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engine rpm surge. help needed.
what can happen (now)
lets say the IAC port is blocked now, with duct tape.
I just thought about this, how to make it safer for engine,. well, clean the bore with carb cleaner, or brake cleaner, it's just metal taped, then run the tape so it extends out the bell, to the out side TB.
put back the Air horn piping locking the tape from sucking in to engine (bad), that big air pipe must be in-place or the MAF gets really upset, (angry limphome ,mode)
on a good IAC , hot, this does nothing at all. (or there is vacuum leaks; others) all we know now is it's not the contributor to illegal air supplies.

but if the engine races still , then that is a vacuum leak, where we don't know yet.

next close (CW turned) the bleed 100% gently, not forcing any cone valve damage. it's just aluminum and thin. (saying to all not just you this)
now the RPM will go to 800rpm or even down to 400 rpm, the ISC dictates this point, if the ECU works an that ISC modulates 200 times a second the ISC goes full open NOW.

The ISC may have trouble rising to 800, and is way lower. RPM. (800 is computer hard coded rule for ISC , AC off)
At this point it can be 400 rpm or stalled or 400 to 700rpm or even 800 now) I expect 400. (the rpm must be ISC only limited)
the bleed screw was closed above so , only the electric ISC is the only source of any air, and is very limited. (about 400 rpms worth of air. (over the bleed)(crude estimates)

(recap, bleed closed, IAC blocked)

If the engine still screams now, (say 1000) then pinch the ISC air hose closed (the cold hose is it) hot engine.
we are now at ZERO AIR:
the engine now has zero air, if not stalled now, there is a huge vacuum leak.(unknown , keep an open mind on where)

that is how i find leaks. there are better ways with the smoke machine.
Some leaks are very hard to find.
seen one TB with ice freeze damage inside it where no human can see or go, sucking air. yeah a beach ! or manifold cracks very very hard to view.
Brake booster leaks, remove hose, plug it with a wine cork (T shaped kind) or best you can. yes the diaphragm here can leak bad.
pinch all hoses.
the FPR vac hose, too. but know that doing so fuel pressure goes nuts)
pinch all vsv hose.
all vacuum hoses must closed to find them leaking air. pinched or using golf tees.
all that was the easy stuff, LOL
I might add, I see no future for IACs now.
it's too old now.
every thermal cycle is stress on it, aging effects.
I know one guy , remove it, and modified it to just and air hose.
then added air valve to the dash and used a knob on it to set idle cold air.
crude, yes, but is no more complex than an old carb choke. manual.

or add and 12vdc VSV valve to that added air port
then have hand made and tuned, up. air orifice there. (a plastic pipe, hand drilled as you want it.)
and a switch marked, hot /cold start.
set it to 1500 rpm cold, or as you like.
In-laws still staying with us till the 3rd. So I'm just about done with the flu... and my wife gives me her cold.. Lol can't catch a break.. I'm excited to see if blocking that iac valve will show results. Good idea about extending the tape to outside the tb mouth and clamped with the air tube. Simple and brilliant.. checking the brake vac booster for leaks. Good idea.. my brake pedal is shit (another day) and an air leak there could make sense. I do know the rear brakes need adjusted as the e brake doesn't grab untill 12 clicks. Doesn't hold the kick till around 18 clicks (e brake clicks). I have a semi seized caliper on the RF and a new... I'm thinking incorrectly bleed caliper from last owner on LF... So that's that lol... So also hearing how that iac is secured to the TB makes me all warm and cozy inside (not) maybe my impact (Milwaukee 1/4 battery impact) with a #2 bit and some heat can do the trick... crap I should have snagged that TB from the 16v kick I found in the bone yard... I did return to that and take the 2 washer motor bottle and 2 washer sprayer (so now I have 4 sprayers lol) and someone already got to it and took all of the interior... I'm sure that 16v is gone by now. Although our 5 degree days may be keeping people away.. man the more I think about it.. and how nice I thought my kick was... the more I realize just how much is wrong with it.. Holy crap. When ever I get this idle figured out... I believe were very very close.. then I'll focus on something else.. of the 10 other problems.. Blast why can't people take care of there shit. So random thought here... that isc i put on from the junk yard car i know closed 100% i tested it with my mighty vac. But I feel that it's being commanded open a bit for no reason... like if I unplug it.. the idle drops by 100rpm or so... So on a hot running engine. In park. With zero electrical loads on... that isc should be 100% closed right? Unless the ecu uses it to maintain 800 rpm. But it's not 800 I'm at 900 rpm. Unless the ecu can't see a vac leak that's already causing a high idle. My idle screw is not closed all the way right now... But it's not far off from closed. Maybe 1 turn (full turn) or less... well good luck getting it to 150 degrees when it's 5 out lol.. diag will be hard now that it's cold.
so true that, he IAC llocated there stinks, because the screws always seize, hope not you.!! (steel screws , aluminum body, love to seize, like motorcycles do.

