Teach me about rail pressure

H

Hoiyay

New member
Mechanic, but gasser...not diesel smart.

Issue: Ive been trying to hunt down an intermittently stuck open injector. Today I spent a couple hours disabling each injector and going for a ride around an empty parking lot.

Surprisingly it didn't matter which injector was unplugged, it would just stick open and roll black smoke and essentially stall and have no power (was still running, but no power and still blowing smoke).

I started looking at rail pressures. I sit at 7700-8000 rail pressure at idle, which I've been told is a bit too high...I need 5500-6000 at idle I've been told.

When the motor smokes out and dies, I sit around 11-12,000 rail pressure at a dead stop idle. If I try to give it has, it will bump to 14-15,000 but the truck won't respond at all...won't have power, will smoke a little more and that's all.

So I don't think I have an injector problem, but a problem causing high rail pressure (which I was told could stick an injector at idle).


So how does the rail pressure build/release?

Obviously the cp3 builds the pressure that gets sent to the rail. I'm assuming there's a regulator or relief valve somewhere on the rail?

I know there's the FCA that is ecm controlled that is on the back of the cp3...is this also the overflow/relief valve?

What would cause high pressure at idle, and cause rail pressure to stick?


Obd 2 isn't working right now, in the process of dealing with that too.

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There is a prv which you can check to see if it is working,

I would check the feed tubes first though (since that is free and relatively easy).
 
ramthiscummins said:
There is a prv which you can check to see if it is working,

I would check the feed tubes first though (since that is free and relatively easy).
Click to expand...

What do I need to check on them?

Engine was just rebuilt, with brand new bosch injectors AND feed tubes. I torqued them exactly to spec according to the book. Was looking today and my torque wrench may have been a little under torque (I used it on something else today).
 
ramthiscummins said:
There is a prv which you can check to see if it is working,

I would check the feed tubes first though (since that is free and relatively easy).
Click to expand...

Where is the pressure relief valve and how does it function? If its just functions mechanically, then I would assume it would only relieve maximum pressure, which I dont think is an issue for me since it doesn't get stuck at max, its getting stuck at 11-12kish
 
Hoiyay said:
Where is the pressure relief valve and how does it function? If its just functions mechanically, then I would assume it would only relieve maximum pressure, which I dont think is an issue for me since it doesn't get stuck at max, its getting stuck at 11-12kish
Click to expand...
It's on the rail, under the return line, next to the rail sensor.
It's mechanical.

Mark.
 
12kpsi won't stick an injector at idle. Best bet is to come up with a plug to remove one injector feed line at a time
 
JWhite said:
12kpsi won't stick an injector at idle. Best bet is to come up with a plug to remove one injector feed line at a time
Click to expand...
That's what I've done though. It seems as if it's an electrical issue. Runs good down the road, come to a stop and it does it sometimes. Turn truck off, turn back on 5 seconds later and it's cleared out.

Did it with ever injector disconnected.


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Could also be your fuel rail pressure sensor. I chased an issue for quite a while that ended up being a bad pigtail leading into the sensor plus the actual sensor. Backprobe the sensor and see what you have for voltage. Do you have the ability to monitor commanded vs. actual fuel rail pressure? Mine was intermittent and then got stuck at 17k psi.
 
Hoiyay said:
That's what I've done though. It seems as if it's an electrical issue. Runs good down the road, come to a stop and it does it sometimes. Turn truck off, turn back on 5 seconds later and it's cleared out.

Did it with ever injector disconnected.


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Click to expand...

Are you electrically disconnecting the injectors or removing the fuel line and capping the rail?
 
Tobin said:
Are you electrically disconnecting the injectors or removing the fuel line and capping the rail?
Click to expand...
Right now just electrically. Since it's not just stuck open I'm just disconnecting it electronically.

It's either not working, or wide fking open it seems. I dont see how capping off the fuel rail would prevent fuel from getting in cylinder since it's not leaking. But I'm here to learn.

