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Discussion Starter #1 (Edited)
Aight, I figure I'll give the ol' forums one last go before I commit arson-based insurance fraud to rid myself of my demon-possessed 99 7.3.

Symptoms: Most of these only apply when the truck is cold. After driving 5 miles or so it improves noticeably, but still lacks power.



  • Idles well. Has a misfire under load (such as low RPMs in overdrive) until it warms up, at which point the miss disappears.
  • Low power pretty consistently, better when warm.
  • Absolutely DUMPS black fuel smoke when cold.
  • EGTs stupid high
  • Fuel economy is garbage. Drinks fuel like no tomorrow.
Tests Performed:


  • No fault codes stored in ECU
  • Buzz test passes
  • CCT shows different results every time I run it
  • All 8 injectors spit oil as expected with valve cover off. Some blowby visible under valve covers. Nothing extreme.
  • Visible blowby from filler neck, but no pressure behind it. Doesn't blow off filler cap or ping pong ball.
  • Fuel in filter bowl clean, not contaminated by oil.
All this adds up to "overfueling, injector issue" in my head. Please share your thoughts.

Every time I look at the truck something breaks. I've been trying to get it to run well enough to sell for months now. Most recent development is that the flexplate cracked sitting in a parking lot. I hate this thing with a holy passion, and I'm tired of working on it. I refuse to spend another dollar on it that isn't absolutely necessary, so replacing all injectors is not an option. I'd like to know the best way to pinpoint which injector is the problem so that I can throw a junkyard injector in it long enough to make it someone else's problem.

Thanks for the help, assuming anyone responds.
 

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All of your symptoms could be explained by lack of boost. Check the turbo for play and do a leak test.


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Discussion Starter #3
All of your symptoms could be explained by lack of boost. Check the turbo for play and do a leak test.


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Will do, that's been on my to-do list for a while. Boost gauge shows 20ish PSI at WOT. Forgive my ignorance, how would a boost issue disappear as the motor heats up?
 

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Have you checked the operation of the Exhaust Back Pressure valve? If it's stuck shut it could definitely cause high EGT's and additional fuel consumption. The PCM is supposed to open the valve when you mash the go-pedal, but if it's getting a false signal (shorted wire applying full voltage to the control solenoid, for instance) the valve would stay shut. Could be as simple as a biased EBP sensor. Try unplugging the EBP sensor and see what happens.
The EBP system stays active until Engine Oil Temp reaches 140F (IIRC).
 

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Discussion Starter #5
Have you checked the operation of the Exhaust Back Pressure valve? If it's stuck shut it could definitely cause high EGT's and additional fuel consumption. The PCM is supposed to open the valve when you mash the go-pedal, but if it's getting a false signal (shorted wire applying full voltage to the control solenoid, for instance) the valve would stay shut. Could be as simple as a biased EBP sensor. Try unplugging the EBP sensor and see what happens.
The EBP system stays active until Engine Oil Temp reaches 140F (IIRC).
EBP sensor isn't connected to anything and the connector at the pedestal is unplugged. The bus motor had a freeze plug at the manifold where the EBP sensor connects on the
Powerstroke motors, and I couldn't get it out to save my life. EBPV valve is confirmed wide open.
 

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I suppose the turbo could be sluggish with cold oil. How many miles are on the injectors? Worn poppets can cause trouble when cold.
 

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Discussion Starter #7
I suppose the turbo could be sluggish with cold oil. How many miles are on the injectors? Worn poppets can cause trouble when cold.
113K, but no knowledge of maintenance history, only a reasonable expectation that the schoolbus donor for the motor was well maintained as part of a government fleet. Turbski seems fine, no play and builds good boost.
 

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Did you use the injector out of the school bus? They are probably single shots. Your stock tune is set up for split shot injectors. Did you look at the injector code on the donor motor?
 

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Discussion Starter #9
Did you use the injector out of the school bus? They are probably single shots. Your stock tune is set up for split shot injectors. Did you look at the injector code on the donor motor?
Bus injectors are ADs. Truck is tuned for ABs, but they are both split shots and can be run on the same tune.
 

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Discussion Starter #10
I got some numbers today. ICP reads 3000ish PSI at WOT, duty cycle around 50% which seems a little on the low side. Symptoms were the same as always today, but I didn't drive it enough to get past the stage of operation where it misses, which would have been good for comparing numbers. Between the oil leak and the fact that the flexplate sounds like it's plotting the worst possible time to shatter into a million pieces, driving it is an anxiety inducing experience.
 

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Discussion Starter #11
Bump, still trying to figure this out. Yesterday I started the truck dead cold. Idled fine, ran rough in drive as per usual. Mashed brake. Mashed throttle. No effect. RPM didn't increase, no pull against the brakes, EGT didn't move. So now we're going back and forth between too much fuel and not enough.

Still need to get a fuel pressure gauge.
 
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