Up & down idle 88 Duke when warm

[Fierofool]

I don't have one out of the car but using this link for description, drill it in the flange, far enough inward that you don't hit the rubber seal. 

https://www.walmart.com/ip/Stant-45819-Superstat-Thermostat/138505932?wmlspartner=wlpa&selectedSellerId=0&adid=22222222227058081778&wl0=&wl1=g&wl2=c&wl3=338615277757&wl4=aud-566049426865:pla-529923070479&wl5=9010757&wl6=&wl7=&wl8=&wl9=pla&wl10=8175035&wl11=online&wl12=138505932&wl13=&veh=sem&gclid=EAIaIQobChMInbacstqY4gIVEtbACh1hDwgNEAQYBCABEgL2j_D_BwE

[Raydar]

It has become a common practice to drill a 1/8 hole in the flange if it doesn't already have a little weep valve pin in it.  I don't remember why, but I pulled both of mine and did the mod.

It just stops the thermostat from trapping air bubbles, which can keep it from opening properly. It also allows just a little bit of coolant to circulate, for the same reason.

[Fierofool]

On the 85 and 86 V6, that could be important.  Because they have the heater line coming off the water pump, when the thermostat is closed, air bubbles would go to the heater core.  Maybe they could be trapped there for a while, reducing the heater's effectiveness. 

But the air bubble issue was why I had heard people were doing it.  I have seen some that have a hole already in them with a little copper or brass pin in the hole.  I don't remember what vehicle they were for, though.  Could have been a Ford or Grand Prix. 

[GTRS Fiero]

The heater line comes off the water pump on a duke?

[Fierofool]

I don't know.  I don't think so.  Neither does it on the 87 or 88 V6. 

[Donster]

Are ther radiator cap and the thermostat cap identical?

[GTRS Fiero]

I don't know.  I don't think so.  Neither does it on the 87 or 88 V6.

That is the return line.

[Fierofool]

I have always thought it was the supply but now that I think about it, the feed goes out on the driver's side. 

[Donster]

OK, thermostat in the car IS a Stant 195F.

I scanned again today, paying attention to what Steve posted regarding to O2 sensor. While it does move around, I'm sure it is not in the correct range. It was one of the first things I replaced when this issue first reared it's ugly head. Just to make sure, I installed the old one again, and it had "0", ZERO readings, then the new one (which I assumed is working) would go from 0.1V to 0.9V to 1.1V to 0.4V and never zero in on a particular value. Can't be right, can it?
So I disconnected the O2 sensor, and whaddayaknow? Engine runs fine, no stumble, no hunting, no idle dropping, no stalling.
Downside is, it will occasionally throw a code 13 (O2 open) and the scanner tells me the engine is running lean. Before, with either O2 sensors installed, it would bounce between lean & rich by the second.

Can we now safely say that it is either the O2 sensor or the (one, purple) wire, going to the ECM that is causing my issue???

BTW: The new O2 sensor was a BOSCH, which you would expect NOT to be bad out of the box.

[Raydar]

The O2 sensor should bounce back and forth. That's perfectly normal. Should be so fast that you can barely read it.
If it settles on one side or the the other of .445 volts, THAT'S a problem. Once it's warmed up, it should always switch around. (The only real exceptions are accel or decel.)

I have had Bosch sensors go bad "prematurely". Can't say that I've ever had a bad one, out of the box, though.
I've had good luck with NTK O2 sensors.

[Donster]

Steve, while I agree with what you say, I have measured the values with O2 sensor disconnected at a constant 0.42 V.
And in the last 2 days, for some reason, the ECM does not throw a code, no CES light....AND more importantly, the engine is running as it should.
I will have to wait until I get a new O2 sensor to see if things return to normal. If not I obviously have an issue with the wiring in the O2 sensor circuit. Should not be to hard to fix, as it is only one wire (purple). No ground wire, as the sensor actually self-grounds to the header. And I don't need to ask for wire diagrams anymore, as I just got this in the mail: 

Brand new! $50.00 with $20.00 shipping from the US! In combination with my new scanner tool I'm ready for just about any problem this Duke throws at me (I know, I've always been an optimist  )
BTW, the scanner tool can be ordered (works on ALL Fieros and shows values in "REAL-TIME"!) as long as you are willing to endure the delivery time.

[GTRS Fiero]

Glad you got the book.

The scanner doesn't reset codes, though.

[Donster]

Glad you got the book.
The scanner doesn't reset codes, though.

Nope, still need to do the manual "Disconnect" with the power to ECM cable next to the C500.
Can't have everything. 

[Donster]

As I have  not received the new O2 sensor yet, we have been running the car with the sensor disconnected and I must say the engine is running great without.
Doesn't overheat, runs stable, no hunting idle, good mileage, good acceleration.
So, what bad could happen not having the O2 sensor connected?

[GTRS Fiero]

Rich or lean, maybe?