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1986 C4 problems
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6th Gear
Joined: Jul 2020
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1986 C4 problems
I have a 1986 C4 Corvette, I had a problem where I'd be driving down the road and it would just die no warning , that was the ignition switch, now that I got that fixed the only way I can keep it running when I slow down is to adjust the idle up high, does anyone know what might be my problem, this is my first Corvette, thanks for any and all help
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Pro
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From: Vancouver Washington
Idle air control is the first thing that comes to mind. I have an 86 as well and when my car comes to a stop the idle is around 1000 rpm for just a few seconds then drops to 800. My engine is modified as well as my ECM. I would have to look up the factory idle setting. There is a good write up in the DIY section of this site about how to set the idle on the L98 engine.
Do you have basic tools like a timing light and a volt meter?
Do you have basic tools like a timing light and a volt meter?
Thread Starter
6th Gear
Joined: Jul 2020
Posts: 6
Likes: 0
Idle air control is the first thing that comes to mind. I have an 86 as well and when my car comes to a stop the idle is around 1000 rpm for just a few seconds then drops to 800. My engine is modified as well as my ECM. I would have to look up the factory idle setting. There is a good write up in the DIY section of this site about how to set the idle on the L98 engine.
Do you have basic tools like a timing light and a volt meter?
Do you have basic tools like a timing light and a volt meter?
Last edited by Nicole Wentz; Jul 26, 2020 at 09:07 AM.
Drifting
Joined: Jun 2013
Posts: 1,590
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From: ft myers FL/Munising MI
Welcome!!!.....all this trouble and no codes?......jump a/b on your aldl below the dash , drivers side, google or search here how to read the codes.....(it's the flashes of your check engine light)
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Drifting
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From: Michigan
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From: Vancouver, WA
As others have stated, I'd check the minimum idle adjustment, then test the IAC valve, then I would check the Throttle Position Sensor (TPS) adjustment.
Here's the procedure to adjust minimum idle speed. My 1985 and 1988 Corvette Factory Service manuals omit steps 4 and 5, which are very important.
http://repairs.willcoxcorvette.com/1...ent-procedure/
FYI: The procedure above says to set the min idle to 450 RPM and the FSM says 400 RPM. I was unable to get it to stay running this low.For the 1985 Corvette, I set the minimum idle speed to 560-570 RPM. The car idles at 610-610 RPM in drive, 630 RPM idle in drive with AC on, and idles at 750 RPM in Park....
One other note, make sure you don't have a Tech 1 or any scanner connected to the OBD1 port, as it will raise the idle. I learned this the hard way.
The IAC test procedure and TPS adjustment is in the FSM. If you don't have one, I or anyone else can post the procedure for you.
Here's the procedure to adjust minimum idle speed. My 1985 and 1988 Corvette Factory Service manuals omit steps 4 and 5, which are very important.
http://repairs.willcoxcorvette.com/1...ent-procedure/
FYI: The procedure above says to set the min idle to 450 RPM and the FSM says 400 RPM. I was unable to get it to stay running this low.For the 1985 Corvette, I set the minimum idle speed to 560-570 RPM. The car idles at 610-610 RPM in drive, 630 RPM idle in drive with AC on, and idles at 750 RPM in Park....
One other note, make sure you don't have a Tech 1 or any scanner connected to the OBD1 port, as it will raise the idle. I learned this the hard way.
The IAC test procedure and TPS adjustment is in the FSM. If you don't have one, I or anyone else can post the procedure for you.
Thread Starter
6th Gear
Joined: Jul 2020
Posts: 6
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Idle air control is the first thing that comes to mind. I have an 86 as well and when my car comes to a stop the idle is around 1000 rpm for just a few seconds then drops to 800. My engine is modified as well as my ECM. I would have to look up the factory idle setting. There is a good write up in the DIY section of this site about how to set the idle on the L98 engine.
Do you have basic tools like a timing light and a volt meter?
Do you have basic tools like a timing light and a volt meter?
Safety Car
Joined: May 2001
Posts: 4,741
Likes: 1,639
From: Fairfax Virginia
Hello Nicole Wentz,
Be careful with your IAC as some of them have a different style and shape of the pintle which makes them not work in my C4. I bought a new one from a NAPA store and the part was the wrong shape. The same part number had two styles of pintle that we found in the NAPA warehouse. I suggest to folks who are going to replace this part take your old one to the store to compare before buying.
