pipefitter

I have a '96 6BT with about 220,000 miles on it. I had a shop fix the crank seal not too long ago, and they discovered my timing cover was cracked. It ended up being a fairly extensive repair. It sat for a few weeks. When I started it, it would fire up just fine, then die. If I cranked on it, eventually it would run very rough, belching blue and black smoke, eventually settling down. I've discovered that if I use the manual primer, it'll start without drama. I've taken it back to the shop that worked on it, and they can't "duplicate the problem". I need it to tow a car next week. It has to be getting air into the fuel system somewhere. Anyone ever experience this before?

cougar

Pressurize your fuel tank and look for leaks between the tank and your lift pump. Other possibilities are a faulty lift pump or a leak at the fitting were the fuel return lines meet behind the head.

texasprd

I've heard of using a shop vac to pressurize the tank and fuel system as cougar suggests above - use the exhaust side and wrap wrags around the hose end so you can get some kind of seal against the filler neck where the cap goes. Or use low pressure (I've heard 5PSI max?) compressed air. You might want to get a cheap cap and drill/tap for a quick-connect fitting for this route.

I wonder why the timing cover was cracked - perhaps the KDP worked it's way out and only took out the cover? If so, I hope that shop replaced the pin!

pipefitter

I wondered about the timing cover also. They said the new cover is an improved design, but I never saw oil leaks except from the seal. I had a really bad PS leak, and the crank seal was leaking as well. They had satisfactorily repaired a minor problem in the past, so I took it to them. It hasn't started properly since. I think they had to have left a fitting loose, (like cougar mentioned), or something simple they overlooked. I wish I had just let it leak.

pipefitter

I picked up the truck last week. They pressure tested the system, and it held. It sat for 5 days, and started without any fanfare. I took it home, and it acted up again after it sat for 4 days. Could a very slight leak at the lift pump cause anything like this?

oliver foster

Yes. any leak in the fuel system / lines could cause this.
Look for leaks at -
Fuel filter
Fuel heater strainer
Lift Pump
ALL line connections

It is most likely between the fuel tank and the LP, if it is after it I would expect to see some wetness from fuel weeping.

pipefitter

Thanks for the response. That's part of what baffles me. I don't see any leaks, or smell any diesel. When it acts up, using the hand prime will make it start fine.

pind

On board with cougar. Your symptoms point to a leaking return line. Air will get in under negative pressure, more easily than fuel will escape under positive pressure. Left even overnight, it can drain the system down and you’ll never see a leak.

oliver foster

the fuel in the system is under a vacuum in a lot of the fuel system when the truck is running. It will not leak fuel, but rather suck in air.
I repeat it will not leak fuel.

You will need to pressurize the tank if you want to try and find a fuel leak, where it is normally sucking in air from a loose connection.
Or just tighten and check all connections.

dguru

Primer pump on lift pump is a common suction leak. Fuel heater is another one.

pind

So if we’re going to chase the upstream side of the system, from pickup tube to injection pump inlet, then I have to ask how the truck runs once it has been started, under normal conditions.

An air leak on the upstream side can cause a lot of drivability issues, from low power, to smoke from late timing, to surging/bucking. There is always some entrained air in fuel. Just a fact of life. A massive restriction in the line can cause this air to become free of the fuel under negative pressure.

The part about it not starting after sitting is what is steering me to the return side allowing the system to bleed down. If there are no other drivability issues.

oliver foster

Parking the truck up hill rather than down hill, or keeping the fuel tank full can help sort out the leak sight as well.

pipefitter

Thanks for all the replies guys, I really appreciate it. I started it this morning and it was the worse to date. In the past, it would run for several seconds then die. Today, it didn't even do that. I primed the lift pump probably 50-60 times, and it started, ran rough for 30 seconds or so then smoothed out. To Pind: the truck runs fine once I get it started, with no hesitation or stumbling. To Oliver: funny you should mention that. I park with the front elevated slightly. Once I started it this morning, I turned it around with the intention of checking it again in a few days. I'll start with tightening all the fuel lines I can find. This is as frustrating as it can be.

texasprd

Sorry, but I disagree with this part based on personal experience. My hard return line rubbed through years ago - you could see fuel dripping on the ground with the truck running. I never had any problems with it starting. I can possibly see your point if the OFV is worn and not sealing and there is a return-line leak - in that case, I can see it might be possible to siphon fuel through the IP.

OP - have you tried pinching the return line while cranking? You don't have a fuel pressure gauge, do you? If you haven't already, get the truck running and carefully inspect the return line with a bright light and mirror from the OFV as far back as you can. Another thing to consider is the sealing washers from the filter mount downstream. When the shop replaced the gear case, they would have pulled the IP - to do that, they had to remove banjo fitting(s) on the IP at minimum and possibly at the filter mount. Those sealing washers could leak if reused.

pipefitter

I haven't tried pinching off the return line, but will. I have a FP gauge coming today for yet another vehicle I own with a fuel issue.Hopefully I can get serious with pin pointing this crap soon.