I want to reset my base idle and don't have my manual.
Which pins?

 

Jumper ALDL pins A & B (two upper right) and turn the ignition On for 30sec to extend the IAC pintel fully.

With ignition still On, disconnect the IAC wiring harness and start the engine.

Then, using the set screw in the left side of the TB, adjust base idle to ~450RPM, backing down from higher RPM if necessary.

Shut the engine Off and reconnect the IAC harness.

Restart the engine and verify correct normal idle.

 

I had to turn the idle up to get it to 450. Even so, it barely would idle there. I think i got it to 475-500rpm, and when it stalled there (for the fourth time) I just turned it a smidgen lower and called it quits. I hooked the IAC back up and plugged in Diacom and my IAC is reading around 130. Before it was around 40. Isn't this high? (temp 170)

The throttle responce is worse than it was. Also, the throttle pos sensor is at
.63v which is where it was when I started.

I have a .515 lift cam with a bored throttle body, big mouth runners and manifold, 10.3:1 comp 383ci...etc...

I thought I remembered ski_down_it talking about lowering the IAC to help control a lumpy cam? What I am trying to correct is a wondering idle.


[Modified by severe-1, 6:01 PM 1/10/2004]

 

I think i know exactly whatyour probelm is.. that is if your running a stroker LT1.. I had this same exact thing happen to me.. you need to increase the size of the bypass hole... read this thread I started for more info.. I think you will find it VERY helpful...
http://forums.corvetteforum.com/zerothread?id=719463

 

Sorry i didn't say.... Its a L98.

Thanks for the info though. I read that post when it originated and thats when
my wheels started turning to readjust mine.

 

I'd do this...but I don't have a set screw in the TB! LoL, it looks like they covered it with a piece of metal at the factory! Should I drill through that metal, so I can get to the screw?

 

:lurk:

 

A couple of things:

I'm told GM issued a service bulletin some years back raising the minimum idle speed setting to 550, up from 450, although my GM Shop Manual shows 450.

My camshaft, while not at all radical, doesn't like even a 550 minimum idle speed setting. Using 550 causes the IAC count to skyrocket

Setting it 100 RPM below the desired idle speed burned in the PROM brings the IAC counts down and allows full control over engine load idle variables.

What does Diacom show as your closed loop, fully warmed up idle speed?

Since you have aftermarket parts, you may have to play with the minimum idle speed adjustment screw to find the setting the engine likes best. Low IAC counts (so that it maintains a large adjustablilty range) AND low enough so that the engine RPMs will drop when you lift from the throttle at speed.

I just went through this not too long ago when I replaced the stock TB with a 52mm BBK. Took several different setting tries before I found the sweet spot.

Hope this helps.

Jake

 

Yes, that's what you need to do to access the adjustment screw.

Jake

 

Well after driving it for a while with the "new" setting, the IAC settled into the
40-60counts range (700rpm). This is probably where the idle was before I started adjusting the idle screw since the throttle pos sensor still reads .63volts.

When I originally set the idle over a year ago, the motor was brand new and I had some difficulties with originally getting it to idle. I thought i may have not quite gotten it right, but I quess i did pretty good.

What originally bothered me about the idle way back then was the BLM of
160(!) I turned the fuel press way up to compensate this. Later on the dyno, I slowly worked fuel press down to about 12.5:1 stoich wot.

At the dragstrip, further leaning produced better et and mph (42psi)

Once again, the BLM's are 160 with the intergrator at 132-136 range w/2.3pw.
This is what I am quessing is my idle prob. (the chip is a TPIS stage V)

As a side test I shut down the IAC and raised my idle screw to 700rpm. The car still did not idle steadily, so I am assuming that it is not my IAC over compensating. (bounces back and forth between 6-800rpm)

Sorry for all the wind...

 

160 means it's running lean and the ECM is adding fuel to the max. Some books I've read say 255 is the max reading, but I've never seen mine go above 160.

Are you setting the TPS voltage with the engine off; key on or is that the reading with the engine idling?

Have you tried different TPS voltage settings to see it makes any difference?

My pulse widths at idle, closed loop are 1.7/1.8, so your 2.3 tells me that you're getting longer "on time" to help compensate for the lean condition.

Is your 02 sensor relatively new? Is it very active during closed loop idle; not slow and lazy?

What are your camshaft specs? 700 RPM desired idle speed may just be too low for the engine to idle comfortably. Mine is 850 in neutral/park and 800 in gear.

Jake


[Modified by JAKE, 10:06 PM 1/10/2004]

 

I am setting the TPS with the engine running.

No, i haven't played with different TPS settings.

I installed a heated o2 sensor approx 3 mo's ago.

I can't remember the cam specs off hand, but I had the cam custom ground for low end grunt. It has no overlap and a 114 LSA.

 

Okay, I've been watching and I'd like to get a little clarification. You guys are saying that I need to drill out the metal that seems to be covering the idle screw? Is this right for a 1990 model?

 

Ras,

I can't remember if I drilled it out or dug it out, but they come sealed from GM.

You need to get it out of there to adjust idle.

 

You can pry it out with a little screw driver. Some drill a small hole in the cover, being careful not to go 'in' too far - only enough to get through the thin metal of the cap. Then screw a screw into the hole just enough for the threads to catch, grab the screw with pliers and pull the cap out.

I like using a little screw driver to pry it out.

 

Thanks guys, I'll try it this afternoon. After the xmas lights come down :U I thought all along i had some different throttle body.

 

The TPS voltage setting - .54 volts +/- .075 - should be set with the engine OFF and the ignition key ON. Not with the engine running.

How much difference does it make? I don't know - never did back to backs - but it's the recommended procedure.

To lower the IAC counts, you open the throttle butterflies more using the adjustment screw on the throttle body. As you open up the butterflies, the IAC counts will drop (closing off the passage more) in order to maintain the desired idle speed setting that is programmed into the PROM.

See, it works like this. The ECM is going to try to maintain the idle speed that is programmed into the PROM. As you open the butterflies, the idle speed increases. The ECM detects this and commands the IAC to close some in order to lower the idle speed back to the speed programmed in the PROM. This closing causes the IAC counts to drop.

If you open the butterflies too much, the IAC will be commanded to close completely and will not longer be able to control idle speed and to adjust to different loads on the engine.

Enlarging the passage in the throttle body is something that I just came across onthis Forum. It involves drilling the passages to a larger side and, from what I read, is sometimes needed when a larger engine and throttle body is involved.

I'm running both (53mm BBK TB and a 415 cid engine) but haven't found drilling out the passage necessary. So I guess it depends on the engine involved.

Be sure the engine has gone closed loop and is fully warmed up when you make the adjustments.

The engine then needs to be shut down, restarted and driven at about 45 MPH afterwards so that the IAC will adjust to the new setting.

Hope this helps.

Jake

 

Jake - Drilling out the bypass hole was nessesary on my engine 48mm TB with a 383cid because I was getting split blms which is a situation that cannot be diagnosed on engines with only 1 02 sensor (such as the L98). Also, My engine is a LT1 which has special idle air ports that plumb directly into the intake runners.. these were needed due to the LT1's short runners to help control the effects of cam reversion.. I think with the L98 the runners are long enough where this passage way wasnt need.. and in turn minimizing the need to enlarge the TB Bypass hole.. :yesnod:

 

The IAC passages in the L98 terminate in the intake manifold.

What are the cam specs? What year car?

To get the cammed motor to idle well, you'll have to juggle TB opening, IAC, commanded idle, spark, and fuel.

Drive to Sarasota, and I'll get it running right.