Just threw in a comp cams .480/.487 206/212 dur 110 deg sep cam last week. The stock chip works fine except this mild "jerking/bucking" that goes on @ low rpm's/ low throttle position. any ideas? Also, while I'm here, before & after doing the cam, I seem to have this annoying 2-stage type of idle. Start the car, and it rockets up to 1500 rpm, then dies down to about 800, and then-after about a minute, drops down to a nice proper idle at about 600-650 RPM in drive. It does this even when warm, when you pull up to a light,high idle- 700-800, then 600-650 after about 7 or 8 sec. BTW, its a 88 T/A 305 Auto.Thanks

 

The low RPM funnyness could be due to the tight lobe seperation of that cam, although for being that small it shouldn't hurt that bad. It could just as easily be a small vacuum leak or some other detail that got passed over.

As to the idle, have you set minimum air lately? Beyond that, it could be the IAC is getting slow. You see, when not idling, the IAC will move to it's 'park' position and stay there. When you slow down below the max vehicle speed for idle the IAC should start adjusting the idle to the right rpm, but if it is a little slow you would see the rpms stay a little high as the IAC park position is probably more open than normal idle opening, and it's slow in clsing.
...ed

 

Thanks, Ed. Would you recommend I replace it, or clean it? is cleaning it a tough job?
thanks, Jeff

 

From what I've read the TPI doesn't work well with cams that have less then 112 lope separation. How well or not it runs as you go below that number I don't know.

 

What's you vac. with that LSA? Also it sounds like a scan would point you in the right direction......


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86 406

 

Its not related to the LSA or the duration on that cam, its too small. Something else is going on, might just need to clean the IAC, adjust the throttle and/or the TPS, something along those lines. Also make sure theres no vacuum leaks anywhere, that could be causing it.

 

What you're describing is perfectly normal.

At engine start with a 1-wire(internally regulated) alternator, the rpm's need to flash up to a certain rpm(typically 1200 - 1500) to create a excitation to an internal sensing circuit which energizes the alternator. On older externally regulated alternators, the excitation voltage came from a switched 12 volt source. This is not possible on 1-wire because the power regulator section uses a network of 1-way diodes. Once the a 1-wire alternator is enegized it will charge at any engine speed(even below the excitation rpm), so the rpm's drop to the next stage of idle.

The next thing you describe - 700 to 800 rpm - is what's referred to as your cold idle. The higher engine speed allows higher exhaust gas temperature(EGT) which in turn helps the O2 sensor. The O2 sensor needs to reach 600° F before it can begin operating and the ECM can go into closed-loop operation. If the ECM does not sense a proper O2 output within a predetermined time from engine start(which I believe is 2 minutes), then it sets an error code.

Btw, a proper idle speed(once the O2 sensor is operating) should be around 450 - 500 rpm. You may have to adjust the "idle speed adjustment screw" on the TB slightly to achieve this.

 

Thanks 88IROCS, that sounds logical. Another quick one for you guys. can I adjust the throttle body idle adjust screw - a very little bit- without doing the "minimum air" procedure? the reason I ask is cause the adjustment seems impossible to get right. Once too high- next time too low. I've heard that messing W/ the throttle screw without doing "minimum air" will confuse the computer. is this true?

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88 Trans Am 305 Auto Red/Silver, MSD coil,wires, ported plenum, gutted airbox, TB bypass, comp cams 08-408-8, everything else stock

 

It's referred to as the "idle speed adjustment screw", and it's purpose is self-explanatory. It adjusts the amount of throttle opening during idle. After you set the idle with this screw, you may have to adjust your TPS so that ECM does not misread the amount of throttle-opening.

While you're in there, it wouldn't hurt to check the IAC for cleanliness and the proper pintle clearance(remember to disconnect the battery before disconnecting the IAC connector). The IAC is meant as a way to compensate for varying engine loads(from changing vacuum, a/c functioning, etc.,) but not as the way to set the proper idle speed.

Btw, are you using a using a 'cold' thermostat? When I tried a 160° 'stat, I had a similiar condition to what you described. When coming to a stop, the idle would stay at 700 rpm for about 15 seconds and then drop to a nice slow 400 - 500(depending on coolant temp) idle. When I swapped in a 180° unit, this condition disappeared.

 

I'm using a 195 thermostat. But I think I'm having some cts issues. The car is hard to start when warm- about 4-5 sec. of cranking- stumble, the starts amidst a "gassy" smell from the exhaust. could my injectors be leaking- if so - wouldn't it be easier to start with the throttle open? well it isn't.
thanks, Jeff

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88 Trans Am 305 Auto Red/Silver, MSD coil,wires, ported plenum, gutted airbox, TB bypass, comp cams 08-408-8, everything else stock