[87iroc355zz4]

If i disconnect the electronic advance it drops down to a constant idle.Once i plug it in it idles higher constanly at a certain rpm ,thats too high for me.Then i put the car in neutral and tap the gas then it drops back down for a little while. What controls the idle control valve? It seems to be idleing up when ever it wants.Its been doing this long before i built my engine.

[Everett#2390]

ECM controls the IAC (Idle Air Control), a controlled vacuum leak. Bigger hole, higher rpm.

Your idle characteristics are dependent upon the camshaft specs. ECM-controlled engines demand short duration@.050 and a wide lobe separation angle to bring in a high vacuum value. Typically, 220*@050" lift & min of 114* LSA are the minumum of stock specs for a stock chip, I believe.

The ECM is correcting this low vacuum signal, from the MAP sensor, by increasing timing and bypassing more air via IAC, to bring up the vacuum, hence higher rpm.

The above assumes you have a TPI intake system and being an 87, no MAF sensor, and a ZZ4 camshaft.

These are just my thoughts, I could be out in left field...........

[JimM]

Sometimes IAC valves get gummed up. Pull it out and clean the part that sticks into the throttle body as best you can. My experience has been that once this starts to happen, cleaning will fix it, for a while, but the problem will come back in a month or two.


fyi, the IAC is a can, about an inch in diameter and an inch long, located on the throttle body.

[87iroc355zz4]

Originally Posted by JimM

Sometimes IAC valves get gummed up. Pull it out and clean the part that sticks into the throttle body as best you can. My experience has been that once this starts to happen, cleaning will fix it, for a while, but the problem will come back in a month or two.


fyi, the IAC is a can, about an inch in diameter and an inch long, located on the throttle body.

I just installed a brand new iac valve and i took the throttle bodie apart and cleaned it but still have the same problem.Ive had the idle problem for years.

[87iroc355zz4]

Originally Posted by Everett#2390

ECM controls the IAC (Idle Air Control), a controlled vacuum leak. Bigger hole, higher rpm.

Your idle characteristics are dependent upon the camshaft specs. ECM-controlled engines demand short duration@.050 and a wide lobe separation angle to bring in a high vacuum value. Typically, 220*@050" lift & min of 114* LSA are the minumum of stock specs for a stock chip, I believe.

The ECM is correcting this low vacuum signal, from the MAP sensor, by increasing timing and bypassing more air via IAC, to bring up the vacuum, hence higher rpm.

The above assumes you have a TPI intake system and being an 87, no MAF sensor, and a ZZ4 camshaft.

These are just my thoughts, I could be out in left field...........

I have a l98 tpi 355 with a zz4 cam ,aluminum corvette heads,flat top with 2 reliefs ,it has a MAF on it.I had the same problem before i built the new engine. Now its more obvious because of the new engine. Do you think it might be the computer? or vacume leak some where?

[BonzoHansen]

Is it throwing codes?

Vac leak comes to mind.

[Everett#2390]

As Bonzo suggests, it could be a vacuum leak. Since you have TPI, the EGR valve underneath the plenum could be leaking, giving a vacuum leak, if the valve is installed, usually removed by owners.

Having a MAF sensor does help your combo, they are more forgiving towards engine mods. Does the engine quit running when you place your hand over the intake opening? Is the tubing/intake runners/plenum between the MAF & throttle body in good repair, meaning no leaks/cuts/holes?

You might try to reset/check the base settings of ignition timing and base rpm. Bring engine up to operating temp. Turn off engine and ign. Short ALDL Pins A to B, turn on ign, no start for 10 seconds. Turn off ign. Disconnect IAC connector and place igniton system into bypass by unplugging a brown wire either at passenger firewall or by the pass kick panel and remove the short between Pins A & B. The ECM has seated the IAC, bypassing the ign system takes out ECM control of advance.

Start engine, base rpm, on a stock camshaft V8 would be around 600-700 rpm. Timing would be around 6*-10* BTDC. See what vacuum gauge reads at this time. A stock engine reading would be 18-20 in./Hg. Yours with a ZZ4 cam, or vacuum leak, may be around 15-17 in./Hg., I'm only guessing. Adjust for best vacuum reading, the higher, the better.

Connect everything back up, disconnect the battery to erase codes from ignition bypass, and hopefully, the engine will rpm correctly. Read vacuum gauge, the ECM is looking for 18-20 in./Hg. If not there, then ECM is correcting rpm to read 18-20 in./Hg.

You might try the trick of using a Mity-Vac hand vacuum pump on the MAP sensor and see if this helps. If it does, then you need a reprogram.

[Buddy_Lee]

my dads mustang did the same thing and he cleaned out the thottle body and problem solved