fall off in gas mileage - 1991 Range Rover

[glenncof]

1991 Range Rover with 173K miles

2001-2004 gas mileage was 14.0 mpg overall (maybe 100 tanks).

I put in synthetic oil in differentials, transfer case, and swivel housings late last year. Replace air filter at the same time. Plugs/wires/rotor/cap fairly new.

MPG has fallen to 12.5-13.0 mpg. I replaced O2 sensors (I think they were originals) but no improvement.

I'm gonna pull a few plugs to see what they look like.

Anyone have this experience ? Recommendations ?

[TerryS]

Depending on geographical location, gas is formulated for winter and summer. They have just started delivering summer formulation in S. New England.
Winter gas yields as much as 10% reduction from summer formulation.

[glenncof]

I have seen the summer/winter variation in gas mpg here in California when MBTF was used on another vehicle. My records on RR don't show this.

I pulled all the plugs. The tips and electrodes were white/lt. gray with some light flaky carbon on the seat of the plugs. Gap was 0.038" on each which is high end of range (owners manual says 0.033-0.038") and the plugs have 19,000 miles on them. As I recall the plugs (Champion RN12YC) were pre-gapped when new to 0.035". This deterioration seems somewhat high as I expect at least 50K on plugs on electronic ignitions.

I mistakenly regapped to 0.030" as my Haynes manual said this was lower range. Oddly the vehicle seemed to run better (???) but I can't believe this change would have any effect.

Any comments welcome.

[OkieRover]

How many fill ups have you had the average be down?
I sometimes notice a drop but the next time it is averaged out.

[glenncof]

2004 (since Aug)
21-Aug 13.4
4-Sep 14.7
2-Oct 14.4
13-Oct 14.4
19-Oct 14.9
24-Oct 15.4 San Francisco trip
2-Nov 12.0 synth. oil in diff/transf/swivels
10-Nov 13.5
3-Dec 12.1
10-Dec 13.3
24-Dec 13.6

Avg. 2004 14.3 (all fillups for year)

2005
1-Jan 12.4
6-Jan 13.5
13-Jan 12.0
20-Jan 15.4
27-Jan 11.0
10-Feb 13.6
26-Feb 12.0

Avg. 2005 12.8

Here is the mileage data. Fluxuations occure due to fumes in tank causing inconsistent shutoff at pump.

[OkieRover]

It's the expedition roof rack and the twelve Hella lamps you installed.
I know that's it.

[mafpolo]

Have you thought of adding an injector cleaner or fuel additive? I had a fall off in fuel mileage ( while still in warranty), and after all the dealer's checking and diagnosing, someone suggested adding a can of BG44.....that fixed it.

Also, check tire pressures, as this makes a difference in fuel mileage as well

[01001010]

Also...your filter box, although your filter might be clean (if it isnt you should consider getting a new one) the box sometimes ends up with a ton of junk in it, especially if you go offroad

[troverman]

Clean your PCV hoses and flame trap (gray canister unscrews on top of pass-side valve cover, and follow and clean the hoses and T-connections.

Any Check Engine light?
Check tire pressures--28 front, 38 rear.
Prem. gas.

[glenncof]

I changed the air filter just before, so it's clean. I recently did the PCV screen and cleaned the nipple at intake end of connecting pipe. No"check engine" and is running like a dream!

I have been thinking about injector cleaner (I use Techron, recommended by Porsche and GM as the only thing to use on my vehicles.) It has never solved a problem for me though (except on my 5.7l Chevy Tahoe that has known injector problem).

The dist. vaccuum advance hose was showing signs of age, I had flipped it end/end. Now I have recently replaced this. The advance unit is supplied very near the throttle plate and small nipple is located ahead of the plate when closed (no vacuum until throttle is slightly opened).

The vacumm advance unit may have a small, pinhole leak as it won't hold suction. They are $85 at Atlantic Britsh so I'll spring for a timing light to check it against spec. if all else fails.

When I connect vacuum advance hose to vaccuum nipple on opposite side of engine (full time vacuum and larger nipple) the engine does rev and backdown as the ECU controls idle RPM. So I conclude advance unit must work at least somewhat.

Thanks for thoughts and comments

[OkieRover]

Originally Posted by troverman

Clean your PCV hoses and flame trap (gray canister unscrews on top of pass-side valve cover, and follow and clean the hoses and T-connections.

What do you use to clean them? Does one thing work better than others?

[troverman]

Unscrew it, take off hose. Take out mesh. I use carburettor cleaner because it comes in a can that can shoot a jet and it cleans great. Clean the mesh, unroll, and let dry. Also clean the inside of the canister and dry out with a rag. Make sure to clean the hoses, too, because they can be more of a problem than the canister.