Chasing causes of large fuel trim

Tugela

Tugela

Well-known member
May 21, 2007
4,760
563
Seattle
My OBD-2 scanner is giving me some far out numbers on fuel trims. This started before I recently replaced my Y-pipe and hasn't changed since. These are the numbers I'm getting:

Short term trims, sensor 1: ranging from -3 to -12 across a range of conditions.
Long term trims, sensor 1: ranging from -3.9 to -5.5.

Short term trims, sensor 2: ranging from -2.3 to -10.
Long term trims, sensor 2: -3.9 to -5.5.

Short term trims, sensor 3: -25 to -30.
Long term trims, sensor 3: -26 to -28.

Short term trims, sensor 4: -24 to -30.
Long term trims, sensor 4: -25 to -26.

My scanner doesn't tell me which sensor is which (i.e. upstream, driver's side), the manual doesn't either. I have a Bluetooth ELM27 OBD-2 dongle on the way I'm going to hook up with Torque Pro to get a more useful readout.

In my research on DiscoWeb and elsewhere I saw mention of other possible culprits, including fuse 3 under the steering column (was fine) and faulty MAF. Here are some data ranges from my scanner during a test drive last night. Again, I don't know what the units are or what range they should take.
  • 0.17 - 0.20 at 25mph, 1,000-1,200rpm
  • 0.45 - 0.55 at 60mph, 2,400 rpm
If my MAF was faulty, wouldn't that affect the fuel trim on both banks?

Recent work done:
  • new Y-pipe and gaskets
  • new driver's side upstream O2 sensor (NGK/NTK)
  • reconnected loose wiring on car side of same sensor connector plug

Not so recent, but also done:
  • new fuel pump
  • new fuel pump wiring harness
  • new O2 sensors on passenger side

I'll probably get some better data with better tools - my scanner is not the greatest - but until then what are some other things I should be looking for or testing to track down causes of my large fuel trim numbers?
 
Tugela

Tugela

Well-known member
May 21, 2007
4,760
563
Seattle
p m said:
Does GEMS even change fuel trims based on downstream O2 sensors? Need to read up on that.
Click to expand...

I'm with you, I didn't think that the downstream sensors factored into fuel trims on GEMS. Part of my confusion stems from not knowing which sensor corresponds to which reading on the scanner. It's possible that the two sensors giving the large readings are both upstream. This might suggest a faulty MAF.
 
Dsport

Dsport

Member
May 29, 2020
15
0
California
DS sensors should not be impacting fuel trim. Check upstream O2. MAF should affect both banks but not out of the question.
 
p m

p m

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Apr 19, 2004
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www.3rj.org
Tugela said:
I'm with you, I didn't think that the downstream sensors factored into fuel trims on GEMS. Part of my confusion stems from not knowing which sensor corresponds to which reading on the scanner. It's possible that the two sensors giving the large readings are both upstream. This might suggest a faulty MAF.
Click to expand...
Nick,

the difference between generic vehicle-specific code readers is obvious: generics go after all possible reportable values, which your vehicle may not have. So... It helps to know (a) what your system actually uses, and (b) what your generic reader is reporting. Without that, it is hard to argue about anything.

Before you jump to conclusions about MAF sensor, have you looked at MAF readings?
 
D

discostew

Well-known member
Sep 14, 2010
7,725
1,022
Northern Illinois
I think you might have switched the o2 sensor connectors from side to side. It makes them run pretty smooth until closed loop. Then they run like shit and smell terrible
 
Tugela

Tugela

Well-known member
May 21, 2007
4,760
563
Seattle
discostew said:
I think you might have switched the o2 sensor connectors from side to side. It makes them run pretty smooth until closed loop. Then they run like shit and smell terrible
Click to expand...

The engine runs smoothly, no suffering performance (other than a recent perceived reduction in fuel economy), and I haven't noticed any awful smells.
 
