Attention: TDR Forum Junkies 
I've lurked for years on diesel forums because I didn't own my own diesel truck, just my brother and father did and I just help them repair them and drive them. In general most of the questions I have had have been answered quite thoroughly, so I never joined. Last summer I bought my brother's 1997 6BT powered truck for a project that I haven't gotten started on, but I wanted to solve any engine issues before that gets underway.
The truck lacks power. It starts easy, but just does not seem to be fueling as much as it should. My brother and I discovered this several years ago because every once in a while the truck will roll coal while taking off from an intersection under modest application of the accelerator and the power increase is evident. Then it might never do that again for several months. So I started learning about the fuel injection system. The first suggestion for troubleshooting was a fuel pressure gauge, so a triple gauge pod was installed with Ispro gauges for egt, boost and fuel pressure. I can see plenty of boost, but really bad fuel pressure.
The symptoms at this point: really bouncy gauge reads just over 10 psi at idle and drops more at higher power demands to about 7psi. Well that needs to be solved. At 2000 rpm the fuel pressure still will not increase with any amount of skinny pedal applied. New fuel filter, no difference. New Cummins lift pump, no difference (made in china, not branded DCFC). It still has low fuel pressure. My brother took the truck East for several years until I bought it from him that past summer. The truck struggled to keep at speed climbing Sideling Hill in Maryland but had no problem climbing the Vail pass in Colorado. Still, I have low fuel pressure. The truck had 340k on it at this point.
I did more research and found that the pressure regulator valve could cause low fuel pressure, so I bought one from TorkTek at the stock pressure setting. The biggest change was at idle I would maintain about 20 psi, but it would drop to 10 psi under modest acceleration and below with heavy acceleration. The power was still not there.
Now my reading took me to suspect air in the fuel. Everything upstream of the lift pump could have a pin hole allowing air in. The fuel heater is a prime suspect. I removed it and there was no difference. I did observe some strange behavior with the fuel gauge. After having to re-prime the system the fuel would read about 30 psi idling. Maybe I had solved it! Nope. After driving around for a bit, fuel pressure only returns to 20 psi. The truck still was weak and under power, fuel pressure would still drop low.
I dropped the fuel tank and pressurized the fuel lines under the truck and in the sender and found no signs of a leak. My next test I thought would be pretty bullet proof. I got a jug with a tap at the bottom and ran fuel directly from the jug to the lift pump right under the hood. I put the return line right back into the jug with some clear hose and found aeration! I drove around a little like this and I always had air bubbles in the return. I did some research and found that some Cummins branded lift pump were cheaply made and the DCFC china made one are preferred. The one on the truck is not DCFC branded, so may that's the issue. I pulled the pump and tried it on the bench. If I kinked down the large fuel hose I had going to the pump I could get air bubbles in the return. So I got a DCFC pump. After installing it I had 30 psi at idle until I drove it around for a bit it returned to 20 psi at idle. At higher power under any circumstances the pressure would drop to about 10psi.
That's where I'm at right now. My next thoughts are to try driving around without the fuel cap on, check again if I have air in the fuel and run the truck off the small container of diesel fuel directly into the lift pump again. I don't want to replace the fuel lines since most of this is getting harvested for my project, and I not convince that that will fix it especially since the whole fuel system was by passed with the under-hood tank.
Check out the pictures of most test tank.
What have I missed?
The truck lacks power. It starts easy, but just does not seem to be fueling as much as it should. My brother and I discovered this several years ago because every once in a while the truck will roll coal while taking off from an intersection under modest application of the accelerator and the power increase is evident. Then it might never do that again for several months. So I started learning about the fuel injection system. The first suggestion for troubleshooting was a fuel pressure gauge, so a triple gauge pod was installed with Ispro gauges for egt, boost and fuel pressure. I can see plenty of boost, but really bad fuel pressure.
The symptoms at this point: really bouncy gauge reads just over 10 psi at idle and drops more at higher power demands to about 7psi. Well that needs to be solved. At 2000 rpm the fuel pressure still will not increase with any amount of skinny pedal applied. New fuel filter, no difference. New Cummins lift pump, no difference (made in china, not branded DCFC). It still has low fuel pressure. My brother took the truck East for several years until I bought it from him that past summer. The truck struggled to keep at speed climbing Sideling Hill in Maryland but had no problem climbing the Vail pass in Colorado. Still, I have low fuel pressure. The truck had 340k on it at this point.
I did more research and found that the pressure regulator valve could cause low fuel pressure, so I bought one from TorkTek at the stock pressure setting. The biggest change was at idle I would maintain about 20 psi, but it would drop to 10 psi under modest acceleration and below with heavy acceleration. The power was still not there.
Now my reading took me to suspect air in the fuel. Everything upstream of the lift pump could have a pin hole allowing air in. The fuel heater is a prime suspect. I removed it and there was no difference. I did observe some strange behavior with the fuel gauge. After having to re-prime the system the fuel would read about 30 psi idling. Maybe I had solved it! Nope. After driving around for a bit, fuel pressure only returns to 20 psi. The truck still was weak and under power, fuel pressure would still drop low.
I dropped the fuel tank and pressurized the fuel lines under the truck and in the sender and found no signs of a leak. My next test I thought would be pretty bullet proof. I got a jug with a tap at the bottom and ran fuel directly from the jug to the lift pump right under the hood. I put the return line right back into the jug with some clear hose and found aeration! I drove around a little like this and I always had air bubbles in the return. I did some research and found that some Cummins branded lift pump were cheaply made and the DCFC china made one are preferred. The one on the truck is not DCFC branded, so may that's the issue. I pulled the pump and tried it on the bench. If I kinked down the large fuel hose I had going to the pump I could get air bubbles in the return. So I got a DCFC pump. After installing it I had 30 psi at idle until I drove it around for a bit it returned to 20 psi at idle. At higher power under any circumstances the pressure would drop to about 10psi.
That's where I'm at right now. My next thoughts are to try driving around without the fuel cap on, check again if I have air in the fuel and run the truck off the small container of diesel fuel directly into the lift pump again. I don't want to replace the fuel lines since most of this is getting harvested for my project, and I not convince that that will fix it especially since the whole fuel system was by passed with the under-hood tank.
Check out the pictures of most test tank.
What have I missed?
