Common Engine Surge Problem

[Tigershark2005]

Ok let me give you the background. I had to rebuild my engine after an old timing belt shattered a piston. New head, valves, rods, pistons, everything except block and manifolds. It runs fine, all speeds, except it doesn't always idle properly. Occassionally I start it up and it idles right at 750. If I turn the car off for maybe 3 minutes it will then start and die. When i turn up the BISSS (base idle speed set screw) I can start the car and by using the screw get it right at 750. Well if I drive around for a while then come to a stop and put it in park, the engine will go up to about 1500 then drop to 1200 and keep going up then droping. Turning the idle screw down will fix this but then if I turn the car off and back on the car will be set too low and will die. Sometimes it will work just fine. Car info is 94 talon 2.0 ES. Also does anyone know what kind of alternator I would need if I wanted to get a new one? Like amp rating.

 

[Quasimondo]

I doubt the alternator is your trouble (autozone will hook you up with the right one as long as you tell them it's for a 2.0 mitsubishi eclipse, btw), but it sounds like you have a vacum leak somehwere. When you pulled the head, did you remove the intake manifold, and if you did, was that gasket replaced? What about vacum lines? Did you put new ones in to replace the ones that may become brittle with age?

 

[steve]

Also make sure that your ISC is working and that you adjust the BISS while grounding the timing adjust connector and pin 10 on the DLC.

On a 1G N/T assuming it's a 2.0L you can use the 75A alternator from the turbo cars or the 90A from the Galant VR4 or Canadian DSMs.

Steve

 

[Tigershark2005]

Well I drove to work this morning and it was great. When I started it up cold it calmed down to 750 after the initial 1400 for a start. During the day however i took it out again and when i started it I had to tap the gas a little to keep it above 300 rpms. I put it in gear and got moving. When I slowed down it started surging. I tightned up the intake manifold and I don't believe it's coming from any vacuum lines. If this means anything, I unpluged the ISC motor cables and it was still surging. Possibly the ISC is stuck open or the TSC isn't working correctly? I'm thinking that it takes the ISC a while to close all the way then when I start the car it will open fully and let air in. After opening it doesn't shut very quickly so the engine keeps adjusting gas mixture since it's getting extra air but seems to have the throttle closed. Think this might be it?

 

[steve]

When the ISC and ISC driver circuit is working correctly the ISC first opens up to help the car start and then quickly closes down to 0 steps while the car warms up. If the BISS is adjusted properly, once the coolant temp settle the ISC will be between 20 and 30 steps while idling at 750 RPM and turing on the AC or turning the steeering wheel will cause the ISC to step open to maintain the idle. Until the ECU learns it's settings the RPMs may overshoot the target speed.

If your idle is sagging and you have checked the ISC coil resistance then it's time to check the ECU.

Steve