A friend asked me to look at his NTC300 because of low power, even after he changed the pump-mounted fuel filter. I added some gauges to measure fuel supply restriction at the filter, and rail pressure at the pump, and boost pressure. Sure enough the rail pressure seemed low, only reaching about 130 psi maximum. I thought it should be up around 175 or so. Fuel supply restriction was around 5 in-Hg, so I believe we're OK there. Boost was low, but I think that is from the limited fueling going on. There is no exhaust haze at all under load. I removed the #27 button and installed a #22, and the rail pressure went DOWN instead of up - now it won't go past 100psi. The #22 button indeed has a smaller diameter recess than the #27 button, which is supposed to increase pressure, but it had the opposite effect for whatever reason. I drilled out the ball in the throttle shaft, and was able to get a pocket screwdriver to engage the screw inside, but the screw doesn't want to turn?? Does it have some type of limiter on it? Any PT pump mechanical engine gurus out there that can offer some pointers?
AFC diaphragm could have hole in it. Did you check fuel screen under large slotted cover on top of pump. To check for air in system--using flair nut wrench loosen fuel feed line to injectors fitting at top of pump-screw back down with fingers to just contact-then with fingers only loosen just less than a 1/16 turn. Just enough to let fuel crown at top of fitting while idling. If air is present you will see the fuel slightly foam at crown, if not it will be clear. That will tell you if air is present before injectors, between tank & pump. Turning the throttle shaft screw really isn't a good way to turn up fuel--that is a static setting set on pump stand. The fact it won't turn is problematic though. What went on when you changed buttons I can't help you with {without being there}. But there are other factors like internal pump leakage at governor plunger, pulsation damper, throttle shaft. The only real way to know what is going on inside is checking pump on a test stand. Stock rail pressure hovers with different cpl's, most 155 to 165. AFC is located on back side of pump/block side. * Check fuel shutoff solenoid for operation and internal obstructions.
I did check AFC diaphragm and it is OK - no signs of rip, tears, or damage
I am wondering if screen is plugged, but the cover screw is stupid tight. Thinking I will have to make a tool to get it loose??
Thanks for this tip, really simple and easy to do. That will be next along with the screen!
I plumb a radiator pressure tester up to the AFC and pump it up to 20 PSI. it should hold. Then with the engine idling pull the throttle to full fuel. Rail pressure should max out and fall off. Max is your rail pressure. I call my local pump shop with the numbers on the pump. They can look up what it should be. You have something going on like Truck Shop said if you went to a smaller button and pressure went down. Be sure the arm on your throttle shaft is breaking over.
Good point - I confirmed that the throttle arm is breaking over.
both tanks' vent lines are clear
Personally I don’t do them.. but I’ve heard that a 17 button and a purple spring will make them fly.. Good luck
Same on ours (L10), I haven't gotten around to it yet but may have to weld up a little tool with tightly fitting full length engagement to the slot.
In the shop they use a bfh on it and THEN a hand impact driver & hammer.
The screen had a small amount of debris in it, but by no means enough to be a restriction. Holding the screen up to sunlight one could see through the mesh.
I used a small file to de-burr the slot and then was able to loosen it with a drag link socket.
I wanted to do that but didn't have the room to get a good whack on it. Used that trick many times for metric o-ring boss plugs!
the 'bubble' had a small amount of air in it at start up, but checking again after a 2 mile trip it was perfectly clear ?!? I think I am going to supply fuel from a bucket next as a test to rule out supply lines... The truck is certainly down on power, I think acts like it has air in the fuel.
Normally when air is a issue, no smooth idle, throttle response is poor, and slower coming back to idle. Not all the time but most. * Check return fuel line to see that the inside isn't swollen.
I notice our L10 has slow throttle response, as well slow back to idle.
that's why I suspect air, throttle response is sluggish next test will be running it from a small tank with entirely different supply and return hoses
While I'm waiting on new lines and fittings to arrive, I backed out the adjusting screw with locknut near the throttle lever that I believe sets the 'no air' fueling. Counter-clockwise increases the 'no-air' fueling, correct?
Idle lever screw adjusting screw?
111.
no, I did not move either of the screws that adjust the throttle lever's rotational travel. I adjusted the screw above the throttle shaft that I understand sets the 'no-air' limits on fueling until the boost moves the AFC. I counted exactly three turns so that I can undo that adjustment if needed.
The fuel control in center of throttle shaft is set with stock button at pump code spec which is {for a 531 cpl--157 psi} fuel pressure at full throttle 2100 rpm with 25 psi to afc. The reason for changing fuel psi with button is it's easier and for every numerical lower button X percentage fuel psi increase. Turning fuel control in throttle shaft without gauges leads to a unknown over fuel psi.
I did not turn the screw within the throttle shaft.
this is the screw that I turned out, I believe it sets the limit of pre-boost fueling?
Correct.