I have read the minimum idle adjust procedure where you jumper out the ALDL, , key on, disconnect the IAC connector, key off, remove jumper, start engine and adjust set screw for 500-600 rpm, key off, reconnect IAC connector.
After I do that my IAC counts on my scan tool are very low, around 3-4 counts with a warm engine. Since the IAC has a range of 0-50 counts I would think the warm rearing should be mid-range or close to mid-range. A count of 4 is either almost fully closed or fully open and doesn't seem to allow for much computer control.
Should I be adjusting the minimum air set screw while monitoring IAC counts or leave it at the low count setting and adjust per the procedure I first mentioned leaving me with a IAC count of 4?
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10:04 AM
PFF
System Bot
phonedawgz Member
Posts: 17101 From: Green Bay, WI USA Registered: Dec 2009
See my cave or read SM? idle stop in general engine section or 700 TBI is l4 engine section.
What trans? Auto in gear or park?
Low IAC count and right idle stop setting can mean nothing or can mean likely has a small vac leak.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
I've been chasing a perceived IAC problem and have been doing basic checks before I throw more money and parts at the car.
No I did not drive it prior to scanning. I adjusted the idle stop and monitored my IAC with scan tool, that's all. I totally forgot about driving it to get the rpm above 2000 and mph above 35 to reset the IAC. Thanks for the tips.
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11:21 AM
Dec 22nd, 2010
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
OK I tried adjusting the idle stop according to the cave and SM. The target seems to be 650 rpm with IAC extended and disconnected.
I cannot get my idle that low and have it steady. It is steady at 1000 rpm, lower to 900 rpm and still steady, 800 rpm is almost steady but once I adjust the idle stop lower trying to achieve 650 rpm it "surges" from 500-900 rpm constantly every 1/2 second. I left it at 850 rpm, key off, reconnected IAC and went for a couple mile drive. When I got back my IAC counts were 7. Seems low, maybe because something else is wrong related to the surging idle when I tried adjusting the idle stop per the manual.
Ideas?
Once again, it's an '88 2.5L 5-speed manual.
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09:22 PM
86FieroSEv6 Member
Posts: 438 From: Navarre, Florida, USA Registered: Nov 2010
Does the throttle body have excessive throttle shaft wear/play? Is the PCV valve the correct one for the application? Is the O2 sensor at operating temp.? To see whether or not you are in closed loop control without a scanner, jump the ALDL as you would for codes while the engine is running. The CE light will flash rapidly if in open loop and much slower when in closed loop. When in closed loop, the CE light will be on above 450 millivolts of O2 output and off when below 450 millivolts. If the light is off more than on you are lean (fuel delivery, vacuum leak, etc.), if it is on more than off you are rich (leaky injector, incorrect fuel pressure, clogged PCV, etc.). A good sensor and system should switch at least ten times in ten seconds at idle. I am always surprised at the number of tech's out there that don't know about air fuel monitor function using the CE light. It really is a handy feature if you don't have a scanner available during service.
[This message has been edited by 86FieroSEv6 (edited 12-22-2010).]
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09:48 PM
Dec 23rd, 2010
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
I'll have to check for throttle shaft wear. That never occurred to me. The PCV, you might have something there. I just grabbed one I had in the garage (new in box). It fit so I used it. I would think all PCV's are basically the same? I'll buy the Fiero specific one later today and report back.
O2 sensor is fine. I have a scanner and it enters closed loop pretty quickly. I can monitor the mv's and they cycle from 100-800 rapidly just like normal. I do know about the CE lights ability to read open/closed loop. I quit using it once I got my scan tool years ago.
The ECM uses both the IAC and ignition timing to adjust idle speed... to set the base idle speed, the IAC must be fully closed and the timing connector disconnected or disabled.
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09:41 AM
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
Replaced the PCV this morning with Fiero part number. No change, still have idle surging with IAC extended and disconnected when attempt to set minimum idle below 850 rpm. Impossible to set 650 rpm.
