Cummins 5.9 12 valve mechanical expert
 

Looking for help with my 1994 RV motor home with the Cummins 5.9 12 valve mechanical engine. Having issues after driving for 2hrs or more , going up even small hills.. RPM,s drop as I go up from 2200-2000-1750-1500.Few seconds later RPM goes back up then down again. OK on level roads and going down hill. Would like to talk to some and give them more info . I was at INDY for the big go and towed a enclosed trailer. Thanks.

Re: Cummins 5.9 12 valve mechanical expert
 

Double check your fuel filter. You could have gotten some algae growth in your tanks that is clogging up the filter and restricting the fuel flow. They are such simple and solid engines that few things will affect them like this, but this is one of them.

I had an almost identical situation years ago with a 1997 12 valve RV. It was algae clogging up the fuel filter.

Re: Cummins 5.9 12 valve mechanical expert
 

KRatcliff, thanks for the reply. I assume that the Algae problem flares up after I drive for a few hours and the fuel heats up , then I put more of a load on the filtration going up the hills. No issues on level ground or down hill.

Re: Cummins 5.9 12 valve mechanical expert
 

It may be that or the filter is just letting enough fuel to flow that you don't notice the lack of power on level ground. The heat may make a difference, but checking the fuel filter would be the first place I would look.

Re: Cummins 5.9 12 valve mechanical expert
 

I agree regarding filters. It can be some bad fuel plugging things up as well. It may be more of an 8.3 liter Cummins thing, but I did have issues with the fuel shutoff solenoid. The solenoid was fluttering and basically shutting the fuel pump on then off. An easy test is to disable it and try it again.

Re: Cummins 5.9 12 valve mechanical expert
 

Filters filters.. hopefully not the lift pump. If by chance you put fuel in mixed with biofuel there is a very good chance you knocked some debris loose for your filters. Bio is an extreme detergent and older vehicles hate it. If you have a algae issue ( starts at the refinery tanks) you will need to dump tanks and clean them. Very hard to get rid of. I haven’t seen an additive for it. Hopefully you just took on a bad batch of fuel. We changed 2 filters from Salt Lake to Bay Area with bad fuel once. We had to leave the diagnostic tool plugged in and clear under way.
Hope you get it worked out.
Find a good hot rod cummins guy close to you. Boys are sharp!!

Re: Cummins 5.9 12 valve mechanical expert
 

Thanks guys for all of the information. Will let everyone know what I find out.

Re: Cummins 5.9 12 valve mechanical expert
 

Do you have a boost gauge? If the boost is down first thing to check is fuel filter. On one trip I drove all night with very low boost checked turbo hose for damage could find any thing. We stop for fuel in early am was looking at engine attendant come up and ask what we were doing told him about our problem with lack of boost. He told us to change filter luckily I alway card a couple. I changed it and were started out and instantly we had full boost.

Re: Cummins 5.9 12 valve mechanical expert
 

Changed both filters . Both were black . Put a fuel pressure gauge on it . Took it out for a test. Cut them, open ad small black dots in the At 2000 had 28 PSI , when I got on it the pressure dropped to 15 . When I got off it cam e back up to 28. Now changing the overflow valve ,and see it that solves it. IF not, then the lift pump. Any other ideas. Thanks.