Car question

[glatt]

I push in the trip odometer every time I fill the tank, and it's usually around 280 miles if I've been driving around town and 310 miles or so if I've been on the highway for the entire tank. I haven't noticed a huge change in those numbers. How big would the change in mpg have to be to be significant? I haven't noticed a change, so I figure the mileage hasn't changed by more than 5%, if it has changed at all. I think it has stayed the same.

[tw]

Originally Posted by glatt

How big would the change in mpg have to be to be significant? I haven't noticed a change, so I figure the mileage hasn't changed by more than 5%, if it has changed at all. I think it has stayed the same.

The difference between crap Liberty gas and Shell gas would (for example) result in maybe an 8% difference without any noticeable change in idle smoothness. If your numbers are as reported, then Liberty gas should have caused a noticeable mileage difference. The fact that you saw no difference is troubling.

In another event, a tank of Mobil destroyed my fuel injectors. I knew something was wrong due to a slight roughness in idle. But the resulting mileage change was from 32 MPG to about 27 MPG in that tank. And 24 MPG in the next tank. (Honda replaced those injectors for free). In that case, diagnostics never reported a problem.

In another case, a problem with an EGR valve kept causing a check engine light. No mileage change. Symptoms that helped me trace the problem to a hard and not easily diagnosed problem. (BTW, EGR valve is not connected to the idle system; so did not affect idle. And that failure was unique so as to not affect gas mileage.)

Examples of different failures that do and do not result in mileage changes. And that isolated the problem quickly to a shorter list of suspects.

Assuming that 5% coincided with road conditions (heavy traffic, summer open road driving, etc), then is we can assume you have a problem traceable to the idle system and that does not affect the other 'normal operation' system.

BTW, remember Cloud's problem. That was also traceable to the idle system and that would not cause mileage changes. That mechanic had one clear advantage. The Civic's diagnostic system pointed exactly to the defect - which that mechanic refused to address. An example of why some people just refuse to learn good diagnostic procedure.

Now, with Shell, what is happening? What are the latest symptoms?

[glatt]

Well, the roughness seems to be not so bad the last week or so, but I'm not sure. The roof used to buzz from the vibration and that's not happening recently, but there is still some roughness in the idle after it warms up. You can feel it in the steering wheel and the cds in the door pocket rattle. It still goes away when you give it some gas.

I have old records of the mileage from when this car was new and I was better at tracking that stuff, and I can compare that to the mileage the next time I fill the tank, but keep in mind that gas didn't have ethanol in it back then, so it would be comparing apples to oranges.

Is it worth it to buy a bottle of fuel injector cleaner and dumping that in the tank? Would that harm anything other than a few wasted dollars?

Also keep in mind that we don't drive much. I take the subway to work, so it might be a couple weeks before we fill the tank again.

[tw]

Originally Posted by glatt

The roof used to buzz from the vibration and that's not happening recently, but there is still some roughness in the idle after it warms up. You can feel it in the steering wheel and the cds in the door pocket rattle. It still goes away when you give it some gas. ...
Is it worth it to buy a bottle of fuel injector cleaner and dumping that in the tank? Would that harm anything other than a few wasted dollars?

Is that a new symptom? Roughness was when warm; not when cold? Or is roughness same when engine is warm or cold at 1000 or 1200 RPM - the cold idle speed?

I would avoid even trying to fix anything. First establish some trends. You still have some Liberty gas in there. And you really do not have MPG numbers. Only mileage for some approximate gallons of gas.

Stated earlier: first get facts. Dumping that miracle fluid in only adds another variable; can create confusion. One step at a time. You describe something that is not serious. You can live with it for a few months until the problem makes itself obvious.

Or you can take a nuclear option. Spend more. Take it to the dealer. My preference is to learn from an anomaly that is minor - will not leave you stuck. Not just learn how a car works. But how to better diagnose any problem. A memorable moment in life is that "Aha-Ha" moment. When that solution is sudden, obvious, and appears to materialize from nowhere.

