MPG frustration

[aaronsb]

Yes, EGT would be higher than expected. But you don't know baseline because you never knew what temps it was like with known good injectors. The correctly equipped shop can check for HC emissions on a dyno, but now you're getting into spending money.

Are there any service records?

One side effect of incorrectly advanced timing is white smoke when cold. It's hard to quantify, but another side effect is more power at high engine speeds, but that's really dependent on the type of pump and governor.

Originally Posted by bread519

If the timing was incorrect would I have any side affects? The rig seems to have power to spare but I've also never driven any other buses

[bread519]

No smoke at all from the bus really. I've been super busy this past few weeks and haven't had a chance to look into it yet. I need to do some research about checking the injectors then I'll probably tackle. I do have a camping trip this weekend with it.

[bread519]

So I took the bus into the local truck stop. My "water transfer"? filter was old. I have two bad tires. One was at 60psi, the other was at 70psi.


Could this be my culprit

[Brad_SwiftFur]

Yeah, those tires sound to be way underinflated.

On 20" or 22.5" tires, my company considers them *FLAT* at anything under 85.

[cadillackid]

if theres water in the fuel that can cause a skip or miss which will make your fuel comnsumption higher in general..

underinflated tires will definitely be a cause for Low MPG..
my bus runs its tires at 100 PSI cold.. and your truck stop probably tested them at 60 and 70 Hot! (opressures read higher when tires are hot).. and thoise tires wouldve been pretty warm after driving them at that pressure..

-Christopher

[jazty]

While 60psi is certainly low, 70psi may be getting close to the recommended pressure, depending on what tires you have and the vehicle weight.

Check out these tire load and inflation tables from Michelin: Load & Inflation Tables | Michelin Truck.

If you look at a G-rated 11r22.5 you can see that the recommended PSI for an axle with singles that is supporting up to 9060lbs is 70psi. 100psi supports up to 11900lbs for an axle with singles. I would wager that few buses have 11900lbs resting on those front wheels. Over inflation will result in reduced traction and increased wear of the center tread.

Also note that most buses have a plaque somewhere (mine was in the fire extinguisher compartment) that will display recommended tire inflation for front and rear wheels.

My tires are 10r22.5 and I shoot for 75psi. At 100psi the wheels provide a harsher ride and have less traction.

[cadillackid]

each tire brand and model is different too... the uniroyals im running, are a higher load rating and the tire guys recommended running 100..

I went wit ha Load range H instead of the 'F' the bus previously had simply because i was into very standard truck tires that offered me all kinds of different choices and better prices.

for me it was pretty easy since I went in shopping for 6 new tires we could match up a nice set of 4 for the back and then a different but best fit for the 2 in the front...

I havenbt played around with my pressures as im still learnign the ins and outs of commercial truck tires


-Christopher

[Tango]

The recommended inflation should be stamped on the tire. My old ones ran at 125psi and the new ones (as I recall) are about 90-100.

[jazty]

Originally Posted by Tango

The recommended inflation should be stamped on the tire. My old ones ran at 125psi and the new ones (as I recall) are about 90-100.

That's usually the maximum pressure, from my experience. The recommended tire pressure needs to be calculated per vehicle: https://www.yokohamatruck.com/commer...ure-calculator

[jazty]

Originally Posted by cadillackid

the uniroyals im running, are a higher load rating and the tire guys recommended running 100..

While a higher load rated tire means you can support more load and run it at a higher pressure there is still good reason to keep the pressure matched to your vehicles weight. Evenness of wear and ride quality are the two big reasons to figure out a proper inflation.

While inflation and load rating for commercial truck tires differ between manufacturers, they really don't differ by a wide margin. Standard truck tires have been mostly... ahem.. standardized ;).

I would trust the plaque that the vehicle manufacturer put inside the vehicle more than the tire salesman. The salesmen know the tires, but they don't know the vehicle. A quick cross-reference from any of the commercial tire inflation tables could help out, even if you changed tire sizes.

For instance if the vehicle information plaque recommends 10r22.5 tires inflated to 80psi for the front axle you can look at a table and see that the front axle is supporting something around 9000lbs. Move to the 11r22.5s and to support around 9000lbs you need somewhere between 70-75psi...

Just sharing what I've learned! It has helped me with ride quality considerably

[Tango]

I absolutely agree. The factory "recommended" pressure is just a starting point. You need to know how your actual vehicles weight is distributed and with all the mods made on a typical skoolie, the real weight bias can be very different from the placard.

