RPM Drop Test

GordMay · 02-01-2005, 05:13

A mechanic passed this tip along to me - but I’ve never tried it.
Does anyone have any experience with an RPM Drop Test?

“Drop Testing” a Diesel Engine:

Start the engine & warm it up. With the warmed-up engine idling at between 800 & 1000 rpm:

1. Loosen the first fuel line connector to an injector (carefully - there’s VERY high pressure here), disrupting fuel flow to that cylinder. Record the level to which the RPM falls for cylinder number 1.
2. Repeat this in turn for each cylinder, one at a time.
3. When all cylinders have been tested, look at the numbers recorded. They should all have a similar (very close to the same) RPM drop.
4. If one cylinder has significantly less RPM drop than the rest, it may have fuel injection, or compression problems.

CSY Man · 02-01-2005, 07:13

Hmm, good idea.
Will try it next time I am playing with the engine.

delmarrey · 02-01-2005, 11:54

Use to do the same for gas engines by pulling the spark plug wire. But it was to check ignition and compression, sometimes carboration. Now it's all done with computers.

Diesels are much eaiser to troubleshoot! Except that in the last few years, diesel manufactures have been adding electronics for monitoring, compression brakeing (trucks) and turbo packages.

wingsofraven · 16-01-2005, 08:46

This is similar to isolating a cylinder on a gasoline engine by removing the spark to the spark plug. You can also identify a connecting rod or piston wall noise by isolating each cylinder, an old diagnosis trick!

utspak · 25-01-2005, 08:55

Yes, it works very well. I do it a little different. I run the rpms up to 1500 or so. Then when you back off the fuel you will hear the difference. If you do not hear a drop in rpm then there is a problem.

Greg B · 26-01-2005, 15:00

I'd like to try this myself. I have perkins engines. The injector pump and injectors are equally accessible but I presume the fitting at the injector is the one that should be cracked loose. How loose? I've never done this b-4 but I imagine fuel spraying all over. How bad will that be?

Greg

delmarrey · 26-01-2005, 22:38

If you have absorbant pads you can cut it into strips and wrap it around the injector just under the fitting nut. And then have another piece to put over the wrench while loosening to keep it from spraying on yourself, that's if it's going to. Rags work too but don't absorb as well. You can also do the procedure at the pump end of the pump feed lines.

You crack the nut just enough to make it leak real good and relieve the pressure to the injector. But don't unscrew it all the way. 1/2 to maybe one turn should be enough.

DO NOT shut off the engine with the nut loose or it may draw air back in the system. And then you'll have to bleed the air out. But that's the same as relieving the pressure................................._/)

JGI417 · 05-03-2005, 14:51

when backing of a line to a ejector and the is no or very little rpm drop that is the problem ejector, veiw marinesurvey,com a lot of good info

02-01-2005, 05:13	#1
GordMay Registered User Join Date: Mar 2003 Location: Thunder Bay, Ontario - 48-29N x 89-20W Boat: C.L.O.D. (Cruiser Living On Dirt) Posts: 12,576	RPM Drop Test A mechanic passed this tip along to me - but I’ve never tried it. Does anyone have any experience with an RPM Drop Test? “Drop Testing” a Diesel Engine: Start the engine & warm it up. With the warmed-up engine idling at between 800 & 1000 rpm: 1. Loosen the first fuel line connector to an injector (carefully - there’s VERY high pressure here), disrupting fuel flow to that cylinder. Record the level to which the RPM falls for cylinder number 1. 2. Repeat this in turn for each cylinder, one at a time. 3. When all cylinders have been tested, look at the numbers recorded. They should all have a similar (very close to the same) RPM drop. 4. If one cylinder has significantly less RPM drop than the rest, it may have fuel injection, or compression problems. __________________ Gord May ~~_/)_~~ (Gord & Maggie - "Southbound") "If you didn't have time/$ to do it right in the first place, when will you get the time/$ to fix it?"

26-01-2005, 15:00	#6
Greg B Registered User Join Date: Jul 2004 Location: Lake Michigan Posts: 85	So how do you do this? I'd like to try this myself. I have perkins engines. The injector pump and injectors are equally accessible but I presume the fitting at the injector is the one that should be cracked loose. How loose? I've never done this b-4 but I imagine fuel spraying all over. How bad will that be? Greg

05-03-2005, 14:51	#8
JGI417 Registered User Join Date: Mar 2005 Location: Tulsa OK Posts: 59	when backing of a line to a ejector and the is no or very little rpm drop that is the problem ejector, veiw marinesurvey,com a lot of good info __________________ John

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02-01-2005, 07:13	#2
CSY Man Registered User Join Date: Aug 2003 Location: Ft. Lauderdale Boat: CSY 33 Cutter, "Rhapsody" Posts: 1,902	Hmm, good idea. Will try it next time I am playing with the engine.

02-01-2005, 11:54	#3
delmarrey Registered User Join Date: Mar 2003 Location: Puget Sound, WA Boat: Choate 40 Posts: 3,921	Use to do the same for gas engines by pulling the spark plug wire. But it was to check ignition and compression, sometimes carboration. Now it's all done with computers. Diesels are much eaiser to troubleshoot! Except that in the last few years, diesel manufactures have been adding electronics for monitoring, compression brakeing (trucks) and turbo packages.

16-01-2005, 08:46	#4
wingsofraven Registered User Join Date: Jan 2005 Posts: 2	This is similar to isolating a cylinder on a gasoline engine by removing the spark to the spark plug. You can also identify a connecting rod or piston wall noise by isolating each cylinder, an old diagnosis trick!

25-01-2005, 08:55	#5
utspak Registered User Join Date: Jan 2005 Posts: 1	Yes, it works very well. I do it a little different. I run the rpms up to 1500 or so. Then when you back off the fuel you will hear the difference. If you do not hear a drop in rpm then there is a problem.

26-01-2005, 22:38	#7
delmarrey Registered User Join Date: Mar 2003 Location: Puget Sound, WA Boat: Choate 40 Posts: 3,921	If you have absorbant pads you can cut it into strips and wrap it around the injector just under the fitting nut. And then have another piece to put over the wrench while loosening to keep it from spraying on yourself, that's if it's going to. Rags work too but don't absorb as well. You can also do the procedure at the pump end of the pump feed lines. You crack the nut just enough to make it leak real good and relieve the pressure to the injector. But don't unscrew it all the way. 1/2 to maybe one turn should be enough. DO NOT shut off the engine with the nut loose or it may draw air back in the system. And then you'll have to bleed the air out. But that's the same as relieving the pressure................................._/)