89 cummins rough idle on grease

can cut it with diesel and make it better, but straight grease will idle fine until i drive it then idles rough. i purge it then back on diesel idles fine, return to grease and it idles great until i drive it, runs great on grease or diesel

I too have a 2001 Cummins in

I too have a 2001 Cummins in my Dodge pick up. I have learned the hard way that what you are describeing is the start of the high pressure fuel pump or the fuel filter on the veggie side is starting to plug up. First off change the filter and see if any improvement. If not take your cummins engine to a GOOD diesel mechanic to check out the vains in the high pressure fuel pump. I have had to replace my HP fuel pump twicw since I had my system installed. $780.00 a pop. What I do now is to run some diesel treatment through and also cut 50/50 between the last 5 -7 galloons in the veggie side to help clear things out.

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