Welcome to the wonderful world of Corvette Ownership!
Welcome to the Corvette Forum!
When you have questions "we" will try to answer them quickly and correctly!
Be careful with your IAC as some of them have a different style and shape of the pintle which makes them not work in my C4. I bought a new one from a NAPA store and the part was the wrong shape. The same part number had two styles of pintle that we found in the NAPA warehouse. I suggest to folks who are going to replace this part take your old one to the store to compare before buying.
Welcome to the wonderful world of Corvette Ownership!
Welcome to the Corvette Forum!
When you have questions "we" will try to answer them quickly and correctly!
Le Mans Master
Joined: Sep 2006
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From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
Tps
I have an 86.
when i first bought it, I thought i would wash the engine area.
after cleaning it, it wouldn't idle at all. Turns out, the tps was never sealed at the factory, so moisture got in it and messed up the signal to the computer.
look at yours, there should be a line where they glued the side on to the tps. There should be a discoloration around the entire curved Edged plastic piece, and it should be the rubber that they sealed it with, You can make impressions in it with your fingernail.
if the glue is not there, u need to replace it anyway.
but check vacuum hoses before spending any more money.
when i first bought it, I thought i would wash the engine area.
after cleaning it, it wouldn't idle at all. Turns out, the tps was never sealed at the factory, so moisture got in it and messed up the signal to the computer.
look at yours, there should be a line where they glued the side on to the tps. There should be a discoloration around the entire curved Edged plastic piece, and it should be the rubber that they sealed it with, You can make impressions in it with your fingernail.
if the glue is not there, u need to replace it anyway.
but check vacuum hoses before spending any more money.
Last edited by coupeguy2001; Jul 30, 2020 at 12:04 AM.
Thread Starter
6th Gear
Joined: Jul 2020
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there is no dash cluster I have to replace the dash cluster they fried the whole thing before I got it
Intermediate
Joined: Jun 2018
Posts: 39
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From: West Seneca NY
Feel each battery cable like right after you start it. I had an issue with mine in which the positive battery cable became less conductive, and when the engine/transmission warmed up it would occasionally stall as well as throw check engine lights, not start all the time, etc. That is where I would start. I spent a year doing all kinds of testing, replacing parts, etc and scratching my head. I might even suggest doing this proactively because the cables are cheap. I had no idea how important clean, proper power was for this engine system. If you decide to go this route, I can tell you how I got the cable in place because it is kind of a pain to fish through. It goes behind the engine like on the top of the transmission area. Here is a link for the cables I used:
https://www.rockauto.com/en/catalog/...ery+cable,2500
https://www.rockauto.com/en/catalog/...ery+cable,2500
Drifting
Joined: Jun 2018
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From: Brisbane Australia
C4 of Year Finalist (performance mods) 2019
Also get a genuine GM Factory Service Manual (FSM) you will need it.
Thread Starter
6th Gear
Joined: Jul 2020
Posts: 6
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Hello Nicole Wentz,
Be careful with your IAC as some of them have a different style and shape of the pintle which makes them not work in my C4. I bought a new one from a NAPA store and the part was the wrong shape. The same part number had two styles of pintle that we found in the NAPA warehouse. I suggest to folks who are going to replace this part take your old one to the store to compare before buying.
Welcome to the wonderful world of Corvette Ownership!
Welcome to the Corvette Forum!
When you have questions "we" will try to answer them quickly and correctly!
Be careful with your IAC as some of them have a different style and shape of the pintle which makes them not work in my C4. I bought a new one from a NAPA store and the part was the wrong shape. The same part number had two styles of pintle that we found in the NAPA warehouse. I suggest to folks who are going to replace this part take your old one to the store to compare before buying.
Welcome to the wonderful world of Corvette Ownership!
Welcome to the Corvette Forum!
When you have questions "we" will try to answer them quickly and correctly!
Drifting
Joined: Jun 2018
Posts: 1,402
Likes: 287
From: Brisbane Australia
C4 of Year Finalist (performance mods) 2019
Last edited by GregMartin; Aug 6, 2020 at 04:44 AM.