Surging? Burst of power??

[morgankshaw]

So cruising the idi 7.3 ats turbo non wastegated zf5. Occasionally I get what feels like a surge of power or a feeling of i don't know like some one was giving me an extra push. Usually seems to happen in 3rd gear between 2500-3000 rpm. I'm not moving the pedal more than say 3/4 and it feels like someone is stepping on the gas well at least that's the sensation. The pedal doesn't move.
It's a noticeable power increase too. I'm thinking the ip is kicking out more fuel, or a leaky injector starts working better i don't know. Because the truck seems to run good and even. Just trying to figure out what's going on.
Also it's not like a run away motor. If I let off the pedal she slows right down just like normal. But the thought has crossed my mind if it could run away on me.....

Has anyone else experienced this?


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[bbjordan]

Yup. That is the classic symptom of air intrusion on the suction side of the fuel pump.

 

[Thewespaul]

Air intrusion x2
Put a clear line between your fuel filter and IP to see any air in the fuel

 

[79jasper]

Yep

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[catbird7]

Mine had same symptom and air leak originated at the fuel selector valve. Try switching tanks and see if you get same results.

 

[morgankshaw]

But it doesn't start hard and it only happens occasionally. I'll check it out


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[morgankshaw]

If I got rid of the intrusion, would that "burst of power" be permanent?


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[catbird7]

Mine only did this during cold weather and only following hard acceleration or during a long climb. I'm guessing the burst of power is due to a "lean" condition which is bad for engine. Correct the air leak and surging should stop.

 

[DaveBen]

Actually diesel engines do not run better lean. Only gas engines do this, right before they burn a hole in a piston. Diesel engines run better when over fueled, again just before smoking the engine.

 

[Thewespaul]

I have an afr gauge on my truck and at idle the truck sits at more than 20:1 that's why diesels are so efficient, they require very little fuel to run

 

[catbird7]

Curious what causes it..... It was impressive on my truck, felt almost like doubling the HP! Figured it had to be bad for the engine. The first couple times it happened to me, I continued driving pulling fuel from same tank and eventually the filter was empty and engine shutoff.

 

[Thewespaul]

When you are getting the burst of power it's probably not the air getting into the engine but when the injector finally gets a full pop of fuel. When there is air in the line it will compress the air and cause the injector to fire later than it should, making it perform poorly along with not getting the full ccs of fuel

 

[morgankshaw]

Ok so find the leak! I'm assuming it could be coming from the switch. I vacuum capped the second tank ports when I installed a 38 gallon rear tank and eliminated the side tank. Should I put a clear line from fuel filter to the ip?


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[AcIdBuRn02ZTS]

From my understanding, the sudden burst of power you feel when air hits the IP is from the timing advance system of the pump. When small air bubbles hit the pump, the timing advance pretty well pegs giving that "burst of power" feeling.

To find your air leak, I'd start by putting a small piece of 3/8 clear tubing between the lift pump and the hard line. There is already a rubber line there... so just replace it with clear tubing from a hardware store and watch for air bubbles.

If you dont see any while running, then the problem is more then likely at one of the olive seals at the ends of the hard supply lines (between the lift pump/filter head or between the filter head/IP)

 

[79jasper]

When you are getting the burst of power it's probably not the air getting into the engine but when the injector finally gets a full pop of fuel. When there is air in the line it will compress the air and cause the injector to fire later than it should, making it perform poorly along with not getting the full ccs of fuel

Not quite. Technically the air isn't compressible. That's why they're so hard to start if you do get air in the injector lines.

The reason for the burst of power is the lack of fuel in the IP is similar to the timing advance being activated.
I can't find my manual.

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