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clear line test, but where is the air coming from?

6629 Views 10 Replies 5 Participants Last post by  willnv
Ok, so my truck started getting hard to start, and it was getting progressivly worse it seemed. It would usually start after a few 10 second cranking sessions. Last weekend I decided I was going to try to fix it but that morning it wouldn't start. It took lots of cranking and ether to get it to start. The night before, I was coming home from work and started to run out of fuel in the back tank so I switched to the front, but that was also really low. Well I started putting clear lines everywhere and heres what I found out:

1) There is no air coming up from the lift pump to the filter.
2) There is no air from the filter return to the #1 injector cap, but I did take out the check valve and I can blow air backwards through it. Its the style with the flap inside not the ball and spring.
3) From the Filter outlet to the IP inlet there is a large air pocket / bubble when the engine is off, but gets pushed out towards the IP when the truck starts. Then you can't see it anymore but when the engine is shut off it returns. It never seems to get any bigger, although I'm not sure.

4) On my truck the two caps at the back go to a brass cross piece like: + . there is a hose from the injector caps that goes into the side of this fitting on each side. The back of the fitting has a hose that goes to the return line and then down to the tank selector. The front of the fitting goes to the outlet on top of the IP. When I initially pulled the hose off the front of this fitting no fuel came out. I put a clear line on and what I found out is that fuel gets pushed out of the IP and with a loop in the line it will fill to the top of the loop and then run freely down the other side to the tank. The line never stays full of fuel. When the truck starts then the line slowly fills and pushes the air out towards the tank, and then stays full. But when I shut the engine off the line imediatly starts to fill with air coming back from the tank.

I was thinking that maybe my problems were because I ran the one tank dry and the other was really low. So I just went and filled them both about half full and drove around for 10 minutes or so. When I stop the engine the line still starts to fill with air coming from tank but not as much as before.

Any idea where this air might be coming from. I'm guessing maybe there could be a crack in the plastic line running back to the tank selector, or maybe on of the connections on it are loose? After writing all this I was just thinking I should put clear lines on all sides of that brass cross at the back to make sure it is infact coming from the tank side and not from one of the caps, but it is a huge leak. I put some pictures up, the reason the fuel is black is because I was running w85 for a while but both tanks have diesel in them now. If what I wrote is to confusing I'll see if I can put up a video somewhere. The first picture is the air from the filter to the IP, it never seems to go away or get bigger. The second picture is the air thats come up from the tank to the IP, before I went and filled the tanks the whole line would fill with air except from the loop down to the IP. The last picture is of the brass fitting at the back incase your not sure what I'm talking about.


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Alright I think I might have found something. I just went out to the truck to put a clear line from the back injector caps to the brass fitting and I found something interesting. The one side is fine, there is no air coming from the cap or runing back to the cap from the tank. However on the other side no fuel is coming out of the cap. I can pull the hose right off with the engine running and nothing comes out. This means nothing is returning from the one side of the engine. So I guess something must be blocking the cap, or could this mean a bad injector? Could this have been my problem all along?
I could be wrong, But it sounds like the return system is not installed right. I have the schematic for the turbo application and I only have 1 line that returns to the tank(s). Maybe someone can post the N/A return system.
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On the NA engine there's no line between 5 & 6, but 7 & 8 connect to a tee at the rear of the engine which returns to tank.
On the NA engine there's no line between 5 & 6, but 7 & 8 connect to a tee at the rear of the engine which returns to tank.
So the line from #2 to the IP is there, and there ISN'T a line from the tee at the back to the IP, right? The rear T should only be a 3 way.
I'll post up a schematic of my installation but I have to go right now. I was wrong about the blocked cap though, I can easily blow through all the lines and caps.
IDI fuel circuit.
Mine is similar to that, but slightly different. Heres a schematic of what mine looks like along with where I'm seeing air. I didn't see any fuel coming out of the lines on the right side, but if you think about it, it would only be excess fuel from the injectors which I think would take a while to see anything. No air comes up the lines at the back, just up the metal tube which goes to the IP. I guess if theres a check valve at the IP then it wouldn't matter, I'll have to check for that tommorow.

If you look on this page then I think it talks about there being different applications on the return system. Ford diesel 6.9 7.3 IDI


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OK, I'm confused. I charged my batteries overnight and I plugged in the block heater about 2 hours ago. She started up in about 3-4 seconds with one cycle of the glow plugs. There was still air in the clear line I have on the IP outlet. I'm going to let it cool down and see what it does tonight. How long of cranking is considered normal for these engines to start from cold in say 40 degrees Fahrenheit? Should it be pretty much instant if everything is in good condition or is some cranking considered normal?

If it doesn't start from cold again I'm going to go over my glow plug system again, checking the resistance on all the glow plugs. I'm also going to get a helper to turn the ignition on while I check voltage at each connector. What is the voltage I should expect to see?
If there is a pigtail curl in the line you will see which way the air is coming from... what ever is in the area of where the air is coming from IE injectors and caps, return line ????

Try re seating the injector caps........they have been known to not seal correctly on the first try.
As I said, the air is only coming up the steel line that runs from the brass collector fitting to the IP outlet. There is no air in the return lines from the caps, or anywhere else. It appears that air intrusion isn't the problem anyways. I checked the glow plugs again and I had 1 bad one and 1 that was about to go. The ones that the previous owner put in were bosch, and I had been replacing one at a time as they were burning out, I already had done 3 and the bosch's just keep burning out so I went and replaced them all with beru's. It now fires from completely cold (around 2C, 36F) in around 6 seconds. With the block heater it fires right away.

Thanks for all the help with the fuel schematics. Its strange how my truck is different then the one Bill posted. I wonder if it would be better to convert mine to that style, would there be any advantage? I have some extra caps with 2 barbs that I could put on the #2 injector and then just run a new hose from the IP to that. Then I would just have to plug the brass fitting where the steel line comes out.
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