Good Morning from Lake Conroe,
Based upon my reading (and always enjoying) this forum, I ordered and replaced the original P7100 injection pump overflow return valve on our 1997 Cummins C8.3 engine. I did so after reading that if the overflow return valve spring was weak it could cause a few different problems, including rough idling, by returning too much diesel to the fuel tank. A little history...several months ago, I began having to put my transmission in neutral at stop lights as the RPMs would drop noticeably low to about 500 - 600 RPMs and also oscillate between 500 and 650. In neutral they would be a bit more stable at about 700. After installing the new valve, which was very easy taking about 15 minutes total, my engine now idles very smoothly - a noticeable difference / improvement. However, when I put the transmission into drive, the RPMs drop by about 1200. I simply cannot recall how much the RPMs SHOULD drop if everything is working properly. It seems that a drop of more than 1000 RPMs may indicate my old faithful C8.3 needs another adjustment and or repair??? Thank you!
Please clarify the amount of rpm drop from neutral to drive. it would seem that if you idle at 700 RPM And when you engage the transmission into drive the drop is 1000 RPM you would be running at -300 RPM.
Ooooooops! The drop is 120, not twelve hundred...the other numbers were correct
That is about the RPM drop when I put my 8.3 in gear.
I replaced the throttle return springs a few years back. Use 2 of these springs. This will ensure that the throttle is returning to the idle set point. Otherwise there is a special tool to adjust the throttle speed on our Cummins mechanical engines. I would let Cummins set the idle speed. By the time you find the special tool and pay for it, you may be time & money ahead. This idle speed set point should not need adjustment any time soon once it is set to the correct setting.
UPDATE: I found directions on-line, went to my rv storage and raised the idle to 750-775rpm AND IT NOW idles perfectly when placed into gear. When placed into gear the RPMs only dropped by about 100. Bottom line, if anyone with a C8.3 experiences rough idling despite fuel filter changes and routine maintenance, replacing the Bosch P7100 fuel injection pump "overflow return valve" is an inexpensive possible fix at less than $40 dollars.
Make sure the service brakes and parking brake will keep the coach stationary when cold and pointed downhill after increasing the idle speed.
A good two micron secondary filter will keep the check valve working well longer. I have used crocus cloth to clean up the little ball. Toothpaste on a pencil will clean up the seat.
Pierce
As noted, it's very easy to adjust the idle. It's even in the cummins owners manual. No special tools required. Just a wrench for the jam nut.
THANK YOU for that suggestion! I am going to go drive it today so can change the engine oil while the weather is mild and sunny, so will test that today :)