Gas mileage help?

[Lokim]

Ok guys, I'm running out of ideas.

I'm generally getting between 14.5-17.5 mpg, and about 70% of my driving is done at freeway speeds. I'm thinking I ought to be getting between 17-21mpg minimum. I do have a heavy right foot, but not so much as to justify this poor mileage as I am a cruise-control junkie.

It runs great, with an occasional very slightly rough idle (I suspect timing chain stretch, as it is at 209K.)

So far I've replaced the knock sensors, O2 sensors, cleaned the intake with the BG intake cleaning system (two-can kit), throttle body, and EGR valve/port. I've also replaced the fan clutch and fan.

I've replaced the MAF and cleaned/bent the connector pins for better contact, as well as re-spliced (w/ solder) the MAF connector and wires. No change in running with a MAF wiggle test.

I replaced the 2 bad injectors it had when I got it, and the rest ohm out fine.

Battery voltage is good, alternator is working fine, previous owner installed Denso double-plats (Their version of our beloved NGK Laser Plats) and they have about 20K on them.

I've got synthetic transmission fluid in the trans, as well as having done 2 BG engine flushes, 2 cans of 44K, and multiple bottles of Techron and Redline injector cleaner. I am running Amsoil Signature 10W30 with an Amsoil Ea038 oil filter @ 7500 mile intervals.

I suspect the right-side cat is cracked inside, as it develops a rattle when hot that I have spent hours trying to eliminate via d!ck around with the heat shield and mount to no avail.

I've also done brakes on all 4 wheels, new rotors, pads, and hoses. None of the brakes are dragging. I re-lubed the caliper pins a few hundred miles ago. The wheel bearings are fine, tires are new and at the correct pressure, and it is aligned within spec.

When I hook up my Snap-On Scanner, the fuel trims are well within spec and the O2 sensors are switching fine. I can't seem to find any vacuum leaks, either.

When I first got the car I got nearly 24mpg on a strictly-freeway road trip, at 10 over the whole time (70-80mph.) Anybody have suggestions for improving my mileage?

[OwnerCS]

Any major changes in tire size (OD) compared to OEM size that would throw off the odometer?

[maxnix]

Denso are the wrong plug. Not known if they will total the engine like Bosch and Autolite. Change soon to correct NGK!

The BG intake treatment is a great maintenance regime, but no substitute (nor very effective) for already dirty path. You don't mention your EGR valve or IAC valve duty cycle. A properly operating EGR valve is worth about 10% economy increase in highway use. And here's a dirty little secret, the BG induction cleaning is n't going to do squat for that tiny length of passage from the EGR valve into the plenum. Only manual labor well, and the plenum needs to be off the engine.

In short, you are 9/10ths of the way there. Forge on!

[Lokim]

Any major changes in tire size (OD) compared to OEM size that would throw off the odometer?

I am currently on 235/50R17's at all 4 corners. According to multiple tire/wheel calculators I am reading 1% slow, as in at 60mph on the speedo I am actually travelling 60.6mph.

Maxnix-
I forgot to mention, the plenum is clean (manually), and I scraped, scrubbed, brushed, and cussed out the EGR valve AND the EGR plenum passage. Have not checked the duty cycles yet. Maybe I'll bring the ol' scanner home this weekend and fart around with it.

[qship96]

Have you tried duplicating a freeway only driving trip to see if you can match your earlier 24 mpg results? If you can match that under the same conditions,then you are OK. One thing not mentioned that I can think of, is what weight,age is the differential fluid currently in there?

[maxnix]

The only way a VH45DE in a G50 cold achieve 24mpg is going downhill. 22 is the absolute tops at 55mph on a level road with real 100% gasoline.

[qship96]

I have got 23.5 easily at speeds of 70-80 on long trips,a/c running,using ethanol laced fuel- many times. Check out my old thread from my round trip from Md to T3 and back, where I got 22.24 mpg at trip speed of 80-90 mph.

[Lokim]

The only way a VH45DE in a G50 cold achieve 24mpg is going downhill. 22 is the absolute tops at 55mph on a level road with real 100% gasoline.

