris in an injector is worth a check, but if that were the cause wouldn't I notice rough running, vibration, some loss of performance? I have none of those symptoms.

Like Carl suggests, I will ask Gildo and Capt. John if they'd be willing to give it a look. If anyone can exorcise a possessed Fiat, those two can!
 
DSpieg said:
Like Carl suggests, I will ask Gildo and Capt. John if they'd be willing to give it a look. If anyone can exorcise a possessed Fiat, those two can!
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I badly want to know! Not as badly as you obviously :)

Hope those guys can work it out.

My last guesses - either the air temperature sensor or a blockage of some sort in the air side [collapsed filter, half full of water/oil/wolf urine or something in the intake trunking].
 
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Update to an older thread. Matters have gotten much worse. Daughter was driving the car a couple of weeks ago on the highway when, she reports, it lost power and tried to shut off. She thought she saw an oil pressure warning light come on, but not positive. She was savvy enough to get the car off the highway, up a ramp and into a neighborhood where she parked it on the street.

I came by later on my way home from work to check out the car and move it home. It would barely start, sounded and felt horrible (as if running on about 2 cylinders), but I was able to limp it the couple of miles to home, where it now sits. Since then I've only had time to remove the distributor cap to check the contacts and the rotor (all looked OK) and check the plug wire connections (ditto). Will start troubleshooting in earnest this weekend.

I came up with a weird theory trying to tie the "lousy gas mileage" (original problem) to the new problem of "horrible running/nasty mechanical sounds", and offer the following theory for your amusement: WHAT IF a fuel injector had failed open (that is, constantly flowing fuel), wasting gas for a while, and eventually the extra fuel in that cylinder caused a hydraulic lock that seized the engine momentarily and broke something in that cylinder?

Obviously I've spent too much time speculating and not enough time wrenching, you don't need to tell me that. But....am looking for a troubleshooting strategy. I guess I should start with spark plug removal and compression checks? Drain the oil and look for metal particles? Check coolant for oil and oil for coolant? Replace distributor cap and test-run? I'm at a disadvantage since I wasn't there when the failure occurred.
 
Dave, the answer to your last question is yes. Also check for the smell of gas in the oil. I don't think the dizzy cap is an issue, yet. Check cam timing.

I would offer to look at it but I don't do fuel injection and I suspect that may be the source of your problem.
 
I will do as suggested. Bummer if it was the timing belt, I had replaced it myself maybe 3-4 thousand miles ago, don't recall any issues with the tensioner at that time. If the timing is off, of course I'll replace the belt again, should I also replace the tensioner bearing and spring?
 
DSpieg said:
I will do as suggested. Bummer if it was the timing belt, I had replaced it myself maybe 3-4 thousand miles ago, don't recall any issues with the tensioner at that time. If the timing is off, of course I'll replace the belt again, should I also replace the tensioner bearing and spring?
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My 1300 had a tensioner bearing disintegrate with under 40K on it. Pretty much took out almost all the valves, head, and dented a piston. If I still had an interference engine, I'd change the tensioner at every belt change.
 
Tensioner bearing is a bearing after all so a couple of quick spins by hand will tell you if it needs replacement. I have never replaced the spring on any of my X or 128 motors. Given the symptoms you present I'm thinking it's not the belt but no harm in checking the cam timing.
 
Yeah, when I replaced the belt the tensioner bearing felt just fine, but hey, while I'm in there....I've ordered a belt + bearing package from Vick (not my favorite vendor, but nobody else seemed to have the bearing in stock).
 
Had just enough free time this weekend to check the cam timing (it was spot on) and pull the spark plugs (see photo, a bit oily but they look normal to me). The only thing that didn't seem right was how easy it was to turn the engine via the crankshaft pulley with the spark plugs still installed. I didn't feel much if any compression. So what's the next step, compression test?
X19 spark plugs_2.jpg
 
Did you check the cam timing against the flywheel marks or the pulley marks? If the latter, have the pulley marks been checked against the flywheel marks since the pulley scale is adjustable for calibration.

The reason I ask is that if the cam timing is off enough, you might come up with a low compression condition.

You could also remove the cam box lid and see if the valves are moving normally. A damaged valve or stem can show up as lack of normal movement from that view
 
The cam pulley mark lines up with the cam cover notch while the crankshaft mark lines up with the metal pointer down there while the distributor rotor is pointing to #4 plug contact - I think we can eliminate the timing belt from the list of suspects. So I did not (and probably won't) change the timing belt or tensioner at this time.

Compression test will have to wait until the timely (pun intended) arrival of a suitable Daughter Unit to crank the engine while I hold the pressure gauge into the spark plug holes. I've tried to train the dogs to assist me in car repair operations but their lack of opposable thumbs has been a real impediment.
 
Let's do this right and use a thread in compression tester. I have one you can borrow and makes this a one family member job. Since you are a working stiff and I'm retired I can even do free delivery. Text me if interested.
 
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