put a propane torch to screw heads cycled hot then cold can un-stick them (a trick that cracks the rust bonding... (but dont let the body go over 200f.)
by 100.. (redacted my comment)
so you got leaks.
(01-11-2018, 07:23 AM)fixkick Wrote: so true that, he IAC there stinks, because the screws ajways seize, hope not you.!!

put a propane torch to screw heads cycled hot the then cold can un stick them(trick)
ok 100rpm test that was good test, if hot engine, that means the IAC is closed and GOOD!, great testing on your part.
"REDACTED"the engine is strong to run at 100 most stall at 200.
but the tach is not very accurate and both numbers are good,. or near stalling..

at idle hot. no loads on alternator, AC off.
the ISC modulates at 50 % or 1/2 open. in fact it does that and holds 800 rpm like rock. even shifted to DRIVER. open more.lots more.

tell me more about what you did (all) to get to 100 rpm or 200.
what hot engine and.....????
tells me there are no leaks. (vacuum)

the question the becomes why is the ECU opening the ISC so much.
one way i did not tell you yet is the the wire to the steering pump, sensor
that tell the ECU the the PS power steering is overloaded and sends idle to 1000 rpm.

1 wire shorts to frame the bad deal.
I well send more data to you later today.

Ha. I think you mis-read what I said. 100rpm. I said unplugging the isc while running hot will drop my rpm by 100. Not to 100. I was just wondering why the isc would be commanded to raise idle if no reasons were present. Like idling in park hot with all loads off. I still have not blocked my iac valve to check if its working right. My fuel mileage is around 19mpg. But I do have the slush box 3speed. No working tcc lockup. And 31" super swampers set to 17psi.
yes, i missed the "BY 100" sorry.

so it has vacuum leaks, and more. where unknown.
the 100 rpm drop means (if ISC does close) that the there are huge vacuum leaks.
the IAC is one of the very first tests.
why? because it works using same technology as a coolant thermostat (wax) not one lasts this long? 20 + years.
a vacuum leak make all other work very very hard to complete , cases in point.
setting spark timing
testing the FPR with RPM wrong (or out of scale with throttle)

there are only 3 normal air paths.;at idle.
1: bleed screw (duty cycle calibration is not for RPM at all and when does, that means its dead, as in cant regulate idle)
2: IAC (closed hot , over 150f closed if the wax magic fails, this fails hard or 150f is way off calibration now)
3: ISC (modulated by ECU 200 times a second hot to control RPM at all times.
4: leaks, illegal leaks.

when you see the bleed screw(and screwdriver) change RPM that means the (ISC/ECU) is out of control HOT.
if you close the bleed gently, CW, and rpm is not at 800 (grabbed by ecu) on the way closed , the same. facts. (leaks)
nature loves to add fresh air to vacuum, that is it's job, (on earth) it wants to find a way and does so with unending talent.(insidious)
answering for this; inline.

that isc i put on from the junk yard car i know closed 100% i tested it with my mighty vac. (isc nor iac use vacuum)? me confused here.

But I feel that it's being commanded open a bit for no reason...
like if I unplug it.. the idle drops "by" 100rpm or so... To what? please use real numbers for rpm ,they matter what.

So on a hot running engine.
In park. (4sp -tcc lock)
With zero electrical loads on... that isc should be 100% closed right? near yes (most old isc leak , look inside it see that rubber ring, and the Pock marks on it sure, that is going to leak. just a bit)

see the word seat here (its a rubber ring, there, and all leak, old , all do but most are just tiny bit, never seen one close 100% used)

Unless the ecu uses it to maintain 800 rpm ( learn that if the bleed is closed (900rpm), and you have air leaks,(vacuum) if the ECu closed the ISC , it cant close further ever. 100% closed is all it can do. The ISC can only hide small leaks, ONLY !
and most of these silly ECU then hunt the idle(surge), in a vain attempt to go more that 100% closed (double silly, it can't and is dangerous to some drivers ,most are A/T drivers, surge is dangerous to some folks " ask Toyata that and get man crying in towel"

But it's not 800 I'm at 900 rpm. (good a number , 900! with ISC unplugged, if yes, and yes hot, that is a vacuum leak, hard proof (bleed closed?)
When I see that, i immediately pinch the AIR line, to ISC, (cold line is AIR) and if RPM drops below 800 the ISC is BAD ! end story (maybe)

Unless the ecu can't see a vac leak that's already causing a high idle.
(It sees all leaks, and can see RPM every second, it will never miss the the fact that IT cant CONTROL IDLE)
the ECU (hot engine) controls RPM , at all times, parked, (not limphome mode) it goes that 1000s of times a second. in fact. it wont miss a thing,
learn that on a modern car it would be puking DTC 'idle controls dead" endlessly.

