Poor Gas Millage/compression check

[macman]

I have had major gas millage issues since I bought my truck last year it has 192000 miles and I get around 11mpg with a 4:10 rear end, Tires are I think 70s and so far I have:

Changed both collant sensors.
Changed the PMD
Replaced the emprom
Re timed the pump
Turbo is ok
New injectors
New glow plugs
New gas cap
Brakes are all ok
Tranny shifts good

There is no smoke on start up, and it starts ok in the cold. It has been in the shop several times but enevn they are puzzled.

Yes I do lose some oil and I dont see it on the driveway yet.

I want to do a compression test on the block, especially on number 8 cylinder. WHAT SHOULD BE AN ACCEPTABLE PRESSURE RANGE?

And can anyone give me some advice or an avenue to look down.

[moondoggie]

Good Day!

If you do a search, you should find lots of info on what folks here consider acceptable compression. Compr. test results appear to have quite a bit of variability, depending on the compr. gauge used & how many "chuffs" (revolutions) B4 you accept the reading.

Our engines are (I think) 21.3/1 compr. ratio; if so, simple math would say ~ 310 PSI (21.3 x 14.7 (atmospheric pressure)). I've heard/read numbers all over the place, usually much higher than this.

It seems unlikely you'd have compression low enough to cause such incredibly poor fuel economy without noticing other symptoms, chief of which would be starting problems. IMHO If it starts OK, it's unlikely your compression is down much. (Experts, please - my experience here is extremely limited.)

Does your truck sound too quiet (in terms of rattle) to be a diesel? How was it timed? Was it actually timed to the harmonic balancer mark, or was the TDC offset simply set? TDC offset will compensate for a worn timing chain, but my understanding is that only partially - it takes ~ 5˚ of timing to make TDC offset change ~ +1.00. My point is, if your truck wasn't actually timed to the harmonic balancer mark & it's real quiet, maybe it's running way retarded.

Is your truck a dooley? From years of reading here on the Page, that seems to depress mpg a lot more than you'd think.

Your mpg is way off the mark. My pickup is identical to yours, except it's a 95. My annual average mpg for 2003 was 19.6, 2004 was 20.1, & 2005 was 19.5. I drive real easy - whatever the little signs on the side of the road say. The last year I did any significant towing was 2003, otherwise this mpg is mostly running no-load. Keep in mind, these numbers were generated in MN - we get pretty serious winter cold here.

Good Luck & Blessings!

[Cowracer]

Not quite doggie.

Your math is correct, but you dont account for thermal expansion as the air is heated during compression, which raises PSI.

The best bet is to do all 8 holes, and look for any more that 5% off.

Raw numbers have meaning, but not as much as hole-to-hole differences.


I have a 3500 dually with 4.10 and I see 13-14.5 mpg average. Your not that far off.

I'd check for torque converter lockup. What RPM do you turn at 65MPH?

Tim

[DennisG01]

Tire size will make a difference. I believe stock size is 245/75. A wider tire has more rolling resistance, a taller tire tricks your odometer into thinking the truck hasn't traveled as far as it actually has (unless it has been recalibrated). Best thing to do would be to use a GPS and verify your speedometer and odometer at highway speeds.

[moondoggie]

Good Day!

"I have a 3500 dually with 4.10 and I see 13-14.5 mpg average. Your not that far off." That's why I asked if he's got a dooley/dually. If he does, he's not that far off; if not & he's running empty, he's way low IMHO - I'd expect 18 or thereabouts. :)

Blessings!

(signature in previous post)

[macman]

NO its just a standard rear end not a dooley.

I have to ask again what kind of timming they did, It was done a little while ago.

And most of the time the truck runs empty. I know kinda of a wast for a truck but thats what it does.

The mechanic said to me I'd would not have a compression problems for the same reasons I don't really have any starting or smoking issues. But I'm sorta at my witz end. I don't wanty to upgrade my exhaust or change the boost because I really don't feel it would do me much good.

my tire size is 275/70 but again I was told it should be affecting the MPG that much.

Maybe I need a new timming pump chain?

