How to compression test

[BOATKID]

how to test compression - UPDATE

after all the superb advice from you guys I had another session with the genny last night :-


No spray from injector (the injector was sent away for ultrasonic cleaning, report back said everything "ok")

fuel being delivered from injector pump (although I dont know if the pressure is sufficient)

Squirted easy start down injector hole and refitted injector - engine fired - hallelujah!!!!!!

I intend to rig up pressure test for injector & pump so any ideas are more than welcome

 

[JohnTH]

If it has fired on easy start It should just go now, well, that is assuming the injector is squirting fuel.

You may like to splash a tea spoon (appx) of oil in the top of the cylinder while the injector is out. Helps compression and lubes the top ring.

73s de
Johnth

 

[BOATKID]

UPDATE

just done air pressure test on injector fired 4.5 bar compressed air into injector but nozzle did not respond. does it need to be a fluid pressure test ??

 

[savageseadog]

just done air pressure test on injector fired 4.5 bar compressed air into injector but nozzle did not respond. does it need to be a fluid pressure test ??

Nowhere near enough pressure, injector tests are done with test fluid at around 100Bar up. You can use diesel fuel but it's both flammable and a health hazard. For DIY work diesel is commonly used however. A basic test of both injector and pump can be done by pulling the injector out and recoupling it to the fuel line from the pump in mid air. Spin the engine and presto, you have some idea of what's going on. You will need to fit new seals and washers or anneal them.

NB............SAFETY Do not put your hands/finders anywhere near the high pressure fuel, the tip of the injector, obviously. It can cause serious health problems.

 

[oldharry]

Nowhere near enough pressure, injector tests are done with test fluid at around 100Bar up. You can use diesel fuel but it's both flammable and a health hazard. For DIY work diesel is commonly used however. A basic test of both injector and pump can be done by pulling the injector out and recoupling it to the fuel line from the pump in mid air. Spin the engine and presto, you have some idea of what's going on. You will need to fit new seals and washers or anneal them.

NB............SAFETY Do not put your hands/finders anywhere near the high pressure fuel, the tip of the injector, obviously. It can cause serious health problems.

Savageseadog is right, 4.5 bar is nowehere enough, and air can be compressed too much to give a proper reading even at high pressure.

The injector fires fuel at more than enough pressure to inject you with diesel, which usually gives you a bad dose of septicaemia (Blood poisoning) which can be fatal. Safe enough if you can support the injector while testing, but not just by trying to hang on to it!

But if the thing is starting and running now, why bother?

 

[BOATKID]

I think I have caused a bit of confusion. the engine fired when easy start was put down injector hole but did not run, I took injector out reconnected it upside down there was no spray when engine was cranked. I then disconected fuel high pressure pipe at injector and noticed spurts when engine was cranked although certainly did not look like 100 bar

hope this is a bit clearer

 

[savageseadog]

I think I have caused a bit of confusion. the engine fired when easy start was put down injector hole but did not run, I took injector out reconnected it upside down there was no spray when engine was cranked. I then disconected fuel high pressure pipe at injector and noticed spurts when engine was cranked although certainly did not look like 100 bar

hope this is a bit clearer

The output from an injection pump doesn't look much until it's forced through little holes. You did the right test but the lack of puffs from the injector means the pump is knackered .....................unless you forgot to bleed the fuel line. You need to slightly unscrew the fuel line/injector union and turn the engine over until the diesel spurts out of the union. Tighten up and retry the test. Could there be a chance that the injector is rusted or siezed solid??? I've not actually seen that one myself and I would have thought that the pressure is so high that it couldn't happen, but I don't actually know.

A word of warning.........

If you put a load of oil, fuel or whatever into the cylinders make sure that you turn the engine over carefully and slowly by hand when you seal up the engine again. You will hydraulically lock the engine and destroy it otherwise.

 

[JohnTH]

Yes, I agree savageseadog, not too much oil in the top of the cylinder or there will be damage to the engine.

It is starting to sound like air in the fuel lines / system BOATKID.

73s de

Johnth

 

[oldharry]

I think I have caused a bit of confusion. the engine fired when easy start was put down injector hole but did not run, I took injector out reconnected it upside down there was no spray when engine was cranked. I then disconected fuel high pressure pipe at injector and noticed spurts when engine was cranked although certainly did not look like 100 bar

hope this is a bit clearer

#


Ah, OK. Easy start will catch even at ridiculously low compression levels. The idea is to get the engine spinning awhole lot faster than the starter motor can, as the higher cranking speed will often raise the cylinder temprature enough to persuade the diesel to fire as well. if it is present at the injector.

Bear in mind that the injector pump will not create 100bar on an open injector pipe. It has to be loaded against the injector to acheive working pressure. Air in the pump or HP line will compress sufficiently to prevent it reaching the high pressure needed to fire the injector.