injector pop?

1bad93

1bad93

that guy
Joined
Aug 10, 2010
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how you all doing, i have i question on injector pop. ive heard about it a few time and sorta understand the basics of what it is, but i dont fully understand. heres why i bring it up. well about a week ago i put some ladder bars on my truck and went to test them out alittle and on a hard 4th then to 5th gear pull once i let off the throttle it shut down. then fired it back up and it was missing and carrying on till i bumped the throttle a couple times then everything was fine. well i figure the low pressure piston pump finally met its match so i replaced it for a airdog 165 and i thought all would be good. well i did it again a yesterday. this time was a hard 3rd to 4th gear pull. now i figure this is happening because of how quickly im letting off the throttle but im asking if anyone heres might know somthing i dont?
thanks jon.
 
1bad93 said:
now i figure this is happening because of how quickly im letting off the throttle
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Exactly.

Here's a VERY common issue with the CTD running a VE injection-pump:
With increasing the fuel (and hopefully the air), the turbo(s) provide more charge-air to the cylinders. The general combustion related pressures in the cylinders go up.

Now imagine this:
- You've got your bombed VE spinning that poor little turbo to over 40psig of boost in a WOT run next to your buddy from the stoplight.
- Out of the corner of your eye, you see the unmistakeable shadow of that Dash-Cam in the unmarked HP cruiser sitting in that side road you just passed.
- You pretty-much side-step the throttle, sharply cutting the fuel to idle.

-> At that point, till the boost dies down, the charge-air cylinder pressures are still close to the peak value of WOT while the fuel pressure at the injector nozzle dropped dramatically.

** It's at that point that the cylinder pressure will literally push gasses into the injector body thus leaving it "Air-Bound". Until that air can be Burped out by normal operation or having to crack the injector lines to let it out, that injector will fail to operate.

Not all injectors will air-bind the same as the one next to it. It just depends on your engine. With a mild case of it, one or two injectors will miss till you rev the engine to clear the air. In a worse scenario, most of the injectors will bind such that when you first get off the gas, the engine will wind down and simply stop running. Attempting to restart it only has the engine weakly hit once in a while at best. You have to get out and crack the injector lines, burping out the air, just like if you ran it completely out of fuel.
(You don't see this happen often with those newer truck running IPs that sport MUCH higher injection pressures compared to the VE).
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Learn to roll off the throttle, Jon. ;)
 
VP trucks are really bad about this too...or ones with 8x0.014's are anyways
 
I just don't see cylinder pressure overcoming injection pressure even in a ve. This happens on VP trucks as well and our pop pressure is higher than a p-pump which this doesn't happen to.

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BC847 said:
Exactly.



Learn to roll off the throttle, Jon. ;)
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That makes alot of sence, I figured that was what was accuring. Just didnt full grasp what was exacly happing. Thanks alot guys for clearing that up.
Jon
 
Cylinder pressure higher than injection pressure? I guess I do not understand how some people come to these conclusions. Exhaust gas entering the nozzle back through the spray orifices is common and known as soot blowback, this is caused by the needle hanging in the guide.

I had this issue with a FASS on a VP truck, switched to an A1000 and the problem was resolved, did the Aeromotive pump increase injection pressure, or was the fuel supply just inadequate before? Seems like a pretty easy question to answer.
 
Smokem said:
Cylinder pressure higher than injection pressure? I guess I do not understand how some people come to these conclusions. Exhaust gas entering the nozzle back through the spray orifices is common and known as soot blowback, this is caused by the needle hanging in the guide.
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Why would the needle hang in the guide only when letting off the fuel quickly from WOT? Just wanting to learn.

I'm running a Walbro 392 with it's IP feed regulated downstream of the filter to 12psig. It might drop to 11psig at WOT.
 
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Letting off the fuel quickly does not cause the needle to hang. Debris or loss of lubricity can, and also if the needle is not seated when assembled.
 
Smokem said:
Letting off the fuel quickly does not cause the needle to hang. Debris or loss of lubricity can, and also if the needle is not seated when assembled.
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The truck runs just fine in any other scenario. Well filtered fuel, Stanadyne Performance Formula, Injectors were disassembled, ultrasonically cleaned and had Pop-pressure set to with in 50psi, well over a year ago. Decent mpgs of around 18 driving like one ought to on the highway, Just shy of 480/900ish on the dyno, and 12.67 in the quarter @ 6400lbs.

At the top of that quarter, if you sidestep the throttle, the engine dies as a result of most of the injectors being air-bound. If you roll off the throttle, all's well.

What's up with that?
 
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We so no pressure drop with the FASS, I just feel it was not enough volume. The truck would flat die/vapor lock after a hard pull, many had an opinion or a reason, some did not believe it, but the new setup resolved it. I tore the injectors down and there was no soot blowback on the needles, this was many years ago.
 
My 4bt did the exact same thing with the ve pump and my old ddp comp injectors when I barked the turbos above 40psi boost. My pod injectors would not do that ever. I've barked it at 75 psi with no issues.
 
BC847 said:
The truck runs just fine in any other scenario. Well filtered fuel, Stanadyne Performance Formula, Injectors were disassembled, ultrasonically cleaned and had Pop-pressure set to with in 50psi, well over a year ago. Decent mpgs of around 18 driving like one ought to on the highway, Just shy of 480/900ish on the dyno, and 12.67 in the quarter @ 6400lbs.

At the top of that quarter, if you sidestep the throttle, the engine dies as a result of most of the injectors being air-bound. If you roll off the throttle, all's well.

What's up with that?
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Its the pump, not the injectors. If you crack the lines and crank it over your bleeding air out of the pump.

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How does the air get in the pump? And why would it only be a very small amount of air? It's not like the entire high-pressure line is full of air. Often, I can crack an effected injector and it will bubble just a bit on its own around the fuel line nut. Tightening it right back up has enough of the air out that the engine will fire up, albeit rough running till the rest clears out.

I am VERY confident it's not coming from the pump. ;)

If it were the pump, it seems all the injectors would be air-bound, not just one or two in a mild case of the problem. I'm not sure how snapping the throttle down would introduce air in the IP.


- I'm not trying to be argumentative. Just trying to consider alternate causes and their validity. ;)
 
If its the injectors, why don't p-pumps do it?
The problem is, rotary pumps do not pump air very well so when you drain the rotor on a hard pull they can't recover. P7100's don't have the same issue because they're a more positive pump and can work the air through.

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Draining the rotor would only occur if I'm starving the rotor of fuel. I'm not draining the rotor.

If such were the case, it seems the problem would occur after every WOT pass regardless if one snapped the throttle off or rolled off the throttle.

Correct?

I have no 1st hand experience with any of the later Cummins IPs, so I can't comment on those.
 
Smokem said:
Cylinder pressure reversion through the nozzle seems valid to you?
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Yes.

Haven't come to a better conclusion at this point.

It seems that perhaps the rolling off of the fuel allows the cylinder pressures to come down a little before the fuel injection pressure drops completely. Dunno.
 
12mm VE and 14mm VE, both fed with an Aeromotive A1000, never had the issue you describe. It concerns me that anyone would think that the cylinder pressure is greater than injection pressure, ever.
 
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