I have a Cummins C8.3 turbo, which is VERY similar to the 5.9 liter Cummins with mechanical P7100 fuel pump.

Background:
I am seeing only 2-3 psi boost. I have a number 10 fuel plate, and star wheel turned up. One week ago I had timing changed to 14 degrees instead of 12 degrees - still had good boost (32-32 psi max). Monday I had the old frozen up PAC brake removed, and a new PAC PRXB installed in the same place just behind the turbo - still had good boost for about 30 miles.

The day after the PAC install, boost gauge went dead, and power on minor hills WAY down. Clearly no boost. Here is what I found upon looking: Out of the P7100 fuel pump is a fitting that feeds straight through to a small copper fitting that goes into the head, plus a 3rd fitting that goes to tubing to the waste gate. So one end into P7100 fuel pump, one into head, one to waste gate.

I had a second T on the above original T, to add the boost gauge sensor that I added a month or two ago. The new T "was" brass. It clearly did not like the weight of the T plus the weight of boost sensor with vibration, and it sheared off flush with the orignal metal T fitting. When it sheared off, the boost gauge went to zero, and at the same time seat of the pants I had much reduced power on even minor uphill grades.

I removed the sheared portion and restored the T back to stock, and put new a barbed T into the tubing between orignal T and the waste gate, which is where I remounted the boost sensor. I now read 2-3 psi max boost on gauge,and seat of the pants I still am lacking power.

I'm getting 2 to 3 lbs of boost at max instead of previous 32-33 psi. Pyro is 700 degrees cruising in 6th gear, 1,000 degrees under max throttle in 4th climbing a grade at 2,000 rpm (normal for the weight on my rig). Pyro sensor is mounted in the exhaust manifold. No sign of smoke from the tailpipe - burning clean. Running great and smooth other than lack of boost (on both gauge and at seat of pants lack of power). No bad sounds from turbo.

As a test, I pulled the control hose at the waste gate leaving the fitting on the waste gate open, and plugging the line. Same 2-3 lbs boost.

Exhaust brake is working freely, fuel filter bowl is clear, air filter clean. CAC to piping and piping to piping clamps look good and are snug, can't see any holes or splits in the clamp bellows.

When the tubing to the waste gate was sheared off, could the turbo have overboosted and created a leak to form in the CAC? If I can pull the CAC, can a radiator shop check it for leaks?

More thoughts (sorry so long):

The power has not been the same since the broken 'T' to the waste gate, even though I have repaired the "t" and three times checked the passages are clear and flowing air in the T.

I have some questions for everyone on how everything works. The original 'T' that mounts on the back of the P7100 fuel pump - one end is copper tubing that goes into the head - the other end goes into the fuel pump (the T fitting that goes to the waste gate was sealed when I blocked the tube to the waste gate as a test). So we now have a connection from the head to fuel pump. What does this do? Does it tell the fuel pump how much fuel to meter base on air flow/boost?

So I don't understand how the fuel pump determines how much fuel to meter based on air into the engine. If this T tells the fuel pump how much fuel to meter, then it has rolled back the fuel to match the lack of boosted air into the engine, and fuel metering is correct for this reduced air flow with no/low boost - thus no smoke and correct metering?

I'm very confused and attempting to educate myself online on how the turbo meters boost and controls it via the waste gate, and how the P7100 fuel pump meters fuel to match air flow. Isn't the pressure in this T and line positive pressure not vacumn? So there is positive pressure into the P7100 fuel pump, and positive pressure into the can on the waste gate. The positive pressure when high enough at the can on waste gate, actuates the waste gate lever to dump excess pressure?

When there was no positive pressure at the can on the waste gate, then could the turbo have over boosted, and damaged either the CAC, or something else - split something to create a leak?

Could be total coincidence to have the broken 't' to the waste gate, then have limited / no boost at the same time - but I sure doubt it. I can't rule out the PAC brake install having created a problem - but I did have good boost for a while after install, and I am 100 percent sure the pac is operating on/off correctly.

I am convinced at this point by seat of the pants that the boost gauge is correct at 2-3 PSI, and also convinced at this time that the fuel pump "knows" there is limited air flow and metering fuel in accord. I don't think I'm wrong on the lack of boost.

I plan to use my air compressor set at a couple different pressures on the T tubing to see what boost gauge reads, and to see if there are any leaks in the tubing/connections, even though I chacked every connection on the T's several times so far.

I'll also push pressure into the waste gate and see what that does. Depending on results, I will be left with only a CAC/tubing leak, or turbo damage of some type.

Unless of course someone has a better idea. I'm not quite ready to go find a diesel repair shop, until I troubleshoot a bit more and try and narrow down what could be happening.

If I'm way off base, I need some education on how the fuel pump meters fuel to air flow, and what could cause lack of boost in coincidence with the 't' shearing off.

pics - orignal T on P7100 fuel pump, 2nd pic sheared off to left of fitting and boost sensor, 3rd pic new T for boost sensor.


Dan

 

boost gauge electric?
dose the trucks computer use a electric boost sensor?

-Brice

 

Hi Brice,

Electronic boost sensor, Isspro EV2.

Non computerized rig, older mechanical engine like the 1994 Cummins 5.9 (though mine is a 8.3 Cummins used same fuel pump, fuel solonoids, etc on both.

