This is a follow up to some other threads here and on the other forum - from the Live Data Analysis, to the Gas Cap Venting, etc, etc. This thread is also over there but I'm looking at as wide a net as possible, because I'm stumped.
Bottom line: Poor mpg in 1996 Land Rover Discovery.
Initial analysis: Engine running lean causing ECU to overcompensate and dump fuel.
Analysis based on: High Long Term Fuel Trims
Mitigation efforts: New plugs and wires. New fuel pump and fuel filter, new front 02 sensors. Replaced MAF with known and tested used one. Replaced Charged Air Temp Sensor (shop stated it was showing -36F even though my OBD scanner shows it working correctly - maybe a stored value I can't see?). Smoke tested intake system - shop declared 0 leaks detected.
Recently - all plugs pulled. All look nice and chocolate. All plugs refit and torqued correctly. All wires checked for closure. Wires sprayed with water to detect arcing. None noticed.
This weekend completed an 800+ mile road trip:
Here are my findings (for purposes of the thread we will assume miles per hour was between 60 and 70 (generally 65), I drive like I have an egg on the gas and brake pedals. Outside of a few hill climbs RPMs stayed around 2200-2500. It only pegged 4000 rpm once, and if it did downshift it was at 3000 or so rpms.
All Gas: EXXON Supreme 93 octane. I started the trip with a full tank and a bottle of Techron fuel system cleaner.
Fill ups: Generally before the Orange light came on but less than 1/4 tank. I know I could get around 235 miles before my light came on so I was going that far each time.
1st Leg: 213.6 miles. 18 gallons. Put in another bottle of Techron. LTFT was at 3.1%
MPG = 11.86
(2 bottles of Techron through the system now)
2d Leg: 234.9 miles. 19.5 gallons. No additives. LTFT was still 3.1%
MPG = 12.04 (negligible difference could be result of elevation changes, no gains -- note: barely 2/10ths of a mile difference, but 1.5 gallons difference)
3rd Leg: 235 miles. 18.7 gallons. (roughly half of this leg was with LTFT of 3.1. About 40 miles were in town, but not really stop and go. LTFT dropped at one point to 2.3. This morning when I started back home LTFT was at 6.3)
MPG = 12.5 However, LTFT 3.1 for half a tank, then 6.3 for half a tank. (Still negligible mileage fluctuation from start of trip. Strange thing is LTFT was 3.1 for 10 hours of driving. Then suddenly switched to 6.3 for 10 hours of driving. Never fluctuated).
Here I put in a bottle of that Berryman Chemtool fuel system cleaner instead of Techron.
It gets interesting here. I had noticed that over the entire trip I could get 100 miles with the fuel showing 3/4 tank full. At about half a tank I'm around 150-160 miles. Empty (or darn near the light coming on) I'm at the 235 or so indicated above. I'm thinking it's just a gauge problem, but this time I stop and test my theory.
Leg 4. 106.1 miles. Stop at exactly 1/4 tank to fill up. It takes only 6.6 gallons. LTFT is still 6.3
MPG = 16.07 miles per gallon! If it were a faulty gauge I should have needed more fuel than 6.6 gallons. My truck is getting 16.07 miles per gallon if the tank is full.
LTFT stays at 6.3. At 3/4 tank again I note I am at 102 miles into the drive. I don't stop because I want to see if maybe overall mpg has improved, or if there really is something going on between 1/4 and 0 tank.
Leg 5. 213 miles. 15 gallons. (tank is now between 1/2 and 1/4 tank)
14.2 MPG.
If I fill up at 3/4 tank I'm pulling 16 miles per gallon. If I go down to between 1/2 and 1/4 tank I'm at 14.2. If I go until just about the time the light goes on I drop to 12 mpg.
So what would cause this? All the driving was the same. All highway miles. Same route there and back. Something is causing the engine to consume more fuel the emptier the tank gets. Note I have taken the gauge out of the question. I know some vehicles have slow gauges, but mine actually has better MPG at a full tank.
This morning I drove it to work. 50 degrees outside temperature. 30 second warm up idle. Closed Loop. LTFT 6.3 still!
STFT did not rise above -- numbers for the entire ten minute drive. I monitored it very closely. A few times it was at 0.0 but mostly it was at -2.3, -13, -14 even, and then -2 to -5 or so. LTFT never changed whatsoever.
So overall high positive LTFT, but during this morning's drive STFT was completely negative.
Once my LTFT went up to 6.3% I actually was getting better MPG.
