2.8 starts bogging with IAC connected? (Page 2/3)
Additivewalnut JAN 19, 06:24 PM

quote
Originally posted by 1985 Fiero GT:


Ok, I have no idea what it may be then



Me neither LOL

I guess this is just gonna be a car that has no IAC. Every car has its quirks I guess
Raydar JAN 19, 08:55 PM
Try to find someone with a scan tool, and look at your sensor readings. Make sure that everything looks normal.
Intake air and coolant temp sensors should show values similar to "reality". TPS should show somewhere between .5 and 5 volts, depending upon throttle opening. Same with MAP, I believe, based upon vacuum.
O2 should show ~375mV when cold. When hot, should bounce back and forth between 200 and 800mV, or thereabouts. The values are not that critical. Just that it's moving from max to min, several times a second.

This is all from memory. If I've messed up, I hope that someone will correct me.
Additivewalnut JAN 20, 03:49 AM

quote
Originally posted by Raydar:

Try to find someone with a scan tool, and look at your sensor readings. Make sure that everything looks normal.
Intake air and coolant temp sensors should show values similar to "reality". TPS should show somewhere between .5 and 5 volts, depending upon throttle opening. Same with MAP, I believe, based upon vacuum.
O2 should show ~375mV when cold. When hot, should bounce back and forth between 200 and 800mV, or thereabouts. The values are not that critical. Just that it's moving from max to min, several times a second.

This is all from memory. If I've messed up, I hope that someone will correct me.




Finding a scan tool is probably gonna be unreasonably hard but I'll look around. Theoretically could I check all of that with a multimeter?
theogre JAN 20, 08:51 AM
In very short...
MSD box is crap "Upgrade" to most cars. Remove that junk. That box will often fry the dist cap & rotor at minimum... If have MSD & other "performance" coils etc often are crap too.
Fix/Replace other ignition parts & check/"reset" base timing after Removing crap MSD box.
See https://web.archive.org/web...t/~fierocave/hei.htm

ECM often won't set codes for various reason.
Could test some w/ a meter but doesn't matter if ECM doesn't see same "data."
See https://web.archive.org/web...cast.net/~fierocave/ ECM section.

IAC & some others Not connected can put ECM in "limp mode" w/o setting codes. In that mode, may "run" w/ many things set to default values & ignoring setting codes.

reddevelriver sells ECM Scan tools. https://reddevilriver.com/aldl.html
⚠️ Warning: If have a "chip" from other then GM, many "Chip" companies remove the data stream to the scanners & more like "deleting" EGR section of code.
So buying a scan tool may not help until you have GM "chip" or have someone else that will making a "custom" chip w/o blocking scanners.

------------------
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)

The Ogre's Fiero Cave

fieroguru JAN 21, 10:30 AM
First, the IAC only controls idle air flow becomes a much less significant air flow path at higher rpms. The 4 wires to it are direct from the ECM and not shared with any other sensors. If RPMs drop when you plug it in, it means the desired IAC position doesn't allow enough airflow between it and the throttle body. If you haven't taken the throttle body off and cleaned all the oil and carbon build up from the bore of the throttle body and the throttle blade, then do that. All that build up restricts airflow and caused idle control issues. If you haven't cleaned out the IAC housing and passages, that would be good to do. The IAC will not learn where it needs to be at cold start again until the car has been driven above 35 mph.

Once you do the items above, manually adjust the IAC, leave it disconnected and get the car started and running so you can start looking at other areas.

The IAC has zero to do with the engine shooting flames out the exhaust at higher rpms or engine performance above idle, so that is an indicator of a different issue(s).
Ignition timing, misfires, overly rich fueling all can cause back fires - and they would all impact idle quality as well.

