Just to give a little background, I just completed an engine swap in my 86 F350. I installed a salvage yard 7.3. I just got it running the last week or so and haven't had much of a chance to let it run or drive it at all until today. I had my truck running outside my shop and I wanted to hear how it ran through the RPM range. It ran pretty smooth up and down. I did notice a little shake about 2000 RPM (if my tach is right, kind of bounces a little). I ran it up again to about 2000 RPM and held it and it was running pretty smooth. After it ran at that point for about 15 or 20 seconds the RPM increased all of a sudden so I let off of the accelerator and it went to idle and was missing and shaking pretty bad. After about 10 or 15 seconds it smoothed out. I did this same thing 2 or 3 times and the same thing happened each time. It seems to me that maybe it is starving for fuel. Does this sound like it or could it be something else? Could a lift pump cause this or maybe a fuel filter? Fuel gauge is reading a little over half a tank. Any feedback would be greatly appreciated. Thanks.

 

Sounds like it is sucking air somewhere.

Do you still have the stock water seperator hooked up?

The 7.3 motors had a water seperator in the fuel filter housing, bypass the stock seperator located on the firewall. They were famous for sucking air.

Lift pump and fuel filter could also be possible causes.

Double check the fuel line connections at the lift pump and the return line at the back of the engine.

 

I've seen a lot of posts about hard to start engines. Most of the responses say that it is probably due to bad return lines...old,cracked, dry rotted. My engine does have to crank over a while to light off but I thought that this problem was limited to starting and I wasn't too worried about it at this time. Could this cause the symptoms that I am having? Also, on the 7.3's, do I need to block off the return line to the filter like is suggested on the 6.9's?

 

Same setup on the 7.3 as the 6.9 for fuel systems except the water seperator location. Same rules apply to both engines.

When I eliminated my water seperator on the firewall I though someone had installed a new engine while I was not looking. Much more power and much smoother running.
Air in the fuel system is bad news starting or driving. The little surge of power is the pump sucking air right before it runs out of fuel. You just have to hope it picks up the fuel again before it stops.

Extra cranking to get it fired off is not good, the starter is very heat sensitive and expensive to replace. 15 seconds cranking, 2 minutes minumum cooling time before cranking again.

 

and to add to what dave said, the return line from #1 to the fuel filter housing should be eliminated. that is the second worst air inlet point, with #1 being the water seperator.

 

I checked and the stock water seperator is totally gone. The return line to the filter head is still there though. I will start by blocking off this line. Seems like the cheapest thing to try first. What was the initial purpose of this line?

 

the thought behind putting the return to the fuel filter was that it would recycle some of the unused fuel to the filter instead of returning it to the tank through the return line on the back of the rail. the problems start when the return lines get old and will let air in when shut down, letting the fuel filter back bleed to the tank. then when you try to start the next morning you fire up on an empty filter housing. ford realized the problem in 89 or 90, and issued a tsb on the line removal as a cure for cold start shutdown due to filter bleed back, but for some strange reason, still put it on the engine.

 

So will blocking off the return line help my engine performance problems?

 

as far as performance goes?? no. but if you are having a cold start problem where the engine will start and run for a second or two and then shut off, and then you have to crank the heck out of it to start it again, yes it most likely will eliminate that problem. i have done the line elimination on more pickups and wreckers than i can count, and there was only one time it did not fix it. and that one had a cracked filter housing that i did not see till after it started leaking.

 

Remove the schrader valve from the filter header and install a fuel pressure gauge.
Remove the FSS wire, front right on the IP, so it will not start.
You should be getting between 5 and 7 PSI fuel pressure while cranking the engine.
Remove the gauge and then catch the fuel in a cup while someone cranks the engine over for 10 seconds. You should have at least 1/3 pint in the cup.

If either of those tests are failed, you need a new lift pump.

If you have fuel in the other tank, you might try switching tanks to see if the problem goes away.

 

I have a simular problem with my 84, I swapped FI pump & injectors from my 86 to my 84 being the 86 pump & injectors were near new. But I seem to have the same "air" problems in my 84 as I did in the 86. But not whenits cold, only when its warmed up, I know its air cause I bleed at the fuel filterhead, now is this caused by my return line to the fuel filter and if it is how do I "block or remove" return line.

 

Well, tonight I removed the return line to the filter. I put a pipe plug in the filter head and stuck a bolt in a short piece of line off of the injector and clamped both ends. This seemed to make the truck run rougher for some reason. I did some more looking and found that where the water seperator was mounted, whoever did this, did not clamp both ends of the hoses connected to the junction fitting. I put a clamp on the end that needed one and started the truck up. Seemed to run better. It held engine speed really well at 2000 rpm but ended up doing the same thing when went to about 2200rpm (speed up and then go to idle and run rough). I did notice that the number one injector return plastic cap is now leaking slightly around the o-rings. Will this cause this symptom? I'm just not sure. After I shut the truck down I pushed the schrader valve on the fuel filter head and was still getting air out of it. Could this be caused by the lift pump not keeping up with the injection pump? I am going to do the flow test on it like Dave suggested. I do not have a second tank to switch to though. Just one. I also think I am going to replace all of the return lines, o-rings, and caps. I noticed when ordering I need to know the year. I do not know the year of the engine because the salvage yard I bought it from did not know. Is there any way to tell what year I have? The tag is not readable at all. Thanks for all of the information you all are giving. This project is starting to wear on me but I am trying to hang in there.

 

O rings are the same for all of them. I am running 94 7.3 injectors, 86 6.9 caps, 92 to 94 7.3 IP and 92 7.3 O rings on mine which has an 87 6.9 motor.

Since you have air in the filter from when it is not running, when you rev the engine and it is asking for more fuel flow from the fuel system, it could be you are just sucking the air out of the filter.

Like I have said before, air leaks into the fuel system can make you crazy till you find them. By the time you can see fuel they are getting to be bad leaks.

 

You are right, these air leaks can make you crazy. At least this forum gives me direction on where to find them. I appreciate the assistance.

 

I did just remember that whoever removed the stock water seperator left the whole length of hoses there. They just used a copper tube to join the hoses together. They are just coiled up into a pile where the seperator used to set. Could this cause a delivery problem to the filter....lift pump working harder or something like that? I will replace this mess with a short piece of hose but was just wondering. I guess I really need to focus on my chassis fuel lines because the salvage yard I bought the engine from said it ran really good. I have also forgot to mention that the fuel lines on cylinders 7 and 8 are slightly bent right at the top of the injector. I don't know if these were bent at the salvage yard or if they have been bent a long time. What sort of problem might this cause? I know this won't cure my air in fuel problem but just curious if it might cause other issues. Last question this time, when I check my lift pump should the fuel flow be solid fuel or will there be some bubbles in it?