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

I've never had an injected fiat. Could a leaking cold start injector cause this issue? If it's fueling each cylinder from that leak it could probably still run well. I don't think that was mentioned yet. It could be one of those things that was disturbed in working on the fuel lines.
 
Here is a photo of the suspect O2 sensor, just removed. Only ran three tanks of gas through it (about 700 miles). Seems like a lot of soot for such little mileage! My Alfa 164S, a chronic oil-burner in the time I've owned it, takes at least 20K miles to get that much soot on its Lambda sensor.

Interesting? Maybe I should look at those spark plugs....
X19 removed 600-mi. O2 sensor_20Dec2023.jpg
 
DSpieg said:
Pardon my frustration with some of the replies, but let me restate the situation: The car was running great, but sprung a leak from a split fuel rail hose. I removed the intake manifold, replaced all the little hoses, and (unrelated to the fuel leak) replaced the 02 sensor because it 'looked old'. Afterwards: The car still starts and idles and runs perfectly, just as before my repairs, except now the gas mileage sucks. BTW there is no catalytic converter on the car (ahem....)

As an engineer, when something changes suddenly, I look first for gross errors (such as unconnected or mis-connected vacuum hoses as Carl suggests), or a defective replacement part which in this case might be the 02 sensor. It does NOT make sense to start poking at gradual degradation-type problems such as dirty injectors, fouled spark plugs, dirty air filters, etc. because any of these would have manifested themselves before my fuel line problem occured, right?

And if the suggested problems were the cause, wouldn't the car run crappy, or idle roughly, or be hard to start? And wouldn't I feel or hear a difference? Like I wrote: It's the disease with no symptoms.

A funny aside: Right after I replaced the fuel lines, the electric windows stopped working. You can imagine how my brain hurt trying to correlate fuel hose replacements with power window operation. Of course there was no relationship, the power windows relay simply failed at that moment all by itself.

Troubleshooting question: Would it be worthwhile for me to hook up a multimeter to the 02 signal wire and rev the engine while watching the voltage output from the sensor? If so, should I leave it connected to the wiring harness or disconnect it first?
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I'm sorry to frustrate you DSpieg. Maybe "as an engineer" you should know how to troubleshoot your own car. How was I to know your car is missing it's catalytic converter? It might serve you better to remember that people are spending their time trying to help you. Were in my case. Best of luck!
 
Steve, about half a year ago I had a nasty running problem with this same car, there is a thread about it here on the BB. Eventually found the culprit to be a failed FI coolant temp sensor. So I don't think that's my current issue, because the car ran terribly after the coolant temp sensor failed back then, but it runs perfectly now. That said, I'll re-check the electrical connection.

Cratecruncher, my frustration was not directed at you in particular. Maybe my descriptions of the problem and my car's configuration weren't very clear.
 
Steve, the fuel pressure no longer bleeds down. I'll speculate that was the start of the split in the one short hose which then started blowing visible fuel a month ago and led to the repairs we've been talking about here. Perhaps I should have inspected the hoses back then, eh?
 
DSpieg said:
Steve, the fuel pressure no longer bleeds down. I'll speculate that was the start of the split in the one short hose which then started blowing visible fuel a month ago and led to the repairs we've been talking about here. Perhaps I should have inspected the hoses back then, eh?
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couda shouda - lots of things we all couda shouda :)
 
Pulled the spark plugs, they look completely normal. Exhaust smells normal to me, not rich. Now have about 35 miles on the new OEM Bosch 02 sensor and will run out the tankful of gas before coming to any conclusions, but with the fuel gauge already down to the 3/4 mark, I'm not feeling optimistic. Yes the gas gauges are inaccurate, but I "know" how mine behaves and it shouldn't be reading this low after just 35 miles.

Will report back to the forum in a week or less. Appreciate everyone's advice, as always. Happy holidays, may Santa put a restored Lancia Stratos under your tree*!

*or in your garage/driveway, that might be better for most of us
 
Here's what I've learned about L-Jet...

The enemy of L-Jet is air leaks.
All of those hoses, diaphragms, joints, o-rings, throttle body bushings, etc. all have to seal.
Once there's a leak, they start sucking air; Unmetered air.
Often, a mechanic will adjust the AFM a.k.a. intake air sensor, either by moving the toothed baseline adjustment, or changing the spring tension, to richen the mixture and compensate for the lean condition caused by the air leak.
Now that you sealed things back up during your recent foray in, it's now richer because you fixed a leak.

Even if the AFM was never adjusted from factory,
The 02 sensor in its feedback loop will do an imperfect job of correcting mixture.
If the car otherwise runs rich or lean, you will see the analog wavering of your AFR on a good gauge,
Oscillating around 14.7 stoich,
But skewed towards the direction that the feedback loop seeks to correct.
Said another way, your air/fuel ratio will change some, even if the o2 sensor is working properly.

So, my money is on your fixing an air leak without knowing it, and now you're running rich.
You tightened up a clamp or seated a hose or o-ring better than it was...
 
Clatter said:
Often, a mechanic will adjust the AFM a.k.a. intake air sensor, either by moving the toothed baseline adjustment, or changing the spring tension, to richen the mixture and compensate for the lean condition caused by the air leak.


