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Poor gas mileage on the Xterra.
- Thread starter saberz
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I hope you folks mean 0.039 and 0.044 inch gaps. Ping is not your first concern with 7/16 inch gaps and dropping to 3/8 inch gaps won't help. NGK spark plugs shows 0.044 inches for all 3 2004 Nissan Xterra engines.
Closing the spark gap retards the timing but the change from 0.044 to 0.039 inches should be too small to notice and should be compensated for by the knock sensor. I've heard tell that some ECM will throw codes if you change the gap from 0.060 to 0.035 inches though its never happened to me.
Testing the oxygen sensor is easy enough. Ensure that it goes above and below 0.450v quickly on the scan tool and ensure the exhaust smells pleasant. Rich smells like rotten potatoes from partially catalyzed fuel. Running lean causes watery eyes from the NOX.
You don't need to test the oxygen sensor. I don't think there is an AFR that could waste half your fuel and have the engine still run. Think about it. The EPA gets 20 highway and you're getting 11. For each gallon you use you are pouring another gallon somewhere besides the engine. The fuel doesn't just disappear. I'd look for something big like a fuel leak or cylinders with low compression. If a spark plug wasn't working your catalytic would melt from burning 3 gallons of fuel per hour and you'd be in the news for catching your part of the world on fire.
As an example I have a 1993 Roadmaster 350 and a previous owner tried to soup it up with 454 injectors and damaged an o-ring in the process. Fuel was streaming in like it was being poured from a gas can. The excess fuel destroyed the oil which wore out the engine. Compression was 70-120. It still got 15 MPG and jumped to 20 MPG when the knock sensor was disconnected. It's rated for 23 highway. I was doing better than you with a destroyed engine.
The easiest way to test the thermostat is with a IR temp gun. A thermometer could work but is much too hard to use.
Closing the spark gap retards the timing but the change from 0.044 to 0.039 inches should be too small to notice and should be compensated for by the knock sensor. I've heard tell that some ECM will throw codes if you change the gap from 0.060 to 0.035 inches though its never happened to me.
Testing the oxygen sensor is easy enough. Ensure that it goes above and below 0.450v quickly on the scan tool and ensure the exhaust smells pleasant. Rich smells like rotten potatoes from partially catalyzed fuel. Running lean causes watery eyes from the NOX.
You don't need to test the oxygen sensor. I don't think there is an AFR that could waste half your fuel and have the engine still run. Think about it. The EPA gets 20 highway and you're getting 11. For each gallon you use you are pouring another gallon somewhere besides the engine. The fuel doesn't just disappear. I'd look for something big like a fuel leak or cylinders with low compression. If a spark plug wasn't working your catalytic would melt from burning 3 gallons of fuel per hour and you'd be in the news for catching your part of the world on fire.
As an example I have a 1993 Roadmaster 350 and a previous owner tried to soup it up with 454 injectors and damaged an o-ring in the process. Fuel was streaming in like it was being poured from a gas can. The excess fuel destroyed the oil which wore out the engine. Compression was 70-120. It still got 15 MPG and jumped to 20 MPG when the knock sensor was disconnected. It's rated for 23 highway. I was doing better than you with a destroyed engine.
The easiest way to test the thermostat is with a IR temp gun. A thermometer could work but is much too hard to use.
severach
"closing the spark gap retards the timing".
Explain how this can occur.
Because is DOESN'T!
If anything, a smaller gap would advance the actual ignition event.
But that requires computing the speed of light over a few thousanths of an inch.
"closing the spark gap retards the timing".
Explain how this can occur.
Because is DOESN'T!
If anything, a smaller gap would advance the actual ignition event.
But that requires computing the speed of light over a few thousanths of an inch.
Have you actually calculated your fuel mileage accurately over several tanks? Do you know how? Miles on a "tank" sounds like an inherently inaccurate way of figuring out exactly what MPG you are getting.
If the coil were to delay longer firing the spark across a wider gap, a narrower gap would advance the timing.
If the size of the spark gap affects initial size of the flame kernel then a narrower gap would retard the timing.
I think the flame speed will have more of an effect on timing than coil firing charge time but neither is much, particularly when the change is only 0.005 inches. I don't see why that change would eliminate ping on a knock sensor controlled engine.
If spark gap is unimportant then why did they all change to 0.060 inches minimum to meet 1996 emissions requirements?
I can find plenty of forum references relating spark gap and advance but no official ones in patents or technical documentation.
If the size of the spark gap affects initial size of the flame kernel then a narrower gap would retard the timing.
I think the flame speed will have more of an effect on timing than coil firing charge time but neither is much, particularly when the change is only 0.005 inches. I don't see why that change would eliminate ping on a knock sensor controlled engine.
If spark gap is unimportant then why did they all change to 0.060 inches minimum to meet 1996 emissions requirements?
I can find plenty of forum references relating spark gap and advance but no official ones in patents or technical documentation.
Before you go crazy have you ever thought maybe the fuel guage has changed in read out. Using a fuel guage is woefully inaccurate as they are never linear.
You need to use your trip meter and measure miles travel by fuel between fill ups to full.
You need to use your trip meter and measure miles travel by fuel between fill ups to full.
Originally Posted By: severach
If spark gap is unimportant then why did they all change to 0.060 inches minimum to meet 1996 emissions requirements?
I can find plenty of forum references relating spark gap and advance but no official ones in patents or technical documentation.
IMHO with OBD-II, pre-cats, iridium spark plugs and what-not, what car manufacturers trying to do is to comply with federal mandated emissions law with much lower emissions and such.
spark plug gap widens? That's mainly to do with stronger/better solid state HV generation (aka "ignitor"). Gone are the days we have to use points, dwell systems with generate poor, if not unreliable HV sparks. Nowadays: sparks can fly over 1mm easily with consistency and strength that would help in getting a slightly flooded spark plug firing once again, and/or burn off any debris quickly.
I never believe that such miniscue plug gaps can cause significant timing issues for the way I see it: once the spark jump gap and started the entire flame propagation process, that officially spells the end of the function of a spark plug until the next firing cycle.
Q.
If spark gap is unimportant then why did they all change to 0.060 inches minimum to meet 1996 emissions requirements?
I can find plenty of forum references relating spark gap and advance but no official ones in patents or technical documentation.
IMHO with OBD-II, pre-cats, iridium spark plugs and what-not, what car manufacturers trying to do is to comply with federal mandated emissions law with much lower emissions and such.
spark plug gap widens? That's mainly to do with stronger/better solid state HV generation (aka "ignitor"). Gone are the days we have to use points, dwell systems with generate poor, if not unreliable HV sparks. Nowadays: sparks can fly over 1mm easily with consistency and strength that would help in getting a slightly flooded spark plug firing once again, and/or burn off any debris quickly.
I never believe that such miniscue plug gaps can cause significant timing issues for the way I see it: once the spark jump gap and started the entire flame propagation process, that officially spells the end of the function of a spark plug until the next firing cycle.
Q.
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