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

Cratecruncher said:
You have some good points Jeff. Have you used any of these products? I picked a cheap pulse unit at random. There are also cleaner reservoirs with pressure supplied from a shop compressor. They also include high pressure fittings for all the major OE connections.
Reservoir unit on Amazon

Combined together they seem to be an affordable alternative to sending injectors out for cleaning.
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I have two cleaning systems that I made. One was mainly for the pre-electronic Bosch systems ("CIS", a mechanical injection system with constant fuel through the injector). That cleaning unit is a lot like the second link you posted with a container pressurized by the shop compressor. That allows a much higher pressure delivery for the cleaning solution. The one I made is significantly larger than the linked one. I used a old fire extinguisher bottle for the reservoir and it allows me to run a LOT of solution through each injector (which I find they need). Initially I also used this cleaning system on electronic injectors (like on the X), with a constant 12V feed to keep the solenoid open. It worked OK for some injectors but not for worse ones. So I made the second cleaning unit. It has a module like in your first link to "pulse" the injectors, and I made a rig to submerge the injectors in my ultrasound (US) machine at the same time. However with that setup you cannot pump cleaning solution through the injectors from a canister (my first system) while they are in the US machine...it will overflow the US tank. So I have a spare Bosch fuel pump that draws solution from the US tank and pushes it through the injectors while they are in the US unit (circulating the solution). This works the best.

I suppose a combination of the two items you linked might work. Use the pressurized canister instead of a aerosol can of cleaner. That also allows you to use any cleaning solution you wish. Feed the solution through each injector while pulsing them with the unit from the other link. Although for the price of the two devices you could just about get a set of four injectors professionally cleaned....I believe it runs about $25-30 each. So it depends on how many times you intend to do it. I have multiple vintage cars, plus a seemly constant influx of additional ones. And it wasn't very costly for me to make my own cleaning systems as I already had pretty much all of it on hand.

Something else to consider when cleaning injectors. They have a small screen filter at the very top of each one. You should remove it before cleaning, and install new filters after cleaning. In fact, in some cases it is the filter that's causing most of the problems. The old filter can be removed with a sheet metal screw and pliers. Replacement filters are available online (cheap on AliExpress). They simply push into the top of the injector.
If I recall correctly these are the right filters (but double check it):
 
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Odd that Dave has not responded since last Thursday. Is he stuck on the side of the road with fouled spark plugs. Guess I better go look for him.
 
I'm here, Carl. I was waiting to see how many tangents the thread would take - quite a few, it turns out. Most replies have not connected what I did (changed a few short fuel hoses and the O2 sensor) to the problem (immediate large drop in fuel economy immediately thereafter). I wasn't expecting any brilliant ideas, so am not really disappointed.

Haven't gotten around to changing the O2 sensor, and right now that's the only plausible culprit, except I can't believe an 8 mpg drop due to that!
 
Maybe you have coincidentally encountered another problem. Are your brakes dragging? Is the odometer correct?
 
Car coasts freely and wheels don't get warm (i.e. brakes are not dragging); odometer was checked against GPS on a long drive and is spot-on.
 
I'm pretty useless in diagnosing problems with fuel injection but I do know there are a ton of hoses in that system. When you were doing whatever it takes to replace injector hoses, did you by chance remove a vacuum hose that never got reconnected or even a fuel hose that got improperly reinstalled? I agree that if you didn't do anything with the cam belt or distributor or noticed any gas leaking that it has to be something that was touched in the process of doing the injector hose swap. Engineers are not big believers in coincidences.
 
Actually most of the reply posts do pertain to your problem. I will repeat my opinion that this does not sound like a O2 sensor issue. It simply does not have that much influence on fuel consumption with the factory ECU. Several great suggestions have been made throughout this thread that I do not see responses to...what have you done so far to eliminate them? :)
 
Catalytic converters can slowly clog causing all sorts of weirdness including weird smells, hard starting, and poor fuel economy.
 
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?
 
Steve Thomas said:
cos why? Where is this idea coming from? The O2 sensor continuously changes mixture except under WOT which is full fueling anyway :)
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You are thinking of a "modern" EFI system. The Bosch system in the X is a very early O2 design. The effect the O2 sensing has in the ECU is minor; it only makes small changes, not the large AFR alterations of later designed systems.
 
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 feel your pain. If this was the Alfa Romeo Giulia Forum everyone would tell you that you have a bad battery...

This is an odd one indeed. If you still have the old O2 sensor, I'd put it back on and see if things change...

I'm sure the power windows stopped working because you removed the intake manifold though...
 
Dr.Jeff said:
You are thinking of a "modern" EFI system. The Bosch system in the X is a very early O2 design. The effect the O2 sensing has in the ECU is minor; it only makes small changes, not the large AFR alterations of later designed systems.
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I am thinking L-Jetronic which ran from 1974 to 1989.... Which is the topic of the doc by Bosch that I posted at #17 in this thread. If you have a cat, you get an O2 sensor and closed loop control.
 
This may not be it but did you disturb the fuel pump when you changed the hoses? or the fuel pressure regualtor?
If the fuel pressure is higher than the computer thinks maybe this could cause worse mpg.
Otherwise did you remove the injectors while you did this, possibly one of them has developed an issue while you where working on it.
Think of things you touched and things nearby if you are investigating a change in behaviour.

Slightly off topic but I once had a carb (UK) x1.9 that had terrible MPG compared to another theoretically identical car. I tried swapping carbs, ignition components. Never got to the cause of it. That car has long been sold and probably split for parts.
 
Before the advent of affordable air/fuel gauges, I would use a single wire oxygen sensor (that was stock for an injected spider) hooked up to a digital voltage meter on my twin IDF spider to set the jetting. I had a chart that compared voltage to mixture readings and voila a cheap oxygen sensor. I thought I read somewhere that you can't run the oxygen sensor to your computer AND a gauge such as a voltmeter at the same time but I don't know if that's a real thing or not.

My understanding of when the oxygen sensor controlled mixture on a Fiat was between idle and full throttle. I assumed that's why it was so critical to set the throttle position sensor correctly.

If you still have it, as suggested, you should put the old oxygen sensor back in since it was really the only thing you changed.
 
Thanks, Carl. Unfortunately the old O2 sensor was brutally hard to remove and it got kinda mangled in the process, so I tossed it.
 
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