Community Forums

Isaac-1 · 08-01-2017, 03:02 PM

I recently acquired a OBD-II Scangauge II for my 2002 Safari Trek with an 8.1L engine in order to monitor engine conditions, and I found I am getting readings ranging from 12-19 on LF1 and 2-7 on LF2 when travelling at highway speed going up slight inclines (I am in south Louisiana, we don't have "hills"). In the course of routine maintenance I have also recently replaced my spark plugs, plug wires, and cleaned my MAF with MAF spray cleaner, and have seen no significant change on the fuel trim readings.

What are your thoughts on this?

Ike

p.s. I have also installed a fuel pressure gauge, fuel pressure runs 58-62 psi

AKIQPilot · 08-01-2017, 05:16 PM

Quote:

Originally Posted by Isaac-1

I recently acquired a OBD-II Scangauge II for my 2002 Safari Trek with an 8.1L engine in order to monitor engine conditions, and I found I am getting readings ranging from 12-19 on LF1 and 2-7 on LF2 when travelling at highway speed going up slight inclines (I am in south Louisiana, we don't have "hills"). In the course of routine maintenance I have also recently replaced my spark plugs, plug wires, and cleaned my MAF with MAF spray cleaner, and have seen no significant change on the fuel trim readings.

What are your thoughts on this?

Ike

p.s. I have also installed a fuel pressure gauge, fuel pressure runs 58-62 psi

Well 12-19 on one bank and 2-7 on the other does indicate a problem. I assume you mean positive 12 to 19? That would indicate a potential lean condition on that bank. Because it's only on one bank it may indicate a manifold vacuum lean near the cylinder head on one side of the motor.

Isaac-1 · 08-01-2017, 05:20 PM

Yes, I am getting positive numbers on both banks, but higher numbers for LF1 vs LF2, though they do seem to generally follow the same trend in their respective ranges.

AKIQPilot · 08-01-2017, 05:26 PM

Quote:

Originally Posted by Isaac-1

Yes, I am getting positive numbers on both banks, but higher numbers for LF1 vs LF2, though they do seem to generally follow the same trend in their respective ranges.

I have the SG2 as well and my LF1 and LF2 are almost exactly the same. They are very near zero any time I'm moving down the road at any speed. At a dead idle the Long term Fuel will bounce between 2 and 7 positive.

My SF1 and SF2 bounce around just a little at idle but average near zero when moving down the road.

Brazel's RV Performance · 08-01-2017, 08:35 PM

Swap the oxygen sensors side to side and see if the high fuel trims move to bank 2 or it remains the same

Isaac-1 · 08-01-2017, 09:23 PM

Thanks will give it a try, do you know if these have 2 or 4 O2 sensors and if 4 do I need to sway both front and back?

thanks

Brazel's RV Performance · 08-01-2017, 10:04 PM

It has 2, one on each side easy peasy

AKIQPilot · 08-01-2017, 10:22 PM

Good call Jon. If the trim numbers move with the swap then its an easy fix with a new O2 sensor.

Jack N Linda · 08-01-2017, 11:24 PM

If memory serves me right, it seldom does, Lft is an adaptive memory which can be reset with your scanner. Why not reset it and drive for a bit, who knows how long it's been gathering info. I think +or- 25% should set a code even on short term trim so that didn't happen or you would have mentioned that, again just my 2 cents....Jack

Brazel's RV Performance · 08-01-2017, 11:52 PM

Quote:

Originally Posted by Jack N Linda

I think +or- 25% should set a code even on short term trim so that didn't happen or you would have mentioned that, again just my 2 cents....Jack

It really *should* but it doesn't. Just like it never sets codes for misfires even when it's blatantly obvious. I think the reason for a lot of 8.1 failures in Workhorse's is the lack of DTC's for lean conditions.

AKIQPilot · 08-02-2017, 01:06 AM

Quote:

Originally Posted by Ultra RV Tech

It really *should* but it doesn't. Just like it never sets codes for misfires even when it's blatantly obvious. I think the reason for a lot of 8.1 failures in Workhorse's is the lack of DTC's for lean conditions.

Yep. Exactly the reason I have started to pay closer attention to long and short term fuel trim and O2 readings. It's nice to be able to predict a burndown before it starts to do real damage.

This forum has helped me a lot in the last 4 months.

TexasTom · 08-03-2017, 06:54 AM

So what should the trim readings be?

Brazel's RV Performance · 08-03-2017, 09:43 AM

0 is the target but +\-10% is within specification.

AKIQPilot · 08-03-2017, 10:07 AM

Quote:

Originally Posted by TexasTom

So what should the trim readings be?

Well in a perfectly healthy engine with no vacuum leaks and perfect injectors and a spotless MAF and brand new O2 sensors and new spark plugs and perfect plug wires and great fuel all warmed and running down the road at a steady 60 mph..............

Short term fuel trim will oscillate between -5 and +5 but it could stay at zero for periods of time and it could spike up to 15 or even higher for short periods of time. The short term fuel trim is managed by the engines computer responding to feedback from the O2 sensors. The engines computer it trying to achieve a perfect air fuel mixture of 14.7 to 1. If the O2 sensor see's this AFR fall below 14.7 it sends a resistance to the computer telling the computer the exhaust is rich and the computer starts pulling fuel back resulting in a short term negative fuel trim. A positive fuel trim number would be the result of the O2 sensor seeing a leaner than 14.7:1 AFR.

As you roll down the road and go up and down hills and around curves and move the throttle open and closed the fuel and air going to the motor will be in a constant state of change.

Long term fuel trim should ideally be at or near zero. Long term fuel trim is the computer learning from the short term fuel trim. If you have constant STFT readings at say -15 or -20 the computer will begin adjusting the fuel to that bank of injectors to compensate for the additional air the O2 sensor thinks it sees. Once the long term fuel trim begins to compensate the STFT will trend back toward zero and the extra fuel for that bank will be managed by the LTFT. If for some reason the STFT starts to trend positive on that bank then the LTFT will back off and reduce the fuel to that bank.

If the STFT continually oscillates on both side of zero then the O2 sensor is happy and the LTFT should remain near zero. When the STFT trends negative or positive for several seconds or even longer the LTFT will kick in and begin adding or taking away fuel trying to compensate for the less than optimum Air Fuel Ratio.