General diesel questions.....

Well, this may sound nooby, but all my time as a mechanic I've worked on approx 3 diesels, mostly oil changes and service crap.

I know some of the dangers involved working on them like high pressure fluid injection.

But, how does one avoid this? on a common rail diesel engine, they run approx 30k PSI, when you shut it off, does the pressure drop?

What if you need to remove a fuel line when servicing an injector, is there a proper way to bleed pressure? run with the lift pump off? just crack the line with a rag around it? what are the actual dangers of high pressure fluid injection? is it common? are modern diesels DIY'able? 

TLDR- how to work on 30,000 PSI fuel systems.

Since most modern diesel engines you'll encounter in the US are German and answers will vary a lot depending on make and model, I would start here. 

I’ve heard you open the filler cap to depressurize the tank then crack open the line at the filter and slowly bleed off the pressure and make sure to prepare for the leaking fuel with a container and plenty of rags.

This is NOT advice, just what I heard! Your safety is paramount so don't trust me!

I have always cracked a line at the injector in question, but that was on older systems. That's also how I usually bleed the systems when they had air in them. I don't see why you couldn't do the same on a common rail system. The pressures are pretty high, but the volume is low. It's not like you will get an explosion of diesel. Wrap a rag around the fitting while you crack it. 

Maybe I've just been lucky, but in the multiple times that I pulled the injectors and rails off my OM642, I just loosed the injector hard lines and pulled them off.  No real drama, just a little bit of fuel to mop up.

wae said:

Maybe I've just been lucky, but in the multiple times that I pulled the injectors and rails off my OM642, I just loosed the injector hard lines and pulled them off.  No real drama, just a little bit of fuel to mop up.

This has been my experience on vp44/cp3 equipped 5.9/6.7 Cummins as well. Of course I make sure it’s not just shut off and break the line loose and break the line loose as fast I can either...

Ranger50 said:

wae said:

Maybe I've just been lucky, but in the multiple times that I pulled the injectors and rails off my OM642, I just loosed the injector hard lines and pulled them off.  No real drama, just a little bit of fuel to mop up.

This has been my experience on vp44/cp3 equipped 5.9/6.7 Cummins as well. Of course I make sure it’s not just shut off and break the line loose and break the line loose as fast I can either...

I think you would find that it because they all run a traditional style injector pump with a fuel shut off solenoid to kill the motor. Modern common rail diesels are a whole different animal. Most will have a high pressure relief valve to allow pressure to be bled off. Or with a scanner you could kill the in tank pump and let the system run itself down.

I haven’t worked on much common rail stuff, one or two tdi’s but I’m sure it’s just like any system. Read up be safe and you’ll be fine.

They hold pressure for a while.  Depending on the diesel in question it might be a few hours or a few days.

But... fluid doesn't compress.  As soon as you break the seal on 30k psi fuel line, it might spit a drop or two of fuel before its at 0 psi.  Last time I did Duramax injectors, they want the engine warm when you do it, so I shut it down and tore into it.  Aside from a slight mess, I had no massive explosions of fuel.

GM techs have a tool (and likely other brands as well) that will open all of the injectors to bleed off the pressure, but they don't usually use it.

1SlowVW said:

Ranger50 said:

wae said:

Maybe I've just been lucky, but in the multiple times that I pulled the injectors and rails off my OM642, I just loosed the injector hard lines and pulled them off.  No real drama, just a little bit of fuel to mop up.

This has been my experience on vp44/cp3 equipped 5.9/6.7 Cummins as well. Of course I make sure it’s not just shut off and break the line loose and break the line loose as fast I can either...

I think you would find that it because they all run a traditional style injector pump with a fuel shut off solenoid to kill the motor. Modern common rail diesels are a whole different animal. Most will have a high pressure relief valve to allow pressure to be bled off. Or with a scanner you could kill the in tank pump and let the system run itself down.

This works on some diesels, but not recommended.  The high pressure pump on some diesels (24v cummins for instance and some Dmax) is very sensitive to operating without adequate flow/pressure from the lift pump.  It makes sense to just kill the lift pump and run it until it dies, but that could cause the injector pump to gall and go belly up.

The low-tech way to do it would be to disable the injector pump (take the belt off) and crank the engine over for two rotations.  That way you're sure all the injectors have fired and dropped most of the pressure, but it's not spinning the injector pump so it can't damage it.

I've always just used the drain at the filter to relieve pressure. But it's always been full size trucks, not sure that's an option on modern diesel cars 

In reply to Curtis :

That does sound a lot smarter than running a system dry that uses fuel as a lubricant.

Wear safety glasses and crack the lines with your sensitive bits covered.  You run a greater risk of a sore back from your wife making you sleep downstairs because you smell like diesel fuel, than you do of injecting diesel fuel into your gizzard.

Most newer HPCR diesels have a pressure relief nut, which diverts the spray downward and away from you. It's a tiny little spray, but if you open the wrong fitting first with your hand in the way, you could possibly get ulta-fine fuel particles shot through your skin, tearing through blood vessels and tendons sometimes without noticeably breaking the skin. You can even lose a finger or hand from the rapidly accelerated infection caused by the tiny particles now dispersed throughout and embedded deep inside your tissue.

https://emedicine.medscape.com/article/1241999-overview#a8