# B14 Gas waste - high injection pulse



## D4m13n (Aug 27, 2010)

Hello, my 1996 sentra *consumes too much gas*, the manual says it should inject *2.3 - 3.3 mseg* and the current injection is *3.8 mseg*. Therefore consumes a lot of gas. 

What should i do?

Thanks.


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## smj999smj (Jan 1, 2006)

Injection pulse is variable and depends on a number of inputs to the ECM. Do you have the long term fuel trim percentage? That gives a better analysis of the fuel system correction given by the ECM. If you are experiencing poor fuel mileage, you may want to look at the oxygen sensor and MAF sensor operation, excessive fuel pressure, potential exhaust leaks near or before the front oxygen sensor, state of tune (plugs, filters, base idle and timing adj.), check for vacuum leaks, exhaust restriction, to name a few.


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## D4m13n (Aug 27, 2010)

Hello, look i just change the *fuel pressure regulator* check the *injectors* (with a multimeter), check *MAF*, *IAC*, *TPS*, *VTC*, *new spark plugs*, check spark plugs *wires*, new *oxigen sensor*, etc.


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## rogoman (Dec 16, 2004)

Have you done an ECU code readout to see what DTCs are set?


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## D4m13n (Aug 27, 2010)

Hello!

Here ir a data info:

TPS SEN................0.62 V
AIR TEMP SEN.........44 °C
START SIGNAL........OFF
MAF POSITION........ON
A/C SIGNAL............OFF
P/N SIGNAL POS......ON
*INJECT PULSE.....3.8 mseg*
COMB PROGR..........1.5 mseg
START REG ADV......4 APMS
SPAL-RPM(REF).......813 rpm
MAF SENSOR..........1.46 V
ENGINE TEMP..........89° C
S O2 C1 (B1)..........0.78 V
S O2 C2 (B1)..........0.01 V
M S O2 C1 (B1).......POOR
AIR/COMB ALFA.......101 %
SPEED SENSOR........O mph
BATTERY VOLT........13.3 V

Thanks.


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## madhatter256 (Feb 2, 2010)

I'd swap out the injectors and redo the readings. If they are reading the same then it's something electrical.


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## D4m13n (Aug 27, 2010)

I already check the injectors and they're ok *10.12 ohms*. (Manual says *10 ohms aprox*.)


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## Jopet (Aug 10, 2009)

Hi! I was wondering if the solution for this problem has been found. I would say that my injectors are spraying at max. I don't have any tests but I have done a lot of replacements and fixes. My worst mpg was around 12mpg, averaging 16mpg 50/50 hwy-city driving. 
What I have not replaced that could be the problem is the front O2 sensor. 

Thanks for the information


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## skeeter123 (Apr 30, 2010)

On the edge of my knowledge of these things, but for what it's worth, I think that the ECU can run in "open-loop" or "closed-loop" modes; open-loop is when engine is first started up and cold, and front O2 sensor is cold. In open-loop, ECU uses default injector-pulse widths to keep engine running and warming up (like having choke on). Once engine/O2 sensor heats up, the system goes into closed-loop mode, and uses the output of the front O2 sensor (and other sensors) to adjust injector pulse width. O2 sensors don't work until they're really hot, which is why many of them have built-in heaters. If the ECU isn't happy with the signal it's getting from the O2 sensor, it may be staying in open-loop mode, keeping your pulse-widths long and ruining your gas mileage. My scanner shows whether engine is running in open or closed loop; was there any indication on yours whether it had reached closed-loop mode?
Hope this helps.


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## Jopet (Aug 10, 2009)

I guess it does go to "closed-loop". Based on my last problem with the distributor. At a certain number of minutes or miles after the temp guage gets to about the middle, #3 spark plug does not fire. But when cold there is no miss-fire. 
That was the last fix I did (good thing the distributor was still under warranty). I'm replacing the front sensor next. The last owner of the car said he had it replaced (about 2 years ago). That's why I had not touched it. 

Thanks for the info


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## jdg (Aug 27, 2009)

In response to #9...and somewhat to #5.
Dead on...with caveats...
In open-loop, the ECU does use pre-programmed pulse widths based on the overall long term fuel trims (which are adjusted during closed-loop operation) at various operating conditions to calculate injector pulse widths. So, to say that the ECU uses default injector-pulse widths to keep running, is very close, but, not 100% correct in the what the ECU does.
The main difference between open-loop (assuming it's not open-loop due to a fault) and closed-loop is the use of the front O2 sensor for fuel injection correction.