Dan

 

i see check gauge and also check all I/C connections if boost is there check for leak

 

You are correct the tube running from the head to the pump is feeding boost presure to the AIR FUEL CONTROL (AFC) housing. It has a diaphram inside backed up by a spring as boost from the turbo comes up it pushes the lever foreward allowing more fuel. If you have a leak anywhere in that system from the head to the wastegate it will not fuel. The wastegate diaphram is prone to leaking. I would check it all with compresed air from the line coming off the head on. The line breaking would not likely cause the turbo to over boost because the system would stop fueling and the waste gate would close together. Unless you are running a lot of fuel closing the wastegate would not add much boost at all.

 

I did some minor trouble shooting a few minutes ago with my air compressor.

I feed 32 PSI into the waste gate can - no leaks and the waste gate moved freely. My Cummins 8.3 was 25-27 psi stock max, 32 psi after adding BD fuel cam kit/elbow

I then fed 32 psi air from the waste gate end of the hose back towards the afc. No leaks, other than the designed in port on the elbow from the BD fuel cam kit. When I blocked the port on the elbow, no leaks. The boost gauge on the dash showed 32 psi both with and without port blocked. Air pressure was identical to max boost I was seeing before, but air volume may be higher than normally fed from the AFC/elbow fitting.

Next I reconnected the waste gate end, pulled the hose from the elbow at the AFC, and fed 32 psi the other direction. Boost gauge again showed 32 psi, and again waste gate moved freely.

Based on this test, I think I can rule out the gauge/sender, the hose, the afc fittings, the waste gate can and waste gate lever.

That I believe leaves a leak in the CAC/tubing, a leak at the turbo, or a leak in exhaust manifold pre turbo. I don't think it's a head gasket, because the engine runs great, just with max psi of 2-3 lbs. It is still strange that all this started when the 2nd T I had mounted on the AFC sheared off. I am now back to stock configuration of the T with the elbow to the waste gate from the BD kit, and I relocated the boost gauge into the tubing from AFC elbow to waste gate - which air compressor test says is ok.

I'll run at idle and check for any exhaust leaks pre turbo/at turbo if I can detect them at only idle.

I'll attempt testing the CAC next. May have to wait until Monday/Tuesday.
I have a method I want to try on the CAC pressure test, I'll post on it later.

Any other/new ideas other than my next plans for testing?

Thanks for the help!

 

A bad Air fuel control diaphram (Aneroid) is a very possible. I have used a portable air tank to activate the air fuel control independant of the engine. You can also relax the spring pressure on it. Jeremy

 

I've been looking on the net, and can't determing exactly where the diaphram would be located? I know where the star wheel is, and know where the cam fuel plate is. I did the air input into the AFC with my air compressor and hear no leaks. How can I visually check the diaphram? Do I take the cover off as if I was installing the cam fuel plate, and then where do I look after that?

Thanks, this is a lead I want to check on.

This all started when the fitting on the AFC sheared off, so little to no pressure at all was going into the AFC, it was just dumping out the part of the fitting that leads to the waste gate. After repairing the fitting, I still get no boost. What damage sould result from no air pressure into the AFC, that would not be solved by repairing the fitting that air flows into the AFC? Only 2-3 psi max now flows from the head into the AFC since the fitting origninally broke and has been repaired.

Also, what exactly is meant by a "blown turbo"? I hear no ugly sounds from the turbo.

Sure could use more advice,






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Could the governor arm in the P7100 have slipped under the fuel cam plate? If so, would it run fine across the fuel range, but without boost? In other words, it drives fine and doesnt smoke, just boost is limited. Later today or tommorrow I may remove the pump cover and visually check it out.

From another thread:
"This arm rides up and down the plate and is responsible for "telling" the pump how much fuel to deliver. You MUST make sure that this arm does not go under the fuel plate once the fuel plate is replaced and adjusted where you want it. If it goes under the plate you'll either need to slide the plate back until the arm hits or adjust the arm level. (Adjusting the arm level is a whole other write up) To check the arm height you'll want to turn the ignition on leaving the engine off then pull up on the fuel shut-off solenoid. After the solenoid is all the way up move the throttle by hand and see if the arm hits the plate. As far as adjusting the fuel plate the basic rule of thumb is towards the front of the engine lets the Governor Arm travel farther forward adding more fuel and towards the firewall lessens the Governor Arm travel reducing fuel. I have found with the plate I use that about 1/16th of an inch from full forward gives me the best power."


Read more: https://www.dieselbombers.com/5-9l-1...#ixzz0o18biBH1

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I THINK IT IS FIXED!!!!!

I'm settled in a campground and haven't moved the coach, but I just put it in drive, left the park air brake on, stepped on the service brakes and gave it some throttle. Boost was climbing fast and came up over 10 before I backed off the throttle.

My theory: governor arm was stuck under the fuel cam plate. Pushing the 32 psi in with the air compressor yesterday freed it up. The 2-3 psi boost I was getting was not enough to free the hangup - but the 32 psi broke it loose. Makes me wonder if I should tighten the star wheel back up just a touch - if it contributed to the arm hanging up. I backed the star wheel way off for quicker fuel response - may tighten it back up a bit.

Won't be 100 percent sure until I go drive it, but I have high confidence since the boost climbed so fast on stationary test. Going to go do the tour thing in the tow car with mama - time for a break.

I was envisioning a big repair bill on CAC or turbo - relief.

Thanks for the help and the leads. I learned a whole bunch about turbos, CAC, AFC, waste gate control, etc. My lead on the governor arm possibly being under the fuel cam plate was at: https://www.dieselbombers.com/5-9l-12...#ixzz0o18biBH1 .

Thanks All!

 

Glad you got it

Brice