Hey all new user here, long time lurker, in need of some serious help. A few months ago I bought an old 95 F-150 xlt 4x4 with a 5.8l with 300k miles and an automatic transmission (E40D) and wanted to fix it up because it was in a sorry state. The engine block was cracked and it had a melted piston but still ran pretty okay. Anyway i pulled the engine and trans and put in a new long block and had the transmission rebuilt. after spending a few weeks putting it back together it started right up and then all the problems started.
It starts up no problem but then dies almost immediately, it'll do that a couple times before it finally sticks but the rpms fluctuate from around 1100 to 400 or less.(once warm it will start just fine without dying). If i give it any gas at all the rpms will bog down before jumping up. If i try to drive it, it struggles and wants to die over just the slightest inclines.(it struggles to go over a bump exiting my driveway that my sedan can just coast over). It runs worse than with the old engine
Since then i have replaced anything that i thought could be the problem. TPS, FPR, IAC, EGR, ignition coil, Distributor, PCV valve, new tanks and pumps, even the computer. and some others i cant think of off the top of my head. I've checked vacuum, fuel pressure, engine compression, and even done a smoke test to it and haven't found anything wrong, it doesn't even throw any codes or overheat at all. I even had a mechanic come and look at it and he was just as confused as i am. the only thing i noticed is that if i pull the big vacuum line off that tree on the upper intake manifold it runs a hell of a lot better, it'll drive fine and has power like it should, but i doubt its good to leave that unplugged.
Im at the end of my rope on this one. Im about to just drop it off at a mechanic and let them have a go at it.
some other things of note about the truck. it has new injectors, plugs, wires, temp sensor, o2 sensor, and the exhaust doesn't have a cat or muffler its just a straight pipe that dumps out just in front of the rear passenger tire.
I don't know anything else to check to try and fix it so any help at all would be greatly appreciated. I want to get this fixed myself instead of accepting defeat and towing it to a shop lol

 

The big vacuum line goes to the brake booster, without that connected there is no power braking assist and that would constitute a big vacuum leak at the motor which would normally cause drivability issues, but in your case it helps so makes me wonder if the air filter is clogged.

Have you inspected the air intake filter and the intake tubes just to make sure there is nothing blocking air flow?

 

Is your truck a MAF engine or a MAP engine. If it's a MAP sensor engine, is there a vacuum line going to it straight from the intake?

 

Well the filter is clean but ill have to check the intake tubes tomorrow to be sure if they're clear

 

Its a MAP engine and it has a vacuum line going to it from the intake

 

Do you happen to have the part number for the long block you bought for it, or any specs on the engine itself?
Was it built specifically for the truck with fuel injection or is it just a generic rebuilt 5.8?
Also, what is the timing set at and did you pull the spout to set the timing?

 

That’s what first crossed my mind. Maybe the camshaft isn’t EFI compatible.

 

Well this is the one i bought https://www.powertrainproducts.net/f...ler-p/1648.htm i cant find any of the paper work it came with at the moment.
And when we set the timing we set it to 10btdc and we pulled the spout when we did it. Mechanic thought the timing might have been off and he did the same thing.

 

I checked the link and it has very generic information, basically this is what it will would have come in truck wise. No actual specs on the engine.



Well, pulling the big vac hose introduces a large amount of air, so it should go lean. But it runs better so maybe the engine is overly rich. Is the fuel pressure regulator new? Pull the vacuum line off of it and see if it is leaking fuel. If it has a bad diaphragm fuel my be being introduced that way. When the engine was installed, did anyone happen to notice if there was actually a roller camshaft installed? Maybe it has the wrong one for the application.

 

The vacuum "test" (pulling the main hose) shows you're probably on the right track. You may have good vacuum, but if you have a clogged tube then that vacuum isn't getting somewhere. Check your vacuum lines, especially any points of repair, and be sure they are open all the way through. I chased a problem longer than I should have because I had "good" vacuum, but a previous repair joint had collapsed blocking vacuum to some activator (I forget which one it was.)

 

The FPR is new and i pulled the line off and no fuel leaked out, also should anything change with the engine when the vacuum line is pulled off of it? I don't know if it should or not but it kept running when i did without any change. just curious. As for when we installed the engine, no i didn't really pay attention if the cam was a roller or not. Is there a way i can check?

 

I pulled out as many vacuum lines as i could and tried blowing air through them all and none of them were clogged or kinked

 

Of air, fuel, spark, and timing, I'd guess it's something related to air since it runs better with more of it. Maybe something related to air measurement or flow causing the problem?

 

Could be a bad MAF sensor causing the engine to run rich. Try leaving the vacuum hose off that makes it run good, then unplug the MAF and see if anything changes. Also try this with the vacuum hose connected, then pull the MAF hose. If nothing changes either time, I would say the MAF is bad.

 

According to post #5 it is speed density.