Surging help

[Hack]

My new build (essentially a '93 Cobra motor but with an aftermarket cam in it) surges some times.

Mainly when I come off the freeway coasting if I push the clutch in the car sometimes dies. Or sometimes it will get down to 100/200 rpm and then it will rev to 1500+ and cycle a few times before finding its proper idle.

The same thing somewhat happens sometimes when the engine is cold and I first start it. It will rev to 1500+, then dip down to 600 or less and cycle back and forth a few times. Once it stabilizes it idles at 700-800 (by the crappy dash tach).

I have the idle set at about 675 with the tps unplugged. The tps reads between 0.8 and 1.5 volts when the throttle is closed.

What should I check next? What is causing this?

[Turbo302]

I would try cracking the throttle slightly and recheck TPS so it is around .95 to .99 at idle with this new position.

I usually like to have idle come down gently after free reving. If you have blade too closed, it will usually fall right down abruptly after free reving and surge as the IAC is trying to find idle.

Give it a try...

Tim

[Stormwalker]

http://forums.stangnet.com/showthread.php?t=698148

[Chrm97PI]

iac, or a vacume leak

[Hack]

I would try cracking the throttle slightly and recheck TPS so it is around .95 to .99 at idle with this new position.

I usually like to have idle come down gently after free reving. If you have blade too closed, it will usually fall right down abruptly after free reving and surge as the IAC is trying to find idle.

Give it a try...

Tim

What do you mean by cracking the throttle slightly? Do you mean set base idle with the TPS disconnected slightly higher? I saw some posts that a larger cam might require a higher base idle setting.

Edit: Can you explain the extremely small range you are recommending for IAC setting? I don't expect I can get that accurate of a voltage reading with my tools. Do I really need it to be that exact?

Thanks for the posts, and especially the link to the Stangnet thread.

I was cleaning the throttle body tonight and I noticed quite a bit of stiction when the throttle is completely closed and you try to open it. Is that normal?

Cleaning the TB didn't help, but I was driving some more and noticed that if I blip the throttle the slightest amount when I push in the clutch there is no surging at all. Also, if I am stepping on the throttle even a little when I push in the clutch there is no problem.

[Turbo302]

What do you mean by cracking the throttle slightly? Do you mean set base idle with the TPS disconnected slightly higher? I saw some posts that a larger cam might require a higher base idle setting.

Edit: Can you explain the extremely small range you are recommending for IAC setting? I don't expect I can get that accurate of a voltage reading with my tools. Do I really need it to be that exact?

Thanks for the posts, and especially the link to the Stangnet thread.

I was cleaning the throttle body tonight and I noticed quite a bit of stiction when the throttle is completely closed and you try to open it. Is that normal?

Cleaning the TB didn't help, but I was driving some more and noticed that if I blip the throttle the slightest amount when I push in the clutch there is no surging at all. Also, if I am stepping on the throttle even a little when I push in the clutch there is no problem.

Hook up a Voltmeter to monitor TPS. If your volts are under 0.99, then just try to open the throttle slightly (via screw). If your TPS goes 1.0V or over, then you will have to loosten the TPS, adjust, and to try and get as close to 1V without going over. You may have to enlongate the holes sightly to get your desired volts.

What you are doing is allowing slightly more air into the engine. You will be on the one side of the IAC counts so the idle might hang a little more, but as long as you feel it is appropriate, and the idle surge is acceptable (maybe 1 drop and settle), run with it.

You could play with fuel pressure within a couple psi one way or the other as well to see if it likes more or less fuel.

Tim