So, my money is on your fixing an air leak without knowing it, and now you're running rich.
You tightened up a clamp or seated a hose or o-ring better than it was...
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Understood but I don't think so in this case - looking at other OP threads gives some history on this car/engine. Past fuel consumption was on a par with other FI cars and consistent. In other words, the FI system was operating correctly for an extended period so I don't think a long standing air leak accidentally getting fixed is likely. Comment on the AFM is interesting tho. Ignoring the idea of deliberate adjustment for a second - how do they fail? Progressively? In any known pattern? If there is a spring involved, then that will weaken over time for example. As y'all may have guessed, I have no hands on time with X FI. If failure is progressive, then that is probably not the problem in this case as OP reports sudden change in consumption.
 
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Steve Thomas said:
Understood but I don't think so in this case - looking at other OP threads gives some history on this car/engine. Past fuel consumption was on a par with other FI cars and consistent. In other words, the FI system was operating correctly for an extended period so I don't think a long standing air leak accidentally getting fixed is likely. Comment on the AFM is interesting tho. Ignoring the idea of deliberate adjustment for a second - how do they fail? Progressively? In any known pattern? If there is a spring involved, then that will weaken over time for example. As y'all may have guessed, I have no hands on time with X FI.
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I have, unfortunately, far too much experience with L-Jet - more than i ever wanted!
Been married to it for years due to smog laws.

The notorious Flapper Door system of AFM can do a number of different things.
Common is the resistor board tracks wearing; As the wiper contacts slide over the board they can wear through.
Even during normal use, because of the large amount of movement from intake pulses, the board gets worn.
A worn resistor board will give different resistance.
This is a reason that they say not to open it - because dirt will be introduced and increase wear where the wiper arm contacts the board.
They supposedly use a very specific grease for the board to lubricate the wiper,
And I've even heard a legend that the lid was glued on with a very special silicone to not promote corrosion inside.
Know that after 40+ odd years this grease does get stiff and slows down wiper movement.
At the same time, the spring gets weaker, so different factors inside the AFM itself will affect running with age.

Backfires can damage the flapper. They have that side cavity, where one of the two blades or vanes on the flapper lives, to (sort of) help prevent damage.
Later flappers are also made of a soft material that can be ingested without damaging the engine (too badly). 🤪
This late in the game it's possible that the soft flapper can have been damaged.
it might not move smoothly and bind or be crooked in its mounting.

Many have purchased NOS AFMs to measure the precise amount of weight it takes to -just- crack the flapper off of its seat.
Once that weight is known, you can use it to adjust spring tension on a used flapper to match.

Something else is that the computer is smart enough to interpret a rapid/large movement of the flapper as an acceleration event.
Like with the pump on a carb, the system will give extra fuel to enhance initial acceleration and drivability.
If your flapper spring is too loose and flails more than it should, it can generate rich running as a result.
You can only imagine what a long-duration cam with lots of overlap will do to idle emissions...

Anyhow, L-Jet can have a whole host of troubles - wiring connector issues, goopy AARs, leaky throttle body bushes, diaphragms rotten from ethanol fuels, and even my favorite - cold start valves that pop apart and spray raw fuel everywhere.
Depending upon the application, the entire engine will have to hold a vacuum in order for the correct signal to be generated.
Things you would never suspect of being intake air leaks, like the oil filler cap or the valve cover gasket or the dipstick boot...

I could go on and on... But know that L-Jet is a persnickety creature and depends upon a sealed environment to work correctly.
For this reason, our esteemed OP is very likely to have a tuning change result from disturbance of fuel and intake components,
And the results he got I've seen on several occasions.
The good news is that it is better than it was, because it was very likely correcting for an intake leak before.
Now you just need to adjust it so it's correct for being (more) sealed up than it was.

I'd love to see what a wideband has to say.
 
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OP can you post a picture of the spark plugs when you have a chance?

Again, not a FI guru, but from what I understand the "read" of a spark plug can be disturbed by even small amounts of idle time. With the L-jet system is it possible the idle will run well enough to give a good read on the plugs, but under load on the high way be dumping too much fuel and sooting out the O2 sensor?

Clatter's points seem to make a lot of sense to me. If the FI system was tuned at a time there were considerable intake leaks, perhaps the system is now running too rich if those were corrected in DSpeig's work.
 
With the new Bosch lambda sensor installed, another tank of gas has been run through, but resulted in another 21 mpg fillup :mad: So the problem doesn't seem to be the oxygen sensor I had replaced a month ago.

A full 50% more fuel is being used somehow, but the car still starts and runs perfectly. I'm about at the end of my rope with this thing.

Should I disassemble and reassemble everything I had done in replacing the old short fuel hoses?

Should I find a local shop with an exhaust gas analyzer and have them measure the A/F ratio at a variety of throttle settings?

Should I sell the goddamned car and forget about it?
 
DSpieg said:
With the new Bosch lambda sensor installed, another tank of gas has been run through, but resulted in another 21 mpg fillup :mad: So the problem doesn't seem to be the oxygen sensor I had replaced a month ago.

A full 50% more fuel is being used somehow, but the car still starts and runs perfectly. I'm about at the end of my rope with this thing.

Should I disassemble and reassemble everything I had done in replacing the old short fuel hoses?

Should I find a local shop with an exhaust gas analyzer and have them measure the A/F ratio at a variety of throttle settings?

Should I sell the goddamned car and forget about it?
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Just pretend that you are running dual DCNFs and 21 mpg won't seem so bad.
 
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