This is a problem on my '91 Firebird 3.1 V6. Recently my car has started to run like absolute crap at idle. It feels like the engine has a terrible miss and it makes the car have these little surging jerks while sitting at idle in gear. I'm really surprised it stays running at idle. There doesn't appear to be any loss of power, it seems to just be an idle problem.

I started out this morning by replacing the fuel filter (already had one and had been meaning to do it) but that didn't fix the problem. Next, I hooked up my AutoXray and took some readings. TPS runs a .68v at idle. IAC shows steps but doesn't fluctuate much if at all while idling. O2 sensor in closed loop jumps between 480 mvolts and 580 mvolts at idle. If I gas it in park, the O2 reading jumps to about 700 mvolts and when I let off it slowly drops back to the aforementioned readings.

I then pulled one plug from each side of the engine and inspected them and they appear to be completely fine. I pulled the cap and discovered lots of deposits built up on the contacts underneath so I replaced that and the rotor and the problem still exists.

Plugs and wires are less than a year old and based on my inspection of the two plugs, I've decided at this time that they're not the likely culprit. O2 sensor is less than a year old as well.

Here's a couple interesting tidbits that may help. The AutoXray shows rich at idle and occasionally will switch to lean and then back to rich but only about once every minute or so, not as often as it should. Also, the engine seems to run a little better when I first start it up and the idle deteriorates as it runs. This doesn't necessarily happen just when the engine is cold either. Example, went to pay my rent today and car was idling like crap. Shut the engine off, took my rent into the office and was only there for about 2 minutes. Went back out, started the car and it idled fine for a few minutes and then started running like absolute crap again.

If anyone has any suggestions on where to go next with replacing things, please let me know.

 

Nobody has any suggestions? I find that hard to believe that no one wants to chime in on this problem.

I guess I'll go change the plugs and wires and see if that does anything. Wires are warrantied so it won't cost anything for them.

 

Not at all like the old days, you only will get replies if someone stumbles onto the board.

Is the mis bad enough to change to rev pattern at idle? Have you checked for vacuum leaks? Have you tried cleaning the IAC (as per Vader's directions)? Does it happen only/moreso in closed or open loop? Is the O2 switching properly? Temp reading correctly?

 

have you replaced your ignition module? what did the plugs look like?

 

Cleaned the IAC when I had the throttle body off about 4 or 5 months ago. Shows steps on the AutoXray so it appears to be working properly. It does idle erratically. Fluctuates about 100rpms at idle according to the AutoXray. It does it mostly after the engine has warmed up but it never actually dies and I can usually restart it right away. Sometimes however, it will crank more than usual before starting when it's hot. The car does switch over to closed loop but I'm not sure if the O2 is working properly. I changed it about a year ago. Haven't checked for vacuum leaks yet, I'm kinda waiting on that one because I'm stubborn and refuse to think it's a vacuum leak. That would just be too simple ya know.

I haven't replaced the ignition module yet, I think I'll have that tested this weekend. When I pulled the plugs they looked fine. My friend thinks the car may be "loading up" on fuel at idle and causing the erratic idle.

Here's the data I got from a recent AutoXray scan while sitting in park at idle in a parking lot. The O2 and open/closed loop reading is what gets me because this was done after driving a good distance from my work to college.

Coolant Temp 205.7*F
O2 - 528 mvolts (it was moving but only about 50-60 mvolts in either direction
Auto Xray also said O2 - Not Ready
Open/Closed Loop - OPEN
Barometric Pressure - 4.66v
Map Sensor - 1.68v
Injector Pulse - 1.6ms
CCP Duty Cycle (what is this?) - 0.0%
EGR Duty Cycle - 0.0%
EGR Solenoid 1 - Off
EGR Solenoid 2 - Off
EGR Solenoid 3 - Off
Air Switch Solenoid - Off
TPS - .64v (Idle)

Do any of these readings look off to anyone??

 

BUMP!

I ran the AutoXray on my car today paying close attention to the O2 sensor and found that at normal operating temps, the engine goes into closed loop with my foot on the gas but when I take my foot off the gas and idle it switches back to open loop again.

Is this normal? And if not, would this cause a jerky idle and a rich condition?

Weird thing is that the AutoXray is showing a rich condition and it smells like it's running rich but I actually got a good 2mpg more out of my last tank of gas than what I usually get. The current tank of gas doesn't seem to be disappearing any faster than usual either.

 

[QUOT
E=86TpiTransAm;3455484]BUMP!

I ran the AutoXray on my car today paying close attention to the O2 sensor and found that at normal operating temps, the engine goes into closed loop with my foot on the gas but when I take my foot off the gas and idle it switches back to open loop again.

Is this normal? And if not, would this cause a jerky idle and a rich condition?

Weird thing is that the AutoXray is showing a rich condition and it smells like it's running rich but I actually got a good 2mpg more out of my last tank of gas than what I usually get. The current tank of gas doesn't seem to be disappearing any faster than usual either.[/QUOTE]

I say check for a vac leak on the hoses and idk of your iac valve is.getting.yhe correct ohms. I am actually having the.same.problem.

 

I'm having the exact same problem.

I don't have anything like an AutoXray though. But it just started doing it about a week ago, mines a 91' Z28 (V8).

 

I'm having the same problem on my 87 formula and I cant find out what it is and its driving me nuts

 

These are old cars. Your best and cheapest starting point is to change all of the vacuum hoses. Including the charcoal canister hoses. On the hoses that are small and hard with rubber ends make sure the ends are tight - if not put a clamp on them.

You can change all the parts you want to, but if the vac hoses are bad it will still run bad.