Boy I have been searching every forum for help, so I joined today hoping someone here can give me some guidance.

1985 stock vette, automatic, 4,000 on new rebuild, alum heads, fresh stock cam, 1.7 roller rockers.

Symptoms:
When cold, easy start but hesitation sputter at low range of throttle. After it is operating temp (165-220), car runs kick butt. Then once you turn it off and attempt restart while operating temp, car will not start without WOT, also hard to get to an idle, once it is settled in, immediate stall in reverse or drive. If I induce limp mode by unplugging something the car is still hard to start but won't die when placed in gear. Without limp mode, can get it going in drive or reverse, IF you can feather throttle just right. Then after that you can go to neutral or park then back to drive or reverse without it dying all you want. Been seeing it show 0's for mileage calc till today it sprang to life for some reason. Dash works fine, all gauges reading, switches work, coolant temp, oil temp, do see low voltage 11.1 (1yr old battery) even though it cranks just fine.

High idle 980 in drive 1120 in neutral, even though previous set (twice) to 450 at idle and then tps to .54. (thank you for this)

Replaced:
Plug wires, cap, rotor, spark plugs during rebuild
IAC valve pre-rebuild
MAT sensor
Cooling fan switch (broke off)
O2 sensor
Throttle body to plenum gasket
All plenum and intake gaskets during rebuild
ECM tried new one no change so put old one back

Repaired:
Vacuum leak (melted tube)
Cleaned throttle body
Connectors on knock sensor and cooling fan switch (brittle)

Checked:
Engine / body grounds
TPS voltage at idle and smooth as throttle opens, No Vacuum leaks sprayed everything down while running nothing no hissing.

All I have is ALDL cable to WinALDL and alldata subscription (meh) and google. FSM on order.

 

Just about the only way to get a high idle is with a vacuum leak.

Did you check for fuel in the vacuum line on the fuel pressure regulator?

 

i would do basic cecking

first of all read for codes stored and report back .

1) fuel pressure,in park at idle,and with the runnin' car snap the gauge on the windshield .85' runs 38 psi with vacuum disconnected at fuel regulator.it has 24 lb injectors.
2) ceck basic factory timing at 6degrees.
3)set minimum idle per factory spec .56 volt at tps, set the idle using the procedure
4) ceck all ignition stuff at the basic wires,coil,plugs.
5) try a NEW battery since battery voltage affects injectors pulse width,there's a specific table on the tune related to battery voltage .,here a snapshot from tunerpro



BTW i see that the rebuilt engine is fitted with aluminum heads since you replaced the heads (stock are cast iron) do you put bigger heads there?if new heads are bigger you need a retune.

as Cliff said,high idle is vaccum related,i would ceck all vaccum lines,the car has to idle at 600 rpm in closed loop,in open loop in the first 240 seconds 1000 rpm is factory,while the MAt is sending temperature incresing data the ECM slows down the idle rpms to 600.

I would ceck the IAC for sure since THIS device controls the idle rmps,may be stuck,the IAC is involved in the procedure for minum idle setting.

 

No I didn't - I will do it today and report back.

 

Yes they are bigger but builder didn't port anything to give more flow to them. Compression is the same. I am not getting any codes shorting aldl and winaldl is not reporting any either.

I will also check fuel pressure today too.

 

OK - Just fired it up, unplugged vacuum to regulator. Let it sit there running for 3 minutes. Reached operating temp. No fuel, no fuel smell on finger. Also as a test shut it off with vacuum off and same trouble restarting and still dies going into gear. So I guess that rules out the regulator? Did I do that right? Thanks for the tip!

 

when you put the car in Drive from Park the ECM commands the IAC to compensate the stall,the same way ECM commands the IAC when A/C compressor kick in,it seems to me your IAC doesn't comes in,are you sure the IAC is workin' fine?

 

I will certainly give IAC another look next. Just got back in from checking vacuum found one more questionable hose, trimmed it and checked how many bars pulling, a steady 14.

Fuel pressure is about 38 when key on, then settles in to 30 when running. If the key is on, it will leak back down slowly. Is that normal? Seems to be right on per spec at (cough) alldata. According to them 34-39 ignition on, 24-36 at idle.

 

Ok Ran the following test on IAC.

