Poor MPG problem
#1
03-10-2017, 08:38 AM
Poor MPG problem
I'm still trying to track down what I think is a very poor MPG problem, even for a Discovery (last tank I averaged 8.3 mpg. Granted that is in in-town driving, back and forth to work, 2 miles each way (no preaching, I'd bike if I could but I live off a road that's 2 miles long, barely 2 lanes wide, no shoulders, immediate drop off into ditches or worse).
To save time: I don't floor it. I coast to stop lights. On highway I run about 65-70 mph, I don't pass unnecessarily, etc.
25k miles ago it got new heads and gaskets, camshaft, hydraulic lifters, timing chain and gears, valve cover gaskets, ignition wires, vacuum hoses and Magnaflow stainless steel Y pipe and catalytic converters
Highway MPG couple months ago (pre new wires and plugs)
800 mile round trip 10.2 mpg average at 2200 rpm.
Scan tool showed Long Term Fuel Trim (LTFT) 16.9%
On December 28 I threw a P1316 #E8 Misfire Excessive Emissions Fault
Cleared it and haven't seen it since.
New Plugs. New Wires. New Upstream O2 sensors.
In January I went to I13-14mpg range (Highway) with the new front 02 sensors (over hilly/mountain terrain up and down through WV and western MD). The LongTerm Fuel Trim has been a solid 3.1 over 300 miles, down from a high of 10.2.
Shortly after it jumped up to 14.7% (no changes in anything on the Disco)
Put in a new fuel pump and fuel filter - Previous Fuel pressure was around 28. New filter and pump has it about 30 psi.
Fuel trim dropped after new filter to 4.2%
Over past couple of weeks drove it and it jumped back up to 9% then has been back down to 4.2 and is now at 6.3%
At this point I'm at a loss.
Took it to a shop for an intake "smoke" test to see if there are any vacuum leaks leading to it running lean and dumping fuel (per Discoweb advice).
Test shows no vacuum leaks. They used a scan tool and wrote up the following
Smoke Test results negative. No Leaks found. 2: Following vehicle data shows Charged Air Temperature Sensor Reading -36 Degrees. MAF Reading at 1.74 GPS should be Between 3 and 4 GPS AIT Temp should be just about Ambient Air Temp.
Ok, cool. Talk to the guy and he says my MAF is slightly out of spec but not bad but my Charged Air Temp Sensor is really out of spec and reading -36 Degrees would make my truck run very rich! I thank them and go home to look for this $7 sensor.
BIG PROBLEM.
When I plug in my Scan Tool my IAT (Intake Ambient Air?) reads slightly warmer than outside air but not much (Yesterday after idling awhile it was at 124 degrees (64 outside temp) but then I drove it around and it dropped to 95 or so (outside temp 60) - so 30 degrees different but it is inside a plastic box in a pretty hot aluminum engine bay).
This morning it was 35 degrees and the IAT was showing around 50 (and it does move up and down as though it's working).
So the mech said the IAT of -36 would definitely cause the engine to run rich (dense air - computer throws more fuel at it). Makes sense.
What I don't get is why my IAT reading on my OBDII is showing a more or less correct IAT reading. It doesn't make sense that they'd make up something (it's a $10 part at best).
Is their tool reading some other Charged Air Intake temperature other than what I think it is (the sensor in the airbox)?
Is their tool reading a reading off the MAF instead?
Did they just completely screw up?
All of this said and done, I still have very crappy mpg, even for a lifted Disco. No vacuum leaks. New plugs and wires. New fuel pump and filter. Proper fuel pressure. New 02 sensors.
Still crappy mpg.
The only thing I could possibly think is that maybe I have a leak at my exhaust manifold to pipe and it's sucking air in there and fooling the 02 sensor to think it's lean so it's dumping more fuel...but those gaskets are new, too.
To save time: I don't floor it. I coast to stop lights. On highway I run about 65-70 mph, I don't pass unnecessarily, etc.
25k miles ago it got new heads and gaskets, camshaft, hydraulic lifters, timing chain and gears, valve cover gaskets, ignition wires, vacuum hoses and Magnaflow stainless steel Y pipe and catalytic converters
Highway MPG couple months ago (pre new wires and plugs)
800 mile round trip 10.2 mpg average at 2200 rpm.
Scan tool showed Long Term Fuel Trim (LTFT) 16.9%
On December 28 I threw a P1316 #E8 Misfire Excessive Emissions Fault
Cleared it and haven't seen it since.
New Plugs. New Wires. New Upstream O2 sensors.
In January I went to I13-14mpg range (Highway) with the new front 02 sensors (over hilly/mountain terrain up and down through WV and western MD). The LongTerm Fuel Trim has been a solid 3.1 over 300 miles, down from a high of 10.2.
