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

D

DSpieg

True Classic
1987 Bertone, 1600 fuel-injected. Recently removed the intake manifold to replace all the short rubber hoses attached to the fuel rail and injectors. While I was in the area, replaced the oxygen sensor (only becaused it had some miles on it and was looking a bit grody). Car starts and runs perfectly, no fuel leaks, no over-rich exhaust smell - but for the last three fillups it's only gotten 21-23mpg rather than the 29-30 it's been getting for years. Same driving routine, mostly highway, some city and stop'n'go.

My first thought is that the replacement O2 sensor (by NTK) could be bad, so I've bought a Bosch-brand OEM replacement which I'll install this weekend. Still, I've had known-bad O2 sensors on other cars and have never lost more than 2-4mpg because of it. Any idea what might be happening here?
 
Uh, no. It's fuel injected, the mixture was not touched, and every injected X1/9 I've owned (four so far) has gotten 28-32 mpg regularly.
 
DSpieg said:
fuel injected
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yep, the numbers I quoted were for an 87 = FI for sure. But maybe it is normal for real world economy to be better than EPA numbers? I do know that in Australia, no way does any vehicle even match the official numbers let alone beat them :)
 
On this type Bosch injection the O2 sensor only affects the mixture at idle, for emissions purposes. It does not calibrate the mixture across the board like other FI systems. So it won't have much influence on fuel mileage and I would not expect it to be a factor here. However I suppose the old fuel hoses to the injectors could. When they get old they might choke off the internal flow, restricting the volume of fuel getting to the injectors a bit. Thus making it run a little lean due to a very mild fuel starvation condition. Especially on a 1600 (I assume performance) engine. That could affect the fuel mileage over time. The only real way to find out how your FI system is handling the load is with a aftermarket wideband O2 setup. Particularly if you are running a stock factory FI on a modified engine (again, I'm assuming it is if it is 1600cc).
 
DSpieg said:
Uh, no. It's fuel injected, the mixture was not touched, and every injected X1/9 I've owned (four so far) has gotten 28-32 mpg regularly.
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Your mileage reflects my experience for all my FI X’s. I don’t know how they ended up with such poor mileage at the EPA, I drive mine quickly every drive. I track my mileage at every fill up.
 
Dave may have misquoted his engine size. As far as I know his is a stock 1500, not a hotrod 1600.

Just to check all boxes, make sure your tires are at your desired pressure. The cold weather will effectively reduce your tire pressures.
 
How much faith do you have in your previous mileage calculations?

Assuming that you had good data, and have good data now, I would first double check for fuel leaks. Note that the pump only runs when the engine is running so it could easily have a fuel leak that isn't obvious.

I would make all of the normal system checks, especially ignition timing. Retarded ignition timing will reduce fuel economy.

One additional item. Since you replaced fuel lines, if the new lines were dirty and weren't properly flushed before installation, you can end up with injectors that are fouled. The spray pattern will cause poor fuel atomization and reduce fuel economy.
 
Fuel injectors can "spit" instead of a proper spray pattern. This happens after some large volume of fuel has been run in-out of the injectors.
Eventually all injectors will need cleaning and replacement of wear parts like the pintle caps, inlet filters and cleaning of the internal parts. This is not a "mileage" thing alone, fuel type and how the fuel system has been run figures into this.

Or why servicing of injectors is a wise maintenance act beyond replacing knackered hoses, replacing the fuel filter and...


Bernice
 
I've noticed for years they change the gas formula from summer to winter; could you have gotten the (lower-mileage) winter gas this time?
 
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Dr.Jeff said:
On this type Bosch injection the O2 sensor only affects the mixture at idle, for emissions purposes. It does not calibrate the mixture across the board like other FI systems.
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I have seen this stated before, but it is simply not correct. Bosch L-Jetronic as used on the X1/9 runs closed loop under almost all conditions. Idle, cruise, acceleration and even brisk acceleration; mixture is adjusted according to the O₂ sensor reading. Coasting in gear above a certain engine speed, the fuel supply will be shut off and the O₂ sensor will read lean. If you step on it enough to trip the WOT switch on the throttle body, L-Jet will run open loop and rich.
 
ng_randolph said:
I have seen this stated before, but it is simply not correct. Bosch L-Jetronic as used on the X1/9 runs closed loop under almost all conditions. Idle, cruise, acceleration and even brisk acceleration; mixture is adjusted according to the O₂ sensor reading. Coasting in gear above a certain engine speed, the fuel supply will be shut off and the O₂ sensor will read lean. If you step on it enough to trip the WOT switch on the throttle body, L-Jet will run open loop and rich.
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Unless Fiat used a different version of the Bosch system that I am not aware of, then according to the Bosch technical training program I attended back when these systems were new the O2 only acts on the idle function. However there were a lot of various changes made for specific applications as requested by some vehicle manufactures, so the X's system may be different in this respect. Therefore I don't doubt your position, but I am curious what your information source is for it?
 
Dr.Jeff said:
information source
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I have read the entire wiki on this topic. It is not helpful that it holds many chunks of data from different makes [Porsche, BMW etc]. I think the non-FIAT ones should be purged.
L-Jetronic is a family of systems fitted by different manufacturers differently depending on market and time and cost. In the most basic form, there is not even an O2 sensor fitted. But, on any car that has a cat, there is an O2 sensor and it operates as mixture correction at all times [once warmed up] with the exception of WOT. The O2 sensor protects the cat. Yes, these are narrow band sensors but the whole system is crude anyway. If the mixture is too rich [from a bit rich to very rich], the O2 senor say "too rich". Not a measurement as such, more a general statemet of condition. The ECU reduces fueling. The O2 sensor reports "too rich", fueling is reduced, the O2 sensor reports "too lean", fueling is increased etc etc. Always chasing its tail
 
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and here is a word or two by Bosch on the topic. The second half of this doc talks about L-Jetronic closed loop lambda control in detail
 

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Cratecruncher said:
On a related aside, has anyone tried the DIY injector cleaning systems now available? Like this one for example:
Amazon link to $27 cleaning system
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The most basic version of that has just the plastic fitting to mount a injector onto a aerosol can of carb cleaner. Then you simply supply a constant 12V feed to the injector to open it while spraying the cleaner. You can get those for well under $10. The one in the above link takes that a step further with a box that pulses the injector....which would be an improvement over the constantly open version.

I guess the big question might be how effective the aerosol can of cleaner would be. For one thing most industrial injector cleaning systems submerge the injectors into a ultrasonic cleaning machine while they are working. And then they pulse the cleaning solution through the injector as part of the process. This also tests the spray pattern to determine how clean it is. From my experience it requires a LOT of cleaning solution going through the injector before it begins working well. That means you will likely go through at least a case of aerosol cans before making much progress. Furthermore the aerosol can won't provide nearly as much pressure as "real" injector cleaning machines, or the regular fuel system on the vehicle. So these DIY units won't be as effective as having it done professionally.

However that might be OK, depending on how bad the injector is. A REALLY dirty/clogged injector isn't likely to get cleaned with these DIY methods. But for routine servicing they should be fine. Someone on Xweb recently mentioned using a DIY set up successfully. Like I said, I guess it depends.
 
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. I hate buying fish when hooks are available!
 
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