High searching Idle

[jss06c6]

Well, just moved #473.. drove it two weeks ago, no issues, ran beautifully. When I started it to move it today, the car sounded like it backfired immediately upon start. But shut down instantly. I cranked it again and it started right up. Took it out for a spin around the block and it ran great, BUT, the idle is now at 1250, not 750 like it is supposed to be.

Pulled into shop and looked around the engine. The PCV plug at the back of the engine was partially popped off. Likely due to the backfire. Pushed it back down and the idle dropped to 1150.

Pulled the IAC and it was clean. Put it back in and hooked up Moates interface and launched TunerPro RT. Started the engine and the IAC is locked at 0. Engine is hunting for idle. My guess is that the IAC cratered.

Input welcomed. Any way to check the IAC before I look for a new one?

Are these IAC's available still? Jerry, do you have one for a '91?

Thanks Guys...

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[secondchance]

I would check MAP sensor vacuum line (unless when you said PCV line partially popped off, you meant MAP sensor vacuum line).

[jss06c6]

Map sensor line is good. The double PCV plug was not fully seated on the PCV valves..

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[XfireZ51]

Map sensor line is good. The double PCV plug was not fully seated on the PCV valves..

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Have u tie wrapped it down?

[jss06c6]

Dominic, it's very tight. I'll check again. When I lost the MAP hose connection about 18 months ago, the car wouldn't hold an idle and was almost undrivable. The car drives beautifully, but now has high idle and it "hunts" about 100 rpm up and down..

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[jss06c6]

It's possible the backfield pushed another vacuum hose loose under the plenum. If it is indeed a vacuum leak, it's a very slight one..

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[XfireZ51]

Just about every vacuum connection on my motor is tie wrapped. I hate vacuum leaks no matter how slight.
When I got my first ZR, it ran great but had a slight hunt. Marc looked at it and said PCV. He was right.

[jss06c6]

Ok, will tie wrap the PCV connection.. I've got a new one too, might just change it out..

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[jss06c6]

Not sure what I can try wrap.. hose clamps on all single pipe connections. I'll put the new o e on and see if that resolves it.[ATTACH][ATTACH]

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[XfireZ51]

Tie wrap the top and bottom halves of the F to each other.

[jss06c6]

Ok, that's clearer..

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[jss06c6]

Old one on the bottom, new one on the top..[ATTACH]

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[jss06c6]

Put the new PCV connector on. Guarantee no leaks there! It's darned tight! Fired up the engine. High idle 1300 rpm, until closed loop kicked in. Dramatic swings in AFR as fueling was dithered. Finally settled down at full coolant temp (193F). Idle still sits at 1175 versus commanded 750. IAC locked at 0 trying to adjust idle to commanded. Verified IAC working. Stepper motor kicked in at 1325 room (4), then rises with increase in rpm. Now suspect something went wrong in the TB. I'll put a borescope under the plenum just to make sure no other vacuum connections are off. Then I'll pull the bellows off and use the borescope to look closely at the TB blades during idle and increasing rpm.

Steve

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[A26B]

Sounds alot like the MAP hose. Off, broken connector, or plain rubber line with low point oil collection in low point. The rubber connectors on each end of the hard plastic pipe are prone to tear due to age. If you can reach, remove the MAP hose from the plenum barb & put your finger over the barb to see if the high idle is due to the MAP hose.

The lower dual PCV hose in mine (94 w/57k) was rock hard on the side towards the firewall & completely rotten on engine side.

[XfireZ51]

Old one on the bottom, new one on the top..[ATTACH]

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U can easily see how out of shape the old one is v the new one.

[jss06c6]

Sounds alot like the MAP hose. Off, broken connector, or plain rubber line with low point oil collection in low point. The rubber connectors on each end of the hard plastic pipe are prone to tear due to age. If you can reach, remove the MAP hose from the plenum barb & put your finger over the barb to see if the high idle is due to the MAP hose.

The lower dual PCV hose in mine (94 w/57k) was rock hard on the side towards the firewall & completely rotten on engine side.
Jerry,

The MAP hose is not off, nor broken. Checked that again this morning. The connections are tight, including the 90 degree fitting on the bottom of the MAP sensor. I'll pull it to make sure it's clean however.

I also checked the PCV hose while replacing the top fitting. Pliable and tight. No issues that I see there.