Staying 3/4 tank and above and I add 4-5 mpg to my best MPG.
Bottom line: Poor mpg in 1996 Land Rover Discovery.
Initial analysis: Engine running lean causing ECU to overcompensate and dump fuel.
Analysis based on: High Long Term Fuel Trims
Mitigation efforts: New plugs and wires. New fuel pump and fuel filter, new front 02 sensors. Replaced MAF with known and tested used one. Replaced Charged Air Temp Sensor (shop stated it was showing -36F even though my OBD scanner shows it working correctly - maybe a stored value I can't see?). Smoke tested intake system - shop declared 0 leaks detected.
Recently - all plugs pulled. All look nice and chocolate. All plugs refit and torqued correctly. All wires checked for closure. Wires sprayed with water to detect arcing. None noticed.
This weekend completed an 800+ mile road trip:
Here are my findings (for purposes of the thread we will assume miles per hour was between 60 and 70 (generally 65), I drive like I have an egg on the gas and brake pedals. Outside of a few hill climbs RPMs stayed around 2200-2500. It only pegged 4000 rpm once, and if it did downshift it was at 3000 or so rpms.
All Gas: EXXON Supreme 93 octane. I started the trip with a full tank and a bottle of Techron fuel system cleaner.
Fill ups: Generally before the Orange light came on but less than 1/4 tank. I know I could get around 235 miles before my light came on so I was going that far each time.
1st Leg: 213.6 miles. 18 gallons. Put in another bottle of Techron. LTFT was at 3.1%
MPG = 11.86
(2 bottles of Techron through the system now)
2d Leg: 234.9 miles. 19.5 gallons. No additives. LTFT was still 3.1%
MPG = 12.04 (negligible difference could be result of elevation changes, no gains -- note: barely 2/10ths of a mile difference, but 1.5 gallons difference)
3rd Leg: 235 miles. 18.7 gallons. (roughly half of this leg was with LTFT of 3.1. About 40 miles were in town, but not really stop and go. LTFT dropped at one point to 2.3. This morning when I started back home LTFT was at 6.3)
MPG = 12.5 However, LTFT 3.1 for half a tank, then 6.3 for half a tank. (Still negligible mileage fluctuation from start of trip. Strange thing is LTFT was 3.1 for 10 hours of driving. Then suddenly switched to 6.3 for 10 hours of driving. Never fluctuated).
Here I put in a bottle of that Berryman Chemtool fuel system cleaner instead of Techron.
It gets interesting here. I had noticed that over the entire trip I could get 100 miles with the fuel showing 3/4 tank full. At about half a tank I'm around 150-160 miles. Empty (or darn near the light coming on) I'm at the 235 or so indicated above. I'm thinking it's just a gauge problem, but this time I stop and test my theory.
Leg 4. 106.1 miles. Stop at exactly 1/4 tank to fill up. It takes only 6.6 gallons. LTFT is still 6.3
MPG = 16.07 miles per gallon! If it were a faulty gauge I should have needed more fuel than 6.6 gallons. My truck is getting 16.07 miles per gallon if the tank is full.
LTFT stays at 6.3. At 3/4 tank again I note I am at 102 miles into the drive. I don't stop because I want to see if maybe overall mpg has improved, or if there really is something going on between 1/4 and 0 tank.
Leg 5. 213 miles. 15 gallons. (tank is now between 1/2 and 1/4 tank)
14.2 MPG.
If I fill up at 3/4 tank I'm pulling 16 miles per gallon. If I go down to between 1/2 and 1/4 tank I'm at 14.2. If I go until just about the time the light goes on I drop to 12 mpg.
So what would cause this? All the driving was the same. All highway miles. Same route there and back. Something is causing the engine to consume more fuel the emptier the tank gets. Note I have taken the gauge out of the question. I know some vehicles have slow gauges, but mine actually has better MPG at a full tank.
This morning I drove it to work. 50 degrees outside temperature. 30 second warm up idle. Closed Loop. LTFT 6.3 still!
STFT did not rise above -- numbers for the entire ten minute drive. I monitored it very closely. A few times it was at 0.0 but mostly it was at -2.3, -13, -14 even, and then -2 to -5 or so. LTFT never changed whatsoever.
So overall high positive LTFT, but during this morning's drive STFT was completely negative.
Once my LTFT went up to 6.3% I actually was getting better MPG.
Staying 3/4 tank and above and I add 4-5 mpg to my best MPG.