How can I improve the idle?
Thread Starter
Advanced
Joined: Oct 2007
Posts: 82
Likes: 0
From: Brownsville Texas
How can I improve the idle?
My 86 normally idles around 600 rpm. I get a slight miss while sitting at a red light and it just drives me nuts. Have new plugs, clean injectors and all the obvious stuff. I switched from Exxon Premium to Shell Premium gas which seemed to help a little. I think Shell puts out a better gasoline. More expensive, but much better. Does anyone have any ideas as to how I can smooth out and improve the idle? Any ideas on timing? Thanks. 
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
I have no idea, mine does the exact same thing and always has since I bought it. Mine idles around 750 though. Could be caused by a number of small things, so its hard to say. I wouldnt waste the money replacing parts to figure it out though. You could hook it up to a scan tool and see if you find anything there.
Burning Brakes
Joined: Aug 2006
Posts: 780
Likes: 0
From: New Port Richey Florida
I had a miss at idle in mine and finally went away after I changed the ign wires, I had checked them and they looked good but one must have had an internal leak. It was driving me crazy it would fluctuate between 600-700 rpm at idle. The good news is while looking for my problem I replaced a lot of other things, re: vacuum hoses, IAC, cleaned t-body and reset per specs, dist cap & rotor, plugs and wires so now it runs really good.
Thread Starter
Advanced
Joined: Oct 2007
Posts: 82
Likes: 0
From: Brownsville Texas
I thought 600 was a little too slow also but the ECM controls that. It idles better if I pedal it up to about 700 but I've been told there's no way to adjust it on an '86.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
5 Best & 5 Worst Corvette Daily Drivers
Joe Kucinski
The Headlights of Every Corvette Generation Explained
Joe Kucinski
5 Best & 5 Most Overrated Corvette Track Packages of All Time!
Joe Kucinski
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
Burning Brakes
Joined: Jul 2007
Posts: 751
Likes: 0
From: Eau Claire Wisconsin
I know yours is an 86, but my 94 was doing the same thing. I did a complete tune up including a new opti and it was still there. I got the computer tuned by pcm4less and it made a huge improvement.
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
First, try a can of SeaFoam, it'll smoke like heck for a few minutes, but really blows out the carbon. I haven't followed the directions for years, I put half can in the gas tank, half through the brake booster vac. line.
You can ohm test the plug wires, but twist them a little while checking, 5000 ohms per foot, but remember they're different lengths, so just compensate.
You can ohm test the plug wires, but twist them a little while checking, 5000 ohms per foot, but remember they're different lengths, so just compensate.
Burning Brakes
Joined: Jul 2007
Posts: 751
Likes: 0
From: Eau Claire Wisconsin
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Idle speed is burned into the Chip. Specs for Actual are 100 +/- rpms from Targeted in Park/Neutral; 50 +/- rpms in Drive. You don't know if it's right/wrong unless you scan it. If you don't have a scanner consider buying one or go over to Scan and Tune to learn how you can use a Laptop with some free software (you'll have to buy a cable).
You can try to isolate a miss by cutting some 1/4 inch vacuum hose into 8 1/2 inch pieces and then inserting a piece between each plug wire and the Distributor Tower. Touch each piece of hose with a grounded Test Light and that will kill the cylinder. Note the rpm drop on your hand held Tach or Scanner. Any that don't drop the same is the problem.
Check compression if you haven't all ready done so.
You can try to isolate a miss by cutting some 1/4 inch vacuum hose into 8 1/2 inch pieces and then inserting a piece between each plug wire and the Distributor Tower. Touch each piece of hose with a grounded Test Light and that will kill the cylinder. Note the rpm drop on your hand held Tach or Scanner. Any that don't drop the same is the problem.
Check compression if you haven't all ready done so.
Drifting
Joined: Apr 2007
Posts: 1,339
Likes: 2
From: port charlotte fl
Don't use sea foam crap,causes more harm than good.
Pull the IAC valve and clean it out with brake cleaner or easy start.
Clean out it's housing in the TB the same way.Set timing 6 degrees before,remembering to disconnect the EST wire.
Set the TPS to .54 volts.
With ignition ON and engine not running jump terminals A&B in the ALDL.
This will close the IAC and block the idle air passage.
Remove the 4 pin connector from the IAC then remove the shorting clip in the ALDL.
Start engine and set idle with the throttle stop to give 500 rpm.
Turn off ignition and reconnect the IAC connector.
Idle should settle to 700/800 after 30 seconds or so.
All this is assuming plugs,wires,cap,ignition module and ALL vacuum lines are in good condition.Anything over 35 thou plug gap can cause a bad idle as well so check that as well.
Pull the IAC valve and clean it out with brake cleaner or easy start.
Clean out it's housing in the TB the same way.Set timing 6 degrees before,remembering to disconnect the EST wire.
Set the TPS to .54 volts.
With ignition ON and engine not running jump terminals A&B in the ALDL.
This will close the IAC and block the idle air passage.
