RPM Surge????

[batesdl]

Hello, maybe someone can help me. Just got me a 93 cobra a few weeks ago and noticed while driving Sunday that when I push the clutch in to gear down the engine surges from 1000rpms down to 300rpms over several times back and forth. Seems about like the car is going to cut off. Any ideas of what may be causing this or maybe this has happened to one of you guys before. Thanks,

Dan

[1993cobra]

any engine modifications? or all stock?

[batesdl]

125000 miles, b cam, 373 gears, msd, full exhuast. This is the metallic teal cobra for sale for 11,500 in Liberty NC.

[Romo93]

Welcome to the site!

Could be an upper/lower intake leak. check all your intake connections. IAC maybe? Sounds like it's probably an air-leak.

Dirty mass-air metter?

Surges can be a pain in the ass sometimes.

[batesdl]

Thanks, the car drives like a dream no kidding. I just noticed it all of a sudden when gearing down. I will check the mass air and go through all that cleaning etc. Thanks for the info.

Dan

[batesdl]

Going to start with IAC and see what happens. I only get surge when first start up and pushing in clutch gearing down.

[Romo93]

Going to start with IAC and see what happens. I only get surge when first start up and pushing in clutch gearing down.

You can also make sure your TPS Voltage is within specs......

[batesdl]

Will do thanks for the help guys.

Dan

[WillyB93]

Welcome to the site!

Could be an upper/lower intake leak. check all your intake connections. IAC maybe? Sounds like it's probably an air-leak.

Dirty mass-air metter?

Surges can be a pain in the ass sometimes.

I agree with this had my red car, surging it ended up being the upper intake gasket.

[buddha]

This was a classic IAC problem back in the day, had a few TSB's on this very thing. Check it first, as well as the other things the others mentioned.

[batesdl]

Ok I cleaned iac and that did not fix so I installed new. Still no luck. Checked my vacuum connections and for leaks, did not find any. I think I am going to try to clean Mass Air Sensor now. U guys are right freaking rpm surge sucks to diagnose.

Good description of what car is doing is surges from 800rpm-1500rpm when cold starting and does this for 40-45 seconds then settles to normal idle. When car warms up while driving I push clutch in for example coming to a stop at a stop light the cars rpms and battery gauage will surge up and down for several seconds then levels out.

[1993cobra]

seems like it takes a few minutes of driving for the computer to relearn after making adjustments. also make sure your egr valve is closing and not sticking open, check timing, and tps sensor.

here is a link on adjusting idle.
http://www.moddedmustangs.com/forums/5- ... reset.html

[93Cobra#2771]

As mentioned above, classic idle issue on our cars, especially when modded.

USUALLY the b cam is pretty much OK for idling, but some cars it gives the surge like you have noticed.

Vacuum or exhaust leaks are the first culprit to rule out.

Then clean your IAC and MAF (assuming you don't have an open air filter element under the hood and that you have stock MAF).

You can also slightly raise idle speed to mask the issue. Sometimes it takes as little as 100 rpm to go from a surging crazy car to a smooth idle. Often, people try to idle a cammed car too low to get that awesome sound, when it really doesn't like that idle speed.

Good luck!

[batesdl]

I will get back to work on it, thanks guys.

Dan

[Cobra Jet]

This exact kit below was the cure for MANY IAC idle surge issues for both stock and modified 5.0's:

http://www.steeda.com/store/ford-idle-b ... stang.html

(Same kit from LMR): http://www.latemodelrestoration.com/ite ... c-Adjuster

It was also available directly from Ford for quite a few years - I'm not sure if it can still be purchased at at Ford Dealer or not, but it can't hurt to call and inquire for price/availability. Those kits were discussed on multiple Mustang Forums and many owners swore by it as ending their surge issues.


There is also this old thread that has useful info too relating to IAC and surging issues:
http://www.stangnet.com/mustang-forums/ ... ed.605187/

Good luck!