# JD 7420 mystery



## rbwiseman (Feb 24, 2016)

I am helping my neighbor who is a dairy farmer debug a problem with his John Deere 7420. The tractor will start ok and run normally but for a very short time, 5-30 seconds. Then it starts running rough like missing on one or more cylinders. Here's the odd part; if the engine is turned off and then restarted, it will run normally and can be used for chores until it is intentionally shut off. If it is restarted within a short time, up to a couple hours (this time limit is not known exactly), it will start and run normally. However, if it sets for several hours or overnight, the problem shows up again.

Last summer all of the fuel lines were replaced. After this the tractor ran normally and was used extensively for all of the fall work. This winter the tractor would not start. After replacing fuel filters and the fuel pump, the tractor would start but began having this issue with running roughly until restarted.

Our thinking at this point is that the problem is not in the fuel system proper i.e., pumps, lines, filters, etc. Since it is hard to understand how stopping and restarting the engine would change the condition of any of these. So, we are left with electrical and/or computer causes. There are no error codes. Any insight or suggestions would be welcomed.


----------



## BigT (Sep 15, 2014)

Howdy Bob,

Good to have new visitors to the tractor forum. Welcome.

I suspect that you have a small suction leak that allows air into the fuel system when the tractor is left overnight. I do not know why turning the engine off and restarting clears the problem. Maybe modern technology???

Next time the engine runs rough, leave the engine run and crack open injection line connections at the injectors one at a time to purge any air at the injectors. 

My neighbor's Kubota Tractor would get into a funk (start missing) like you are describing, and purging at the injectors as described above would cure the problem for a month or more. We never did find the source of air entry.


----------



## rbwiseman (Feb 24, 2016)

Thanks for the quick reply. We suspected air leak into the system but couldn't reconcile that with the re-starting the engine effect. We haven't tried bleeding the injector lines while the engine is running so we will give that a try. If that works and we can hopefully find the source of the air leak, then we will just write off the re-start issue to good ol' Murphy.


----------



## Fedup (Feb 25, 2014)

The 7420 was supplied with more than one fuel system. By the parts book there is a Mexican version which shows either a Lucas or a Stanadyne pump, and a worldwide version with an electronically controlled system. The symptoms you describe don't exactly fit common problems with mechanical pumps. You should first establish which system your friend's tractor is equipped with before deciding on a troubleshooting strategy.


----------