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[jss06c6]

U can easily see how out of shape the old one is v the new one.
Yes, agreed, but replacing it had no effect on the problem. Going to check under plenum with borescope for all the usual suspects and check the TB as well.

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[Ccmano]

At the risk of stating the obvious. You checked both MAP sensors, right?
H
:cheers:

[XfireZ51]

Yes, agreed, but replacing it had no effect on the problem. Going to check under plenum with borescope for all the usual suspects and check the TB as well.

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Have u done the ole 3ring binder in front of the TB trick yet?
Could point to either vacuum leak, or TB issue.

[jss06c6]

Not yet but definitely part of the diagnosis..

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[jss06c6]

At the risk of stating the obvious. You checked both MAP sensors, right?
H
:cheers:
?? Hose under the ECM ?

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[Ccmano]

?? Hose under the ECM ?

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Yes, the other map sensor is under the ecm on the 90-92
H

[jss06c6]

Looked at it Hans. No issues. I'll recheck to make sure hose is good.

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[mike100]

well, since you have eliminated the outside hose connections, it sure sounds like the hard plastic curly-q under plenum hose has popped off. It goes to the gas tank purge solenoid. The leak is big enough to idle at 1200 rpm as you described. My car came like that and was the reason for my first plenum pull.

[jss06c6]

well, since you have eliminated the outside hose connections, it sure sounds like the hard plastic curly-q under plenum hose has popped off. It goes to the gas tank purge solenoid. The leak is big enough to idle at 1200 rpm as you described. My car came like that and was the reason for my first plenum pull.
That's my guess too after reading Marc's summary of known offenders! Will know this afternoon when I get under there with my borescope..

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[jss06c6]

Found it.. unfortunately. Borescope clearly shows the hose on the curved pipe under the TPS going to the gas tank purge solenoid is off. Backfire must have blown it off. Will put hose clamps on these hoses when im in there this time.. happy I found it and glad it's not the TB. Well, needed to pull plenum and IH's to Port them anyway!

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[XfireZ51]

... happy I found it and glad it's not the TB. Well, needed to pull plenum and IH's to Port them anyway!



The Lord works in mysterious ways.

[Ccmano]

Glad you found it, Had to be something related to the backfire. Unlikely the throttle body would go that suddenly.
H
:cheers:

[jss06c6]

The Lord works in mysterious ways.
Indeed!

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[jss06c6]

Glad you found it, Had to be something related to the backfire. Unlikely the throttle body would go that suddenly.
H
:cheers:
More labor, but easier fix!!

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[Demps]

Permatex 2 is your friend.

Ted

[XfireZ51]

Permatex 2 is your friend.

Ted

It certainly is for the coolant passage between the IH and cylinder head.

[jss06c6]

Got it done without pulling plenum. Pulled TB, which gave me just enough room to get the hose fitting back onto the end of the pipe nipple. Made a wire hook with coat hanger wire and pulled it into the pipe. Put a tie wrap around it and cinched it up.

I'll be changing out the throttle set screw while the TB is out.

When i get the plenum and injector housings I got from Constantine ported, I'll pull the top end down, including the valve covers and get everything powder coated gloss black with red lettering. Going to re-plumb everything under the plenum when I build it back up.[ATTACH][ATTACH]

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[Demps]

Sweet. I wrongly assumed it was the other end. It will come loose at the purge solenoid connection sometimes if you raise the front of the plenum.

Glad you fixed it.

Ted

[jss06c6]

I should have known to look here first since the PCV connection at the PCV valves was popped up. These two connections are right next to each other at the front of the plenum..

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[jss06c6]

Got the TB reinstalled and fired up the engine. It settled down to about 900 rpm, cold and in open loop. Let the engine warm up and it dropped down to my "normal" 750 rpm in closed loop. Got the laptop, launched TunerProRT and began to log the engine. I replaced the throttle set screw with a hex head and lock down nut. Installed it the same offset distance as the original one I removed. IAC sits a 0, versus previously at 4 with a hot engine. Have not set minimum air yet, but will get that done with a warm engine.

Took the car out for a drive. Each time I came to a stop, the engine would idle down to about 700-750 rpm and IAC would return to 0. Throttle position at 0.53 volts.. all good. After about a 5 mile test drive, I noticed the idle was again about 1200 and hunting again. AFR was swinging from about 13 to 15.5, not quite same frequency as normal dithering. Let it sit in the driveway for about 5 minutes, idling, no change.

