1984 Fiero SE 2M4 Sudden Idle Spike/Surge (Page 1/3)
Sanhino JAN 16, 09:23 AM
First off I want to say thank you for everyone that's posted before since this a treasure trove of information! I'm 26 and got my Fiero in August and have slowly been picking away at the issues. It's my first project car so it's been a great learning experience so far, however one of the last things I haven't been able to figure out however is this idle surge.

Occasionally when the vehicle is idling it will spike from about 1,300-1,400 rpm to between 2,500-3,500 rpm without any notice (at least from what I can tell). I've replaced the plugs, checked the spark plug wires, replaced the fuel filter, the spray seems consistent without any visible large droplets, I have checked the vacuum hoses visually and although they're old I haven't seen anything, I've held an unlit propane torch along the hoses to see if it would spike and it seems fine, I replaced the EGR gasket and tested the EGR and it slides fine, I replaced the IAC valve, the TPS seems to be working fine, and I have a new MAP sensor, so at this point I'm more or less out of ideas, unless it's the ECU.


When it idles normally it runs fine (albeit loud due to the exhaust leak), and is able to drive 60+ miles at a time seemingly without issues. It sounds like other people have run into this issue but a combination of the things I've already done have been their solution.

Any help/suggestions would be appreciated. Thanks!

Bonus Pic: The Project
Gall757 JAN 16, 01:41 PM
I would be suspicious of a bad ground. ECM or a sensor or ignition.

[This message has been edited by Gall757 (edited 01-16-2020).]

theogre JAN 16, 06:09 PM
Start w/ Check/fix all grounds on engine and in engine bay. And battery and Alt cables too.
Iffy power can cause idle problems.

note that 84 has alt wires going thru C500 and contacts there can burn etc.
Other + wires go to the Starter.
Is not like later versions that have + wires under c500

------------------
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

Sanhino JAN 16, 08:01 PM
So this is what I'm working with.. I would imagine if it's a grounding issue it would be the wiring to the frame rather than the engine.
pmbrunelle JAN 16, 09:09 PM
That entire grounding setup looks crusty...
Sanhino JAN 16, 09:28 PM
yeah it does. I'm thinking I'll just ground it somewhere else in the interim before cutting and filling in that hole
pmbrunelle JAN 16, 09:50 PM
The factory battery-to-body ground uses this screw:


Dremel/clean the contact area to bare metal as required for a good contact. You don't have to use that screw, but it's there for that purpose.

Everything else should be sanded / wirebrushed as well...

Since the wire strands themselves on the fat negative cable are corroded-looking, probably the cable should be changed as well.

[This message has been edited by pmbrunelle (edited 01-16-2020).]

Sanhino JAN 21, 05:42 PM
Just as a quick update: I haven't had much time or daylight to work on it but I did clean the terminals, wire brushed the ends, undid the negative terminal and cleaned the grimy copper. I haven't yet replaced the negative cable entirely but it didn't surge when I had it idle for a few minutes, so it might be promising..
Sanhino JAN 25, 02:59 PM
I spoke too soon.. it'll idle normally for the first few minutes and then it'll want to stay at about 2500-3000 rpm
Patrick JAN 25, 03:20 PM

It would help to get a scanner (or WinALDL) connected to see what the ECU is being told the coolant temperature is.