Like the title says when the truck idles it will lope and when I'm cruising at around 45 and start to slow down it will lope a little. I also started getting worse mileage. No check engine light and the truck is at 200,000 miles.

 

How old are the injectors?

The lope is likely from the fuel pressure being too far out of range or too slow to compensate. It may be a bad fuel pressure regulator on the CP3.

Check all of the easy things first, fuel filter, air in the lines, bad fuel, etc. All of these can do weird things to the truck.

 

Mine IS doing the same thing and I just replaced the FPR

 

As far as I know the injectors are good, I just bought the truck so I dont know when injectors were done. I did just replace the fuel filter and it seemed to help go away for a day or two but it came back. I'm thinking its the fuel pressure regulator, is there anyway to tell definitely that it is the regulator? If i hooked it up to a scan tool am i going to see something that points definitively to the regulator.

 

yes. i hate to say it but it involves replacing the FPR and watching the problem go away, then you will know for sure if it was the regulator or not

with the scan tool you will want to watch desired and actual fuel pressure. they should stay close together. when they diverge too much then the ECM tries to compensate for the difference and causes over and undershoots in the fuel pressure because the regulator is not giving the ECM what it is asking for. this difference in fuel pressure causes the engine to surge or lope. you will want to watch for fluctuations or consistently low pressure

if the regulator is good (just replaced) and you still have problems then i would say the CP3 is weak or there is excessive fuel return from either the CP3 or injectors causing a drop in pressure.

 

I put a scan tool on the today and everything read fine. It still sounds like its loping, maybe i'm just hearing things. So i think Im just gonna run it through a couple of tanks of diesel with stanadyne and see if that helps it at all.

 

most duramax experts will say if you cant see it on your tac (engine RPM) dont worry about it

many have had good luck with lubricating the fuel and it helping with engine lope. you may want to look into 2-cycle oil too

 

ATF always works ( dont worry , it wond dye your filters) .. I run 1gallon to 1 qt each tank in my cummins and my old IDI ford , sure cleans up stuff quick ... gained 2 mpg alone in the Ford ... love it !!! and its cheep and easy to find anywhere ..my .02

 

I got my lb7 back together and had a lope for a while. Strange as it sounds, I think it had to do with air in the cooling system. I would open my bleeder on the tstat housing and it would idle with no lope, as soon as I tightened the bleeder it would lope again! Once I got the air bled, lope went away. I too use 2 stroke oil, 16 Oz per fill up

 

any progress either way ???