[DennisG01]

Between having a taller tire AND a wider tire, they might be having more of an affect than you think. Somewhere on this site someone posted a link to figuring this out. I couldn't find it - maybe someone else knows? I would check your speedo and odometer with a GPS. You can figure out how much % you are off. Although not as accurate, you can check your speedo and odometer on the highway against the mile markers, at 60mph.

Good Luck! I went through something similar to you - it can be frustrating!

[JohnC]

If you have bigger tires and the VSSB has not been corrected, then you're going more miles than you think, which will make your mileage calculations low.

The mile markers on a federal highway are at least as accurate as a GPS reading. GPS readings can be quite inaccurate as the GPS measures straight line distances while the road may not be straight...

[macman]

I think I found another place too look. I hooked up a pressure guage to the in fuel water drain valve and found I was getting around 8-9 psi. Good apparently. But when I started to drive normally and even a bit hard the pressure dropped to sometimes 0 thus indicating the injector pump may be starving for fuel. I've run a short line from the inlet to the lifter pump into a small diesel gass can and drove around. I noticed the pressure dropped very little and stayed above 2 psi. I think I may have a plug some where in the tank. Maybe the sock has to be replaced.

But thanks for the tire tip I'll ask around.

[moondoggie]

Good Day!

"But when I started to drive normally and even a bit hard the pressure dropped to sometimes 0 thus indicating the injector pump may be starving for fuel." Definitely worth fixing, but very unlikely to have any impact on your mpg. :( Think about it - a diesel draws all the air it can, all the time - there's no throttle plate. Reduced fuel simply reduces power.

The most commonly-noticed symptom of an IP starving for fuel is a miss under heavy load that feels almost EXACTLY like a gasser with an ignition miss. There are certainly other symptoms, but unfortunately I doubt reduced mpg is one of them. (Please, if someone knows this to be wrong, chime in; please include a reasonable explanation so we all can learn. ;) Thx!)


Blessings!

(signature in previous post)

[DennisG01]

That's true that a GPS measures straight lines - but it does it in a way that's like drawing a thousand short, straight lines. If used properly, a GPS is VERY accurate - if it's receiving a WAAS signal, it can be accurate to less than 10 feet, anywhere on Earth.

Macman - sounds like you're getting closer and closer - good luck!

[DmaxMaverick]

Your GPS accuracy depends on the resolution of the device. Most consumer GPS have about a 1 or 2 second resolution, which means it will update every X seconds. In a curve, that could be significant enough to skew the MPH and distance. My Garmin has options to configure the resolution, but I leave it at 1 second, which seems to be ideal for accuracy and battery life. Many GPS models claim rediculous battery life, but it is usually at the expense of resolution. If it's not a setting you can change, then you got what you paid for.

Another thing that can effect resolution is the number of sat's it can track simultaneously. If you are only getting 3 or 4, then the resolution will be low. More than 4, and it should be pretty high. Most GPS, even the cheaper ones, have an option to track at least 5 or 6. Check it and see how many it's tracking.

[jspringator]

Macman, I think those pressure readings are pretty standard with stock diameter fuel lines. Mine does exactly the same thing and I have a 30 micron prefilter at the lift pump. I don't think that constitutes a defect.

[Bnave95]

I have had major gas millage issues since I bought my truck last year it has 192000 miles and I get around 11mpg with a 4:10 rear end, Tires are I think 70s and so far I have:

Changed both collant sensors.
Changed the PMD
Replaced the emprom
Re timed the pump
Turbo is ok
New injectors
New glow plugs
New gas cap
Brakes are all ok
Tranny shifts good

There is no smoke on start up, and it starts ok in the cold. It has been in the shop several times but enevn they are puzzled.

Yes I do lose some oil and I dont see it on the driveway yet.

I want to do a compression test on the block, especially on number 8 cylinder. WHAT SHOULD BE AN ACCEPTABLE PRESSURE RANGE?

And can anyone give me some advice or an avenue to look down.

What TDCO timing are you set at? Miles on pump? T-stat deg. are you running?
When oil fill cap is removed,how does the vapor look coming out? Do you use any diesel fuel condisher?
It's hard to think that's all you can get unless the OD is way off.
When I had to reset my VSSB,my OD will gain 1/10 mile every 8 miles. Road markers,interstate.