Poor MPG follow on

[luckyjoe]

Same road (going opposite direction), same throttle activity, temperature, payload etc.

Do the same road test but calculate on full tanks. See what your MPG average is over the full tank. My guess is it will stabilize. Oh, since your diving in deep, make sure your odometer is accurate too.

My old D1 got about 13mpg regardless of how/where I drove it. My LWB/4.2L is also about 13-14mpg local driving.

 

[Dave03S]

Just as an FYI if you are using your odometer to determine your MPG your are very likely introducing a false value into your calculation.

Both the speedometer and the odometer are most likely inaccurate just like the fuel gauge... but for different reasons.

Even with factory size tires the odo/speedo were wrong on my 2003.

The only way to actually know is to use a gps or speed gun to determine your actual mph, and also don't forget use a map app or gps to determine your actual distance traveled.

After recently changing tire size I wanted to find out how many mpg I had lost going to a bigger tire so I set out on a lonely stretch of highway only to find out that my odo/speedo were wrong on both sets of tires!! Reality was somewhere in between. Also interestingly on the stock size tires the Ultragauge was off from the dash display by 5mph. BIgger tires and UG still reports 5 mph lower even though reality is I was actually going 7mph lower than what the dash said with stock tires and now 3mph higher than the dash says with 265/75's! Meanwhile the UG stays 5mph lower than the dash with both sets of tires but there is an actual 10mph difference. The same relates to the odometer. On a trip I take often, old tires the ODO read 335 miles traveled. New tires read 297 miles same exact trip! that would be a 10% difference in the ODO for those of you keeping track, introducing an error in your mpg calculation to appear like you are getting less mpg with bigger tires. Interestingly I found that my MPG did not really change with bigger tires... just my readings changed effecting the math. I checked the exact path on google maps and they say it is actually 320 miles

My suggestion would be to start by getting actual data not from your instrument panel or OBD port and do your calculations over and see where you are actually at before trying to diagnose anything.

 

[AbnMike]

My odometer is dead solid perfect. MPH actual is irrelevant as long as I keep my speedometer in the same range (always 65-70 indicated, regardless of if it's actually 55 or 75).

 

[bsa_m21]

Re depressurized fuel tank:

Check your fuel return lines and fuel cap as the tank may not be venting properly. I had an '95 Rangie that would occasionally die on the side of the road, then 1/2 hr later start fine and run for a while, then do it again. As well, sometimes it would run out of fuel, although the gauge showed 1/2 full.

After MUCH debugging grief, including changing ECM's, fuel gauge, filter and pump, turns out the tank was depressurizing, forcing the fuel pump to work against the vacuum in the tank and eventually being unable to, the engine would die. After sitting for a while the pressure gradually equalized, enabling everything to work again. Finally by accident, I opened the fuel cap right after the engine died and heard a massive woosh as the tank equalized.

I replaced the filler cap and the problem went away. Strange, but true.

As well, the tank was plastic, so would collapse under vacuum, preventing the fuel sender float from showing the (relatively) accurate fuel level.

You may have a bit of similar problem, to a lesser degree, causing some of your issues. Something to check.

Re a good writeup on STFT/LTFT, go to:
http://gmtune.ru/tuning-fuel-ve-table-using-fuel-trims

M.

 

[Frobisher]

If you're still looking for a shop in the Pittsburgh area, my go-to guy is Harold, owner of Jagspeed Motorworks in Latrobe, PA. It's about an hour east of Pittsburgh but he's totally worth it. He knows these rovers inside and out.

His shop number is Phone: (724) 520-1881. He's part of the Ft. Pitt Land Rover Club as well.

I've been following your thread to see if I can apply any of it to my 95 since my mpg is pathetic at best. You're certainly doing your homework on this. I only wish I could offer something productive.

By the way, that awning I picked up from you has been tremendous!

 

[p m]

Bottom lines...
Using gas gauge for measuring the difference in gas mileage between 14 and 16 mpg is moot.
Relying on gas station shut-off valves for measuring the difference in gas mileage between 14 and 16 mpg is moot.

I've been following this and earlier thread out of curiosity, but man... Mike, you are misguided.

I've been tracking my gas mileage by full tanks on long trips, and yet it varied between 11 and 19 mpg. That 8 mpg difference was recorded on the same stretch of a road, familiar for Utah folks (I-70 between Green River and I-15), driven at the same speed, in the same direction. The difference: head wind or lack thereof.

 

[AbnMike]

Bottom lines...
Using gas gauge for measuring the difference in gas mileage between 14 and 16 mpg is moot.
Relying on gas station shut-off valves for measuring the difference in gas mileage between 14 and 16 mpg is moot.

