I have had your exact issue with numerous vehicles from the 80's when fuel injection was still sorta new. The solution for me, was a clogged EGR valve. You can replace it, but they are cleanable also. They get a build up of carbon on the little piston that is vacuum actuated so it stops moving, then causes it to act like it is overheating even when it is not. If you have the vacuum actuated kind, go get a fresh EGR gasket first - very cheap, then remove it, spray the end where the rod goes up & down with good penetrating fluid, then if yours has the rod that protrudes from the bottom, whack it gently on a hard table top then air blow it out, then lube it again, then re-instal & see it that helps. If you also have the early fuel injection with the air pump, remove that hunk of a turd & you will get better mileage too!

 

check your spark ? ig coils are notorious for failing under heat on the 80s 318s. might idle good but then underload it could eff up.

 

I'm having the same issue with my truck. The cause is the coil overheating. There is a universal plug wire kit from Accel that comes with a 60" wire and several wire ends and boots. It runs about $16 from Advance auto. Worth a try to relocate your coil to the fender and get it some airflow.
I would also suggest if you do move the coil, go with an aftermarket performance coil like the Accel Superstock or MSD Blaster. A little more power and you'll notice an increase in fuel econ!

 

Thanks everyone. Ran in to a few more problems such as a leaking brake master and blown heater core. It will be a while before its back on the road.

 

So, a little update.

I havent been really caring about the truck lately so i left it until now. So today I changed the coil, leads and plugs. Still having the same problem.It idles decent with a miss here and there when cold. When it starts to warm up it sounds like it has a big cam in it. Once its fully warm, it wont run. I have to mash the gas peddle to the floor and it basically sounds like its running on one cylinder. The truck runs pig rich when warm and cold.

Im getting really frustrated with this truck. I love it, its 100% rust free so i dont want to get rid of it, i just want to drive it. lol.
I think im going to try and find the EGR valve and clean that tomorrow. see how that goes.

any other suggestions?

 

Here is a video of how it runs once warm

http://www.clipshack.com/Clip.aspx?key=9D41970A879DBEF8

 

Ok, update. What a stupid mistake. My buddy was helping me with the truck and noticed a broken fitting from a line going to the valve cover. Well that is the hose to the PCV valve. He blocked it off with a bolt. I took the bolt out and BAM! Runs like a top again. SOOO happy. Thanks everyone for advice.

 

So i insured the truck for the summer and guess what, it wont run when hot still.

Ive changed; Coil, EGR, Vacuum lines(that were brittle), battery, alternator, air filter, coolant temp sensor on intake manifold and a whole new TBI(started after the TBI swap). The reason for swapping TBI was because it wouldnt idle and the injector spray patter was less than ideal.

The symptoms...

Runs excellent when cold. Lots of power and goes like stink. When it warms up it start sputtering, black smoke comes out the tail pipe and it REEKS of raw fuel. Hardly any power at all and is very hard to start once it stalls out. I let it sit for 5-10 min and i can start it again and drive. It sounds like an ignition or fuel problem to me.

any ideas of where to look ? I just want this thing to run so i can enjoy it.

 

Decided to pull codes and coolant temp sensor came up twice. sounds like it could be the culprit.

 

When you get a code for the CTS, the problem may not be the sensor itself. Codes represent the whole system. In this case that includes the sensor, associated wiring, the associated computer internals, and all connectors. Wiring and connector problems are very common on our trucks. Voltage and resistance checks need to be done for trouble shooting any problem.

Have you looked at the injector spray pattern? I understand you replaced the TBI but if there was trash in the fuel system even new injectors can give you problems very quickly. Have you changed the fuel filter? How old is the fuel in the truck?