I've had my '86 coupe for a while (4+3 tranny, basically stock engine) and its never been perfect. Just when I thought she was starting to be a daily driver, that rough idle that I had always noticed got a bit worse and now it's to the point that the car is basically not driveable (stalled 3 times going 1 mile to the supermarket last thursday). In addition to the rough idle, the car seems to have a hesitation at highway speeds, and a tough time accelerating when it's at highway speed and will sometimes buck when you try to accelerate. so i borrowed a scanner last weekend and I'm trying to debug my issue. The car had a code 43 (knock sensor-ish failure), and since I cleared the code, the code has not come back (though i've only let it idle in my driveway and taken it for one spin around the neighborhood since I cleared the code). The best information that I've gathered is that the idle is fine as the engine starts cold, but as soon as it transisitons from "open loop" to "closed loop" mode, the idle becomes unstable (lopes) and eventually stalls. My understanding of those two modes (which may be incorrect) is that when the coolant temp is low, the idle speed is derived from a look-up table in the eeprom, and in "closed loop" mode, the idle speed is controlled by the computer and the feedback signals from various sensors. Does anyone out there know what sensors are used in the "closed loop" mode that may be causing my issues?
Thanx in advance.

 

How old is the fuel filter?
Start with basics.

 

You need to look at the TPS voltage first, assuming there are no vacuum leaks, that would be the biggest culprit if no other codes are showing. O2 is used in closed loop but primarily affects gas mileage and not the idle so much, but that is possible.

 

fuel filter is a year old (has less than 2000 miles on it), fuel pump is same age, I had that issue last spring... O2 sensor is maybe 2 1/2 years old, and when it went it set a code & turned on the "service engine soon" light, so i'm less worried about that. vaccum leak is a possibility... any quick way of testing for that?

 

Similar problem on my '89... butterfly valve needed to be cleaned with some carb cleaner.

 

O2 failures rarely set a SES light, its one of those things that may be within spec for the code, but not really working fine.

 

I am a new member and soon to be an owner of an 85 with the same problem. I have looked up a few threads that might be helpfull. I have them saved on my PC at home I'll post them tomorrow morning for you. The threads that I found basically center around some of what has been mentioned(IE throttlebody cleaning) and also IAC and one other sensor. I'll get you the links tomorrow.

 

I have similar issues with a warm idle surge. Once the car reaches about 170°-180° the engine gives a big shudder then starts the idle surging or what sometimes seems like a miss.

I assume around that temp is when it changes circuits.

Today I was running the car in the garage after re-filling my coolant and I got a service engine light up for code 44 (lean mixture, check O2 sensor). This one cleared itself after I started the car back up. What is the average life of an O2 sensor? I imagine, judging by the rest of the maintenance levels on my car, that the sensor is original with 120k miles on it now.

I also occasionally get a hesitation on acceleration (just like one you would adjust your carburetor accelerator pump to eliminate)

The seal on the idle screw has been broken so I assume the idle screw has been adjusted.

If there was a vacuum leak then would it not be noticed during cold idle (open loop)? Or is cold idle rich enough to work within the parameters with a small vacuum leak that would affect the closed loop?

If it is not a vacuum leak then I guess the hunt would be through the electric sensors of the closed loop, right?

 

The car has already been in closed loop for awhile when it gets to 180, if you get an O2 code, then its probably bad, no matter whether or not its causing your idle or surge problems.
Let me reiterate that I doubt that the O2 is causing these problems, but it is one that is read during closed loop. More than likely its going to be TPS, dirty TB/IAC, or bad vacuum leak.

 

Hi I have an 85 and had the same problem. I did a lot to find out. Now it seems good.

1. Clean the throttlebody Butterflies and the walls for carbon deposites.
2. Clean the IAC valve and the connection from the throttlebody to the valve.
3. I had a bad EGR valve. Vacuum membran was bad.
4. Then after not finding the reason for the bad idle I searched for the vacuum leak. Should have done that first. I used start gas and spryed some over the runners at the intake manifold and the engine increased the idle. So, I had to take the runners off and with new gaskets it became good and the idle is now .ok.

 

Here are the links I found
http://forums.corvetteforum.com/show...m_id=48&arch=1

http://forums.corvetteforum.com/show...m_id=48&arch=1

http://forums.corvetteforum.com/show...m_id=48&arch=1

https://www.corvetteforum.com/techti...D=55&TopicID=2