I've been following this and earlier thread out of curiosity, but man... Mike, you are misguided.

I've been tracking my gas mileage by full tanks on long trips, and yet it varied between 11 and 19 mpg. That 8 mpg difference was recorded on the same stretch of a road, familiar for Utah folks (I-70 between Green River and I-15), driven at the same speed, in the same direction. The difference: head wind or lack thereof.

True, I know it's not exact, and admit it might be an anomoly but like I said, I notice it every time - about 100 miles on 1/4 tank. And 1/4 tank is +/- 6 gallons (which corresponds almost to 6 gallons per 1/4 tank for a capacity of 23 total - again, not exact, but close) To remove the gauge out of the equation I filled up at 1/4 tank and took 6 gallons. Even pump discrepency would only be about half a gallon.

I'm still driving and will test it again. After that I'll drive it until empty, put in 6 gallons, drive it until empty and see how far it goes. If it gets 100 miles in 6 gallons on a full tank, but far less on the same 6 gallons on an empty tank we'll have a problem, but no solution still.

Combined with the abnormal tank pressure it's probably not -the- problem but it seems to be contributing.

If I can replicate 100 miles in 6 gallons repeatedly, and then less once below half a tank (to remove the gauge I'll measure miles and gallons filled, and ignore the gauge) then there's something there....but might not be. Lot more driving to do.

On a full tank, at 19 gallons, I've pulled repeated 11/12 every time. Never higher. City driving only in winter with long idles in the morniing I get 8. I'm just not understanding why 6 gallons in a 23 gallon tank nets 100 miles. But overall tank (20 gallons since I never run dry) only nets 234/5.

I've now identified a potential cat problem, so maybe that's it...but I'm down to these two things....unless there are valve problems - but have a vacuum gauge coming (old one broke) to do a real motor test.

 

[AbnMike]

Vacuum Gauge Test: Normal. About 17 on the gauge. Does not fluctuate. Does not decrease under load (2500 RPMS) but increases and pops right back to normal. Showing good motor condition and no intake leaks.

Heat Gun to Cats Test:
Bank 1 (driver's side) 442 degrees pre cat. 448 post cat.
Bank 2 (pass side) 44x pre-cat (I forget) 492 post cat.

Based on some readings on cats, the Bank 1 isn't doing its job. Should increase 30-100 degrees post cat. It's also the downstream 02 sensor that stays at 1.005-1.010 volts and does not fluctuate. I know that downstream 02 sensors don't affect fuel trim but could it be affecting mpg?

Also a "working" Cat downstream 02 sensor should stay steady (mine is) but it's also not increasing in temp.
Bank 2 sensor fluctuates, but the temp is in range for a working cat.

So maybe both are bad?

Months ago I had noticed my manifolds were loose at the engine and the Ypipe to Manifold was loose. I replaced the gaskets at the Y pipe and have stayed on top of keeping the manifolds tight at the engine. However I don't know how long they were loose. I'm guessing that's when my Cat may have burned out.

I may try replacing the downstream 02 sensors on both sides first though.

 

[p m]

How would loose exhaust manifold connection cause cats to "burn down?"

 

[AbnMike]

How would loose exhaust manifold connection cause cats to "burn down?"

No idea. Just a theory. Sucking in extra air? Causing 02 sensors to enrich the fuel trim dumping gas into them that burns up?

I've got a tuned up, perfect motor (according to vacuum test) that gets barely 12 mpg driving like a little old lady....something is amiss.

 

[p m]

First, it is rather unusual for an exhaust manifold to create a vacuum.
Second, extra air won't do anything wrong with catalytic converters - as a matter of fact, extra air is injected in many vehicles into them to fully burn the extra fuel and oil vapors.

 

[bsa_m21]

An exhaust leak at the manifold will NOT add air to the exhaust pipe. It actually decreases the flow and pressure in the pipe, as fumes are leaking out.

M.

 

[AbnMike]

Pretty much every site I've read on catalytic converters stays an upstream exhaust leak will cook the shit out of them.

Regardless of the cause it seems they're both bad?

 

[mearstrae]

If you can get the system checked with and exhaust analyzer, this will tell you the exact mixture exiting the pipes. And a loose exhaust manifold or down pipe will cause a rich mixture to occur, this will essentially clog the cats after a short period of time and lower the temp. A rich mixture will also cause a miss reading by the o2 sensors if the carbon collects on them. Check the Evap system also, this might show a leak or a clog causing the tank to pressurize in negative direction (high vacuum). A hiss is OK but a whoosh isn't. I modified my '95 R.R.C. Lwb, with Buick electronic ignition (8.8 wires and wide gapped platnum plugs), K&N filter, and Magnaflo exhaust. Made sure all was well before making the changes and ended up, on a trip from Pittsburgh to Raleigh NC and back, getting an average of 22mpg, from a truck usually struggling to get 16mpg. I know D1 with coil packs is a different animal but, similar changes may work on a D1. I'm going to try this on my wife's D2, using a D.U.I. coil pack, and the other changes.

