Might be nothing ……

[Richard Kirshy]

Since the warmer weather started ( a few weeks ago ) I’ve noticed something.

To begin with:
2002 Nissan Xterra 3.3L non-sc 4×4 automatic 86,000 miles.
No CEL or pending codes.
All scheduled maintenance services have been done.
All spark plugs (4 years old ) look like they were just taken out of the packages.
Ignition wires are new and double insulated.
Fuel filter is changed yearly
Oil changed every 3 months.

Now after driving (10 – 30 min) , when turning on to a side street from a stop, I feel a flat spot in the acceleration at low throttle.
As soon as you apply a hair more throttle the vehicle speeds up.
I’ve gone through all the obvious possibilities:
Fuel pressure is to spec. and stable
Fuel pressure regulator is functional.
Timing is in spec.
Cleaned the MAF and throttle body, though neither were dirty.
Washed and re-oiled the air filter.
Inspected all the plugs.
Silicone pasted all the ignition wire connectors.
Removed and cleaned the distributor cap and rotor.
Even went as far as dismantling the distributor cleaning all the components and oil the shaft bearing. ( FYI no moisture in the cap)
I have done a PVC reroute mod several years back and check that none of the fittings were leaking.
PVC valve is functioning.
Oil catch can is not leaking.

The only other thing I noticed is that prior to noticing this…
Fuel trims were:
LTFT 1 & 2 were stable around 7.8 – 9.8
STFT 1 & 2 fluctuated from -8 to + 8

Now LTFT on bank 1, goes from +5 to +15
LTFT on bank 2, goes from +5 to +12
STFT on bank 1, – 11 to +15
STFT on bank 2, -9 to +14

Now for the topper…
Gas mileage has gone up from 13.2 to 14 city driving.
Is this all just due to warm humid weather?
Any thoughts?

Topic

[Its me]

Have you put a scan tool on it and checked the Throttle position readings. Especially when this issue is happening?

[Richard Kirshy]

Yes, one of the benefits of using TORQUE.
TPS readings swing smooth up and down.
No binding in the mechanical action either.
Engine runs and idles rock steady, no surging.

[Mike]

Is the flat spot just past idle? Could be a slack/stretched throttle cable giving you some wasted motion at the pedal.

[Richard Kirshy]

Yes, just past idle as you apply throttle, but easily passes it with a hair more throttle applied.
Thanks for the idea.
Checked the cable, linkages and attachments, no excess play between the pedal movement and throttle body plate linkage..
Even lubed up the return spring and cable ( just in case ) but made no difference.

I might try re-calibrating the TPS as a last resort.

[Mike]

Summer blend gasoline only recently started on Long Island.
With recent wide temperature swings that could be it–or not.

Are the air and coolant temperature sensors giving accurate readings?

MAF could be clean but failing
Test #4 here may be useful
http://easyautodiagnostics.com/nissan/3.0L-3.3L-3.5L/maf-sensor-test-3

O2 sensor could be getting sluggish.
http://troubleshootmyvehicle.com/nissan/3.3L/p0130-and-p0150-diagnostic-tests-1#3

Vacuum leak or injector issues.

I’m not sure if your posted fuel trims are all at idle or if they are the total range of fuel trims under various throttle conditions.
Comparing the at idle to the sustained 2500-300 rpms fuel trims may yield a clue.
What are the fuel trims at 20-30 seconds of idle ?
What are they at 20-30 seconds of 2500-3K rpms ?

[Richard Kirshy]

[i]Summer blend gasoline only recently started on Long Island.
With recent wide temperature swings that could be it–or not.[/i]

I would believe that except, last fill up was 3 1/2 weeks back ( still has 3/4 of a tank )
This started ~ 2 weeks ago with the first of the warmer / humid weather

Are the air and coolant temperature sensors giving accurate readings?
All temp sensors working like a charm

[i]MAF could be clean but failing
Test #4 here may be useful
easyautodiagnostics.com/nissan/3.0L-3.3L-3.5L/maf-sensor-test-3
[/i]
Tested the MAF reading show it’s healthy

O2 sensor could be getting sluggish.
troubleshootmyvehicle.com/nissan/3.3L/p0…diagnostic-tests-1#3

All 4 O2 according to scan data are working as they should

Vacuum leak or injector issues.
I considered these, did do a smoke test no signs of a leak.
Engine idle is rock solid (675 – 687 rpm in gear, 700 – 712 rpm in park), No sign of any vibration in the engine compartment.

[i]I’m not sure if your posted fuel trims are all at idle or if they are the total range of fuel trims under various throttle conditions.
Comparing the at idle to the sustained 2500-300 rpms fuel trims may yield a clue.[/i]
What are the fuel trims at 20-30 seconds of idle ? LTFT 1,2 14.5, 11.5 // STFT 1,2 + 2 / – 8 both banks
What are they at 20-30 seconds of 2500-3K rpms ? LTFT 1,2 11.5, 5.5 // STFT 1,2 + 3 / – 5 both banks

One item to note:
As you release the throttle from revving all fuel trims appear to max out then fall back.
The other oddity is that I have never seen the LTFT vary as quickly as it has in the last 2 weeks, they are both usually very stable @ 9.8.
Now they vary at about half the rate of the STFT.

Here is a sample of what the plugs look like straight out of the heads

Engine is burning clean, rock solid, fires off with in a single rotation of the starter, no smoke from the exhaust.

Attachments:

• 100_1607.JPG
• 100_1609.JPG
• 100_1613.JPG

[Richard Kirshy]

Update:
Well list this under the heading of strange and bizarre.
After a morning of extremely heavy rain and a reasonable dry afternoon and early evening.
Decided to take a drive.
Had the scan tool hooked up and running along the way.
Keeping an eye on the STFT / LTFT as I went.
Up to the first 1/4 of the drive I see that the long and short trims are reading ( IMO ) extremely high:
STFT +10 > +25 both banks
LTFT +15 > +25 both banks
I thought for certain I should see a CEL, but none came up.
Then as I passed through 7 – 8 miles out, all the trims dropped off.
It was now reading on both banks:
LTFT ~ +6 > +8.3
STFT ~ +5 > -11

Basically equaling out to about 0 total.
Also the flat spot appears to have also cleared itself.

I’m quite pleased that the numbers have fallen back into a normal range and that the flat spot has cleared.
But a bit concerned to the root cause.
Perhaps a ECM glitch or a hung up sensor.

[Author]

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