Untrue. I drove to Oregon last spring and before I left, I filled the tank to the tippy-top. I was running factory size tires at the time. I could see the fuel resting in the filler neck. When I got down to 1/8 of a tank, I filled it up again to the same level. IIRC my calculation ended up at around 23.7 mpg. Here in WA you CAN NOT get pure gasoline at the gas stations. You can only get ethanol blends, varying up to 10%.

[Lokim]

Have you tried duplicating a freeway only driving trip to see if you can match your earlier 24 mpg results? If you can match that under the same conditions,then you are OK. One thing not mentioned that I can think of, is what weight,age is the differential fluid currently in there?

I was planning a day trip this weekend, so we shall see...

As far as the diff fluid, it was changed last may w/ conventional 80W90 LS additive fluid. It was really clean when I drained it, but it's cheap to do and I figured it was better to safe than sorry. It'll be getting Amsoil 80W90 w/ a separate LS additive (Amsoil as well) before the year is out.

[qship96]

synthetic diff fluid will give you a very slight boost in mpg,especially when it is cold. No need for any ls additive, as the Q limited slip is viscous based and sealed off from the diff fluid one adds to the diff. case.

[OwnerCS]

For non-spirited highway driving (e.g. holding it steady at 65 mph) mine consistently delivers 22.5 MPG running 93 octane E10. Also running Royal Purple diff fluid.

[Lokim]

synthetic diff fluid will give you a very slight boost in mpg,especially when it is cold. No need for any ls additive, as the Q limited slip is viscous based and sealed off from the diff fluid one adds to the diff. case.

I had been wondering about that, as neither Alldata nor the FSM mentions it as a requirement. Again, I figured better safe than sorry. Nice to know that I don't have to spend that extra bit of $$. Do you think I oughta stay with 80W90 or is 75W90 a better choice where I live (Seattle?) Occasionally gets above 80F, and rarely above 90F in the summer, and rarely gets below 25F in the winter.

[Lokim]

Denso are the wrong plug. Not known if they will total the engine like Bosch and Autolite. Change soon to correct NGK!

If they were anything other than Denso they would have been gone by now. I know they're not ideal and they will be changed soon, but other issues were a bit more pressing. I've been a mechanic for 10 years, and while I am a staunch supporter of using OEM plugs (they did spend a buttload of money figuring out which plugs work best) I just can't see the brand difference (since they are both double-plats) causing that drastic of a mileage drop.

[Lokim]

So, any other ideas?

Wes?
Heath?
Paul?
Brian?
Jeff?????

[djwarner]

As someone with little to no experience compared to you, might I suggest looking at the bad catalytic converter causing back pressure and poor breathing. I've had media inside the converter crumble blocking the flow of gasses.

[qship96]

swapping the newish plugs will not give you any difference in MPG at all. as far as 75w-90 vs 80w-90 gear oil, really no big difference, other than the 75w is slightly thinner at very low temps- with a synthetic fluid,either will work perfectly fine for you. Until you take a long highway only trip and carefully measure mpg and compare to your historic result, I would not stress over the issue.....too many variables in just looking at weekly short jaunt trips mpg.

[maxnix]

Denso are the wrong plug. Not known if they will total the engine like Bosch and Autolite. Change soon to correct NGK!

If they were anything other than Denso they would have been gone by now. I know they're not ideal and they will be changed soon, but other issues were a bit more pressing. I've been a mechanic for 10 years, and while I am a staunch supporter of using OEM plugs (they did spend a buttload of money figuring out which plugs work best) I just can't see the brand difference (since they are both double-plats) causing that drastic of a mileage drop.

Its not for mileage drop but interference compared to OEM, especially at high RPM. It is disastrous.

Does Canada have ethanol? If not, go tank up there and report back.

Annual fuel filter change? Top Tier Premium?

[Lokim]

As someone with little to no experience compared to you, might I suggest looking at the bad catalytic converter causing back pressure and poor breathing. I've had media inside the converter crumble blocking the flow of gasses.

Near the top of my list. Already formulating a plan for replacement...