I would avoid that additive until current trends stabilize - ie enough tanks to accurately identify gas mileage or to see roughness stabilize – become predictable. Currently roughness is different at different engine temperatures? Or just at different idle RPMs?

[glatt]

Originally Posted by tw

The difference between crap Liberty gas and Shell gas would (for example) result in maybe an 8% difference without any noticeable change in idle smoothness. If your numbers are as reported, then Liberty gas should have caused a noticeable mileage difference. The fact that you saw no difference is troubling.

In another event, a tank of Mobil destroyed my fuel injectors. I knew something was wrong due to a slight roughness in idle. But the resulting mileage change was from 32 MPG to about 27 MPG in that tank. And 24 MPG in the next tank. (Honda replaced those injectors for free). In that case, diagnostics never reported a problem.

In another case, a problem with an EGR valve kept causing a check engine light. No mileage change. Symptoms that helped me trace the problem to a hard and not easily diagnosed problem. (BTW, EGR valve is not connected to the idle system; so did not affect idle. And that failure was unique so as to not affect gas mileage.)

Examples of different failures that do and do not result in mileage changes. And that isolated the problem quickly to a shorter list of suspects.

Assuming that 5% coincided with road conditions (heavy traffic, summer open road driving, etc), then is we can assume you have a problem traceable to the idle system and that does not affect the other 'normal operation' system.

BTW, remember Cloud's problem. That was also traceable to the idle system and that would not cause mileage changes. That mechanic had one clear advantage. The Civic's diagnostic system pointed exactly to the defect - which that mechanic refused to address. An example of why some people just refuse to learn good diagnostic procedure.

Now, with Shell, what is happening? What are the latest symptoms?

For the last two months we've been putting nothing in it but Shell. No improvement. I put a bottle of fuel injector cleaner into it. No change. Still a rough idle after it warms up.

So I bought a CarChip Pro so I can monitor RPMs and other stuff.

I just started it and let it idle for 8 minutes or so. I didn't touch the accelerator. The first 4 charts I got are below. It started off fine, and the idle got progressively rougher, until at 8 minutes in, the roof liner started vibrating it was so bad. Then it was dinner time, so I turned it off.

Chart 1 is RPM. You can see it gradually slows down. I didn't touch the accelerator ever.


Chart 2 is the throttle position. I had no idea what this would show, but here you go. It shows little when you don't step on te gas.


Chart 3 is the first of several oxygen sensor readings it can take. I ahve no idea what this means, but this is O2 B1, S1


Chart 4 is also an oxygen sensor reading. It's called O2 B1,S2, whatever that is.


I'll have to double check the shop manual, but if I recall correctly, it should read between .1 and .9 volts. It goes a little below .1 volts.

There are a whole bunch of other readings I can have this make, but you can only do 4 at a time, and you have to take it out of the car and hook it up to a PC to change what you are monitoring.

It's a neat little gadget.

[tw]

A car at idle runs completely different from one at normal engine speed. At idle, a car must dump more gas in the engine while making the car work so inefficiently as to run at only 800 RPMs. Most pollution occurs during idle. Also why cars sitting in bumper to bumper traffic consume so much gas (another reason why hybrids are so necessary).

IOW a better test of your O2 Sensor and other factors is to be moving at constant speeds as indicated by little or no throttle movement.

Now, some facts. In the first minute, an O2 sensor must get warm before it reports anything. So newer technology sensors have an internal heater to make them hot (work) sooner. In your case, sensor one got to temperature quickly. Sensor 2 seemed to be doing nothing until engine exhaust finally heated it. Sensor 2 may have a defective heater or for some reason is taking longer to get the temperature. (BTW, I am assuming this is a 6 or eight cylinder since fours only need one O2 sensor. If this is a four, then the second sensor may be after the cat converter making this analysis bogus.)

