85 Lynx - Random Engine Misfire, Fine When Choked
This problem has been stumping me ever since i've had the car (a year ago). When the engine is cold, it will not start. I barely touch the gas pedal and the choke kicks in and it idles at about 1200 rpms. After some time, i tap the pedal again and it goes down to 1000 and pures like a kitten. No misfires at all. The only thing i notice is that the tach seems to hunt between 950 rpms to 1050 rpms. Not enough to audibly or physically notice, but the needle shows it. However, when the engine reaches operating temperature, and the choke is totally disengaged, it idles rough at about 400-800 rpm. The rough idle seems to be caused by a misfire. The tach needle is very jumpy. It starts going up for a few seconds then you can hear the engine misfire and it goes back down and hovers for a second there and then back up, then misfires and back down it goes... you get the idea. When i apply too much brake the engine almost stalls, or does stall if the defrost is on (because of the draw of the A/C compressor). Now, here are the things i have had people tell me it is:
1. Vacuum Leak
2. Bad Spark Plugs
3. Bad spark plug wires
4. Bad distributor cap and rotor.
Here is how i eliminated each one:
1. Checked vac hoses. Checked EGR valve. Engine has no PCV valve, so that isnt it. Did the "starter fluid test" where you spray fluid around the intake manifold to test for gasket leaks there. Does anyone know a good way to test the brake booster, i dont think this is it, but ill try it.
2. new plugs.
3. new wires.
4. new cap and rotor.
What i am wondering is what other part could fail to cause a random lean misfire. I think it may be the fuel pump but i dont know how it would relate to the choke. The reason i suspect the fuel pump is because when i first got the car it kept stalling and wouldnt let me get the idle up to higher than 1200 rpms in gear. Then, I changed the fuel filter. It did it once more in the first month i had it. However both times were after long trips and i think it may have just been vapor lock since it has a mechanical low pressure fuel pump.