Starting Issues…Fuel system?

[MotherTrucker]

I have a 2008 de-mandated ISX. Just hit a million miles, previous owner did an out of frame on it at about 750k. Bought it in Jan, and have been “getting it right” since. It’s almost there! I took it to Mr Hag about 6 weeks ago and had him go through the tune (and rebuild the jakes and do an overhead). It’s always been a little sluggish to start, although not to the point that anyone but me really notices. Just takes a second or two longer than I feel like it should. On cold start the rpm’s will vary 25-50 a few times until it starts to warm up (noticeable on gauge at low idle, less at high idle). Every now and then when I turn the key it will act like it’s starting for a second, then quit…have to turn the key off and try again (this has happened maybe 4 times since Jan). Within the last week, it has given me the orange check engine light 3 times on startup for 3-5 seconds after firing up, then it goes away. I haven’t had the code read, but will try to get that done this week. I had the check valve replaced about 2 months ago. Last thing to note…my fuel gauge is wonky. Will go from full to 3/4 after 50 miles, to 5/8 after 100-150 miles, 1/2 at about 400 miles (tanks are 75 gal tanks, and I generally get around 6 mpg). Below 1/2 it’s fairly accurate. If I stop driving and let it idle, the gauge will go up to a more accurate reading of fuel left vs miles driven. The truck drives well, has plenty of power, haven’t noticed any lack of power or surging while driving, but my fuel mileage does seem to be more variable than I would expect. For example I got 7.3mpg yesterday through Washington with a 14k load on, through MT and ND today was only 5.7, but it was windy. So…my question is, are all these things related? Obviously the starting issue is my biggest concern, but any advice is appreciated

[Rawze]

could just be the starter getting weak. That is a common issue with them when they have more than 500k miles on the starter.

[Nilao]

My Guage tends to fluctuate as well. My sending unit is in my rrivers tank. It's the crown of the road. Some of the driver tank fuel goes to the passenger tank from gravity. The passe ger tank is lower than the drivers on most highways when cruising in the right lane. Once parked and idling the tanks equalize once on level ground causing the level reasing to be correct again.

The hard/slow start could be the gear fuel pump. It has to be able to get the psi up to a specific amount before it will fire. You need to get insite and watch the measured fuel psi during cranking. The rpm fluctuation cold could be this same problem or it could be the actuators are worn out allowing unmetered fuel to be injected.

The cranking issue I am presuming to be the starter engages then just quits cranking if I'm understanding what your trying to say...that could be the ignition switch, relay, solenoid or starter TBH.

Fuel.mileage is going to be dependent on several factors. You being number one, and second equipment condition/specs. Take mine for example. I know I most likely have a bad injector and a leaking actuator as I have a noticeable miss at cold start and around 1300 rpm I have a vibration like a misfiring injector. I need to put it down and fix it but it's not in the cards right now. But if I was to fix those issues I could probably pick up a bit of fuel mileage.

[Mrkentee]

Also check battery cables. I just replaced all of them on my 2010, and it made a big difference in cranking.

[JMBT]

I am voting “weak starter” on the cranking issue as well. It happened to mine at about 700k.

The sluggish start makes me think a bit of air in the system? Like a loose fuel filter or separator?
Or maybe your lift pump is getting weak? Cycle the key once or twice for a minute or so each time before you crank it and see if it’s still sluggish at the start.

Everything else seems pretty normal to me…

[MotherTrucker]

Thanks for all the replies! I’m going to keep running it until it gets worse, or I get back over towards one of my trusted mechanics! I’m a worrier, so anything that isn’t quite right makes me nervous, and this is my first ISX, so I’m still getting used to what’s “normal”