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Old 06-20-2020, 10:21 PM   #1
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Purchasing an RV with ISX engine

Looking at purchasing a 2004 Monaco Signature with the ISX 525hp. I'm a little worried about the ISX dropping valve issue reported on the forum. Is it a problem worth my trouble or should I move on? If I purchase, is it worth it to have a new head and EGR delete done (electronically)?

Thanks
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Old 06-20-2020, 10:51 PM   #2
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We have owned two coaches with the Cummins ISX 525hp engine. Both have been exceptional.
How many miles are on the coach?
An EGR system delete may be an option. Some may disagree, but it has worked well for others.
So far the ISX 525 engine has been reliable. I don't think Cummins even offers an updated cylinder head for this engine.
I installed the Ugly Fix on both of ours and that has worked well. It tells the engine ECU that the coach is above 7000' and locks out the EGR.
https://www.theuglyfix.com/Cummins-I...duct-s/123.htm

The big problem engines are the 2008-2010 ISX 650hp twin cam versions. Some folks have had very expensive issues caused by the EGR system recirculating exhaust gases containing acid and the acid has compromised the #6 intake valve. An RV owner with this engine should consider the new head replacement and/or a complete EGR delete. Plan on spending $15-25k on the new head and associated parts and labor.

The ISX 600hp engine may get by with just the Ugly Fix or possibly consider an EGR system delete.

I would personally purchase a coach with either the 525 or 600hp ISX engine. I would hesitate purchasing a coach with the 650hp ISX unless I could budget $25k for the updated cylinder head with Inconel stainless valves and special seats.
Hope this helps.
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Old 06-21-2020, 05:41 AM   #3
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What Vito said. When anyone states ISX I have to ask if it is a CM870 or a CM871. In your case a 2004 should have a CM870 Using the engine serial number on Cummins Quick Serve will tell you what the engine is.

The CM870 with 2003-2006 emissions (EGR only) is relatively bulletproof.
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Old 06-21-2020, 09:39 AM   #4
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The coach has just under 50k miles...

Is there anything I should look for in particular on the engine to see if something may mechanically go wrong?

Is there any negatives that affect the engine by using the ugly fix? (I hear EGT temps go higher due to the EGR not being used).

I have found a couple ISX 525 dropping valves on the forum. Were those just isolated incidents?

And why are the valve drops occurring at stop lights and at idle? All the posts I'm reading show its not dropping at the time of heavy use.

Thanks for all your help!!
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Old 06-21-2020, 11:16 AM   #5
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I would have an oil analysis done on both the engine and the transmission. Also have the engine coolant checked.
You could take it to Cummins for a check up. They always find a few things.
You yourself should check the condition of the belts and hoses and also look for oil leaks.

Your issues are probably not going to be with the engine. I would set aside $10-20k for updating and repairs. That's if you are willing to do some or most of the work.
Most of us have had to rebuild the engine driven hydraulic pump.
The Sauer Danfoss cooling system electronic-hydraulic fan controller is probably bad. This will cause the twin cooling fans to default to full rpm all the time.
I would bet the drivers side slide out locks have been neglected and are almost locked up.
One or more of the original A/C systems are probably bad or will go within the next few years. The thermostats are probably dim.
The roof fiberglass deteriorates and will need coating with Rustoleum deck paint.
The TV's will need updating.
If it still has a Norcold refrigerator, it will need to be replaced.
The Aqua Hot system will need servicing.
The interior lights will need updating to LEDs.
The three water tank quantity sensors (fresh, gray, and black) will need replacing.

Not trying to scare you just trying to tell you the things we have had to address. Most are not show stoppers and can be fun projects.
Let us know if you need more info?
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Old 06-21-2020, 11:26 AM   #6
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As far as why the valve drops are occurring at idle I think it's the temperature. When the engine is working hard it is very hot and the acids are immediately burned off. When it idles, acid is at it's highest point. But remember, these issues are with the ISX 650HP twin cam engine. There are literally hundreds of thousands of lower horsepower ISX engines out there in all sorts of RV's, trucks, and machinery that are doing well.
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Old 06-21-2020, 03:32 PM   #7
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Vito.a, thanks for all of your input! I am not afraid of any of the projects you described. I enjoy that kind of small time work. I can't rebuild a diesel engine, so that's the scare I've had. If the ugly fix will lessen my chances of failure then I'll spend the $150. This RV has been stored indoors, and has tons of service records. I plan on replacing that norcold anyway because they suck! I feel better about the engine since you have posted, so I will see what cummins has to say.

Regards
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