On my '98 200SX-SE, at idle, the engine will stay in closed-loop.
On my '97 Sentra, at idle, the engine goes to open-loop as soon as the throttle hits the idle mark, and goes closed loop just above idle.
The O/P's vehicle is a '96 and I see in post #5 that the throttle is practically closed, therefore at idle (and the rpms are at 813). That would lead me to assume the ECU is operating in open-loop (like my '97) and therefore will likely be pumping out a bit more fuel causing the pulse widths to be higher.
But I digress...
I'd be wondering what the base timing is set at...


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## skeeter123 (Apr 30, 2010)

jdgrotte. thx for clarifying... 
D4m13n. Have you tried disconnecting the battery for ~20 minutes? Again on fuzzy end of my knowledge but you've changed the O2 sensor and FPR; as I understand it the ECU maintains a table of values for pulse-widths, depending on rpm/load/etc.... and when you change things that have to do with fuel metering, you can either wait a while (days?) for the computer to re-adjust all the table settings, or clear out all the old settings and let the computer generate new ones by resetting it (disconnecting the battery). I've heard that sometimes the ECU can get lost and not lock-in on a new set of table-values.
FYI for comparison, I've been tweaking my daughter's 98 Sentra GA16, replaced plugs/wires/fuel filter/air filter/a couple vacuum hoses/distributor cap/rotor/front seal/belts/thermostat/O2 sensor/oil sending unit/flushed transmission; at 157K miles, its getting 30 mpg (with A/C on in Florida).


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## jdg (Aug 27, 2009)

Again, practically dead on with with everything...except for the computer resetting itself.
Back in the day, simply disconnecting the battery would reset the 'tables' (cells), whether it be for a few minutes, hours, or days. Clearing the codes will reset those tables in some vehicles, not others, disconnecting the battery, same thing, works on some, not on others. Kind of a crap shoot sometimes.
As far as the ECU getting lost, it's not that it's necessarily lost, it's that the ECU's last known calibration values in those cells (Long term fuel trim and Short term fuel trims) may or may not be correct depending on what was done to the engine (or the ECU) to fix a problem, whether real or perceived.
Consider these 2 cases:
-If an engine had a problem that the ECU had to compensate a fair amount for, and the ECU was reset without the correct problem being fixed, the next time the engine was started, the ECU might be back to 'stock' calibration values, which for an engine with a problem, may be out in the weeds, therefore, the engine runs like crap.
-If an engine had a problem that the ECU was compensating for, and the ECU was reset after the problem was repaired, the engine would run very close to how it should run because the 'stock' values are likely close enough to correct to run the engine right.
There's a number of permutations on these scenarios, but overall, suffice to say, if the ECU is reset and the engine is repaired correctly, all other things being equal, it should run right after it's started.

As far as setting those 'cells' to the correct calibration, yes, it can take some time to do that. None of those 'cells' get tweaked without the ECU being in 'closed loop'. And to set those 'cells' to where they need to be, the engine has to be driven to hit each of those 'cells', as you say, depending on calculated load, rpm, temps, throttle position, and so on. That process could take hours, days, maybe even weeks. I notice it in my '97 Sentra after I clear the recurring 'P0440' code (I'll fix it someday when I get good and ready). The engine is a bit old, 177K miles, so those 'cells' need a bit of tweaking to be right on. After I clear the codes, the engine idles a bit rough, hesitates ever so slightly on acceleration, and just plain doesn't feel quite right for a day or two. Nevertheless, I drive it like I'd normally drive it. That way the ECU tweaks the 'cells' that normally get tweaked on a daily basis. Since it's a 4 door, the 'cells' for cold engine, wide open throttle, high rpm, maximum load, don't normally get tweaked, but I don't worry about that, 'cause I don't normally drive in that area of operation.
In short, if a guy drives the way he would normally drive, the ECU should get those 'cells' tweaked within a few miles and the next time around, the ECU will know what's going on and won't have to wait around to do the tweaking.

Whew...I'm spent. Anybody else getting this? Where'd I put that hammer?


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## Jopet (Aug 10, 2009)

Hey guys, found my problem. It was very surprising. Since you were talking about the ecu, I decided to clean the wire harness terminal that goes to the ecu. I found oil there. There was no oil anywhere on the carpet nor the ecu cover. 
cleaned everything with electronic cleaner and plugged it back in and made sure it was snug and firm.
Oh man, car runs better, shifting more smooth, idles smooth, accelerates more, and best of all, mpg is better. After 2 years since I purchase the car.
I was wondering how oil got to the ecu??


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## skeeter123 (Apr 30, 2010)

Glad you found the problem. Maybe previous owner tried to put dielectric grease on the connector....


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## jdg (Aug 27, 2009)

Happens all the time in the aircraft maintenance field.
You'd be surprise how many problems/issues are fixed by simply removing and replacing an offending connector, not even cleaning it...which we'll do the majority of the time whether it needs it or not.
Corrosion...it's a wonderful thing eh?

How did oil get to the ECU? Uhhhhhhhh... Love?


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