I noticed one thing. during the short time I was in open loop it did idle very smooth at a very low 500 rpm. Once closed loop occurs (with IAC still disconnected) the surging idle happens (500-1000 rpm every 1/2 second).
Once I reconnect the IAC the idle is pretty steady at 900-1100 rpm. Not perfectly steady but certainly drivable.
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10:59 AM
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
The ECM uses both the IAC and ignition timing to adjust idle speed... to set the base idle speed, the IAC must be fully closed and the timing connector disconnected or disabled.
You're going to have a hard time disconnecting the timing connector on the 88 2.5 with DIS. The manual does NOT say to disconnect the timing connector or to disable it. The manual states to jumper A-B so the ECM closes the IAC passage, and then to disconnect the IAC's electrical connector AND to remove the A-B jumper. So setting the minimum idle speed is done with the ECM' s advance enabled.
Is your scanner using 10k mode during monitor? This causes the vehicle to attempt to hold engine speed at 1000rpm to keep the O2 and cat. conv. up to temp., so if you are using the scanner to watch rpm while trying to set minimum idle, this may be an issue.
[This message has been edited by 86FieroSEv6 (edited 12-23-2010).]
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07:58 PM
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
Truth, just because it is swinging within limits does not mean that it's calibration isn't off relative to the actual exhaust mix. Also, how old is the coolant? Old or contaminated coolant can cause a ground side voltage that will shift sensor output, but that should show up in your fuel trims (block learn and integrator counts).
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10:43 PM
Dec 24th, 2010
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
Test power... bad power will hunt the idle. Check and replace/repair: battery and cables Alt and alt wires main dist box under C500 any grounds bolted to engine and trans bell housing
To test grounds... Take off Tstat cap. Meter set to dc volt. (Auto Range is good) Put probe on frame. (Bare metal is needed.) Other probe in coolant. Not touching Tstat or Tstat housing.
Meter Should read 0 volts. Any volts = bad grounds. (Get new radiator hoses... voltage on coolant eats hoses from inside.)
Test with engine on and off if possible. (Take Tstat out. See cave coolant fill... Some cars can't run with Tstat cap off... read "More on Step 9..." or very small wire to make a probe extension, slip it in hose connection and clamp it, and keep Tstat cap on. )
------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
I put the Fiero back in the condition that I see the surging idle (adjusting minimum air) and while the engine was surging between 500-850 rpm I disconnected the O2 sensor. Nothing, no change at all. While I was set up I disconnected the MAP sensor (the only sensor I have not changed yet due to failures). Once the MAP was disconnected the surging immediately stopped and the idle was rock steady. I am going to grab a new one today and see if it fixes the surging idle during the above condition.
Ogre, Power is good, 14.4 volts while running. Battery cables and battery are new too.
I have never heard about checking grounds with a volt meter and coolant so I tested as you described. I got 0.159 volts dc with key off and 0.25 volts with key on. Close to zero but not exact. I then compared to my other two vehicles and got the exact same readings. Must be good.
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11:03 AM
PFF
System Bot
phonedawgz Member
Posts: 17101 From: Green Bay, WI USA Registered: Dec 2009
Problem is getting worse now. I get the surging idle almost all the time now, not just when i am adjusting the minimum idle (IAC disconnected). Now it happens every startup with all sensors connected.
Scan tool first read IAC counts of 80 at idle fully warmed up. A couple re-starts later that number climbed to 125. Again a couple re-starts later (and driving around the block) the IAC counts are over 200 and I set a code of 35. Go figure.
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03:29 PM
phonedawgz Member
Posts: 17101 From: Green Bay, WI USA Registered: Dec 2009
CTS - compared to what you think it should be and compared to the gauge.
IAT (MAT) (ACT) Intake/Manifold/Air Charge Temp
MAP if it stays steady enough to hold
The surging occurs with the IAC disconnected right? So that pretty much takes out the IAC as the cause or even in the any part as being part of the problem. Yeah I know the IAC is supposed to control idle speed. But if this is occuring with the IAC unplugged, this is a mixture problem or perhaps a spark advance problem. right?
