Having trouble getting air out of system. I have been told several different methods, but not sure how reliable. I'm not sure these guys know that much about diesel injection systems. I found out I don't know as much as I thought I did either. Been working on this for a couple of days now. It's not even trying to fire. If I keep cranking will air finally make it's way out? I'm stuck and need help!!

 

How to remove air from the fuel system and/or find an air/fuel leak on a 6.2L



This thread covers bleeding the air out of the fuel system on these engines, which is necessary after running out of fuel, after the vehicle has been sitting for a long time, or after replacing any part of the fuel system.

It also includes a method for finding an air leak (or a fuel leak) in the fuel system. Symptoms of an air leak include surging, hard starting, no start, rough running, rough idle, and stalling.

These methods should not be used on trucks with very rusty fuel tanks, as it is possible to blow a hole in the tank.



To bleed air (or old fuel) out of the system on early models (1982-1984):


Hook an air compressor hose into the return line at the injection pump (IP). The line you want is the rubber one that goes into the front of the IP. Alternatively, you can stick the air hose into the filler neck, but that is not as effective. You can also modify a fuel cap with a valve stem off of a tubeless tire and use an air chuck to supply the air, which works good. Whatever way you choose, the next step is to loosen the fitting on the outlet side of the fuel filter. Loosen it enough that fuel will come out readily, but don’t take it all the way off as you will need to be able to tighten it fairly easily. Put a pop bottle with a funnel stuck in the neck under the fitting to collect the fuel. With the fuel cap on tight, turn on the air to about 4 psi. Give it some time to fill the fuel tank. Fuel will start to run out of the fitting and into the pop bottle. When the pop bottle is about full, tighten the fitting. Then turn off the air. Disconnect the air line, wait for any remaining pressure to bleed out of the tank, and then reconnect the return line to the IP. If you used the modified fuel cap or the hose in the filler neck, you can ignore these last few steps.

Once you have bled the air out of the system to that point, you may need to crank for a while to get it to start. It is not necessary to loosen injector lines to bleed the system to the injectors on the 6.2. Simply crank until it starts. You will need to do this in several cranking sessions of about 15-25 seconds in length so that you do not burn out the starter.




To bleed air (or old fuel) out of the system on 1984 and later models:




These trucks were equipped with a combination fuel filter/water separator/fuel heater ("square" filter). If your truck is equipped with a "round" filter, see the procedure for early model trucks.

Hook an air compressor hose into the return line at the injection pump (IP). The line you want is the rubber one that goes into the front of the IP. Alternatively, you can stick the air hose into the filler neck, but that is not as effective. You can also modify a fuel cap with a valve stem off of a tubeless tire and use an air chuck to supply the air, which works good. Whatever way you choose, the next step is to open the air bleed valve. It is located on the top of the fuel filter mounting block. There is a hose fitting right next to it. Hook a hose onto that, and run the hose into a pop bottle (this will collect whatever fuel you bleed out). With the fuel cap on tight, turn on the air to about 4 psi. Give it some time to fill the fuel tank. Fuel will start to flow out of the bleed valve and into the pop bottle. When the pop bottle is about full, close the bleed valve. Then turn off the air. Disconnect the air line, wait for any remaining pressure to bleed out of the tank, and then reconnect the return line to the IP. If you used the modified fuel cap or the hose in the filler neck, you can ignore these last few steps.

Once you have bled the air out of the system to that point, you may need to crank for a while to get it to start. It is not necessary to loosen injector lines to bleed the system to the injectors on the 6.2. Simply crank until it starts. You will need to do this in several cranking sessions of about 15-25 seconds in length so that you do not burn out the starter.