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xDiesel03 said:
Could also be your fuel rail pressure sensor. I chased an issue for quite a while that ended up being a bad pigtail leading into the sensor plus the actual sensor. Backprobe the sensor and see what you have for voltage. Do you have the ability to monitor commanded vs. actual fuel rail pressure? Mine was intermittent and then got stuck at 17k psi.
Click to expand...
I don't have a tuner and obd2 is dead for now.

I could back probe somewhere, but I wouldn't know how to interpret the volts into "asked for" rail pressure

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Hoiyay said:
That's what I've done though. It seems as if it's an electrical issue. Runs good down the road, come to a stop and it does it sometimes. Turn truck off, turn back on 5 seconds later and it's cleared out.

Did it with ever injector disconnected.


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Click to expand...

Hoiyay said:
Right now just electrically. Since it's not just stuck open I'm just disconnecting it electronically.

It's either not working, or wide fking open it seems. I dont see how capping off the fuel rail would prevent fuel from getting in cylinder since it's not leaking. But I'm here to learn.

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Click to expand...

You need to cap off the rail to isolate the injector from pressure. This 100% proves whether it's the culprit.

And on the rail pressure sensor, it's 0-5v with .5v being zero and 4.5v being max (so probably around 26k)
 
JWhite said:
You need to cap off the rail to isolate the injector from pressure. This 100% proves whether it's the culprit.



And on the rail pressure sensor, it's 0-5v with .5v being zero and 4.5v being max (so probably around 26k)
Click to expand...
I can cap off the rail. But educate me how that would make a difference please. If injector is "dead"...unplugged, it's shut closed. Why would it matter if it's got pressure behind it? I can see if it was just leaking a little, but this is dumping.

Got it on rail pressure! Thanks.

Where should I measured for what the ecm is asking for? At the fca?

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Hoiyay said:
I can cap off the rail. But educate me how that would make a difference please. If injector is "dead"...unplugged, it's shut closed. Why would it matter if it's got pressure behind it? I can see if it was just leaking a little, but this is dumping.

Got it on rail pressure! Thanks.

Where should I measured for what the ecm is asking for? At the fca?

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Click to expand...

If everything works like it should, you'd be right. But the part that actually seals the fuel while the solenoid is de energized might be problematic.

You'd be best to watch desired pressure on a scanner, not any straight forward way to measure that with a volt meter on the engine.
 
JWhite said:
If everything works like it should, you'd be right. But the part that actually seals the fuel while the solenoid is de energized might be problematic.



You'd be best to watch desired pressure on a scanner, not any straight forward way to measure that with a volt meter on the engine.
Click to expand...
Got it.

Injectordirect already gave me an RMA for the injectors. Going to send in and have them tested since I have to wait on my cluster to get here anyways. Hopefully that fixes the obd2.

I replaced fca and rail pressure sensor...one at a time. Not a single change.

When changing out rail sensor....I noticed bubbles in there. Not sure If it from opening rail pressure or making them.

Checked all over, no leaks. Even at high idle.

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Isolating the injector by capping the rail removes its pressure source so that no fuel can access it.
Isolating it electrically only eliminates the solenoid and armature, but does not disable the valving inside the injector.
Generally speaking, the armature on an injector isn't powerful enough to meter the fuel directly when at operating pressure. The solution is to use pilot operation, a smaller quantity of fuel at lower pressure, in order to shift the pintle open. This action is what actually meters the fuel used for combustion.
An injector can malfunction mechanically and still meter fuel when it isn't being electrified. Isolating the suspect injector is the only way to be certain it isn't metering.



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Not sure if this was mentioned but do you have a rail pressure gauge?
I would not recommend getting one as they can mess up the wiring if not done correctly. If you have one check the wires and check the connections at the rail.
 
Hoiyay said:
What do I need to check on them?

Engine was just rebuilt, with brand new bosch injectors AND feed tubes. I torqued them exactly to spec according to the book. Was looking today and my torque wrench may have been a little under torque (I used it on something else today).
Click to expand...


so you are saying you leave your toque wrench torqued up after use?
 
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