[cadillackid]

when i ran the tires at what is stamped on the 'Carpenter' Tag i had a soft , mushy ride and a lot of rear-end sway in the bus as well as lower MPG...

and YES i set the pressures with the tires cold.. 60 degrees f, thats when I went back to the tire store and they said 'on a school bus id run 100 all the way around'.. so I do and the bus drives wonderfully.. perhaps a harsher ride but I dont have that sway and I dont go broke buying diesel

-Christopher

[Tango]

As a side note...just about every trucker I have chatted about tires said they always run their pressure a little bit over what the weight and book numbers call for. They do it for both MPG and swear they get more miles out of the tires (?).

[cadillackid]

as long as I wont blow up in a ball of fire for running my tires at 100 then im going to keeop doing so because running them at the 80 / 85 stamped on the bus label makes it a real Chore to drive... at 100 its all smiles for me..

the sidewall says max 120 PSI cold..

-Christopher

[EastCoastCB]

Originally Posted by cadillackid

as long as I wont blow up in a ball of fire for running my tires at 100 then im going to keeop doing so because running them at the 80 / 85 stamped on the bus label makes it a real Chore to drive... at 100 its all smiles for me..

the sidewall says max 120 PSI cold..

-Christopher

MIne are all at 110psi.

[jazty]

Originally Posted by cadillackid

when i ran the tires at what is stamped on the 'Carpenter' Tag i had a soft , mushy ride and a lot of rear-end sway in the bus as well as lower MPG...

Yeah, there's definitely some room to play with for personal preference. The trick is to not run 'em low, but over probably won't kill ya

Here's a nice read about tire pressure courtesy of truckinginfo.com: The Magic Number - Article - TruckingInfo.com. It showcases several opinions about tire pressure.

To get back to the original question about fuel economy, I wouldn't attribute any huge drop in MPG to that low of tire pressure. 60psi is too low and 70psi is possibly low, but if the rears were properly inflated you would likely be looking at a reduction in fuel economy of less than 3% not a whopping 30%.

Perhaps its worth focusing some attention on the turbo. Check the boots for cracks and the exhaust up-pipe for black soot (which would indicate an exhaust leak). Might be worth doing a boost pressure test. I think these engines want to be around 15-20psi of boost.

[2kool4skool]

Our BB's handling and comfort is optimized at 105psi steers and 85psi drives cold.

The wt. dist is 50-50 at 9700# per axle w/265/70/19.5 tires.

Our boost at max pull is just above 18psi per scan gauge D on a 5.9 Cummins.

11 mpg avg over 6000 mi.

[cadillackid]

fuel pump timing can be a huge factor in power and fuel economy... it is the ignition timing of a diesel.. there are lots of articles and yest data out there on various engines as to what brings the best of both worlds...

I dont k ow on a diesel with a PCM how you check or set the actual timing.. on a mechanical its easy to do .. just remove the cover plate and look, and adjustment is usually very easy too..

I definitely didnt have 30% MPG loss when my tires were run lower so I wouldnt expect the OP to either.. now running my bus fast I definitely had huge MPG loss...
running a diesel at its max rated RPM and the extra wind resistance makes it thirsty for sure..

as for my own bus I havent experimented with running my rear pressures lower than my fronts.. I do know performing some practice near-panic stops on wet pavement that its Pretty easy to lock the rears up on my bus while still having plenty more braking in the front... im guessing the weight transfer rear to front must be quite high on a CE style bus during a stop... esp since its an empty short bus... perhaps a little lower tire pressure in the rear would alleviate that a little..

Turbo leaks will surely kill MPG like mentioned.. not only look at the pressure side of the intake but also make sure the exhaust manifold is not leaking or has loose bolts.. since exhaust spins the turbo any exhaist leaks pre-turbo will reduce the amount of boost it produces..

-Christopher

[Stu & Filo. T]

Originally Posted by Tango

The recommended inflation should be stamped on the tire. My old ones ran at 125psi and the new ones (as I recall) are about 90-100.

I've read twice now that on RV & semi truck tires the psi stamped on the tire is a minimum pressure, This was mentioned twice now in motorhome magazine.

[cadillackid]

mine actually has the word Max as 125 PSI COLD.. its assumed the pressure will be measured between 50-70 degrees F (tire temperature)is what I was told..

-Christopher