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06:58 PM
phonedawgz Member
Posts: 17101 From: Green Bay, WI USA Registered: Dec 2009
It might even be helpful to unplug the IAC at some setting that is about the correct idle speed. That way you know you aren't fighting the IAC/ECM trying to correct the wacked idle speed, that has to be caused by something else.
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06:59 PM
phonedawgz Member
Posts: 17101 From: Green Bay, WI USA Registered: Dec 2009
What does the scanner report as the fuel pump voltage. Does that very? Try running the engine with a battery charger connected at the same time and plugged in. That way there is no way what your chasing is a changing voltage perhaps caused by the alternator and picked up by the ECM and trying to be adjusted for by the ECM. The ECM is supposed to adjust the pulse width when the fuel pump voltage is lower. The ECM however doesn't have a wire that connects to the fuel pump to sense voltage. It only senses battery voltage.
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07:03 PM
86FieroSEv6 Member
Posts: 438 From: Navarre, Florida, USA Registered: Nov 2010
Aim a strobe light (timing light will do, just hook it up to the coil wire) at the fuel spray from the injector. You should NOT see any droplets, only a nice cone shaped spray pattern. If you see droplets then it's possible your injector is fouled or the lower injector o-ring is leaking. I bet you haven't rebuilt the throttle body since it was sitting all that time.
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10:31 PM
Dec 25th, 2010
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
Lets back up some. Before it started running "worse" I did take the throttle body off, disassembled the lower end and soaked it in a gallon of carb cleaner. I removed the injector and flushed the upper section with spray carb cleaner. The injector and o-rings are new.
When I assembled everything the idle seemed worse then ever with the current idle stop setting and IAC connected. It's was surging from 500-900 rpm and IAC counts pegged at over 200. This tells me the IAC is fully retracted allowing as much air to bypass as possible.
I ended up just adjusting the idle stop further to get the IAC counts mid-range (around 25) and my idle is pretty smooth with everything connected. I think I may have had some minor vacuum leaks at the throttle body allowing un-metered air to get in the engine. Once I rebuilt the TB those small leaks went away and the old idle stop setting was too low for a steady idle so the IAC fully retracted as much as it could to compensate.
I have about given up on adjusting the idle stop per the book.
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08:19 AM
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
CTS - compared to what you think it should be and compared to the gauge.
IAT (MAT) (ACT) Intake/Manifold/Air Charge Temp
MAP if it stays steady enough to hold
The surging occurs with the IAC disconnected right? So that pretty much takes out the IAC as the cause or even in the any part as being part of the problem. Yeah I know the IAC is supposed to control idle speed. But if this is occuring with the IAC unplugged, this is a mixture problem or perhaps a spark advance problem. right?
CTS reads actual coolant temps, normal.
IAC is outside air temp, lower then coolant temp, normal too.
MAP reads 0.8 volts, again normal.
Yes the surging occurs with IAC disconnected attempting to adjust the idle stop (per the book). I only get surging when I try to adjust the idle stop to 650 rpm, if I leave the rpm's at 900 or so there is no surging. So my "problem" is the engine won't hold a very low idle during idle stop adjustment. Is that truly "bad"?
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08:28 AM
PFF
System Bot
86FieroSEv6 Member
Posts: 438 From: Navarre, Florida, USA Registered: Nov 2010
So, all else being the same, it got worse after disturbing the throttle body assembly . . . . . . hmmmmmmmm. Even with all my years at doing this stuff, that would make me rip it back off and go through it with a fine tooth comb to be certain something wasn't amiss, like an o-ring out of place etc. Did you use a TB overhaul kit? As to how low the idle can go, that affects decelleration performance and engine vibration effects at idle. It can also have a minor effect on idle and low speed emissions (but I don't think we have any spotted owls as members here . . . lol)
[This message has been edited by 86FieroSEv6 (edited 12-25-2010).]
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08:31 AM
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
It got "perceived worse" till I re-adjusted the idle stop due to the un-metered air that was bypassing the IAC and throttle blade. Now I'm back to original problem, can't set idle stop per the book without surging idle. Car runs great otherwise.....
