Skip to main content ↑ ↓
Topic: C8.3 Idle Question (Read 901 times) previous topic - next topic

C8.3 Idle Question

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!
Jeff & Sandy
1997 American Eagle 40' Slide-less
2013 Toyota FJ Cruiser 4X4 Toad
Interested in Foretravel Coaches

Re: C8.3 Idle Question

Reply #1 –
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.
Peter & Beth Martin
No Forrest? What have you done?
MC# 15890 until Dec 2016; FMCA #F329677
Cincinnati, OH

Re: C8.3 Idle Question

Reply #2 –
Ooooooops!  The drop is 120, not twelve hundred...the other numbers were correct
Jeff & Sandy
1997 American Eagle 40' Slide-less
2013 Toyota FJ Cruiser 4X4 Toad
Interested in Foretravel Coaches

Re: C8.3 Idle Question

Reply #3 –
That is about the RPM drop when I put my 8.3 in gear.
Mark Freeman 1995 grand villa, Brimson Mn
8.3 Cummins  G95 4709 40 WTBS U280 WE21

Re: C8.3 Idle Question

Reply #4 –
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.
Peter & Beth Martin
No Forrest? What have you done?
MC# 15890 until Dec 2016; FMCA #F329677
Cincinnati, OH

Re: C8.3 Idle Question

Reply #5 –
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.
Jeff & Sandy
1997 American Eagle 40' Slide-less
2013 Toyota FJ Cruiser 4X4 Toad
Interested in Foretravel Coaches

Re: C8.3 Idle Question

Reply #6 –
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
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: C8.3 Idle Question

Reply #7 –
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.
1998 U270 34'
 

Re: C8.3 Idle Question

Reply #8 –
Quote from: Pierce & Gaylie Stewart –
Make sure the service brakes and parking brake will keep the coach stationary when cold and pointed downhill after increasing the idle speed.

 
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 :)
Jeff & Sandy
1997 American Eagle 40' Slide-less
2013 Toyota FJ Cruiser 4X4 Toad
Interested in Foretravel Coaches