My idle screw is not closed all the way right now... why not? if I say 900 with isc unplugged that would be next. (after all that is all I can control on this engine (easy)
Do yo know the story about the Dutch boy and Dike leaks. (dam) he has to plug leak with thumb. same deal here but negative pressure air. (vacuum)

But it's not far off from closed. Maybe 1 turn (full turn) or less... well good luck getting it to 150 degrees when it's 5 out lol.. diag will be hard now that it's cold.
150f takes driving to get there, idling can take for every even 2 hours. use cardboard to block the radiator opening like the do in Sweden (endless times)
My buddy (ASE mech) up in Alaska has to spray foam on the engine block or it never gets to temp, -55f there. or like many there,,never turn off the engine ever in dead of winter, there..... hard times.

the ECU never shuts the ISC closed 100% (unless massive air leaks are happening) it does in fact run 50% and is the 50% idle dutycycle rule. (the bleed screw is that calibration POINT)
the FSM covers this but what you do is. (in a nut shell ) and need tool to measure duty cycle, as most do not. Ill cover this 2 ways. (a man with tools or not)
1: end all illegal vacuum leaks (air) find them end them.
2: engine hot. 150f , 180f , 195 f (195F thermostats work best in cold climates) 150f is the IAC calibration point set at the factory and what the ECU expects to see !!!
2b: a/c off, (on 1989/90 more things must be off , ask) we are doing 1992 + 16v only here.
2c: steering pressure switch not closed, (or wire shorted there causing 1000 RPM. fast idle mode, to be in effect, fix this next and a/c errors,for same.
3: make sure iSC unplugged, idle drops well below 800 rpm, if not close the bleed down. if you can not attain below 800 now, see #1 again. or pinch the isc air line next !
4: using the duty meter, tune the bleed screw until 50% duty is reached as seen here.
we set the bleed on a scope (1 tool example there are many tools to do this) set the bleed until the high time equals the low time. 2.5mS high and 2.5mS low, even Steven.
5: done.

with no tools, turn the bleed closed CW, until the ECU grabs RPM from you and hits 800 (+-50rpm) that is called ECU idle regulation now working
then close the bleed 2 more turns for luck,(we all this by seaman's eye, or guessing) if regulation fails at other times, close it more. until 800 is hit again.
many quality DMM meters today have duty mode, on the dial , a good idea that. it reads out in %

finding a leak is not too hard, sure daunting.
if the IAC is not leaking hot,
then its a simple gasket leak some where.
1: brake diaphragm booster (block the hose with cork, dont drive just test)
2: all hoses.
3: all induction gaskets in the whole plenum and injector cushion gaskets sucking air.
4: all vacuum operated diagrams are suspect , booster, FPR, VSV valve broken or stuck open.
some cars this old have all 4 bad, some do, expect it and win. (we test and prove other wise ) or even get a smoke machine to find it , watch you tube on engine vacuum smoke machines.
my bud uses a cigar to do that.... wow. flash light in hand look, and looking for leaks.
when you turn on the head lights and heater blower , (not AC) or shift to drive.
the ECU sets the duty higher, it opens the duty cycle in the ISC from say 50 % normal to say 70% or more to add engine power to match the added loads on the engine.
the method is called PWM (pulse width modulation)
all modern EFI cars do exactly this. (and is by law too. it must) the ecu controls idle , not people by EPA law. ( but can fail, easy , with vacuum leaks)
on modern cars add spark timing, (fixed and only ECU can set it) the CMP and CKP are precision machined and not adjustable at all on modern cars.
the ISC on modern cars has a learn mode and calibrates it self. and if it can not?, it puKES DTCs for the same.

knowing how it works is key to diagnose.
one key test is the pinch vacuum line test
smoke tests.
leak down tests with vacuum hand tool on all vacuum nipples, (all vacuum devices, even FPR,egr main, brake booster) even vacuum lines them selves can be tested with this tool.
in all comments, the idle switch is closed and reads 0vdc.

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