720D cylinders firing unevenly when not under load

kssheets · Dec 7, 2012

Bought 720D last year, using it for various things around a small farm. Never had it on a dyno, but seems to have about stock power and sounds good at all times under load, and at idle before it warms up. But after it warms up, both at fast or slow idle it sounds like it"s hitting harder on 1 cylinder. Then when governor kicks in under load, it sounds fine again. Also seems to me and my Dad who was around them growing up that it smokes more than it should when warmed up.
I don"t know the rebuild status of the pumps, fuel delivery pump, or injectors. It seems like something gets out of balance with either the pumps or injectors after it warms up. I"ve thought of several possibilities- one pump is leaking past the plunger when warm; one pump not timed properly or drifits when it"s warm, something wrong with the rack, injector warms up and plugs. It doesn"t use oil, but would it explain it if I was losing compression on a cylinder after it warms up?

Hopefully someone else has gone thru this or has suggestions for most likely causes I can track down. Thanks all.

buickanddeere · Dec 7, 2012

There is a procedure in the service manual to time and balance the pumps.
Is one side of the cylinder head, just below the port. Warmer or cooler than the other?

Mike M · Dec 7, 2012

I know when I sent my pumps and injectors off they come back marked as to # 1 & # 2 so as they match up as they were when tested on the bench. I was told that is the proper way they are to be done also. Be wary if yours aren't after you get them back.
Your tractor could have mismatched stuff in it now ?

F-I-T · Dec 7, 2012

(quoted from post at 10:25:24 12/07/12) ...after it warms up, both at fast or slow idle it sounds like it"s hitting harder on 1 cylinder. Then when governor kicks in under load, it sounds fine again. Also seems to me and my Dad who was around them growing up that it smokes more than it should when warmed up.

After making certain things like the filters are serviced, the screen in the sediment bowl is not clogged over with old diesel goo, I would check the timing on the pumps. Then I would run it, and loosen either injector line first, see if the miss goes away, then try it with the other one to confirm.

Or, the better way is to just take the injectors out to have them tested, since it is cheap and easy to do. If you don't want to make that trip yet, swap them left for right and see if the miss follows the injector.

After that, look something silly like the decompression lever riding on the valves and causing problems. The front end of the service manual has a section with good tune up information that you should go through, as I normally forget to do something.

F-I-T · Dec 7, 2012

Sorry for the double post guys. Database is responding slow today.

BTW Glenn, my 70D manual (SM-2017) does not cover synchronizing the pumps. That is in a separate diesel service book, SM-2018.

DONMYEIL · Dec 7, 2012

You are correct the nozzles are set to each pump and must stay with it. Sounds like you have injector and pump issues and the only way to fix it is pull them and have them run and calibrated. Change fuel filters and check fuel pressure .

richardinnz · Dec 7, 2012

....."It doesn"t use oil, but would it explain it if I was losing compression on a cylinder after it warms up?"
I have been caught out on this before ,no,or normal oil consumption after keeping oil-v-fuel used records for a period of 100hrs or more.
Your oil control rings may be fine, but you can get this loss of compression with out increased oil consumption.
Carry out dry and wet compression tests when the engine is hot. You won"t need to know what the correct compression should be,It is not a new tractor and there will be wear in the engine, what you are looking for is a difference in pressures between the two pots, more than around 10% between them and you need to be looking for trouble. If you get a difference you will need to carry out a wet test in case you have a compression loss through a valve or valves
Do your testing after checking the valve clearances, and pull the air inlet pipe off the inlet manifold. Test must be done engine hot.
If all is o.k there then you can turn your attention to the fuel system which may need to be done by a Diesel injection specialist knowing that the base engine is satisfactory.

kssheets · Dec 7, 2012

Thanks all for the suggestions. I"ve tried to feel for a temp difference between cylinders just to know which one wasn"t hitting hard, but couldn"t tell. Will need to bring an IR gun home and check. Will run through the pump timing and balancing, and try opening the fuel lines to see which side is weak.
I just hope it isn"t a compression issue... thanks guys

mjbrown · Dec 8, 2012

My 730D ran a little off and it turned out to be a small head gasket leak on one cylinder. Take off the radiator cap and sniff the coolant to see if it smells sooty.

cushman · Dec 8, 2012

It sounds to me that the pumps are not balanced,or matched, to each other.That it determined by the rack ajustment between the two pumps.I have adjusted them with the engine running on several occasions.I lenghten or shorten the rod untill it seems to fire evenly hard on each cylinder.It is wrather tricky to do with the rack ajusting nuts down where they are ,and the wrench can get caught opening a pump up causing a runaway, making the decompress feature a valuable unit.,

buickanddeere · Dec 8, 2012

Perhaps swapping injectors side to side would verify if it's a dribbling injector or a pump calibration issue?

F-I-T · Dec 8, 2012

(quoted from post at 14:22:58 12/08/12) Perhaps swapping injectors side to side would verify if it's a dribbling injector or a pump calibration issue?

The spec is to calibrate the pumps to be 105 cc +/- 2 cc. I cannot imagine anyone getting this close with the pumps on anything but a test stand. Glad I had Albert at Renaissance Tractor calibrate mine. Bolted them on, fired it up, standard output on my dyno first try.

720D cylinders firing unevenly when not under load

kssheets

New User

buickanddeere

Well-known Member

Mike M

Well-known Member

F-I-T

Well-known Member

F-I-T

Well-known Member

DONMYEIL

Member

richardinnz

Member

kssheets

New User

mjbrown

Member

cushman

Member

buickanddeere

Well-known Member

F-I-T

Well-known Member

Similar threads