1. Recorded RPM of 1200 in park at normal operating temp (203 degrees)
2. Turned engine off, disconnected IAC valve, got engine started again (hard start as usual) recorded RPM of 1800 in park at idle, increase in RPM, as expected per test procedure.
3. Turned engine off, reconnected IAC, started it back up and idle settled back down to 1200 as before.

So I guess IAC is operating correctly.

EDIT: Also checked park/neutral switch at the ECM harness no issues.

 

Fuel pressure is ok.

you set the minimum idle but you still have highier rpms than expected:

in your IAC test do you force 1200 rpm's or it was idling at 1200 waarmed?

when warmed 185-190 F the car simply CAN't idle at 1200,she has to slow down to 600 in 4 minutes max.

I have the same issue in my 85,she doesn't hold steady pressure with engine off:normally guys here say their cars hold presssure for about 15-20 minutes and then it slow down.My pressure drop instantly after engine shoot off,it drops to 10's psi in less than 30 seconds.i'm chasin' this issue :
i ceck for
1) ruptured fuel pressure regulator diaphragm / gas coming from vaccum feed tube into regulator
2)1 or more injectors leakin'

1)primed the rail with key on, had 38 psi vacuum disconnected ,no fuel into vaccum hose and reg vaccum feed tube.
2) clamped return fuel line into tank,primed the rail ,pressure jumped well over 65 psi and started to drop istantly
So,if i'm not wrong,being regulator diaphragm OK,and not flowing down into return pipe,it may be 1 or more injectors leakin'.

 

Seems reasonable might be injectors leaking causing ours to leak down.

It is idling at 1200 warmed. When I cleaned throttle body I didn't take cover off IAC passage, I only pulled the valve. I think I will pull it today, inspect IAC passage and valve. Anything I should look for on the pintle? Main thing is that is backoff enough for installation right?

 

I've clened my TB yesterday.The torx bolts on the lower TB may be stucked.Even with plenty of Wd40 i was able to breack 2 of them.Changed my approach and used carb cleaner through the IAC housing and compressed air ,the pintle is usually just a little blacked,no big carbon built.My TB was in good condition,very clean at all.

if you have the FSM look at the section 6E3-93,it says to adjust IAC pintle shaft if this is extended for more than 28 mm,before install into housing.

 

OK. You have 1.7 rockers so it is letting in more air. Has the ECM been set to compensate? What is the command idle? What is the IAC count?

Your fuel pressure is low.

Also have you checked the spark quality and gotten a blue flame?

 

subbed

 

No mods to ECM. But, just fyi... This problem appeared suddenly. After rebuild, car ran awesome with no problems for about 6 months. One night problem started out of the blue. Originally it was a bad surging problem with it hard to hold it at a light. After cleaning throttle body and resetting idle that disappeared and it was running good again for a couple days. Then this dying when warm going into drive issue started. I just got done completely redoing the throttle body tonight a second time. Cleaned it up with throttle body cleaner, this time new gaskets all around. Checked IAC. Clean as a whistle. Reset TPS to .54 and test drove it. Still has same issue.

I don't have anything to read command idle from ecm. IAC count on WinALDL is 44. I will try some new plugs, check spark and I will also try resetting idle/tps again tomorrow when I got some help. Anyone know the definition of insanity?

Here are some screenshots from tonight. I went out and restarted it (WOT required) got it settled in, it was not running like it usually does. Definitely rich tonight and hunting for an idle as you an see. Dash read about 170F coolant temp. Winaldl shows 9.9F. Dunno if that is a bug in software. Fans and gauge seem to work fine.

Again take this data with a grain of salt, cause it was richer than usual after this restart.

 

Thanks! I did checked pintle length, just under 1 1/8" and visually saw it was not bottomed out when installed. Got my FSM Monday. I broke 5 of the seven torx holding IAC cover. No problem getting new ones - thank you fastenal!

 

This is exactly what I was going to say before reading your last post. Your CTS is not accurate. The engine runs richer when cold, adds more fuel during cranking when cold, higher idle when cold, retards timing when cold, etc. When your engine is hot, the CTS is telling your ECM that it's cold! That's why it's saying 9.9*F. Your problem is in the CTS. Either the sensor itself, the wiring, or the ECM. One of those is your problem.

 

Also your ECM uses a different sensor than your dash does.

 

Well alrighty then! It will be changed out tomorrow. Thanks that would make sense.

 

Not sure. I am using AutoXray and it tells me the desired idle that the ECM is trying to achieve.