I have a 1986 C4 Corvette that I've had for about 9 months that's developed an idling problem. I live where its hot and it can sometimes get up to 110 degrees. I started having the idle problem when the temperatures started getting above 95 degrees outside. The idle would start going up and down while at a stand still until the car died. I could avoid the car from dying most of the time by slightly depressing the accelerator to bring the idle up. The problem got worse the hotter it got. I was short on time and didn't have the necessary equipment to diagnose it so I took it to a local auto shop. They determined it was the MAF sensor. They replaced the MAF sensor and wire pigtail along with the MAF burn off relay. I originally asked them to install a rebuilt MAF sensor from Napa, since its the original Bosch brand that came with the car and I've read about problems with other brands. They accidently installed a new one from O'reillys, which was a Spectra. The car seemed to run fine with it, but I asked them to put on the Napa rebuilt like I had originally asked. The car ran with the Napa, but kept throwing a code saying there was a problem with the burn off relay. I went back and forth with the auto shop and agreed for them to put the Spectra back on. The burn off relay error went away. The car seemed to run fine for about a month or so. It then started having problems again with the idle. It will sometimes die when I start off from a stop light or sign. The car seems to run fine as long as the accelerator is depressed and I'm driving. The problem occurs when I stopped and its idling. The air temperature outside has been cooling off and has been between 90-95 degrees and I'm not sure how big of a part heat plays into the problem now although it seemed to initially be a factor. I was able to replicate the problem last night. I start the car from being cold and its idling fine in my driveway. The car starts to warm up and the coolant temperature gets to about 160 degrees. The engine starts to adjust the idle and the idle starts dropping a little, then it drops a little more and then it starts oscillating by going up and down and it gets so low it sounds like it could die. I give it a little gas and it seems to correct the idle and it stays at a higher idle. I depress the accelerator and the idle drops a little lower again. I still have contact with the guy that I bought the car from. He said the car had a idling problem about 5-6 years ago, but an auto shop fixed the problem. He gave me all of his invoices and it looks like they changed the TPS and IAC valve. I suspect the TPS or the IAC valve now. My parts book says my year of Corvette has a MAP sensor. I don't see it anywhere. I called a local dealer and they confirmed that it should have one, but again I don't see one anywhere and the dealer said his schematic doesn't show where its located at. I saw pictures of the throttle body of a 1985 and its clearly on top of the throttle body. I read posts where it might be on the firewall for other years of C4, but I see anything there either. The previous owner said he replaced the throttle cable a few years ago and had to lube the wire inside of the cable at times because it will sometimes stick. I put some lubricant inside of the cable and don't think its related to the current idle problem. I replaced the battery a few months ago before the idle problem occurred and my hand accidently hit the connector on the MAF burn off relay. It seemed loose and I pushed on it and it seemed to be not connected. I'm wondering if someone purposely disconnected it. I'm not sure if it was loose or purposely disconnected, which I think should of generated a code. I'm tempted to replace the TPS and the IAC valve, but I don't feel real confident about adjusting the TPS voltage and I don't want to make things worse. I was also considering changing the MAP sensor, but I can't locate one anywhere. I've added a link to a video of my car at an idle with the distinct oscillating sound that I was talking about. Please take a look. Have any of you experienced this problem and do you have any ideas of what could be casing the idle problem? This issue is causing major problems for me and I would greatly any advice that you can give me.

 

I know this may sound far afield but have you checked fuel pressure. I had an 82 that did exactly what you describe. Found out the rubber hose in the tank had split and the computer kept trying to correct fuel pressure. It got worse the warmer it got. Replaced the rubber hose and the idle problem went awat. Good luck finding the answer to your idle problem.

 

Wrong section but having 3 c4's without me reading your long paragraph first check your O2 sensor and your IAC . Probably one or both have carbon build up.Try cleaning them first...

 

Thank you Prysmian46. No, I haven't checked the fuel pressure yet, but good idea. I had the fuel filter replaced. I've been replacing rubber hoses and I ask the auto shop if there were any rubber fuel lines to replace. They said no, because they're all steel lines because of the high pressure fuel injection. It sounds like you're saying there's a rubber line directly inside of the fuel tank. Is that right? Is it connected to the fuel pump to suction gas? I would think it would also affect the engine while I was driving if it was a fuel supply issue, right?

 

Not sure about your 86 since it runs at a higher pressure than my 82 did.

 

My 85 does the same thing. Exactly the same symptoms. I can put in a can of Seafoam in the fuel tank, and the surging idle stops for a while, but comes back a few tanks later. It also goes away when I use premium fuel. I'm thinking it might have to do with the IAC. A quick blip of the throttle usually stops it until a few stops later. Also, I live in Canada, and it's much cooler than where you are. New fuel pump and filter recently.