 

[AbnMike]

If you can get the system checked with and exhaust analyzer, this will tell you the exact mixture exiting the pipes. And a loose exhaust manifold or down pipe will cause a rich mixture to occur, this will essentially clog the cats after a short period of time and lower the temp. A rich mixture will also cause a miss reading by the o2 sensors if the carbon collects on them. Check the Evap system also, this might show a leak or a clog causing the tank to pressurize in negative direction (high vacuum). A hiss is OK but a whoosh isn't. I modified my '95 R.R.C. Lwb, with Buick electronic ignition (8.8 wires and wide gapped platnum plugs), K&N filter, and Magnaflo exhaust. Made sure all was well before making the changes and ended up, on a trip from Pittsburgh to Raleigh NC and back, getting an average of 22mpg, from a truck usually struggling to get 16mpg. I know D1 with coil packs is a different animal but, similar changes may work on a D1. I'm going to try this on my wife's D2, using a D.U.I. coil pack, and the other changes.

Thanks for that. I'm due to have it inspected here (never had to in Montana) but I was kinda hesitant. Don't want them to flag me then give me X days before having to fix it. I guess I can take it in and see if it passes the sniff test.

I think the high vacuumm in the tank after using 1/2 tank of fuel is causing this. I put in a tested and used evap canister and a new purge valve that meets resistance but I haven't tested it while running yet. I'll try that tomorrow.

Also have a new gas cap coming that may stop the extreme air whoosh (I mean it's a LOT) when I open the cap.

 

[AbnMike]

46 miles. All city driving (about 7 was hauling a 760lb trailer with 1000lbs of crap in it). No idle in am. Start and go within 30 seconds.

LTFT 3.3
Gallons 7

MPG 6.5

LOL Anyone want a goddamned Discovery?

 

[AbnMike]

Any other ideas before I leave it running with the keys in it downtown?

Seriously. 6.5 mpg is less than dragsters get. It's not even something that can be believed in a tuned, tested, new plugs and wires vehicle driven like a fat lady in Wal-Mart on a motorized cart.

Something has to be missing. Someone has to have had this problem before. Anyone?

 

[kk88rrc]

Another test: engine off, key on: 38psi fuel pressure.

Engine on: idle 30 psi
Engine on: 2500 rpms 25 psi.

Clamp fuel return line either running or idle: fuel pressure 100+ psi (gauge only goes to 100)

Vacuum detached while running: 36-38

Ten minutes sitting engine off: 28 psi

20 minutes engine off: 20 psi

1.5 hours: 0 psi

Any other ideas before I leave it running with the keys in it downtown?

Seriously. 6.5 mpg is less than dragsters get. It's not even something that can be believed in a tuned, tested, new plugs and wires vehicle driven like a fat lady in Wal-Mart on a motorized cart.

Something has to be missing. Someone has to have had this problem before. Anyone?

Correct me if I'm wrong but those fuel pressure reading look suspicious. That plus your tank pressurizing would suggest a venting issue or an over zealous pump or your FP regulator. 100+ psi with the return line clamp can't be right. Also, your basic FP looks low. Shouldn't it be 34-37 psi.

 

[AbnMike]

Correct me if I'm wrong but those fuel pressure reading look suspicious. That plus your tank pressurizing would suggest a venting issue or an over zealous pump or your FP regulator. 100+ psi with the return line clamp can't be right. Also, your basic FP looks low. Shouldn't it be 34-37 psi.

I read somewhere it should be around 30ish and then go up to 38 or so if the fuel regulator vacuum line is removed. That's what it does.

I'm not an expert, just things I've read, that's why I've been trying to document and post as much as I can, rather than the "my truck is broke, please help" posts... I was hoping someone would pick up on the FP number days ago and let me know whether something was wrong or not.

When I clamped the return line it definitely shot up to over 100 each time. I didn't leave it clamped for fear of busting something. I clamped the line to the front of the motor. The line to the back is the feed as I understand it and clamping that made the motor stumble.

 

[AbnMike]

37.7 miles
5 gallons

7.54 mpg. City driving but 3/4 of that was probably coasting.
LTFT 3.9

Interesting: my P1317 Rough Road Line fault that I was throwing almost every day has stopped though I've done nothing to correct it.

New fuel cap does not stick like the first replacement.