No/Very Low boost problem - Diesel Bombers

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Old 05-14-2010, 09:10 PM
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Default No/Very Low boost problem

I have a 1994 Cummins C8.3 turbo, which is VERY similar to the 5.9 liter Cummins with mechanical P7100 fuel pump.

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 (32-33 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? Could the now working versus broken Pac with the resulting back pressure have created a leak somewhere to affect boost?

Turbo is Holset TC-WH1E-3530994, which is the mother of the newer Holset HX40.

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.

Thanks for any advice, education, or help

---AutoMerged DoublePost---

I have the thread running on the help ticket area now. See for info.


Last edited by AC7880; 05-14-2010 at 09:10 PM. Reason: Automerged Doublepost
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Old 05-14-2010, 09:24 PM
Begle1's Avatar
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Join Date: Mar 2007
Location: Capo Beach, CA
Posts: 3,910

You think a lot.

Only 3 PSI of boost pressure is VERY low. If you have low boost with no black smoke, it's a sign of insufficient fueling. A blown turbo, faulty wastegate or air leak would usually cause black smoke.

The injection pump is equipped with an AFC; air-fuel control. The more boost pressure it sees, the more fuel it injections. No boost on the AFC, very minimal fueling.

There is a little bit of a chicken-and-egg diagnosis that can go on. No boost gives no fuel, but no fuel gives no boost... But it sounds like you don't have a smooth enough pressure connection between the AFC on the pump and the head.
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Old 06-09-2010, 05:02 PM
Join Date: May 2010
Posts: 8

First, I'd make sure that it wasn't something I messed up working on it. It looks like you've done that.

Next test for a boost leak. You can make a boost boot from parts you buy at the hardware store. Cost is around $10. Put the boot on the inlet side of the turbo and pressurize to 25# or so. Listen/feel for air leaks. Putting air at the turbo tests the whole system. You can block certain parts by sticking a can in the piping. I don't have pics but all the parts are sold in the plumbing section & a google should turn up the details.

Last edited by junkman; 06-09-2010 at 05:25 PM.
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Old 08-04-2010, 08:05 AM
Join Date: Jul 2010
Posts: 1

I have what seems to be the same problem with my 94, 36' Dynasty by Monaco. I had the pump rebuilt, turbo rebuilt. and exuast gaskets replaced. Last night we by passed the air cooler. When I gave it full throttle in 2nd gear, the boost gauge went pass
20 lbs. and blew off our buy pass hose. It seems I have a restricted air cooler
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