I would:
  1. Check ohms to ground on MAP pin A and TPS pin C. Both of these are grounded through the ecm, so if these show high resistance to ground, then the ECM has a grounding issue - which will change the MAP sensor reading (controls fueling), TPS readings (controls transient fueling), and coolant temp (controls fueling).
  2. Check ignition timing with the IAC unplugged and the ALDL jumpered.
  3. Check the plug wire placement on the distributor - looking for 2 cylinders to be swapped in the firing order sequence.
  4. Pull 1 plug wire and corresponding plug at a time - check the plug for any damage or signs of misfiring (oil buildup). Then clean, gap and reinstall the plug.
  5. Take the plug wire and measure the ohms through it while spinning it like a jump rope - if it shows OL anytime during this, replace the plug wire. Reinstall a plug wire, move to the next cylinder - and remove the spark plug and ignition wire. Doing it this way keeps you from installing the wires wrong.
  6. Restart the car and see if it runs better and isn't shooting flames.
  7. If it is shooting flames, remove the MSD system. If I am remembering correctly it has a built in rev limiter, which will cut ignition timing (that is the only thing it controls), which would send fuel into the hot exhaust, if there is enough air, in the exhaust it will ignite.
fieroguru JAN 21, 10:38 AM

quote
Originally posted by Additivewalnut:
Finding a scan tool is probably gonna be unreasonably hard but I'll look around. Theoretically could I check all of that with a multimeter?



A scanner or scan tool is the only way to see the IAC counts at startup, when driving and when idling, what the KOEO MAP reading is and what it is while the engine idles and cruises, if the ecm is going into closed loop and how much fueling the ecm is adding or subtracting... lots of valuable info to help get the car running its best.

https://reddevilriver.com/home-page
Dukesterpro JAN 22, 12:04 PM
Only Ogre would find MSD "Crap".


Its not crap, period, BUT it is a unknown variable that is usually installed on a case by case basis, that can make a ignition / timing issue hard to diagnose. It would be good to have stock system in place to work as a sanity check before you work in upgrade like MSD
theogre JAN 22, 03:33 PM

quote
Originally posted by Dukesterpro:
Only Ogre would find MSD "Crap".

Its not crap, period, BUT it is a unknown variable that is usually installed on a case by case basis, that can make a ignition / timing issue hard to diagnose. It would be good to have stock system in place to work as a sanity check before you work in upgrade like MSD

Wrong. People that knows hows MSD "Boxes" actually works & more say the same.

And That's Way Before MSD Accel etc moved to most to all parts made in Mexico China & worse factories.

Big Hype Machine Example: "Multiple Spark" is not a Feature but a Requirement to have enough Spark Dwell for the MSD Spark to have Enough Time to Ignite the Fuel under Compression. MSD Hype this as a Feature to sell High $ part to Thousands of Fools w/ stock & many mod engines for Decades.

Many here alone have had big problems w/ MSD products. Either buying a car w/ it/them or installing them then die often on the highway or far from home. Accel HEI & DIS coils (& other Dry coil types) have problems too like I wrote the first pages covering DIS 20+ years ago.

Also Mounting MSD Box in the Trunk in Fiero & others often Does Not Help because Trunk gets Hot & little Air to flow to cool the box. IOW When the Trunk is @ 90-150°F then MSD box is a lot higher temp while running often pushing 180-200+°F. Worse when you have a lot of other things in that too taking air space. You often can't put many Food types in there for same reason & that's even if have all OE heat shields etc blocking engine &/or exhaust heat. Car Trunks can kill people too even when park just because of sun heating to the point NHTSA & others require safety release parts inside for years now. FMVSS 401 Interior trunk release. (Cornell law copy)

"But racers use them..."
History: First "Developed" Decades ago to "Fix" Breaker & Condenser in "old school" distributor engines that die from engine heat, high amps needed by "performance" coil(s), etc, for racing...
● Some in the Race world actually use the boxes for various reasons & often mount them in the Cabin far away from engine heat. Even then often have Two units installed & switch in the spare because first unit often dies during a race lasting minutes to a few hours. I know many that Hate them but have little options to have any ignition in their race car.
● Huge amount of Teams have MSD & more Stickers etc only because they get money to have the marks. Most don't actually use the products but Depending on Race Org setup, having a sticker & drive laps or better win a race can generate a lot of money even when Sticker company don't fully sponsor a team. Because Stickers are Cheap Ads on a race car to show everywhere the car appears.
Dukesterpro JAN 23, 03:33 PM
Ogre, Im not gonna get into this with you. Its the same stuff you always push. "Muh stock components superior"

1. Multiple spark is a gimmick for most applications, but does have its purpose in some high compression engines. Writing it off because you listen to boomers at the car shows who know next to nothing about ignition system design (I do btw, not for cars but for industrial applications) explain why single shot style ignition is superior cause thats what the engineers went with isnt really relevant to the quality of MSD.

