My 85 has a rough idle.  It's not stalling or anything like that.  It just idles kind of rough - moreso than I think it should.  When idling, the RPMs jump around a little bit.  I'm not sure if that's entirely normal or not.  They may jump, for example, from 550 to 560 down to 540.  If I sit long enough and it really warms up, the idle will drop down to about 500 and jump a bit between 480 up to 520.  I have checked the TPS voltage.  It was at .62, so made the adjustment per the FSM to .54.  It still does this rough idle, though.  Otherwise, the car runs great and is a lot of run to drive.   

Looks like your idle speed is really low, to begin with. However, the specs are 450 rpm for standard trans, and 400 for the auto(in Drive)... There's no WAY my '85 would idle that low in gear....mine runs right at 600 rpm in gear, and prolly 650 or so in neutral.
A vacuum leak would cause a higher than normal idle. Possibly the IAC(Idle Air Control) is crudy, the throttle body is dirty....or even the MAF(mass air flow) is dirty.
Maybe it just needs a good trip down the hiway...real quick...at full throttle....clean that puppy OUT!

|UPDATED|7/26/2013 9:59:58 AM (AZT)|/UPDATED|

A few weeks ago my 85 was running very similar after sitting for several weeks. A quick rip down the freeway and it took care of it. It is a shame how fast todays fuel can sour. Still looking for a station in the area that sells non-oxy premium.

I drive this car 3 or 4 days a week.  While I take it on some lengthy drives, I can't say I really push it.  But, that's not specific to this car.  It's how I am with all my cars.  I mean, I think the highest I've gotten it up to is about 85-90, and I had it in overdrive.  And it's rare that I drive anything that 'fast.'   We tend to be covered in cops around here (larger city.)  It's definitely got its benefits to have a good force, but not conducive to fast driving.  Some day, I do want to get one of my Vettes really going, but I'll have to find a better place to do it.

M.A.F. is easy on 85' also just a little trick so you don't get a ticket. once the car is at operating temp find a nice open parking lot put the car in park and put your foot in it to about redline and hold it for 10 seconds. Let off and let it idle for twenty seconds then do it again. After about three or four times it should have cleared its throat.

Laurie:

I wouldn't put anything in the MAF sensor. I last bought one over 15 years ago. It was a one-year only (85) part, that cost about $525 IIRC, available from the dealer only. Don't risk damaging an expensive MAF with a $5.00 bottle of "feel good".

The 400-450 RPM figures Joel mentions are the base idle RPM that is set with the throttle body screw, with the IAC fully closed. The programmed idle (with functional IAC) is in the low-mid 600's (I'm not going to look it up right now). If your's is in the 500's, something is wrong with IAC response. It should be in the 600's for an automatic. Look at the emissions sticker on the radiator shroud for the program idle speed.

If it runs fine and doesn't stall, drive it. My 85 has never had a smooth idle since new. The only GM EFI engines I've owned that did idle smoothly were ones I tuned myself with available software. GM idle programs run lean, with retarded timing for emissions and economy, and don't idle very smoothly. Its probably running just the way it did when new.

DaveP


|UPDATED|7/29/2013 11:06:11 AM (AZT)|/UPDATED|

Several mentions of IAC, I noticed.  I read through the FSM on IAC, and will see about testing it.  Dave, I saw the timing specs on the radiator shroud, but not specs regarding the program idle speed.  I may be looking in the wrong place.  But, you and others noted that the idle I'm seeing is unusually low.  Now, I do get idle speeds in the low to mid 600s when the engine is cooler, especially when the whether is cooler.  But, when the engine gets good and warmed up, I can expect it to drop to the lower, and incorrect, idle speeds.  While I feel much better about the rough-ish idle after reading, I would like to get the idle consistently up to where it should be.  We've discussed looking into the "health" of the idle air controller to see if it needs to be replaced, or perhaps cleaned.

I give up. I just spent 15 minutes looking for an idle speed spec in my 85 FSM, and never found it. I thought it was on the under hood sticker, but it probably says something like "idle speed is ECM controlled, and is not adjustable" and doesn't give the spec.

My 85 has had a few episodes of "rough start, stalling, etc" over the years. Most recent was in 2008 when it sat all day with the battery disconnect open during its last NCRS judging at Chapter for my Founder's Award. When we went to leave, it wouldn't start at all. It farted, and tried, but wouldn't start. I disconnected the battery again, waited a minute, turned it back on and then if fired right up. I don't know what it is, but it's only done it a few times in 28 years, so I ignore it. I also now never let the battery die, and use a "memory saver" when changing the battery, or doing electrical service work with the battery disconnect open. The ECM has never been de-powered since the 2008 incident.

There was a Service Bulletin for an updated PROM change that addressed idle quality issues on the 85 Automatics. I don't have any further particulars, as my car is a manual trans and I've never needed to learn about it. I know of this from several other knowledgeable 85 OO's in NCRS, and awhile back I stumbeled into a GM site online that had a database of recalls and TSB's and I found it there. (I somehow lost that link, and have never found that site again :sad:)

There is a switch on the shifter that tells the ECM when its in Park or Neutral. This switch changes the way the IAC behaves when its in gear. If interested, I will look up and tell you the pin-out of the ECM for this wire and it can be checked with a VOM. I know the wire color is orn/blk.

Once again, I think I'd just drive it.

Thank you, Dave.  I guess that some of this stuff is simply common quirks for the car.  Overall, the car is a great runner, and I haven't had to do much to it for all the enjoyment I've been getting.  I will try not to worry so much about the small stuff.  

|UPDATED|7/30/2013 12:56:01 PM (AZT)|/UPDATED|

My 90 L98 idles very smooth.  Not sure why all L98s can't.