To find an air/fuel leak:




Utilize air pressure to test your fuel supply system for leaks. The procedure for connecting the air is the same as the one used to bleed the system, however you don’t loosen the fitting at the fuel filter (on 1982-1984 models) or open the bleed valve (on newer models). Once you have the system under pressure, crawl under the truck and look for fuel leaks (it might be a good idea to wear safety goggles when you do this). Work slowly from the fuel tank to the injection pump. You need to look very closely, because any leaks are likely to be very slow leaks. Check every connection, every hose, the fuel pump, the fuel filter(s), and the tank switch (if the truck has dual tanks). In short, check everything. When you think you’ve checked everything, go back to the fuel tank and start over. A good thorough inspection should take about fifteen or twenty minutes. Anything that is just barely seeping fuel needs to be repaired. Fix any leaks, test it again, and when you get to the point where it no longer leaks, bleed the air out of the system. Replace the rubber hose on the return line with a clear hose before you start the engine. Start the engine and let it run for a while. If it runs good, take it out on the road for a short distance. Then look at the clear hose and look for any bubbles. If there aren’t any, you are probably in good shape.

If everything looks good, but the engine won’t start the next morning, you probably need to replace the fuel pump. Replace the pump, bleed the air out of the system, and you should be in good shape.

 

Thanks Crazy! Will this method work on 93 gmc 6.5 td ? I'm finally starting to get a little smoke from exhaust while cranking. I was told that is a sign that fuel is starting to reach the injectors. Is that true? I would tell you what I have been through with this truck in the last four months, but I don't think either one of us has that much time. I will say I'm about $6,000.00 in to engine and trans costs. Not to mention the work I have lost. But I like my truck and just can't seem to give up on it and throw in the towel. Family and friends think I have lost my mind. But, I have been hitting the books and asking anyone who will listen, trying to learn about diesel engines. Anyway, Thanks again Crazy. I'm gonna try it now>

 

it does take lot cranking to get the air just remember gave the start some breaks or you will burn out and may want to put battery changer on to keep the batters up

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Starting a fresh engine/IP and inj change

I have been reading posts from several here that have had issues with getting a fresh engine and or an engine thats seen an IP and injector change started again.

FOR A DS4 system DB2 very similar

The process is simple.

The diesel system is a fuel system designed to run completely air free.
This is not all that different that your brake system (well actually a lot different) in that air compresses and fluid does not.

The Injection pump pushes fuel through the lines to the injectors until it reaches about 2000 PSI and then the injector POPS open and the fuel sprays into the firing chamber.

When starting fresh the system is full of air and this has to be removed before the engine is going to run.

Turning the engine with the starter is about the only option that we have to do this.

Long crank times with the engine fighting compression will kill the starter and the batteries as well as your patience.

Here is the easiest way to get this task done without killing anything.

I will skip some of the obvious steps like "Hook up batteries etc"

Depending on your rigs model year use what ever procedure is needed to get the lift pump running.

Get fuel to the Injector pump inlet (Hose in a cup until fuel flows.

Hook up fuel line to IP

Turn key switch to "ON" only

Run lift pump for approx 45 seconds or so. This will actually purge fuel through the IP and back to the tank.

Now WITH ALL GLOW PLUGS REMOVED (the RH side can be easily accessed via the inner fender access hole with tire off)

NOW
Have a helper watching the glow plug holes (LH side is easy)
Spin the engine over (will spin fast with plugs out)

Usually takes about 30-45 seconds to have fuel mist coming from the plug holes.

Stop and shut off the key switch.

Install the glow plugs and hook up the wires.

Take a deep breath ):h

Turn the key on and allow the glow cycle to complete.

Turn key to start and crank the engine over.
The engine should start in 3-4 seconds providing that all the goodies are working properly. (glow plugs, relays etc)

The engine should start right up and run fairly well using this procedure.

Every set of injectors and every IP is slightly different but thing should go smooth and fairly quick without the usually hastle of bleeding the lines and fooling around with a smokey rough running engine for 30 minutes.

With the engine spinning with glow plugs removed, the IP does not have to fight the compression to force fuel though the lines and out the injectors.

There are small clearances in the injectors that will allow air to escape readily as long as there is no pressure against it.

The engine will spin at around 400 RPM without the plugs in and this greatly speeds up the process.


I hope this helps any who need to restart an engine that has been freshened and or seen the IP and squirts replaced.

Plus while the engine is spinning without compression (Fresh build) you are getting oil pumped up into the system.

