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Old 08-01-2021, 07:02 PM   #1
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Need Help with Low Power on 6C 8.3 Cummins

Hello all! First post. Iíve searched and read extensively on what Iím posting about, but Iím not making progress.

I just bought a 1998 Monaco Windsor with a C6 8.3 Cummins. I donít know much about itís history but I think it has been sitting a lot. It has 68k miles. The issue I am experiencing is very low power. It starts easy and idles smooth. When I put it in drive and attempt accelerate, it is very slow/sluggish. I have not been able to get it past 35 miles an hour. It is shifting fine up and down but it can only accelerate on flat or downhill. It makes very little boost, never getting over 5 psi.

What I have done so far:
- Changed primary and secondary fuel filters
- verified the exhaust brake is not stuck closed
- verified that the waste gate is not stuck / seized
- sprayed soapy water on most of the couplings with noting obviously bubbling

It had a quarter tank when I bought it which is about 25 gallons. I put 25 more gallons in it with a cetane booster additive.

I have a feeling the fuel has some algae build up I found this when I pulled the primary filter(see attachment)


Any suggestions on what to do next?
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Old 08-01-2021, 07:14 PM   #2
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Quote:
Originally Posted by cmexlr8 View Post
Hello all! First post. Iíve searched and read extensively on what Iím posting about, but Iím not making progress.

I just bought a 1998 Monaco Windsor with a C6 8.3 Cummins. I donít know much about itís history but I think it has been sitting a lot. It has 68k miles. The issue I am experiencing is very low power. It starts easy and idles smooth. When I put it in drive and attempt accelerate, it is very slow/sluggish. I have not been able to get it past 35 miles an hour. It is shifting fine up and down but it can only accelerate on flat or downhill. It makes very little boost, never getting over 5 psi.

What I have done so far:
- Changed primary and secondary fuel filters
- verified the exhaust brake is not stuck closed
- verified that the waste gate is not stuck / seized
- sprayed soapy water on most of the couplings with noting obviously bubbling

It had a quarter tank when I bought it which is about 25 gallons. I put 25 more gallons in it with a cetane booster additive.

I have a feeling the fuel has some algae build up I found this when I pulled the primary filter(see attachment)


Any suggestions on what to do next?
I would take it somewhere that could pump your tank out and refill it with good diesel. You might have to replace the filters a few times.
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Old 08-01-2021, 07:48 PM   #3
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You maybe have some luck putting some algae treatment in, centane booster ain't going to do anything for it...need something to kill and breakdown that algae....then adding some more fuel and changing filter several times. Or...See if there's a drain on tank, perhaps a pipe plug, I'd try to drain the fuel....the yuk in the filter is a red flag. Maybe other issues as well, but that's got to be dealt with 1st....the fuel flow restriction is also your low boost issue.

I have had to to this a few times - If tank ain't got a drain, you can remove primary filter, put a rag/towel around a air blow gun and pressurize the tank at the filler neck, it'll push it out. But the yuck algae will still be in there.
You'll be good to blow out the fuel lines from tank as well.

I'd also remove the secondary filter and blow those lines as well...

It'll take a couple of filter change to get it running good, so don't leave home w/o a couple!

Run it at home at 1200 or so rpm for a while, like a hour, pull the filter, see if it'll clear up before road testing...
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Old 08-01-2021, 07:51 PM   #4
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I've got the same engine. It has a third fuel filter, RH side tucked under the intake manifold transmission end. Not all have it but it's worth a look.


Have you checked/changed the air filter? Checked after the turbo for leaks?
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Old 08-01-2021, 09:26 PM   #5
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What I would do is this:
Replace Air and All Fuel filters.
Disconnect fuel supply line at lift pump and install a line going in a 5 gal fuel can full of fresh fuel.
THEN take for a short drive and see if the power improves.
This will determine if your problem is in the tank-pickup-lines or NOT.


