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wlrdlr · 05-24-2023, 07:10 AM

I have a cat c-7 engine in my tow vehicle. It has gone into LIMP mode w/no dtc,s on my scanner. Please help if you have any ideas. THNX

Jim_HiTek · 05-25-2023, 09:35 AM

A clogged fuel filter will cause a Cat engine to throttle back so it's hard to get up speed. So you'll be limping. I couldn't get my Cat 330 HP 3126b (the father of the C7) above 25 MPH for 30 odd miles on a hilly highway. It had been behaving oddly for many miles before that. It slowly dropped down to 25 MPH and then sort of stayed there. I initially thought it was the hills as it would speed back up on the downslope.

Then when the road flattened out I could tell it wasn't getting up to speed, but I was able to inch it up to 45 MPH. Drove straight to a diesel truck shop, they changed out the water/fuel separator filter, and I was on my way at full speed.

So have that checked.

wlrdlr · 05-25-2023, 10:33 AM

I will check that.
Thanks Jim

Iceclimber · 05-27-2023, 05:13 PM

Any gotchas or best practices to swapping that out ? Mine has roughly 5k miles on it but this thread scared me.

Jim_HiTek · 05-27-2023, 05:42 PM

Some say to fill it with fuel others say it's not necessary but I'd say check your users manual first, then set it up so you'll do it all at once. On my 3126b, I pre-filled the time I changed it myself.

Have the pre-filled filter ready if you decide to go that route, a dump bucket to pour the old fuel and filter into, and have at it, all at once. Mine is in an odd place and with my knees and poor eyesight lately I'm happy to pay a tech to do it for me. I usually carry one with me but this last time I'd forgotten to buy one. Many many miles later, boom. Limp mode.

BTW, I've since put 2220 miles on it since the new filter was installed and the Cat purred like a kitten the entire time so I don't think limp mode harms anything...it prevents harm.

2004Horizon · 05-27-2023, 05:52 PM

Your water separator has an electric probe that this "normally open" which means it will trigger or complete the circuit when water is present, which almost never-ever!

If the probe is bad you don't have to swap out water-separator switches to test it. Just disconnect the wire. Also, I doubt the engine will limp along if water is ever detected so this is not the problem.

My old C7 experienced odd problems and even engine stalls. It turned out to the the Transmisson Position Sensor (TPS) under the fuel pedal, which is just a big a rheostat, whicih is a variable resistor which is used to control current.

If your vehicle has one, and many do not, then it's possible the electrical lead is frayed or it's all gummed up. Look and see. If you don't have one, "nevermind." It was just a shot in the dark anyway.

Do you have any trouble codes to refer to?

gatorcq · 06-04-2023, 07:44 PM

Did the Check Engine Light come on? The only way Limp mode comes on is when a Check Engine Light comes on.

If power is degrades as you describe, then properly it is a mechanical issues.

Turbo, Turbo Clamps, filter, etc