1984 Fiero SE 2M4 Sudden Idle Spike/Surge (Page 2/3)
Sanhino JAN 25, 04:01 PM
So I figured I would mess around with some things just to see what would happen, and included a bunch of pictures. As it stands it normally idles at about 3000 rpm.

Here's the broad engine bay right now:




The throttle adjustment screw was messed with previously but I believe I have it set more or less correctly. It's only sticking out just a small amount and the throttle valve is pretty closed.




I had unplugged the vacuum line from the MAP sensor and the idle had decreased to approx. 2000 rpm. Every time I reattached the vacuum line it immediately shot back up to 3000 rpm, and when I unplugged the MAP sensor all together the engine died.

Also with the vacuum line from the MAP sensor unplugged I also unplugged the vacuum line from the EGR valve, which made no difference. I had previously replaced the gasket and checked to see if it moved under vacuum and it did.



I also played with the TPS, and idling if I presses the sensor arm in it idled more roughly. I know the connection to the TPS is sketchy but it seems to work for the time being. When it's completely unplugged it doesn't do anything, whereas when it's plugged in it seems to be working properly.




I also took out the IAC Valve entirely and it didn't seem to make much of a difference. While it was running I unplugged the IAC Valve and re-plugged it in and could feel it slightly vibrate, but the end didn't particularly shoot out or retract.



I also checked the vacuum lines for any obvious leaks, which they seemed okay. I also checked the line from the break booster and that was fine too.

So the only thing that's actually decreased the idle for me is to unplug the vacuum line from the MAP sensor, which clearly isn't the correct thing to do. Maybe someone with more experienced eyes can spot something that I wouldn't think to look at or pay attention to.
Sanhino JAN 25, 04:57 PM
I just retried starting and got a code 22 so maybe the TPS isn't working the way it should, which makes sense looking at that god awful connection. Would that cause the high idle though?

[This message has been edited by Sanhino (edited 01-25-2020).]

Sanhino MAR 12, 04:36 PM
So I replaced the TPS and set a multimeter to it, and it's reading 0.5V and increases when the throttle increases as it should, and I also replaced all the vacuum lines to the throttle body. I also put a pressure gauge on the fuel line just for the heck of it and it's reading about 12 psi consistently whether under load or not. Sometimes from a cold start it idles at about 1,500 rpm for a minute and then decreases to 1,000 rpm, which it should be doing, and runs fine for a good 10 or 15 minutes, but then other times it idles fine and then just surges seemingly without any input. Still have no idea what's causing it so seemingly randomly surge.

The last time the idle was fine at 1,000 rpm I disconnected the temperature sensor sender wire to the ECU and it ran rougher, not stalling but just a bit more rough, which makes sense to be doing to me, bu the last time the idle surged I disconnected the wire and it didn't make any difference. I'm not sure if that's a good thing or a bad thing.
AsaBergman MAR 13, 03:43 AM
This may sound odd but have you verified you have the correct ECM in the car? When I got my 84 it had strange issues that didn't make sense. It drove fine but on startup or coming to a stop the idle was 2800-3000 rpm before it would go back down to 1k. Turned out the previous owner put an 85 automatic ECM in my 84 manual Fiero.

Those battery wires and clamps should be cleaned or replaced with proper side-post wires. Those connectors with the exposed clamps always cause trouble.
Sanhino MAR 13, 08:07 PM
I'll have to check into that, is there a way to tell if it's an ECM from another car? For example is the vin on the OEM ECM?
Sanhino MAR 13, 08:08 PM
Never mind i just saw http://gafiero.org/ECMgood/fieroprom.html#L4

I'll check tomorrow since it should be nice out!
AsaBergman MAR 14, 07:16 AM

quote
Originally posted by Sanhino:

Never mind i just saw http://gafiero.org/ECMgood/fieroprom.html#L4

I'll check tomorrow since it should be nice out!



It's a stretch but I learned by now to never assume the parts that came on the car are correct.

That wiring at your TPS needs to be fixed. Even butt splices would be better than whatever is going on there. Edit: it could indeed cause idle issues.

Also I noticed in the picture you still have that plastic cover over the temperature sensor in the water neck which makes me think it's ancient. Have you ever replaced the temp sensor?

[This message has been edited by AsaBergman (edited 03-14-2020).]

Sanhino MAR 14, 01:23 PM
So first thing today was to replace the TPS connector.




And for the hell of it I turned the car on and let it run for a bit, it cold started and idled at approx. 1,500 rpm, then decreased to about 900-1,000 rpm for maybe 2 minutes, then without pressing the gas pedal or messing with anything the idle increased to about 2,200 rpm and wanted to stay there.

I took off the center console to check the ECM. From what I saw the ECM number is 7-6156, calibration code 0301HJ, which from what I can tell doesn't correlate to any of the Fiero ECM numbers. I noticed the paper being ripped at the top and saw the PROM, and it looks like from the serial number inside that this is a remanufactured ECM but from searching the internet I can't seem to figure out if this is right for the car.





I haven't replaced the temp sensor with the wiring to the ECM but I had previously taken it out and wiped it clean on the end. Also last time I was messing with it when the car was idling correctly I had unplugged it, and the car ran slightly more rough, and when the idle was high I also unplugged it and it didn't seem to make a difference in the idle. I'll likely end up replacing it anyways just to know that it's new.



And that's the latest and greatest. Here's the engine bay as it stands now, a slight amount cleaner but not great.
AsaBergman MAR 15, 06:31 AM
Correct PROM and computer.

That temperature sensor looks to be ancient. I don't even know if you can get them with that connector anymore. Without using a scanner to see what the ECM is reading(like Patrick suggested) the only option is to just replace it. Note that the 84 has three coolant sensors: the one in the water neck is for the ECM, the one further back on top of the head is the gauge and temperature light, and the last is on the back of the head near the exhaust is for the radiator fan. The other two wouldn't cause the idle issue. You'd want to replace both the sensor and the connector with one like ACDELCO 21352 or a cheaper version.

Also if you have any knowledge of electronics you can build your own 'scanner' cable for a couple dollars.
AsaBergman MAR 15, 06:35 AM
Also don't get discouraged. It took me two months to get my Fiero on the road although I had far worse issues.