For a stock L98 (1985) I believe once the coolant temp reaches 180 deg the ECM goes into closed loop: Y or N ?

Once it's in closed loop if the coolant temp drops below 180 deg does it still stay in closed loop or does it go back to open until the temps go up past 180 ??

My reason for this question is this:
I am concerned that I may have dropped the temps down to the point that the ECM is cycling in & out of open/closed loop. There are no "symptoms or problems" but I got some info the other day that has caused me to question what I did !!

About 4yrs ago I had installed a 180 t stat & replaced the factory thermo switch with one that trips on at 200 & off at 180 to keep the temps down to a more reasonable level.
The engine at idle, would run up past 220 before the fan would cut in & that seemed a little high. It always ran over 200 on the highway. Now it runs at 180 +/- 2deg on the highway.

TIA.

 

From my 90 FSM:

ECM goes into "closed loop" when all following occur:

1. o2 sensor heats up (has varying voltage output)
2. CTS (coolant temp sensor) is above 104F.
3. A specific amount of time has elapsed..

Installing a lower thermostat will not cause the ECM to jump out of closed operation..If you have or can borrow a Tech1 or any datalog software it will tell you whether the ECM is in open or closed loop..

 

According to Datamaster my 86 goes closed loop at about 130* coolant temp.
You can see if you are in closed loop without a scanner or laptop?software. http://members.shaw.ca/corvette86/No...it%20Check.pdf
Well,on my 86 you can

 

People worry about this thermostat open/closed loop thing too much.

The temp dependent timers all go off within a couple minutes, the coolant needs to reach about 130-140F, and the O2 has to reach 600F. If your car is at 180, and you dont have headers, then youre in closed loop barring a sensor failure.

For cars with headers, which may move the O2 further away, the car can drop out of closed loop at idle because the O2 doesnt stay hot. If you have stock exhaust manifolds then its not.

 

Thanks, I guess I just started to 2nd guess myself.

 

Well, what makes my car slip back into open loop mode on long idle periods? Is that supposed to happen????? This happened on a 5 minute idle w/ a 191 coolant temp and 199 oil temp.

 

- True.


- Not true.


Probably true about the header owners but not true about stock exhaust owners. Gordon Killebrew told me that extended idling can cause the system to drop out of closed loop and go into open loop mode even though the coolant temp is at or above 180* and the timers have all passed.

Additionally, my 88 FSM agrees with Gordon and says in a several places that even short periods at idle may cause the ECM to drop out of "closed loop" mode due to the O2 sensor not maintaining a high enough temp to sustain closed loop mode. See Pages 6E-3 and 6E3-A-8. "On some engines the oxygen sensor will cool off after only a short period of operation at idle." and "To restore Closed Loop operation run the engine at part throttle..." Restore... i.e. return to.


Your car slips back into open loop because it is normal for it to do so. See above or consult your FSM. You should be able to find the same info I did in the 89 FSM in the same sections.


1-MORTOY, you should be able to put your ECM into Field Service mode with the engine running and see for yourself if the ECM is in closed loop operation. Your manual is probably similar to my 88 FSM which states in Section 6E, page 6E-5:
If the Diagnostic terminal is grounded with the engine running, the system will enter Field Service mode.
... In "Open Loop" the "Service Engine Soon" light flashes two and one-half times per second.
In "Closed Loop" the light flashes once per second. Also, in "Closed Loop" the light will stay OUT most of the time if the system is too lean. It will stay ON most of the time if the system is too rich.

The ALDL has 2 rows of terminals with the top far right being "A" or ground and the terminal to its left being "B" the Diagnostic Terminal. Short them with a paperclip or a wire with tiny clamps on the ends. Doing this with the engine running it goes into Field Service mode. With the engine off it puts the system in Daignostic Mode and displays any Diagnostic Trouble Codes (DTCs) stored in the ECM.

Now, without a scan tool, you can do a limited scan of your system that tells you if the ECM is in closed or open loop mode and when in closed loop if it is compensating for a lean or rich condition.

 

I've never experience my car dropping out of closed during extended idle but that's just one car with headers.

 

To the original poster's question I think we are all basically agreeing the variable to keep the ECM in closed loop is o2 temp, not operating temp..

My experience is my car, stock exhaust can idle all day and o2 never cools off, according to Tech1...

 

Yes that is all correct, but I dont give paragraphs of explanation when its not asked for. It tends to confuse people more when they just want the simple answer.

My car goes into closed loop at 140ish and stays there until extended idle, with headers and with exhaust manifolds. The ability of the iron exhaust manifolds to retain heat doesnt allow the O2 to drop out of closed loop quickly, so when his coolant reaches 180, the pipes are well above that, and the O2 will stay in closed loop unless youre idling for a very long time barring a sensor failure. Therefore for most people, its not an issue so I wont mention it.

 

Actually, I appreciated all the info that Mike_88Z51 provided ( as well as yourself & the others too) because I like to know the "why's & how's".

(but then, I am a bit of a detail orientated person anyway. )

I do have a complete set of service manuals but they are in the car & I can't get to it. I will run that diagnostic once it's out of winter storage.
Thanks again guys.

BTW: I have considered getting a scanner, which one would be recommended & would it also work on an C3 (82 CFI).

 

As far as scanners go-you may want to look in the scan and tune section. I just bought a used laptop for scanning purposes-some of the scanning programs are free for awhile-I think Craig Moates, anyway, computer-download program,simple cord-whole thing runs about-$75-$100. Plus you have a screen you can actually see. Most of the scanners you buy run around $300.plus, and have those small screens. Check out Morleys site-nice guy and very helpful forum member.

 

OOPS, wrong pdf
http://members.shaw.ca/agent86/Diagn...ice%20Mode.pdf