Shortly after it jumped up to 14.7% (no changes in anything on the Disco)
Put in a new fuel pump and fuel filter - Previous Fuel pressure was around 28. New filter and pump has it about 30 psi.
Fuel trim dropped after new filter to 4.2%
Over past couple of weeks drove it and it jumped back up to 9% then has been back down to 4.2 and is now at 6.3%
At this point I'm at a loss.
Took it to a shop for an intake "smoke" test to see if there are any vacuum leaks leading to it running lean and dumping fuel (per Discoweb advice).
Test shows no vacuum leaks. They used a scan tool and wrote up the following
Smoke Test results negative. No Leaks found. 2: Following vehicle data shows Charged Air Temperature Sensor Reading -36 Degrees. MAF Reading at 1.74 GPS should be Between 3 and 4 GPS AIT Temp should be just about Ambient Air Temp.
Ok, cool. Talk to the guy and he says my MAF is slightly out of spec but not bad but my Charged Air Temp Sensor is really out of spec and reading -36 Degrees would make my truck run very rich! I thank them and go home to look for this $7 sensor.
BIG PROBLEM.
When I plug in my Scan Tool my IAT (Intake Ambient Air?) reads slightly warmer than outside air but not much (Yesterday after idling awhile it was at 124 degrees (64 outside temp) but then I drove it around and it dropped to 95 or so (outside temp 60) - so 30 degrees different but it is inside a plastic box in a pretty hot aluminum engine bay).
This morning it was 35 degrees and the IAT was showing around 50 (and it does move up and down as though it's working).
So the mech said the IAT of -36 would definitely cause the engine to run rich (dense air - computer throws more fuel at it). Makes sense.
What I don't get is why my IAT reading on my OBDII is showing a more or less correct IAT reading. It doesn't make sense that they'd make up something (it's a $10 part at best).
Is their tool reading some other Charged Air Intake temperature other than what I think it is (the sensor in the airbox)?
Is their tool reading a reading off the MAF instead?
Did they just completely screw up?
All of this said and done, I still have very crappy mpg, even for a lifted Disco. No vacuum leaks. New plugs and wires. New fuel pump and filter. Proper fuel pressure. New 02 sensors.
Still crappy mpg.
The only thing I could possibly think is that maybe I have a leak at my exhaust manifold to pipe and it's sucking air in there and fooling the 02 sensor to think it's lean so it's dumping more fuel...but those gaskets are new, too.
#2
03-10-2017, 09:38 AM
#3
03-10-2017, 10:02 AM
#4
03-10-2017, 10:24 AM
#5
03-10-2017, 10:49 AM
There's only one sensor for air temp. The MAF only measures volume, no provision for temp. The coolant temp for the ECU is most important and then the air temp sensor. Pretty sure you get a check engine light if that one is out of spec. Any dragging calipers or bearings not being greased?
#6
03-10-2017, 11:19 AM
There's only one sensor for air temp. The MAF only measures volume, no provision for temp. The coolant temp for the ECU is most important and then the air temp sensor. Pretty sure you get a check engine light if that one is out of spec. Any dragging calipers or bearings not being greased?
The coolant temp stays around 180 or so.
No caliper drag that I'm aware of. New bearings all around. Rolls easily enough for a 6,000 vehicle when on a hill...
I'll take it out for an Italian tune- up and see if I can't clear out some of the coked up crap and use an injector cleaner in it and see what happens...
I guess I need to go back to the shop and ask them why my IAT shows correct intake air temp but theirs shows it at -36. Maybe they're reading something different?
Last edited by AbnMike; 03-10-2017 at 11:31 AM.
#7
03-13-2017, 07:36 AM
#9
03-13-2017, 02:10 PM
So does it sound like I need new injectors? I hate to start just throwing parts at a problem but I'm not getting any answers really.
Was told it was a result of a vacuum leak. No vacuum leaks.
Told to replace plugs and wires. Replaced plugs and wires.
Maybe it's a bad fuel pump. Replaced fuel pump and fuel filter.
Bad MAF? Ok the MAF is apparently a bit out of spec but that's the same shop that said my IAT was showing -36 except my OBDII shows it operating properly. I guess I could replace the IAT sensor for the hell of it.
The only thing left is injectors....
Was told it was a result of a vacuum leak. No vacuum leaks.
Told to replace plugs and wires. Replaced plugs and wires.
Maybe it's a bad fuel pump. Replaced fuel pump and fuel filter.
Bad MAF? Ok the MAF is apparently a bit out of spec but that's the same shop that said my IAT was showing -36 except my OBDII shows it operating properly. I guess I could replace the IAT sensor for the hell of it.
The only thing left is injectors....
#10
03-13-2017, 02:18 PM