Sensor 2 appears to be reporting inaccurate voltages. An O2 sensor is typically between .6 and .7 volts when the engine is running optimally. When dumping gas excessively, then the voltage is typically about 1 volt. Your sensor two appears to be reporting numbers that are low. Or one side of the engine is using more gas than the other.

However, how electronics measure those voltages might explain different numbers. Your particular car may measure or report those voltages differently. Normal voltages for an O2 sensor must be confirmed from the shop manual. But something is different on both sides of the engine.

Throttle clearly remained closed the entire time. But after 8 minutes, your idle was not yet at 800 RPM? The car should have been hot and at idle after 8 minutes. You might repeat the test while monitoring engine temperature. Your thermostat might not be fully closed (partially sticking) when the engine is cold. Therefore the engine is taking too long to warm up; wasting gas in the first ten minutes.

What you are looking at? Fuel injection is a switch (much like Cloud's idle adjustment valve) that is turned on and off quickly by a microprocessor. Time of each opening (called fuel trim) determines how much fuel is dumped into each cylinder. An O2 sensor measures how much got burned. Microprocessor reads the O2 sensor to determine whether to lengthen or shorten that opening time (adjusts short term fuel trim).

Graph is each O2 sensor number (maybe) sampled only at 5 second intervals. The actual numbers vary significantly in milliseconds. Actual O2 voltage is varying many times more often and more gently. Your chart simply shows some of the highest and lowest voltages. Therefore the steep changes.

Those variations may be normal for idle. But if you see variations that large for normal RPMs, then that may also be symptoms of a problem. An engine at normal speeds should see O2 sensor voltages varying little between 0.6 and 0.7.

Notice that at seven minutes, the variations decrease to what I am more used to seeing.

Voltage should increase when, for example, when a throttle is pressed. An open throttle means intake manifold has no vacuum. So the computer dumps more fuel into cylinders (short term fuel trim increases). More fuel comes out unburned. The O2 Sensor (that I am used to seeing) reports 'more unburned fuel' at about 1.0 volts.

Again, large variation may be normal for your engine. If it was any of my Hondas, I would be concerned. Approaching eight minutes, the variations decreased. Whereas your seven minute voltages to me are too low, still, the O2 sensor varies more like I am used to seeing it.

Meanwhile, the 'Shell' test implies gasoline was irrelevant to your original problem.

You can view a relationship between short term fuel trim, throttle angle, O2 Sensor voltages, and intake manifold vacuum. To see what the microprocessor sees and responds accordingly.

Another parameter is long term fuel trim. I have never confirmed this by doing it. But a long term number would increase if the gasoline is inferior. If gas (ie WaWa) does not burn as efficiently, then engine software may respond by increasing (over a long time) the amount of fuel it dumps into the engine. Long term trim is how a microprocessor learns and adjusts from engine overall performance. Short term fuel trim is how it responds due to immediate road, throttle, load, and other current variations.

O2 Sensor is not the only input. But it varies according to how much gas remains unburned. Other inputs include a sensor that measures atmospheric pressure (are you one mile up in CO or at the shore?), and one that measures manifold vacuum. When does it know to dump more fuel into the engine? When vacuum decreases (due to an open throttle or a sudden air leak). Ignition timing also changes (advances) when more fuel is dumped. You can also view that relationship.

I don't remember what your car's technology is. However older cars adjusted engine timing mechanically (centrifugal force and manifold vacuum). So a sticky or binding ignition timing could be seen on that chart. I have seen where distributor grease got sticky after ten years causing engine timing to intermittently stop changing. That was also indicated by a slight knock. However some auto companies (ie GM) decided knocking is normal rather than fix an inferior engine designs. Knocking alone might not report a problem. But sticky mechanical ignition timing might be seen on the chart. Unlikely. But an example of what the microprocessor can see. But the failure is not serious enough to report. An example of why many detected problems can remain unreported by engine check lights.

Hopefully this successfully introduces simple operations inside a computerized engine. Computerized engines are simpler than carbureted. But involve fancy nouns (ie short term fuel trim) that at first sound confusing.