 

Thanks again Crazy, I tried the compressed air process that you suggested and found two leaks on two injectors where the return lines connect on top of injector. I guess while opening and closing injector line, I must have broken hose barb with wrench. Would this leak on the low pressure return line cause the trouble I'm having getting air out and fuel to injector? Anyway, Thanks again, I'm going out there now to remove glow plugs and try that process that you listed. !!

 

any air in the system going gave you hard time

 

I gotta chime in here. I'm sorry but I disagree. The reason you probably saw leaks is you used wayyy too much pressure. You should not pressurize a fuekl system like that. You could distort the fuel tank among other things as well as damage the fuel syste. The fuel system is designed to run about 7-8 psi on these things. On a mech IP you could damage the cold advance as it works off fuel pressure.. When looking ofr a suspect fuel leak remember this, It's almost always going to be between the lift pump and the tank. If it's post lift pump you would see signs of it leaks generally. If you suspect air in the system the best way to find it is to sight glass it it. Remove the fuel line at the IP and install about 2-3 feet of clear tubing(it does not have to be rated for fuel as it's a temporary thing). Purge it by either cranking (mech LP) or running the LP(electric). Then connect and attempt to start the engine if the leak is minor the engine should start. While it's running you should see NO air bubbles at all. If you do it's most likely either the line along the frame between the tank and the LP or more commonly if you live in the rust belt, the picktube itself. They tend to rot right at the bend no matter what vintage.
Pressurizing a the fuel system with shop air is a very bad idea. Also if it's a DB2 look for signs of wet or fuel loooking grime around the throttle shaft. Seals are common failure and will lead to the truck having backflow issues.

As far as starting it...Yes, if you hold your foot to the floor with 2 solid batts it should eventually start and I have heard of the removing the glow method as well. However pulling glopws on some are a PIA. I prefer to loosen the inj lines. ON a 6.2 it's easy. ON a 6.5TD the two by the turbo are a bitch so leave them and do the rest. as soon as you get some dribble, lock em up and foot to the floor. A DB2 will purge pretty quickly once running but a DS4 can sound really horrible and take a bit to smooth out because the Optical sensor sees the air in the fuel and goes beserk trying to balance out the motor by cutting fuel which makes it worse. You can either disconnect it temporarily which makes it a bitch to start or just take the truck out on the road and give it a good 15 min pounding.
ON an OBD1 truck pin g on the aldl connector is the LP bypass so you can put 12v to it . On an OBD2 truck just keep clikcing the key on and off.

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Also as a diagnostic tool I keep a small cheapo elec LP handy and if all else fails I'll drop a hose in a 5 gal can of diesel and connect it right to the IP. That eliminates the whole supply system. If that does it you work backwards towards the tank with diagnosis and as far as the return lines laeking, that will not affect how the truck runs. You could almost pull them off and the truck would still run. If the inj bodys themselves are leaking that is different.

 

Thanks for the info. Let me give you some more background. I changed out a head gasket. I removed ip lines to remove head and loosened all lines to get to the ones that I had to remove. Replaced gasket and put all back together. I thought everything went well. The truck was running when I parked it for repair. Goin on three days now and I can't get it to fire. I'M LOST and running out of time. It just seems like I can't get fuel to cylinders. LP is wired direct and appears to run fine until pressure gets built up.. ANY IDEAS.... PLEASE

 

Well, you should get fuel to the injs pretty quickly. I would crack as many lines as you can get to easily and see if you get fuel coming out at all. Also two fully charged strong batts is a must. If after cranking fior a few minutes you don't see any fuel at the injs at all something else is wrong. Make sure the wires to the IP are all hooked back up correctly. These engines are generally not too difficult to purge. Also before you do anything else if you have an elec LP and it's hardwired, Pull the supply line off at the IP and run some fuel into a can or something and make sure you have nice strong flow.

 

Thanks for the info. I have tryed everything you suggested and now I think it's not an air issue. I'm not getting fuel from IP to injectors. It's a mechanical IP. I am told to check ops, I can't find out what that is or what to do about it. Also, the solenoid on top of IP is getting power,using test light. Do I need to pull top off IP and see whats going on? Does this problem sound like anything you have ever heard before? I appreciate your input!!