Cummins revs randomly ;)

[Tomon4winds]

Ahoy! I have a cummins motor, she wasn't running when I got here and we found the injection pump was not working, we got it serviced. Now she's running but she kept revving up for a second or two before going back to a comfortable idle. Air? Bad injectors? She doesn't struggle, just revs up! What's going on?

[jimbunyard]

It's generally called hunting. Which engine? Does it do it regularly and constantly, or just at random; revving up to a specific point and then dropping back to a steady idle?


How much have you run it after getting it going? Sometimes these kind of things work themselves out after a bit...

[Bruce K]

Mechanical or electronic injector control?

[Tomon4winds]

Originally Posted by jimbunyard

It's generally called hunting. Which engine? Does it do it regularly and constantly, or just at random; revving up to a specific point and then dropping back to a steady idle?


How much have you run it after getting it going? Sometimes these kind of things work themselves out after a bit...

Haven't run it much, I think I'll try cracking the line at the injectors while it's running to see if any air pops out... it's a 6bt a. Thanks, I love it when things just sort themselves out after a while!

[Tomon4winds]

Originally Posted by Bruce K

Mechanical or electronic injector control?

Mechanical, there is a stop solenoid and a lift pump in the line. It's a walbro pump and beats like a heart I guess it's pressure controlled?

[ranger58sb]

Originally Posted by Tomon4winds

Mechanical, there is a stop solenoid and a lift pump in the line. It's a walbro pump and beats like a heart I guess it's pressure controlled?

Fuel filters replaced during that service? If not, a clog?

If so... what density? Too dense a filter (combination) could maybe impact the lift pump's ability to provide a steady flow...

As far as I know, the Cummins recommendation for 6BTAs is still probably 30 and 10 for primary and secondary filters.

-Chris

[mvmojo]

Had a similar problem. More than likely you've got a small air leak somewhere before the Walbro pump such that it's sucking a little air into the line. Mount the pump as close to your tank(s) as you can so that the fuel to the on-engine fuel pump is pressurized. With the engine turned off, wrap a strip of paper towel around every connection from the pump to the engine and run the pump overnight. You'll probably find one of the paper towel strips damp with diesel the next day.

[svthreecheers]

Originally Posted by Tomon4winds

Ahoy! I have a cummins motor, she wasn't running when I got here and we found the injection pump was not working, we got it serviced. Now she's running but she kept revving up for a second or two before going back to a comfortable idle. Air? Bad injectors? She doesn't struggle, just revs up! What's going on?

Check your Turbo Charger for oil seal problem on shaft bearings
Engine will run faster on the leaking oil in idle and under no load
Cheers
Sig

[Bruce K]

Originally Posted by Tomon4winds

Mechanical, there is a stop solenoid and a lift pump in the line. It's a walbro pump and beats like a heart I guess it's pressure controlled?

Possible a bit of wear in the pivots of the governor, is this is the case, the hunting wil often stop when the engine warms up. Sometimes a bit of sticky dried oil will make guvernor jump speed about, usually resolves itself in time.

[a64pilot]

I think we need to know what are the RPM differences, Cause he says it revs up, makes me think more than hunting?
Hunting to me is more of an unstable idle, 100 maybe 200 RPM

[Tomon4winds]

I made a video but can't post, she's running up, not sure how much cos rev counter not working but it's quite big, happening every minute or so, sounds just like someone's revving it up...

[Taipe]

I don't what you mean by "beats like zzz heart". That could mean regular or irregular. Let's drop that analogy. Your title for this thread says "random", some answers to these following questions might clarify whether ongoing fuel injection problems or burned valves/low compression. I'm not familiar with that engine, but assuming it Does not have common rail injection:

Does it have a 2-speed injector pump?

Does it rev up only at operating temperature?

When ideling, does the normal diesel knock occur regularly?

Have you pulled the injectors and had them tested?

[a64pilot]

Originally Posted by Tomon4winds

I made a video but can't post, she's running up, not sure how much cos rev counter not working but it's quite big, happening every minute or so, sounds just like someone's revving it up...

To me that’s not what I call hunting, hunting is like a DD rack out of adjustment, a steady, rhythmic rump, rump, rump, RPM varying only slightly.

[a64pilot]

This is I think actually just a sound file of a cold engine hunting until it warms up slightly.
https://youtu.be/XviBswlMUFw

[Diesel Bill]

A Cummin's 6-BTA with mechanical P pump is one of the most reliable of all types of injection pumps. It's called the million mile pump for a reason. More than likely as MVMOJO says you have a small air leak somewhere in the line between tank and pump.
My Perkins 4-236M port side engine was warming up a few weeks ago and started surging RPMs . Turned out to be poor connection in fuel line. The lift pump was sucking air and causing surging till engine ran out of fuel. It can be a challenge to find it.
Good luck.