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08:59 AM
86FieroSEv6 Member
Posts: 438 From: Navarre, Florida, USA Registered: Nov 2010
TPS? Sometimes a live scope is necessary to catch glitches in the TPS signal. Is this a manual trans' car? Manual trans' cars put a good deal more wear on the TPS than do automatics.
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10:57 AM
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
Intake gasket leaks? Have you done a leak down test on the engine to look for uneven cylinders? Four bangers are particularly sensitive to uneven compression at low engine RPM's.
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11:36 AM
sportcoupe Member
Posts: 329 From: Savannah, GA Registered: Oct 2010
Might have an intake leak. Engine is a new short block just installed. The original 2.5L locked up years ago (2005), that's why it was parked for so long.
I do have all the vacuum ports on the TBI capped off except the MAP sensor port. Do I need one open to the charcoal canister for a vent?
[This message has been edited by sportcoupe (edited 12-25-2010).]
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11:54 AM
phonedawgz Member
Posts: 17101 From: Green Bay, WI USA Registered: Dec 2009
Lets back up some. Before it started running "worse" I did take the throttle body off, disassembled the lower end and soaked it in a gallon of carb cleaner. I removed the injector and flushed the upper section with spray carb cleaner. The injector and o-rings are new.
When I assembled everything the idle seemed worse then ever with the current idle stop setting and IAC connected. It's was surging from 500-900 rpm and IAC counts pegged at over 200. This tells me the IAC is fully retracted allowing as much air to bypass as possible.
I ended up just adjusting the idle stop further to get the IAC counts mid-range (around 25) and my idle is pretty smooth with everything connected. I think I may have had some minor vacuum leaks at the throttle body allowing un-metered air to get in the engine. Once I rebuilt the TB those small leaks went away and the old idle stop setting was too low for a steady idle so the IAC fully retracted as much as it could to compensate.
I have about given up on adjusting the idle stop per the book.
Well the throttle stop is to keep the engine running during the short times that the ECM is kinda wacked out and has the IAC too extended/closed. It really doesn't come into play too much and is rarely adjusted. Most Fieros still have the factory cap over the adjustment.
The adjustment procedure is to jumper A-B and then uplug the IAC. Then start the engine and adjust the idle at 500-600 which is well below the 1,000 target of the ECM. So you know what the ECM was trying to do during this time right? It kept on sending pulses to the IAC to open it up more. The IAC is a stepper motor. It has two coils, each with two wires. The ECM 'turns' the stepper motor by applying either positive or negative voltage to one of the coils to 'step' the motor to the next position. So if the ECM left the motor in a position pointing 'East' if the next pulse on the N-S coil is positive the stepper motor will turn one click and point North. If instead the ECM wanted to move the stepper in the other direction it would have applied a negative pulse to the N-S coil and the motor would have been turned to a South position.
So while you had the IAC disconected the ECM tried opening and opening the IAC. The ECM counts where it is at and remembers it. So when you are finally done with adjusting the throttle stop the ECM is way off of where it thinks the IAC is at. So yeah plugging in the IAC at that time might result in the ECM finding the 1,000 rpm target idle at some number way off of 'normal' Now actually as long as it's still within the 'normal' range for the ECM everything works fine. If you reset the ECM, it will again re-learn the IAC position and the reported position of it should be more normal.
[This message has been edited by phonedawgz (edited 12-25-2010).]
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12:21 PM
phonedawgz Member
Posts: 17101 From: Green Bay, WI USA Registered: Dec 2009
IAC is outside air temp, lower then coolant temp, normal too.
MAP reads 0.8 volts, again normal.
Yes the surging occurs with IAC disconnected attempting to adjust the idle stop (per the book). I only get surging when I try to adjust the idle stop to 650 rpm, if I leave the rpm's at 900 or so there is no surging. So my "problem" is the engine won't hold a very low idle during idle stop adjustment. Is that truly "bad"?