[Lokim]

swapping the newish plugs will not give you any difference in MPG at all. as far as 75w-90 vs 80w-90 gear oil, really no big difference, other than the 75w is slightly thinner at very low temps- with a synthetic fluid,either will work perfectly fine for you. Until you take a long highway only trip and carefully measure mpg and compare to your historic result, I would not stress over the issue.....too many variables in just looking at weekly short jaunt trips mpg.

That's the thing! They're not short trips. I drive 20 miles each way to work 5 days a week. I ought to be getting at least the city rating on average. About 85% of the time my mileage hovers at right about 15-15.5mpg.

[Lokim]

Its not for mileage drop but interference compared to OEM, especially at high RPM. It is disastrous.

Does Canada have ethanol? If not, go tank up there and report back.

Annual fuel filter change? Top Tier Premium?

I get the motor to redline a few times a month, so it doesn't look like that's an issue. When I finally get my Nico ECU I'll definitely swap to the Laser Plats.

The filter is due for replacement. Maybe I'll do that this weekend.

ALWAYS, WITHOUT FAIL, Chevron Supreme or Shell V-Power.

[maxnix]

And does Canada dope their petrol with ethanol?

[Lokim]

And does Canada dope their petrol with ethanol?

No idea. I can't go to Canada, legally speaking...

[Lokim]

Quick update - I checked my fuel trims again, and both short- and long-terms are in spec. 100-105% across the board. The limit is 20% rich or lean, so for all intents and purposes the motor runs perfectly according to the ECU. I guess a compression test is in order...

BTW, I found a way to do my cats for FREE! At the shop I work at, we have a guy who comes by to buy used cats, and I asked what he'd pay for the Q's. He told me likely $80 each. I asked if he bought the guts alone, and he said yes, $5/lb. So if I cut the old cats off, there's $160, and if I gut the pre-cats that's likely another $20+. So approx. $160-$180+ for the cats on the car. I can get Magnaflow or Walker universals for wholesale through work, so around $70-$90 ea., possibly even less. In the end, I could get PAID to replace my cats. That ought to eliminate both the annoying rattle AND eliminate clogged cats as a possibility in my poor fuel mileage.

[maxnix]

Unless abused, I doubt the catalytic converters are under performing.

Almost all after market catalysts have far less of the precious metals and thus have much shorter life than the OEM catalysts.

A few years ago, $100 per converter was standard. Has to be much more now.

[Lokim]

Its not an issue of under performing, but of possible mechanical flow restriction due to a broken matrix inside the cat.

I know in the past the car was run rich for an unspecified length of time (I have the service records of the prev. owner) which can definitely ruin the cats.

Plus, that rattle at idle when hot is driving me nuts!

[qship96]

do it for the rattling, but my gut says if the converters were actually plugged and restrictive enough to affect your mpg, you would be also experiencing acceleration and performance issues also.

[Lokim]

do it for the rattling, but my gut says if the converters were actually plugged and restrictive enough to affect your mpg, you would be also experiencing acceleration and performance issues also.

You're right about that, I'm just grasping at straws.

It also turns out you may have been right elsewhere as well. I took a trip over the mountains to Wenatchee today, and it looks like I got around 21-22mpg. I topped the tank off before I left, and the needle is now at half a needle-width under half a tank and I went around 240 miles. I guess Seattle's traffic is just as crappy on gas mileage as it is on the psyche.

Green city my A$$!

[maxnix]

Leaving Seattle traffic is a plus. Austin traffic is getting to be as bad.

Tap the converters and if you hear a rattle, bad news.

[Lokim]

Leaving Seattle traffic is a plus. Austin traffic is getting to be as bad.

Seattle traffic blows goats. Imagine being stuck on the freeway surrounded by multitudes of elderly Asian women wearing hats, talking on cell phones, and driving Subarus...

[maxnix]

While DWA is a real hazard in Seattle (no concept of time and space relationships), here it is the trophy wife in her extra large SUV drying her nails, talking on her hand held cell phone (using BT and NAV is beyond the skills of most of these women) while drying their nail job and changing lanes by eminent domain with no signal.