Hello everyone,
I just picked up a free 1985 f-250 4x4 6.9 c-6 from a guy I work with. I just had a few quick questions.
The truck seems to run really good, (after you pour about a cup of fuel down the intake!) and I was just wondering what would cause this?
He says that the rebuild under the hood has about 100k on it. How many miles can you expect out of a rebuilt motor?
Which trans was more capable in the towing department? I currently drive an '85 supercab 4x4 6.9 with the 4 speed manual and am planning on making the two trucks into one?
Thank you for any input on the situation

 

The cast iron 4 speed is hard to beat for strength.
It is very easy to beat in the mileage department though by the weaker 5 speeds and E4OD automatics that have overdrive.
The C6 and T19 four speed have the same 1 to 1 top gear ratio.

Pouring a cup of fuel in the intake is asking for trouble.
You have an air leak in the fuel system that is allowing the fuel to drain back to the tank.

 

so the guy that gave the truck to me said that the injector pump was bad. That is why he had to put the fuel down the intake. He swears he has done it for 3 months now. What damage has he done?

 

If the IP was bad it wouldn't run very well. Not just the startup. Look at the return lines on top of the motor. If painted gray then there is the problem, Air will leak in letting the fuel return most of the way back to the tank. But the line will not leak fuel out as the fuel molecule is larger than an air molecule. Replace all lines and o rings.

 

The pistons at top dead center are very close to the heads. If to much fuel went into one cylinder, the fuel will not compress like air will. At 22.5 to 1 compression, you are taking 54.8 Cu.In. and squeezing it down to 2.3 Cu.In. during compression. If more than 2.3 cubic inches of liquid enter a cylinder during the intake stroke, you have damage. Even if it is less that 2.3 Cu.In. of liquid, head gaskets and head bolts will take a beating from elevated compression pressures. Head gasket, head bolts, pistons, connecting rods or crankshaft damage would be the result.
The name of this condition is hydraulicing the cylinder or hydrolock. The piston running into a cylinder full of liquid is like you running into a brick wall at a full run. Something is going to break, and it probably will not be the wall.

Also the fuel poured into the cylinder will not be sprayed in at the correct time. I would be surprised if it even burns since it is still in a liquid state. If it does, it will not be at the correct time, the engine could run backwards.
What fuel does not burn will wash down the cylinder wall and wind up it the oil which will over time dilute the oil. That reduces the shock loads the oil is able to absorb before bearing damage.

Pouring gas in a intake is a gasoline motor trick. But they have spark plugs to ignite the mixture in the cylinder at the corret time.

Diesels use compression to heat the air to a level that the fuel will ignite when injected. That is why injection timing is critical on diesels. The 6.9 is running 22.5 to 1 compression. Fuel will autoignite at around 470 degrees F. Where that occurs in the compression stroke may not be the correct place for the fuel to be ignited.

Starting problems with the IDI diesel fall into three areas.

Electrical:
Not enough battery amperage from dying batteries, engine cranks to slow.
Failing starter, engine cranks to slow.
Bad electrical connections, engine cranks to slow.
Either of the two starting relays failing, if the engine cranks this is not it.
No voltage to the fuel shut off solenoid, if the engine runs this is not it.

Glow plug system problems are almost electrical problems since they are elecrtically operated:
Bad connections to the relay or glow plugs.
Bad glow plug controller
Bad glow plug relay
Bad glow plugs

Fuel problems:
Blocked fuel filters or lines, if the engine runs this is not it.
Air intrusion is a biggie here, air will seep in holes to small for fuel to leak out. Makes them very hard to find, but lets the fuel needed to start the engine drain back to the tank. Return line tee's and the return lines are common problems. The water seperator on the early (Pre 87) engines was known for air leaks. There is also a return line from the filter to the #1 injector return line that causes many problems.
Bad lift pump
Bad injectors
Bad injection pump

If the truck runs good after it is started, a lot of the above problems can be eliminated.
My main areas to look for the trouble on this truck would be the lift pump, air intrusion, the battery system, and the glow plug system.

I would start looking at the glow plug system.
The pre 87 trucks used the old style glow plugs and controller. It was great when it worked, but when it started failing it was a nightmare.
First questions, does the "Wait to Start" light come on?
How long?
Do you hear a click under the hood when you turn the key on and the engine is cold?
Have you tested the glow plugs?
Have you ever seen the "Wait to Start" light come on while driving when the engine was warm?

The IDI is a simple engine, very precise but simple.
With a little trouble shooting it is easy to fix until you have internal damage.
Then it is still easy to fix, but expensive.

I almost forgot, do not crank the starter for more than 15 seconds or so without a 2 minute rest between cranking cycles. The starter will overheat and die.

 

[QUOTE=caseyatbat]
The truck seems to run really good, (after you pour about a cup of fuel down the intake!) and I was just wondering what would cause this?


I would think this is the air in fuel system.

 

the pump is probly not the source of you prob mostly it is a line leading from tank to pump next time before pouring the fuel down the intake take the time to pull the fuel filter off and see if it is full if so then see if the fuel pump is working in my case i just installed a eletric pump turn key on and let it preasure up and found that i need to replace fuel line from tank switch to fuel filter and the return lines and o rings and returnline from pump to tank switch

 

i have an 83 and i have starting issues. i put an electric fuel pump on it,o rings,return lines, seperater bypassed and stuff like that on it. still no start without help from ether. i am in the process of putting glow plugs in it now but i ran into a huge problem. i really think my problem now is the injector pump because with the fuel pump it has plenty of psi. also it takes 10 seconds for fuel to even start to trickle out of the injector pump. it runs good after it is warm but then after driving for a few minutes it will die and not start back up. i am pretty sure my injector pump is bad. i am getting power to the solinoid and it works when hot. i think you probably just need an electric fuel pump or possibly an injector pump like me.

 

F100guy,

Have you checked the fuel lines under the cab?
It could easily be a small hole in the fuel line letting air into the fuel system.

 

yes they did not look cracked but i am going to replace it one of these days. also the injector pump says it is a remanned unit. i just got this so i know nothing about it.

 

They might not looked cracked but has a lot of rust on it in certain spots. I found my leak under a clip holding down the fuel line to the frame. It wasn't leaking fuel but was very rusty, pulled on it just a little bit and boy did the fuel flow.

 

when i put the fuel pump on down they i bent one to fit and was jerking it around a bit. i know it is not bad. that was after the tank switch. i can't do anything to the truck until i remove the heads and get the glow plugs out. so that is going to wait a month or so.

 

I had a 6.9 that the dip tube in the tank had a little hole in it. It would not start with out a little either. Once running it ran great until the next time you let it set for a few minutes or so.