mpg issues on my truck

[sc5952]

Let me tell you what I have to start. 1998 Volvo with condo sleeper, 500 hp N14 Cummins, 18 speed. It goes 70 mph at 1520 rpm or so. We pull flats and haul 75-80k gross frequently. My problem is no matter how my driver drives it 4.5 mpg is all we can achieve. I know not going to be 7 mpg but should be at least in 5's. Any secrets? Only problem right now besides mpg with truck is thermostat is stuck open and only getting 165 max under load. Would that cause more fuel consumption? I am to witts end on this. Something has to give. I already treated for algae. Fully services with overheads ran. Please help!

[Chunker]

A stuck open thermostat that doesn't let the engine reach the optimal operating temp will cause the engine temp sensors to read a cold engine and cause the fuel system to run rich.

[sc5952]

So for sure will possibly be my problem. I know cooler it has been outside worse it has gotten. Thanks!

[Musicman]

A stuck open thermostat that doesn't let the engine reach the optimal operating temp will cause the engine temp sensors to read a cold engine and cause the fuel system to run rich.

Possibly, but not guaranteed. My 2006 ISX had the thermostat stuck open for a few months before I fixed it and I never saw a change in fuel economy. Still, it's a problem that should be fixed. Sounds to me like there's something else going wrong though.

[sc5952]

Im to the point where I'm open to anything. What mpg you think it should get under average driver? Only 478k actual miles.

[crazyeyes]

Do you trust your driver??? I have heard storys how drivers selling fuel.

[sc5952]

Its gotten same with me driving. This is my best, most trust worthy driver.

[Musicman]

Im to the point where I'm open to anything. What mpg you think it should get under average driver? Only 478k actual miles.

Have you had your charge air cooler checked? Any leak in the turbo system (most common in the charge air cooler) will kill your fuel economy.

Are you accurately calculating your economy? Meaning that your odometer is accurate and you are dividing miles on the tank by the gallons it takes at the next fill-up?

How much idling does the truck do?

[sc5952]

It idles under conditions where heat or ac needed while sleeping, otherwise it's off while off duty. The odometer is correct and calculate my actual miles divided by actual gallons. I will check air charge lines tomorrow before he takes off for the week. Thanks for that idea.

[Chunker]

The best way I've seen the CAC checked is disconnecting one side, plugging the other and pressurising it to see if it leaks down in the cooler, much like you would a coolant system. For a visual check, making sure none of the clamps are loose or broken is the best you can do.

[firebird_1252]

go to a dealer and get the CAC checked. mine was cracked and it made a huge diffrence with the mpg. my mbn cat climbing hills running 65-70 has been anywhere from 6.25-6.5.. before that it was 5.5-6 mpg

[rank]

well......we used to have a 1995 model year 435hp N14 (Celect) and I could only manage 5 mpg driving 60 to 65 mph on flat terrain, so I'm not really surprised you're getting 4.5 doing 70. We now have a 2008 14L ISX and it gets even worse mileage.

Idling will kill your mileage pretty good also.

What makes you think something is wrong? Did you used to get better mileage with this truck?

[sc5952]

Previous owner got 7.3-7.5 so he claimed. Onboard stuff agreed but you can reset it and manipulate that. i just want to get in 5's, nothing huge. My driver did inform me this morning that when he initially took off he thought he heard air leak on of air charge system but once he got rolling couldn't hear. When it gets back in shop I'll thoroughly check it.

[rank]

Previous owner got 7.3-7.5 so he claimed.

I don't think I buy that. They must have been awful light loads. I'm driving an M11 Cummins, which is a 10.7L engine vs the 14L of the N14, and I only get 6.75 - 7.0. IIRC, I lost ~.75 when I put the new lugged drives on. You have 30% more cubic inches than I do.

[sc5952]

Thats why I put the he claimed part, lol. I don't believe it either but positive more there than 4.5 mpg. He did haul light van loads he said.

[Musicman]

Previous owner got 7.3-7.5 so he claimed. Onboard stuff agreed but you can reset it and manipulate that. i just want to get in 5's, nothing huge. My driver did inform me this morning that when he initially took off he thought he heard air leak on of air charge system but once he got rolling couldn't hear. When it gets back in shop I'll thoroughly check it.

You can't easily reset or manipulate the ECM. Have you looked at a print out of what the ECM itself shows for miles and fuel consumption? I'm not talking about what the in-dash fuel computer shows, I mean take the truck to the dealer and have them print out everything logged in the ECM... there's a big difference.

[sc5952]

I put new computer in (well Volvo did) when I bought truck. Previous owner I believe hooked charger up to it backwards and killed it. Guy had sitting because heart problems. Has about 30k miles since then. However when I took in shop to have thermostats changed I told them to pressure check air charge cooler and it has lots of little leaks not noticeable when not under boost, I ordered new air charge cooler that showed up little while ago. Hopefully with correctly working thermostats and sealed intercooler it will get at least 5 mpg, would be nice to get a lot more but would be happy with 5. Only time and miles will tell.

[Musicman]

Hopefully with correctly working thermostats and sealed intercooler it will get at least 5 mpg, would be nice to get a lot more but would be happy with 5. Only time and miles will tell.

Keep us posted. I'll be curious to see the results for sure.

[sc5952]

Well I got thermostats in and was going to put air charge cooler in but they sent wrong one so I sent truck down the road the way it was. So far after 1500 miles or so it seems to be .50 mpg better but hard to tell since not many miles. He has filled twice (wasn't closed to empty). First fill he got 4.75 mpg, but he sat idling for like 10.5-11 hours on that tank. He filled in Fountain CO today and was 5.27 mpg sitting 7-8 hours idling. Keep in mind it was getting 4.5 mpg religously. When he gets back tomorrow night I am putting new correct air charge cooler in (well actually will be Saturday when I swap). Hopefully gets little better yet, only time and open road will tell true story of this beast. Highly considering putting APU on it after first of year. How much fuel you think 500 hp n14 Cummins will burn idling?

[solo379]

How much fuel you think 500 hp n14 Cummins will burn idling?

Anywhere between 1-1.5 gallons per hour, depending on a load. ECM shows that wrong. For heat, i'd recommend Espar. Will keep you warm 24 hours for about a 1 gallon. Pays for itself in a less than one year at today fuel cost.