I pulled the bellows and got a piece of cardboard to block off the TB. Took about 5 seconds before the engine began to bog down. I let it get to about 400 rpm and then pulled the cardboard off (difficult with that much suction!). The engine RPM came back up and the AFR was now swinging pretty hard from 12.5 to about 16.5, but the idle was now back to 750! Repeated the sequence two more times with the cardboard, with same results. IAC still at 0 at idle and TPS still at 0.53 volts.

I'm going to try to set the minimum air this afternoon while I've got good weather. I looked very closely at the TB when it was out and I cleaned it. There was virtually NO light coming past the TB blades. Only spot was near the shaft on one of the blades and this was extremely small. There is no DAG in this TB. Makes me wonder if this TB has been worked over in the past, possibly by Marc and prior owner had it with him..

So, I've either still got a small vacuum leak under the plenum, or the TB is allowing some air to pass through at idle. The fact that after the TB was blocked off caused the idle to return to 750 causes me to think that a vacuum hose connection may have been leaking slightly and the increased manifold vacuum "pulled" the connection in enough to re-establish integrity?? Don't believe the TB was sticking, it was clean and free. I put a very small amount of oil on the shafts where they come through the outside of the TB casting..

[jss06c6]

Reset minimum air and TPS to 0.55 volts. Got it good and warm. Throttle was sticking slightly. Cleared that. Idles at 725. IAC still at 0 when hot and at idle. Three pulls to 7250. Smiles all around!

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[mike100]

These throttle bodies and the pcv vent bypass too much air an it is hard to get a positive IAC number. I was able to get a value of 10-20 on the iac counts after I restricted the diameter of the pcv hose to the manifold. To make up for the pcv flow at high rpm where it needs the full tube, I tee'd off to a mini filter atmospheric vent.

[Ccmano]

Reset minimum air and TPS to 0.55 volts. Got it good and warm. Throttle was sticking slightly. Cleared that. Idles at 725. IAC still at 0 when hot and at idle. Three pulls to 7250. Smiles all around!

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Your going to want the IAC around ~10 or so. Otherwise you will run the risk of stalling as the car comes to a stop. Zero means the system is out of adjustability to compensate the idle speed. I’m going through that right now in the tuning process with Dom. Mine is dialed in pretty well now, IAC went to Zero and I started getting isolated stalling issues. Dom might be able to pipe in with a more detailed explanation.
H
:cheers:

[jss06c6]

Hans,

Yes, ideally this is the case. I run my idle at 700 rpm. If I move up to 850, IAC will be at 10-15. When I set minimum with the IAC pintle fully closed, I set it to 600 RPM. Then reset the TPS to 0.55 volts. Plugged the IAC back in and it settled in and slowly walked back to 0. Can't get the idle any lower with the IAC disengaged. If I moved my idle speed up, no problem holding 10.

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[jss06c6]

Oh, and I don't have any issues with stalling. Used to, but tuned that out. No trouble any more.

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[jss06c6]

These throttle bodies and the pcv vent bypass too much air an it is hard to get a positive IAC number. I was able to get a value of 10-20 on the iac counts after I restricted the diameter of the pcv hose to the manifold. To make up for the pcv flow at high rpm where it needs the full tube, I tee'd off to a mini filter atmospheric vent.
Mike,

What do we gain trying to hit IAC of 10? If the car idles butter smooth and doesn't stall coming to a stop, what is the "win" with IAC sitting at 10? Just don't understand why we should chase this and suffer top end starvation like you describe.

Steve

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[XfireZ51]

Hans,

Yes, ideally this is the case. I run my idle at 700 rpm. If I move up to 850, IAC will be at 10-15. When I set minimum with the IAC pintle fully closed, I set it to 600 RPM. Then reset the TPS to 0.55 volts. Plugged the IAC back in and it settled in and slowly walked back to 0. Can't get the idle any lower with the IAC disengaged. If I moved my idle speed up, no problem holding 10.

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How are you setting the Min Air? Let’s review to make sure its correct.
The objective of Min Air is to require a small amount of IAC enough so that it has margin to intervene and prevent a stall but not so much as to override what should be mechanically done by TB opening.
IOW, u set the idle speed somewhat below the desired idle so that the IAC actually controls idle speed, but with few counts, ie 10-15.
BTW, I have situations where my IAC drops over time. There are modifiers in the cal that pull IAC counts based on MAT. I try to set Min Air with the motor hot and with fans ON.