S

Stocksuspension66

Well-known member
Mar 26, 2020
104
9
California
Seems super rich. Check coolant tempt sensor and wiring to it. I would unplug the battery and hold both positive and negative together for at least 10 minutes. That drains all the internal capacitors in ecus and wipes out memory. Start fresh and see what returns
 
Tugela

Tugela

Well-known member
May 21, 2007
4,760
563
Seattle
What's the relationship of the fuel trim to the coolant temp sensor? I did a cooling system refresh a couple years ago (new radiator, water pump, fan clutch, thermostat) and my live data consistently shows temps in the range of 192F - 196F.

What are the other consequences of draining the ECU capacitors? I'm about to take a 700-mile road trip on Wednesday and I don't want to create any new problems before then.
 
E

_ExpeditionMan

Well-known member
Dec 11, 2017
295
34
Texas
Are you sure you are exiting open loop? That's the relation to the coolant temp sensor. My short and long trims are similar to yours when in open loop (smells of gas) but once I enter closed loop everything cleans up to the +/- 3 range.

Stolen from the internet...

Open Loop
When the engine is first started the system goes into open loop operation. In open loop, the ECM calculates the Air/Fuel ratio based on inputs from the coolant and MAF sensors. The system remains in open-loop until the following three conditions are met:

    • Coolant temperature above 105°F (whatever value is set by your manufacturer).
    • Oxygen sensor has varying output voltage (indicating it is hot enough to operate properly).
    • A specific amount of time (preprogrammed into the MEM-CAL) has elapsed.
Closed Loop
When all of the above conditions are met, the system goes into closed loop operation. In this mode, the ECM calculates the Air/Fuel ratio based, mainly, on input from the oxygen sensor and maintains the Air/Fuel ratio at the desired 14.7 to 1.
 
Icannap1

Icannap1

Well-known member
Mar 3, 2015
123
21
OC, CA
I had similar issues running extremely rich and replacing the cat twice on passenger side once on driver side. After resetting the adaptive values in the morning when the vehicle is cold running an older 2016 Samsung Android tablet with GEMS Reader Lite my issues went away now the rover is running great and values are +/- 3-5 on fuel trims. Before resetting adaptive values I was replacing parts here and there 02 sensors and so on. Fuel trim values, engine load, and throttle position sensors were reading high compared to my other D1. One thing that always threw me off was once the rover was in closed loop and you decelerate when coming to a light or climbing a steep hill and RPM's are at 4000 Torque Pro app would show open loop then it would read closed loop once RPM's were lower or you come to a complete stop and begin driving.
 
seventyfive

seventyfive

Well-known member
Jan 3, 2010
4,280
100
over there
Can you change the MAF reading to kg/h?
If not you have to do the math but at idle you should read 13-17 kg/h. Then watch it as you slowly accelerate up to 2500 rpm. Or have someone watch it while you drive. It should be a nice linear increase.
long term is multiplicative which usually points to a maf. Short term is additive which usually points to vacuum leak.
if it’s a d2 you should get a lean fault and a transmission fault if the maf is dead.
if you can get a hold of a known good maf install that, clear faults, and road test.
 
Tugela

Tugela

Well-known member
May 21, 2007
4,760
563
Seattle
Icannap1 said:
I had similar issues running extremely rich and replacing the cat twice on passenger side once on driver side. After resetting the adaptive values in the morning when the vehicle is cold running an older 2016 Samsung Android tablet with GEMS Reader Lite my issues went away now the rover is running great and values are +/- 3-5 on fuel trims.
Click to expand...

Thanks, I've got that app on my list for when my new(er) diagnostic technology arrives.

Also, this is a 98 D1, no fault codes.
 
4Runner

4Runner

Well-known member
May 24, 2007
663
111
Boise Idaho
Have you done a fuel pressure leak down test recently? How old are your injectors? Your ECU could be cutting back on the fuel due to at least a couple of injectors that are leaking since it’s on both banks. Could be the MAF, check to make sure it’s clean. Those seem to be pretty reliable though.
 
WaltNYC

WaltNYC

Well-known member
Mar 3, 2010
710
138
NYC
use RobertF's GEMS tool and reset the adaptive values. No downside and I'd be willing to bet a little bit that it works to solve your problems.
 
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