Community Forums

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

jhanan · 06-12-2023, 07:46 PM

Fuel filter and fresh engine oil for the cat heui and injectors. Always fill the fuel filter to the top before installing

Shad034 · 06-26-2023, 03:24 PM

Posting potentially for posterity, as the OP has probably fixed this by now.

I JUST went through a similar situation couple of weeks ago. I was headed up to Southern UT from NV for the first trip of the year. Coach felt fine pulling out of the driveway, or at least I thought it did. It's always hard to judge when I haven't had it out in some months.

Rolling along at freeway speed until about 60 miles into the trip when the first decent grade occurs, about a 6%. I usually expect to drop speed down to 45-55 up the ascent to control temps and not run things too hard. This time I found myself getting put rapidly down to 45 and then to 35 and having a hard time getting even that much. That, I felt was pretty odd but it has been a while since I drove..maybe I forgot this hill speed.

However, things came into sharp focus when I pulled off the freeway in St George to go through town on the way to the highway that would take me to my campsite up in Pine Valley. Sitting at the off ramp light waiting for my green. Green comes, push the go pedal and coach no go.

It tried to go, but RPM would not climb, turbo wouldn't spin up and I cleared that intersection at about 5 mph but it did start getting going after some rolling. Found similar behavior at the other two lights that I encountered. I just could not get any RPM out of the motor without putting pedal to the floor and just waiting for things to slowly start creeping up. Not good.

However, once rolling, it seemed to be OK-ish. I noted that on the climb into the hills, I could get about 25 MPH where I was pretty sure I usually held myself down to 45 by choice previously.

Where it really stunned me was once I got to the campground and had to make a slow turn right and go up an incline in the campground to my loop, I got 600 RPM and 5 MPH with the pedal to the floor. No RPM rise, no response to pedal, just pushing up the hill at basically idle and maybe a tail wind.

Landed it in the campsite and figured I now had a week to figure it out or plan to have it towed out of the site. Either way, at least I made it here. A bit of research (this site and everywhere else I could) said that its probably injector issues, a HEUI pump, turbo wastegate stuck open, exhaust brake stuck closed or something similar. Most of these guesses being under the assumption that you have already changed your fuel filter, because who would need to be told that, right?

Well, me apparently. My new-to-me coach had fuel filters (primary and secondary) with 8/2021 written on them and I figured that is probably fine given the amount of miles I put on yearly (which is maybe a couple of thousand). I figured filters is something that I could do something about here and now, so down into town I went with the wifes car to source some new filters. Swapping those out in the campsite was a bit of a black-op, but results were very educational.

I will say though, all the of the horror stories about having the fuel system drain itself back into the tank or lose prime in other ways had me a bit paranoid. I filled the filters (from the outside holes, not the inside center) prior to putting them up there and I did as quick a change as I could between the old and new. I'll never be sure if I needed to be that cautious, but I'll say that the CAT fired right up for me.

While the coach still wasn't quite running right, felt like it was "missing", it would respond to pedal input and RPM/turbo would ramp much more normally. I did manage to get it home at the end of the week, definitely down on power, but nothing like it was coming up with it starving for fuel.

I have it in the shop now, and as expected, I have two dead injectors that certainly did not help the situation. I'm not sure if the injectors were dead (or maybe 1 was and the 2nd is when I actually felt it) or if the filter starvation contributed to their demise, but I can say with certainty that fuel filters will definitely contribute to this lack of power issue.

jhanan · 07-04-2023, 04:03 PM

Replace fuel filter and put in fresh oil as the HEUI uses engine oil to run the injectors - must do both

FIRE UP · 07-11-2023, 11:00 AM

Quote:

Originally Posted by 2004Horizon

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?

Water-separator has an electric probe??????????

May I ask, WHAT ELECTRIC PROBE? And are you talking about the clear plastic bowl on the bottom of the fuel filter that's by the radiator on about 99.99999% of the C-7's out in RV land? If so, I've changed my one and only fuel filter and water separator oh, maybe 20+ times in the 11 years we've owned our '04 Horizon 36GD with the CAT C-7 330HP and there NO electric probe in that water separator on that filter. There is no electrical lines anywhere near that filter/separator.

Now, I also DON'T have the secondary fuel filter that hangs off the side of the engine at the top like some do. But, I've not heard of any electrical probes in any of those secondary filter/separators either. Can you explain?
Scott

2004Horizon · 07-12-2023, 07:10 PM

The 2nd 10-micron filter has a Water In Fuel (WIF) sensor built into it. It's that black thing in the picture below.

The clear glass bowl is a 20-micon filter and does not.

wolfe10 · 07-12-2023, 07:23 PM

With two fuel filters:

Primary filter (first one from tank) is usually the water separator and has a coarse filter element (usually between 10 and 30 micron).

The secondary filter is the fine one (2 micron on your Caterpillar engine).

I am a STRONG believer in using the Caterpillar Corp secondary filter.

Primary-- use whatever the OE brand of primary filter is-- Fleetguard, Racor, etc.