Remove the 4 pin connector from the IAC then remove the shorting clip in the ALDL.
Start engine and set idle with the throttle stop to give 500 rpm.
Turn off ignition and reconnect the IAC connector.
Idle should settle to 700/800 after 30 seconds or so.
All this is assuming plugs,wires,cap,ignition module and ALL vacuum lines are in good condition.Anything over 35 thou plug gap can cause a bad idle as well so check that as well.
Safety Car
Joined: Oct 2007
Posts: 3,816
Likes: 0
From: :45 minutes from everywhere / E-I-E-I-O
If it's a spark miss, timing will have no effect on it. It's gonna' be 'missing' at any rpm's.
Increasing idle speed would only make it less noticeable.
Are you referring maybe to 'loping' idle?
Increasing idle speed would only make it less noticeable.
Are you referring maybe to 'loping' idle?
Thread Starter
Advanced
Joined: Oct 2007
Posts: 82
Likes: 0
From: Brownsville Texas
Don't use sea foam crap,causes more harm than good.
Pull the IAC valve and clean it out with brake cleaner or easy start.
Clean out it's housing in the TB the same way.Set timing 6 degrees before,remembering to disconnect the EST wire.
Set the TPS to .54 volts.
With ignition ON and engine not running jump terminals A&B in the ALDL.
This will close the IAC and block the idle air passage.
Remove the 4 pin connector from the IAC then remove the shorting clip in the ALDL.
Start engine and set idle with the throttle stop to give 500 rpm.
Turn off ignition and reconnect the IAC connector.
Idle should settle to 700/800 after 30 seconds or so.
All this is assuming plugs,wires,cap,ignition module and ALL vacuum lines are in good condition.Anything over 35 thou plug gap can cause a bad idle as well so check that as well.
Pull the IAC valve and clean it out with brake cleaner or easy start.
Clean out it's housing in the TB the same way.Set timing 6 degrees before,remembering to disconnect the EST wire.
Set the TPS to .54 volts.
With ignition ON and engine not running jump terminals A&B in the ALDL.
This will close the IAC and block the idle air passage.
Remove the 4 pin connector from the IAC then remove the shorting clip in the ALDL.
Start engine and set idle with the throttle stop to give 500 rpm.
Turn off ignition and reconnect the IAC connector.
Idle should settle to 700/800 after 30 seconds or so.
All this is assuming plugs,wires,cap,ignition module and ALL vacuum lines are in good condition.Anything over 35 thou plug gap can cause a bad idle as well so check that as well.
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
Don't use sea foam crap,causes more harm than good.
Pull the IAC valve and clean it out with brake cleaner or easy start.
Clean out it's housing in the TB the same way.Set timing 6 degrees before,remembering to disconnect the EST wire.
Set the TPS to .54 volts.
With ignition ON and engine not running jump terminals A&B in the ALDL.
This will close the IAC and block the idle air passage.
Remove the 4 pin connector from the IAC then remove the shorting clip in the ALDL.
Start engine and set idle with the throttle stop to give 500 rpm.
Turn off ignition and reconnect the IAC connector.
Idle should settle to 700/800 after 30 seconds or so.
All this is assuming plugs,wires,cap,ignition module and ALL vacuum lines are in good condition.Anything over 35 thou plug gap can cause a bad idle as well so check that as well.
Pull the IAC valve and clean it out with brake cleaner or easy start.
Clean out it's housing in the TB the same way.Set timing 6 degrees before,remembering to disconnect the EST wire.
Set the TPS to .54 volts.
With ignition ON and engine not running jump terminals A&B in the ALDL.
This will close the IAC and block the idle air passage.
Remove the 4 pin connector from the IAC then remove the shorting clip in the ALDL.
Start engine and set idle with the throttle stop to give 500 rpm.
Turn off ignition and reconnect the IAC connector.
Idle should settle to 700/800 after 30 seconds or so.
All this is assuming plugs,wires,cap,ignition module and ALL vacuum lines are in good condition.Anything over 35 thou plug gap can cause a bad idle as well so check that as well.
Good tips: you can also adjust the TPS's position all the way up to .63 volts --which will slightly increase in the engine's idle rpm too. In normal conditions my car idles around 600; and 500 when engine is really hot (my TPS sensor is set @ .61 -- my car runs better at this setting).
Good luck!
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
Don't use seafoam???? Really, there have been more posts praising this product than any I can remember! I've been using it in all my cars and boats for over 10 years, with fantastic results.
Le Mans Master
Joined: Aug 2007
Posts: 8,735
Likes: 414
From: WV
Safety Car
Joined: Oct 2007
Posts: 3,816
Likes: 0
From: :45 minutes from everywhere / E-I-E-I-O
SEAFOAM???
Maybe they shoulda' called it seascam!
What is it really? Chemically speaking, that is...
I got a little chem engineering in school, although I don't get a weekly check for it...
Maybe they shoulda' called it seascam!
What is it really? Chemically speaking, that is...
I got a little chem engineering in school, although I don't get a weekly check for it...