Hope we can solve this..
Car idles fine at 750 rpm at 0 mph. When I drive away everything is fine but when i take the foot of the gas it idles at 1500 rpm and WONT drop under it until I stop the car! So I can drive under 1500 rpm unless I stop and drives away again.

I have a scan tol but cant find anything wrong!


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GTA -89:Free flow muffler,dual 3" HF cats, Edelbrock headers, Vette heads, 1.52 RR 7/16, ZZ-9 cam, Big Mouth, AS&M LTB, ported plenum, 52 TB, MSD 6AL, TPIS level V prom, Mod .MAF, Superior shift-kit and some minor mods. and more to come!
http://www.itv.se/tacs/members_cars/m279/m279.html (Swedish Trans Am Club)

 

I had an 89 TA that did the same thing. it was the TPS. the ecm would throw a code 21 though.

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92 Trans Am/WS6/Blue/305/5Speed/T-Tops
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69 Camaro/427-currently awaiting restoration

 

Not necessarily .. my TPS was WAY off and it performed similar with no engine code. Start with doing the IAC and TPS adjustments ... see what that does for ya.

 

My TPS is at 0.54 v when I take the foot of the gas, and my idle is at 750 at 0 mph. No error codes. No vacuum leak (as far as I know...)

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GTA -89:Free flow muffler,dual 3" HF cats, Edelbrock headers, Vette heads, 1.52 RR 7/16, ZZ-9 cam, Big Mouth, AS&M LTB, ported plenum, 52 TB, MSD 6AL, TPIS level V prom, Mod .MAF, Superior shift-kit and some minor mods. and more to come!
http://www.itv.se/tacs/members_cars/m279/m279.html (Swedish Trans Am Club)

 

Try driving with the scanner hooked up and see what the TPS shows when the problem occurs.

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"84Z28/'91 305TPI comp cam flowmaster TB coolant bypass/shutoff valve to heater core MAT relocated Ram-Air(home-made) gutted air-box IROC ground effects/wheels GY245/50's 700R4 superior shift kit/ corvette servo(excellent shift kit BTW) Rebuilt engine and transplanted it myself. Love this engine!
LOW performance people should drive low performance vehicles!

 

If you have access to a scanner that can read the IAC stepper counts, check that too, if I remember correctly it should be about 15 at 650rpm. I had that same kind of idle problem a while back, it would idle around 13-1500 until the ECM swap a couple months ago when after scanning with Diacom we found that the IAC was out of whack... just a suggestion, but it worked for me. We were able to cure it by just backing out on the throttle screw. Not saying that's the way to fix yours, but give that a try.

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Kevin Irving
85 Trans Am WS-6, 305 TPI, custom burned '86 PROM with '87 "165"ECM, Accel Supercoil & 8.8 Wires, MSD 6AL, Aluminum Driveshaft, TB Coolant Bypass, Ported Plenum, Modified MAF, Cyclone Fuel Pump, JET Airfoil

Veiw my car at: http://www.geocities.com/transam85tpi/TransAm.html

 

should try adjusting your tps ,
maybe till its about .68 at idle

 

Proper TPS voltage is 0.54vdc +/- 0.075v.... 0.68 is WAYY too high. .65 is the highest I would go.

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Kevin Irving
85 Trans Am WS-6, 305 TPI, custom burned '86 PROM with '87 "165"ECM, Accel Supercoil & 8.8 Wires, MSD 6AL, Aluminum Driveshaft, TB Coolant Bypass, Ported Plenum, Modified MAF, Cyclone Fuel Pump, JET Airfoil

Veiw my car at: http://www.geocities.com/transam85tpi/TransAm.html

 

My TPS is at .54 v when it occurs.

Tried to record the event yesterday so I can post all the data but my scanner could'nt connect to the vehicle???? Will try it this weekend again after cleaning the connectors....

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GTA -89:Free flow muffler,dual 3" HF cats, Edelbrock headers, Vette heads, 1.52 RR 7/16, ZZ-9 cam, Big Mouth, AS&M LTB, ported plenum, 52 TB, MSD 6AL, TPIS level V prom, Mod .MAF, Superior shift-kit and some minor mods. and more to come!
http://www.itv.se/tacs/members_cars/m279/m279.html (Swedish Trans Am Club)

 

in alot of cases adjusting the tps higher to compensate for more airflow due to modifications is normal. i've seen settings as high as .72. alot of buick grand national guys adjust theirs up to gain a richer air fuel ratio. max voltage can be is 5.0. if adjusted at .54 you aren't even close to max adjustment. it's another way of moving up higher on the mapping programmed into the chip. fairly simple the chip is a large programmed map. how you take advantage of it is up to you. the better question i have for the situation in question is what is the iac counts at during this high idle and what is the desired idle rpm shown by the ecm????? this is the more important info.

 

Des.idle is 750 rpm
IAC is at 47 steps
TPS .54 V.

 

This is downright strange...mine started doing this also after swapping runners this Saturday. As long as I'm not moving, it idles down to desired idle 750rpm @ 49 IAC counts. However, if I'm driving it "hangs" at ~1500rpm @ ~75 IAC counts. After a few seconds of not moving, the ECM knocks the IAC down to ~49 again and I'm back to 750rpm.

Vacuum gauge shows 14in/Hg at 750rpm idle (LT4 HOT cam on board which is what I've been seeing since I built the engine last year.

I'll check the "Desired Idle Speed" tonight on the way home and see if the ECM "wants" it to idle this high. Any other ideas greatly appreciated!

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1982 Z-28

See http://www.mycar.net/mafb/registry/detail.cfm?id=276 for details

 

I found the problem: sticking throttle shaft. I had the TB off while swapping runners this weekend, hosed it down with TB cleaner and apparently de-lubed the bushings. Quick cleanup and lube w/ white lithium grease and I'm back to normal!

How I found it: scanner shows TPS=0.54, IAC=50, Desired Idle=750 while I'm idling normally. After I touched the gas and let off, TPS=0.72, IAC=64, Desired Idle=750. The ECM sees the throttle is slightly open and doesn't command the IAC to close.

-Kevin

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1982 Z-28

See http://www.mycar.net/mafb/registry/detail.cfm?id=276 for details