[jss06c6]

Dominic,

I set the minimum air by locking the pintle in the full open position (ignition run with IAC unplugged). Starting the car.. setting TB to 625-650 rpm. Reset TPS to 0.55 volts. Shutting it down. Pulling negative battery cable. Wait 5 minutes. Reconnect IAC harness. Reconnect battery. Start engine. Observed IAC at 140 upon start. Slowly worked it's way down and within 2-3 minutes it was back at 0. Engine idling at 725-750, as commanded. Hot engine during setup.

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[XfireZ51]

Dominic,

I set the minimum air by locking the pintle in the full open position (ignition run with IAC unplugged). Starting the car.. setting TB to 625-650 rpm. Reset TPS to 0.55 volts. Shutting it down. Pulling negative battery cable. Wait 5 minutes. Reconnect IAC harness. Reconnect battery. Start engine. Observed IAC at 140 upon start. Slowly worked it's way down and within 2-3 minutes it was back at 0. Engine idling at 725-750, as commanded. Hot engine during setup.

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Just to be clear, you put the ECM in ALDL/Diag mode, turn ignition to ON before
disconnecting IAC motor, turn ignition OFF, then disconnect ALDL, then re-START the motor. I let it idle until fans kick in, then adjust TB.
Have u modified the IAC tables at all?
The stock calibration has an idle of about 650. I’d be interested in hearing what someone w a stock motor has for IAC counts at idle. I know the commanded counts are not 0.

[jss06c6]

Yes regarding ALDL set to diagnostic mode.

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[jss06c6]

Need to look at IAC tables. I haven't changed them. Not sure if Marc's tune changed them, but doubt it.

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[XfireZ51]

Need to look at IAC tables. I haven't changed them. Not sure if Marc's tune changed them, but doubt it.

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I don’t think he plays w them. I just refreshed my memory re: stock cal, and it commands an IAC count of 30 at 0 mph with an idle of 650rpm.

[jss06c6]

Let me look at mine..

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[jss06c6]

Only two tables in my XDF..

[jss06c6]

where are you seeing the table you're referring to? may need to add this to my XDF.. I'm using $AFA for my '91. AXFK...

[S.hafsmo]

Just to mention it, my idle drops 50 rpm (750 to 700) if I plug the bore to the primary blade (IAC obviously still at 0). I'm assuming the air to the remaining 50 RPM are leaking around the secondary blades. These throttle bodies are not the best design.

Primary blade:
http://imagizer.imageshack.us/v2/800x600q90/924/jyMoKh.jpg

[jss06c6]

Hot engine..

Idles smoothly at 750 rpm
MAP rock steady at 34 kPa
MAT 44 C, 111 F
Coolant temp 93 C, 199 F

[jss06c6]

Just to mention it, my idle drops 50 rpm (750 to 700) if I plug the bore to the primary blade (IAC obviously still at 0). I'm assuming the air to the remaining 50 RPM are leaking around the secondary blades. These throttle bodies are not the best design.

Primary blade:
http://imagizer.imageshack.us/v2/800x600q90/924/jyMoKh.jpg

Yep, checked my primary when I had the TB out. It was pretty dirty, so cleaned it up a bit with brake cleaner.. Didn't have quite as much daylight around it as I see in your picture.. Secondary blades had virtually NO light showing around them.. I feel like I'm chasing a rabbit here for which there is no real symptom.. The engine idles well, never stalls when I come to a stop and pulls like crazy all the way to 7500 (no courage to go higher than this!).

[XfireZ51]

where are you seeing the table you're referring to? may need to add this to my XDF.. I'm using $AFA for my '91. AXFK...

I think you are using the wrong .xdf for AXFK. When I use AFA on that bin, the cylinder volume is 114. I read AXFK correctly using $AF.

[jss06c6]

Cylinder volume reads right on mine.. Marc may have swapped to another BIN..

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[XfireZ51]

Cylinder volume reads right on mine.. Marc may have swapped to another BIN..

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I use BRXB for the $AFA Mask. GM changed the Broadcast code for the correct calibration depending on whether it was for the original Mask or if it was for the updated Mask. So $AF would use the AXFK cal, while $AFA would use BRXB for the 91s. As another example, the 92 $D0 bin is AZZC, while the updated $D0A uses BPPA.
Actually, I do think that Mark uses the latter version of the Mask for his cals. No real reason to use the earlier version. As I look at both 91 stock bins, they did play a bit with the IAC and then changed it in the 92. But it shouldn't affect the idle portion.