My motorhome is a 1994 model. I've had it 11 years now. It has made 9 trips to CO and UT from Ohio. IT is used 1 month of the year. rest of the time it sits in the yard. I always Fill the fuel tanks as soon as I get home. a Full Fuel Tank has a much less chance to grow algae since it grows on the surface. I can't tell you how many years it has been since I've put a new fuel filter on it. (I do keep New filters with me in the tool box. )
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Old 08-01-2021, 09:53 PM   #6
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Quote:
Originally Posted by Mekanic View Post
What I would do is this:
Replace Air and All Fuel filters.
Disconnect fuel supply line at lift pump and install a line going in a 5 gal fuel can full of fresh fuel.
THEN take for a short drive and see if the power improves.
This will determine if your problem is in the tank-pickup-lines or NOT.
Yep, good idea as well.
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Old 08-02-2021, 06:41 AM   #7
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When I first bought my coach it was in Florida, a repo, no idea how long it sat and/or any maintenance history. I did take it to a Cummins shop for a service and inspection to give me a base line to start from.

Used it on and off for 5 months before taking it on a long trip I pulled the primary filter and the top was covered with algae, much worse then yours. At that point I started using Biobor, that kills the algae, and Startron, which is an enzyme treatment that breaks down the dead algae.



In late 2015 we took a long trip. Got home and I filled the tank and used the Biobor and Startron and parked the rig. I then started clearing a lot and building a new house, doing a lot of the work myself. Took me until late 2019 to finish (I will never ever build another house myself).

I would occasionally move/exercise the coach but no trips. In early 2020 I started pumping fuel out of the tank in anticipation of actually using it. Then the pandemic (say no more). I did refill the tank, pumping in 85 gallons of fuel back in, added the biobor and startron again. Did a full service on the rig and changed fuel filters, >>>> no algae. Just got back from a 4700 mile trip with no real issues as to engine performance.
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Old 08-02-2021, 09:43 AM   #8
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Thank you for all of the suggestions. I will work on the algae problem and report back. The suggestion to test with a fresh tank of diesel with fresh filters had crossed my mind - thank you for the suggestion.
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Old 08-02-2021, 12:57 PM   #9
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I've had the same issue a couple of times and both were related to air. Once the filter was clogged with ice/snow and collapsed the rubber intake boot on the engine. The other time was after some maintenance I had lost one of the clamps on the silicone hose between the turbo and the intake. This hose kept blowing off and it wouldn't allow the turbo to generate boost.

I'd check all of your intake piping/plumbing and see if there's any restriction.
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Old 08-06-2021, 12:18 PM   #10
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I am just now being able to devote some more troubleshooting time. I pulled off the fuel cutoff solenoid and tied open the fuel cutoff and started the engine. I let it warm up and did a couple test takeoff's in my driveway and there was no change. Another test I did was to leave the park brake set and put it in drive. The RPM's barely move and no boost builds. It is odd to me that the engine won't even attempt to push against the brake.
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Old 08-06-2021, 12:50 PM   #11
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I also just verified that with the ignition key on, when the accelerator pedal is pressed down all the way, the throttle is fully open at the engine.
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Old 08-06-2021, 01:20 PM   #12
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Im sure you've probably already checked, but have you looked for a restriction before the air filter? I had to add a mesh screen to my body intake grille to keep the critters out.
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Old 08-06-2021, 01:23 PM   #13
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Yes. I actually have the elbow upstream of the air filter completely removed so I can see in the air filter clearly. It is a little dirty but I don't think it is bad enough to cause these problems.
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Old 08-06-2021, 02:55 PM   #14
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Quote:
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What I would do is this:
Replace Air and All Fuel filters.
Disconnect fuel supply line at lift pump and install a line going in a 5 gal fuel can full of fresh fuel.
THEN take for a short drive and see if the power improves.
This will determine if your problem is in the tank-pickup-lines or NOT.
Do I also need to run the return to the temporary fuel can? Also, connecting it this way bypasses the primary filter - is that ok?
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