2. I agree with you. Dealing with other peoples rat nested half assed installs is not a good thing. Hence why I said in my original post. Remove it and install a proper stock system to verify all your compoenets before you worry about adding in a MSD system.

3. Using the hot Fiero trunk against MSD is a bad argument, since the stock component suffer from the Fieros blistering engine bay, hence why GM added that half assed little cooling fan setup they came up with. Similar solutions are available to people who want MSD ignition.

4. yeah, most people understand how sponserships work. I don't expect Jimmy Spencer to be blasting down a pack of Winstons while racing. It all depends on the league rules as to what ignition systems teams are allowed to use.

The point of the matter is that MSD is a fine ignition system, when installed properly, and when extra care is given to the application in a Fiero . I've used it and abused in 12 separate vehicles. Its not going to magically give you a 20 percent boost in HP or turn your iron duke into a small block. But for a readily available ignition system to replace the flawed stock one that may or may not be in production much longer, its fine.
Additivewalnut JAN 23, 04:27 PM

quote
Originally posted by fieroguru:

First, the IAC only controls idle air flow becomes a much less significant air flow path at higher rpms. The 4 wires to it are direct from the ECM and not shared with any other sensors. If RPMs drop when you plug it in, it means the desired IAC position doesn't allow enough airflow between it and the throttle body. If you haven't taken the throttle body off and cleaned all the oil and carbon build up from the bore of the throttle body and the throttle blade, then do that. All that build up restricts airflow and caused idle control issues. If you haven't cleaned out the IAC housing and passages, that would be good to do. The IAC will not learn where it needs to be at cold start again until the car has been driven above 35 mph.

Once you do the items above, manually adjust the IAC, leave it disconnected and get the car started and running so you can start looking at other areas.

The IAC has zero to do with the engine shooting flames out the exhaust at higher rpms or engine performance above idle, so that is an indicator of a different issue(s).
Ignition timing, misfires, overly rich fueling all can cause back fires - and they would all impact idle quality as well.

I would:
  1. Check ohms to ground on MAP pin A and TPS pin C. Both of these are grounded through the ecm, so if these show high resistance to ground, then the ECM has a grounding issue - which will change the MAP sensor reading (controls fueling), TPS readings (controls transient fueling), and coolant temp (controls fueling).
  2. Check ignition timing with the IAC unplugged and the ALDL jumpered.
  3. Check the plug wire placement on the distributor - looking for 2 cylinders to be swapped in the firing order sequence.
  4. Pull 1 plug wire and corresponding plug at a time - check the plug for any damage or signs of misfiring (oil buildup). Then clean, gap and reinstall the plug.
  5. Take the plug wire and measure the ohms through it while spinning it like a jump rope - if it shows OL anytime during this, replace the plug wire. Reinstall a plug wire, move to the next cylinder - and remove the spark plug and ignition wire. Doing it this way keeps you from installing the wires wrong.
  6. Restart the car and see if it runs better and isn't shooting flames.
  7. If it is shooting flames, remove the MSD system. If I am remembering correctly it has a built in rev limiter, which will cut ignition timing (that is the only thing it controls), which would send fuel into the hot exhaust, if there is enough air, in the exhaust it will ignite.




Okay so a couple things here:

1. My map sensor has some goofy ass resistor or capaciter or SOMETHING on one of the wires, I'll post a pic of that later. Seems to be absent on every other Fiero I've ever worked on.

2. The IAC being connected makes little to no difference in the idle, and in the cases where it does make a difference, it opens itself all the way up and then I've got a 1500 rpm idle. Generally, it gets connected, and then something happens that goobers up the AFR. It smells rich. Disconnecting it allows the AFR to come back to something halfway normal and it starts revving out fine again. I did spray the throttle body down and cleaned out all the passages I could find, but it didn't seem to help.

3. checking ignition timing in this engine bay SUCKS! I couldn't see a damn thing. Granted I didn't try that hard, it was ~3 degrees outside.

4. Plug wires are a-okay. Checked firing order, looked solid.

Would it be worth throwing the spare ECU I have in and seeing if it's computer related?