[jss06c6]

Thanks!

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[XfireZ51]

BTW, Hans tells me his stock 90 idled at 750rpm w 6-14 IAC counts.

[Vette73]

Couldn't help but read this entire thread on engine idle......One question I have is, how accurate is the tachometers on these cars? I have a 91.....

Mine idles at between 900-1,000 rpm...Motor is fully ported with full exhaust and intake cams...

Even when my car was stock it seemed to favor anywhere from 850-950 usually..

I do notice at times when I pop it into neutral coming to a stop, it may take 5-10 seconds for the idle to " settle in" at around 900-1,000....

Forgive the ignorance , new bee here still learning...👍

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[XfireZ51]

Couldn't help but read this entire thread on engine idle......One question I have is, how accurate is the tachometers on these cars? I have a 91.....

Mine idles at between 900-1,000 rpm...Motor is fully ported with full exhaust and intake cams...

Even when my car was stock it seemed to favor anywhere from 850-950 usually..

I do notice at times when I pop it into neutral coming to a stop, it may take 5-10 seconds for the idle to " settle in" at around 900-1,000....

Forgive the ignorance , new bee here still learning...👍

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Not at all. Your questions are welcomed. That's the purpose of threads like this.
To your observation, the IAC controls have "delays" which will hold rpms up a bit using the IAC until you are at a stop. Those delays can be reduced. But IMO, the LT5 has a tendency to want to stall. It reacts quickly to inputs which makes it responsive, and when the motor is modified and events happen quicker, the stock settings represent somewhat of an "overreaction".
So it needs to be "toned down" a bit.

[Vette73]

Hey X fire thanks for getting back...I know this is common in the ZR-1,coming to a stop with the car in neutral , the idle finds a way to "settle" in ....

My other Corvette I have is a 73, no computer of course and a grand total of about 5 fuses,don't even have seat belts and its a convertible, go figure....

Anyway as far as the idle settling in at times, is that common among late model corvettes?

I think I mentioned in a PM to you, I drove a 15 C7 at Ron fellows school.....Granted, your hammers to hell around that track but didn't really notice any idle searching so to speak..

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[XfireZ51]

Vette,

I had a highly modified 84 Xfire Z51 before owning a ZR-1. I never had a hunting idle. When I depressed the clutch, motor would drop to 800rpm and stay there even as it was rolling. Not so much w the LT5. But I have been able to mitigate that w tuning but not completely. Witha modded motor, something like that can setup an oscillation that can get out of hand.

[Ccmano]

Couldn't help but read this entire thread on engine idle......One question I have is, how accurate is the tachometers on these cars? I have a 91.....

Mine idles at between 900-1,000 rpm...Motor is fully ported with full exhaust and intake cams...

Even when my car was stock it seemed to favor anywhere from 850-950 usually..

I do notice at times when I pop it into neutral coming to a stop, it may take 5-10 seconds for the idle to " settle in" at around 900-1,000....

Forgive the ignorance , new bee here still learning...👍

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Most likely your tach is off. They tend to show on the high side with age and they are all old. I would only trust a Tech1 or scanner of some type. I must say using ALDLDriod has been a positive experience.

Looks like your car was modified. I would check the chip and see how it was set up, if at all. 5-10 seconds for the idle to settle down when at full operating temp is not normal. It should linger at higher rpms as you come to a stop but should be back to normal within a second or two.
H
:cheers:

[Vette73]

Most likely your tach is off. They tend to show on the high side with age and they are all old. I would only trust a Tech1 or scanner of some type. I must say using ALDLDriod has been a positive experience.

Looks like your car was modified. I would check the chip and see how it was set up, if at all. 5-10 seconds for the idle to settle down when at full operating temp is not normal. It should linger at higher rpms as you come to a stop but should be back to normal within a second or two.
H
:cheers:
I would think that as well,tach being a little off with age.....As far as 5-10 seconds to return to idle, its only really done that a hand full of times.....Basically,it is a few seconds to come down to idle....

Salute........

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[jss06c6]

I would think that as well,tach being a little off with age.....As far as 5-10 seconds to return to idle, its only really done that a hand full of times.....Basically,it is a few seconds to come down to idle....

Salute........

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That's completely normal. The tach in my '91 reads about 50 rpm high. I trust my scanner... Not enough difference to worry about for me.

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