A 4 speed auto? 1987 came with either a 4 speed OD manual or a 3 speed auto.

My 1987 van with 4 speed OD manual averaged about 17 MPG for its first 100K miles and now at 199K miles it is closer to 15 MPG. This is mostly a daily drive of 17 miles each way and the occasional road trip in the mountains. All stock, I wonder what I could get if I...?

There are some real common reasons why your slant 6 can't pass emmisions. Your carburetor might need rebuilding. The holley 1945 on my truck had internal leaks which at times made it run rich and while accelerating it had a lean miss. This can lead to your catalytic converter to get plugged up and reduce engine power. Another thing to check is the voltage at your ignition coil while the engine is running . A weak alternator or bad ground can cause an erratic engine misfire. Also check you coil voltage while its running with all the lights on and the ac on high. Good luck.
D150

My van is a 3 speed auto, and currently i'm getting abpout 14-15 mpg.
The theory about a bad ground makes sense, because most times when i put my signals on, the radio/clock display is flashing.
Would that mean i have to tighten-up the negative terminal on my battery?
I also tried to look for the gas/air mixture screw to see if it is maybe out of adjustment, but couldn't find it.
Also, i talked to one friend last week, and he said, that i better have a spare ballist resistor with me at all times, in case it goes bad.
Where would it be most likely located on my van, and how does it look like?

Your gas mileage sounds about right, might be a little better if it was running better. Is your smog airpump hooked up? It is on the front left of the engine, has a belt on it and a pipe that goes to the cat. The cat can't burn up those excess HC without oxygen. Others have made good suggestions about parts to check, here are some answers to your specific questions:

Would that mean i have to tighten-up the negative terminal on my battery? -That and the other ends of that ground cable that go to the body and engine. Might be some other ground and ignition + connections to clean,tighten, and test. I like d150ram's idea of checking the + voltage at the coil while running.

...tried to look for the gas/air mixture screw to see if it is maybe out of adjustment, but couldn't find it. -If its a Holley 1945 of the 1987 era it is low on the carb at about the 5 o'clock position. You adjust it from the passenger seat with a small allen wrench. I have seen a 1945 carb with a screwdriver adjusted idle mixture.

Where would it (ballast resistor) be most likely located on my van, and how does it look like? -There isn't a ballast resistor on this engine if it has the ESA computer. The ESA computer is mounted on the engine side of the firewall and has two electrical connectors and a vacuum hose going to it.

Let me know if you have any specific questions, I have the 1987 service manual.

Thanx Fred, i also like the idea of checking the voltage while running, but un-fortunately i don't have any mesauring equipment, so i ewill have to wait till i can get it into the shop.
I have re-calculated my mileage and actually it is a little worst than i stated, around 12 mpg if i'm lucky(no load).
In the meantime i will try the air-fuel mixture screw.
Here is my question:
Which way do i turn the screw to allow mor air and less fuel?
Also is there any standartized way these things are supposed to be set?
I remember once someone told me that in general, the best way to set the screw is to tighten it all the way, and then loosen it for 3 1/2 turns,
but i'm not sure how reliable was the source.
And last question: What is the ESA computer?

Wolfears,
I think it is a 3/64 inch allen head and you adjust the idle mixture by turning the screw in (clockwise) to lean it out and turn it out (counter-clockwise) to make it rich. Make sure it is warmed up and you may have to adjust the idle speed (a screw on the carb linkage that bottoms out at idle) if it goes too high or low. Find the spot where the idle speeds up and the engine runs smooth. That's it, basically. I would shoot for the leanest, slowest idle that will idle smooth, start easily, and not stall in gear, maybe 750 RPM in neutral.

The ESA is the Electronic Spark Advance computer which controls the timing and some other things. Open the hood and it is located dead center on the firewall.