 

There is no MAP sensor in the 86 corvette. Manuals are sometimes for multiple models of vehicles.
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Thank you for the reply LiveWire1. I like the picture of your Corvette in your profile picture. Looks nice. I added a couple of bottles of fuel injection cleaner to the fuel tank over the past few months, but I haven't noticed a difference. I'm also suspect of the IAC, but the one I currently have was replaced only 5-6 years and about 8,000 miles ago. Doesn't seem long enough for it to go bad so early, but it can definitely happen. My valve guide seals in the head do leak some oil that gets burnt off. I wonder if some of the residue from that could gum up the IAC valve? Maybe not, since its away from the exhaust system. Do you have any issues with your 85 dying when taking off from a stop sign or light from idle at the wrong time?

 

Thank you Vets-Vet. That puts my mind at rest. I was going nuts looking for the MAP sensor. I was getting the feeling that a MAP sensor isn't used with a MAF sensor, but I had nothing definite. From your picture, it looks like you have a great resource for older Corvettes. Is it available to the public Internet or is this something you must have a subscription for?

 

Yes, it is a PDF of the Field Service Manual and it is invaluable as is the ability to see info from the ALCL. Did you see TommyFox's suggestion of O2 ? Check the air bellows between the MAF and throttle body and make sure it is air tight and not folded under on the ends. The MAF reports an accurate count of air passing through it and that bellows should not be allowed to let any extra in. Tune up ? Air filter clogged ? Vac hoses OK ?
https://www.zip-corvette.com/86-87-s...yABEgJj6_D_BwE
$29.

 

Very nice! Thank you for the link Vets-Vet. What does ALCL stand for? I missed the comment from TommyFox (thank you as well), but I see it now. I'll take a look at the bellow to see if there are any leaks. I didn't know the bellow could get folded/pushed in under the weight of the hood. Your talking about the accordion looking rubber boot between the air filter and throttle body, right? If that looks good, I think I'm going to install a new IAC sensor to see if that fixes it. If not, I'll try the O2 sensor. My O2 sensor was replaced at the same time as my IAC sensor, which was about 5-6 years ago. That's not very old, but again, that doesn't mean it can't fail. It has a new air filter and spark plug wires. The spark plugs look good and the distributor cap and rotor was changed only 5,000 miles ago. The easiest way to check for vacuum leaks is a vacuum gauge and a visual check, right? Is there any other tricks to check for a vacuum leak? I use soapy water for pressure leaks. That will always create bubbles to identify the hole. Is there a similar trick for vacuum lines?

 

"I didn't know the bellow could get folded/pushed in under the weight of the hood" No not the hood. Yes, the rubber boot between the MAF and the engines TB (throttle body).

No, not the hood. It gets folded under sometimes when being installed, as it is pushed onto the throttle body or onto the MAF sensor it can catch the lip and fold under leaving a gap. Feel around the underside as well. Check it for a tear also.

ALDL or ALCL is the cars OBD1 computer connector above your right knee sitting in the seat.
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No, I don't have those problems. My car shows only about 70,000 miles (128,000 kilometers) and uses ZERO oil, so I don't think that's going to be your problem.

 

Thanks Vets-Vet. That description of the MAF boot made it much clearer to me. Thank you for the vacuum diagram. It looks like I need to check the red vacuum lines in the diagram, right? What's the vacuum ball that you're describing? What are the black lines for? You seem like you're a sharp mechanic and I appreciate your help. I've always done some of the work on my own vehicles, but diagnosis can sometimes be tricky as you know.

 

To figure it out you really need to use a scan tool that gives you live data. You log the live data as it surges and stalls and the problem will more than likely jump out at you. It could even be the CTS-but you have to scan it.

 

Thanks drive it. CTS = Coolant Temperature Sensor, right?

 

Yes, correct. There is currently an auto xray ez link 4000 on ebay for 80 bucks with all the cables. That will give you live data. Also a great book that will explain all the live data.

 

Great information drive it. I have an ODB1 code reader to to pull codes, Does the ez link 4000 plug into the same port?

 

Yes, and then you have live data, not just codes.

 

My 87 uses the 10k ohm resistor across pins A & B, it is in the 86 FSM as well (I believe it does require it) but I am not sure it is required maybe someone with an 86 can chime in. Maybe the newer USB cable style data readers with Win ALDL or Tunerpro on windows will be more user friendly (with a learning curve) to him, but that is his call. Red River or OBD allinone.com. Just don't fall for the expensive peperclips that only read codes.
https://obd2allinone.com/products/aldlobd1u.asp
https://reddevilriver.com/
https://www.corvetteforum.com/forums...obd1-data.html

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