Throttle plate stop screw: how to adust?

JD Adams opined in news: snipped-for-privacy@4ax.com:

Your symptoms point to too much air into engine after cold high idle drops back (controlled by IAC)... which means the stop is adjusted in just a tad too much.

I assume the cold idle (after a minute) had been good before?

First look for a vacuum leak; finding none, then adjust the screw out about

1/8 at a time.

Run several day/cycles between adjustments. If you back it out much, you MAY have to start over.

Here's what I've done in the past..

Clean throttle body and plate

Get engine to operating temp

disc IAC with port closed / or block ports

Adj idle speed to 75-100 RPM below nominal (550-575 for 5ltr), goose several times to ensure it returns.

check tps for .95 - .98, adj if necessary

Quick check, on warmed engine, what is RPM with IAC ports disc/closed?

Too close to 650, there's your problem.

** Your symptoms point to too much air into engine after cold high idle drops ** back (controlled by IAC)... which means the stop is adjusted in just a tad ** too much.

That's that I thought too. The cold idle before was fine, and the stop adjustment was set to about 3.5 turns past point of contact. I cleaned the TB well before doing any of this.

** First look for a vacuum leak; finding none, then adjust the screw out about ** 1/8 at a time. ** ** Run several day/cycles between adjustments. If you back it out much, you MAY ** have to start over. ** ** Here's what I've done in the past.. ** ** Clean throttle body and plate ** ** Get engine to operating temp ** ** disc IAC with port closed / or block ports ** ** Adj idle speed to 75-100 RPM below nominal (550-575 for 5ltr), goose several ** times to ensure it returns. ** ** check tps for .95 - .98, adj if necessary ** ** Quick check, on warmed engine, what is RPM with IAC ports disc/closed? ** ** Too close to 650, there's your problem.

Gotcha. I've used this method in the past but have since forgotten about it. I call this the 'unassisted idle' with the IAC and ports out of the loop. I'll check that out tonight when he gets home and the engine is warm.

-JD

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JD Adams wrote in news: snipped-for-privacy@4ax.com:

I posted this a little while ago; it's from an e-mail I received as a result of this topic being bandied about in here years ago. I'm sure this stuff is all in Google, but here it is again:

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The EEC 'likes' or expects to completely control the idle. To properly position the throttle plate, disconnect the wiring harness from the idle control solenoid. (This disables the computers ability to control idle). Adjust the screw closing the plate to the point that the car slowly stalls or is idling very low (below 675 RPM). Reconnect the wiring harness. This puts idle control completely in the hands of the computer. Opening the plate will allow more air to bleed around the plate raising the idle beyond 675 RPM. It may also move the TPS voltage above the idle mode mark near 1.0 volts putting the computer into off idle mode. Either way, the computer will not maintain a steady idle as the demands of the accessories change or other external factors change.

I should comment at this point. My car had the throttle plate positioned at the factory and the threads were sealed with blue paint. I broke the seal to experiment with different positions. Afterwards I found myself trying to put it back a close as possible to the factory setting regretting ever messing with it in the first place. If your car has this marking, think twice before re-adjusting the plate.

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FWIW, this worked beautifully on my car and several friends' cars.

Joe Calypso Green '93 5.0 LX AOD hatch with a few goodies Black '03 Dakota 5.9 R/T CC

Well stated, Joe.... if there seems to be a need to adjust the throttle opening, it is time to find out what the problem really is. One thing often overlooked is the fact that the PCM knows how big the motor is and how much air is going into the motor by computations using BARO, MAF, MAP, RPM, TPS and sundry other readings. Some of the things we do quite innocently can upset the equalibrium.

FWIW.... in most cases, mixture problems will be from lean mixtures rather than rich. There are very few problems that create a rich mixture but a plethora of conditions that can drive mixtures lean.

Back in the early to mid 80s, PCMs didn't have E-Proms... what you gat from the factory was pretty much what you were stuck with other than chipping the thing. 85/86 302 pick ups could develop a real bad idle surge... some I could cure, some I couldn't but it boiled down to the PCM "chasing it's tail", never being able to catch up to it and establish a smooth idle.