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High start up idle


LarryK 87

New info, I had posted this once before, and like most others i assumed a possible Vacuum leak somewhere.

When first started the engine has a very high idle 1700+ rpm
it will hold this rpm for 2 -3 seconds, then idle back down to normal range. It does this warm or cold

Have replaced all flexable vacuum lines
Checked and reseated all Vacuum connections
T.B. disassebled and cleaned
TPS adjusted to spec.
Throttle linkage is not binding

what i have since found is this. If i put the ECM into Diag. mode, Key On, Engine off,disconnect the IAC, turn ign off, re-connect IAC, remove Diag jumper, The very next time the engine starts it will start with the expected idle, and maintain just as it should. when i shut the engine down and restart again, Idle is right back to a High RPM for 2-3 seconds, then idles down again. This condition is repeatable.

My question is: Does the ECM hold some sort of memory setting to command the IAC Steps to a specific position on start up? This would be upset when doing the steps listed above, and then re-set it'self for the next start?

Is there a set of steps for reseting the IAC "Home" position?

LarryK 87

got ya all stumped on this one? ... me to:confused


Well-known member
May 9, 2002
1984 White Z-51/ZF6-40/Shinoda body
No codes set??

I'd check for stored codes, then reset the ECM, and try again. On the '84, there is an orange wire from the + battery terminal,that had a plug, inline. Pulling the plug for 10 seconds or more would reset the ECM. I don't have time, right now, to check if the '87 is the same.

All connectors and wires in the area of the TPS and IAC appear okay?

I have not found anything in the '87 manual yet, but I wonder if the condition would improve by disconnecting the cold start switch (ppl & tan wires). It is supposed to run for 8 seconds, below 35*C.coolant temperature, controlled by a variable bi-metallic strip, heated also by an element, which allows less time if the engine is warmer. Perhaps you have a fault here.

Then, some sensor may be telling the engine it is cold.

In Ft.Myers, one of my fav FL cities, BTW, I doubt you ever need the cold start feature, which was eliminated after '88, I believe, in favor of longer injector pulses, ECM controlled.

Jun 20, 2002
Tallahassee, FL
none right now :(
My guess is the computer thinks it needs extra fuel on startup... once it registers the high idle it lowers things back to normal... I am pretty sure '87 has the 9th injector for cold starting... try disconnecting the wires to the injector and see if it still does it.

LarryK 87

the 87 does indeed have the 9th injector, i have run the diagnostics per the helms manual involving that circuit, about 2 months ago ( elect. and fuel pressure tests). That of course does not mean there is no problem there. The only other odd part is that i can make it start properly by simply putting the ECM into diag mode, disconnect the IAC, then key off. reverse the steps, car starts perfectly every time, warm or cold.

But i must also admit, in doing this i have never reset the ECM ( BTW ECM has no error codes stored), just gives me the standard 12-12-12.

i suppose i should finish building the diag cable kit i ordered and hook the laptop up to the ECM, Maybe that will give me more info?
i would assume it will at least give me IAC steps, expected and actual ( Craig Moates ECM software )


Motor head!!!!
Feb 13, 2001
Valencia, CA,USA
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Larry..sorry it took so long

Do this

Start car
Shut car off ( within 10 seconds )
restart car...

It will idle high.. then drop back

Drive car over 40 mph 5 min..

Pull in driveway.

Park car.

Shut off

wait 2 min


Idle cool?

Turn off car

Go inside

Have beer

Try next morning

Post results



Active member
Aug 11, 2002
Washington, PA USA
2004 Black Coupe
I had idle problems with my 89. After swapping out relay, MAF, IAC, TPS, checking for vacumn leaks, cleaned throttle body, replaced the ECM and that did the trick. Picked up one at the bone yard for $10. Best investment I made. It seems that the idle circuit was fried in the old ECM. If you connect the diagonistic cable to the car, with a laptop and correct software, the idle immediately goes to 1000 rpm. When I connected this to my car with the old ECM the idle did not change. With the new ECM the car idles and runs GREAT. I am now testing a second ECM I got at the salvage yard. I now have a backup! Mine uses a 1227165 common in S-10 4 cylinder, standard shift trucks. Good Luck.

LarryK 87

ok, tried vigman's steps, but i did alter one of the steps,( had a baccardi's silver instead), no change. So ok, Tried again, after a package store run for beer, it actually works better now. I wonder if the brand of beer will help anymore?

But seroiusly, the only thing i changed on the second try, was finished a repair on my curtsy/clock circuit, and installed the R.V. mirror(the one with map lights), and re-installed the fuse (has been removed for the last 2 months). Interior lights now operate as expected, and even time out like they are supposed to. I also did some fixing on the door panels ( one speaker/amp assy was floating around loose inside the door, and disconnected a broken door ajar switch. Now the circuit in question ( the ctsy/clock) also controls the Radio and clock memory, i have chased that circuit through the schematic, and it does not appear to have any tie in with the ECM, But now when the engine is started, it goes to a high idle ( not anywhere as bad as before, maybe 1700 - 1800, before was well over 1800) and drops right back with in 1 second, Stumbles for a while( a few seconds, then clears up, and all is cool. ( sounds like the cold start circuit is acting up, will have to disconnect that this week-end).

so the saga continues.

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