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5/21/10 2:35am - Original Message: 'Fast Idle'
tb2k82ce Lifetime Member
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Lemon Grove, CA - USA

Vette(s):
1982 C3 Collectors Edition 44000 miles, sat in the sun most of it's life, My wife purchased it for me for fathers day in 2007 from her girl friend that had it for 19 years. It is on the road again. I'm retired but it is now my daily driver.

Joined: 10/17/2007
Posts: 2007

This is talked about a lot for crossfire.  I finally got tired of chasing this issue around and took it to a mechanic that was recommended.  He seemed to be familiar with crossfire.  He found a few items that were wrong and a small vacuum leak.  He still cannot get it to idle below 1000 RPM and is trying to give up on it.  He wanted me to pickup the car from the shop.  I avoided that as I wanted to get some advice first. 

I had a tune up and spark plug wires replace while it was in.  I look at the plugs the pulled out and they were all carboned up after only 400 to 500 hundred miles.  Is that any help in guessing what the issue is. 

No check engine light comes on but it does light when you start the car.

I am told it does not have any vacuum leaks and nothing they have tried seem to be able to get the idle down.  Could it be that the computer needs to learn the car again like I've been told sometimes happens with the newer cars?  If so how long should that take and how do I do it correctly? 

If not, what should I ask him? What are some of the things he should have done?  What else could be wrong assuming that all of the normal checks were done. I do not believe for example he checked the balance between the injectors.  

A little background may help.  My wife purchase the car from her girlfriend for me it has been sitting for 3 years but only has a total of 44K miles on the engine.  I talked to her and ask if she could remember if she ever had high idle issues.  She said she thought that it had a fast idle.  The way I asked could have led to that answer but??. 

Any help would be nice.  If I just drive it with a fast idle but what are some of the issue with that?  

Tb2k82CE2010-05-20 23:37:10

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5/21/10 6:20am - Reply: 'Fast Idle'
crossfire1982
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Cramerton, NC - USA

Vette(s):
black 1982 coupe slate gray interior, 350 crossfire, 1985 fuel pump, Steeroids R&P conversion

Joined: 8/22/2006
Posts: 1094
There are a multitude of things that can cause the fast idle but a vacuum leak is one of the prime suspects.  Has the Coolant Temperature Sensor ever been changed on your car?  This sensor is located in the front of the engine block not far from the thermostat housing.  This sensor, especially the original ones, are notorious for going bad and not sending proper data to the computer which causes the computer to think that its freezing weather and it needs to make the engine idle fast to warm up.  If the sensor doesnt send accurate information, the computer never knows the engine is warm. That is one of the first things Id try, the sensors are not expensive. If you do replace the sensor, dont order the "original" style unless you are an NCRS guy, the newer style ones are more reliable.  Originals are available though through the corvette suppliers, new style sensors are available at any parts store. Balancing the throttle bodies isnt a bad idea but on a car with only 40K miles I doubt they are out of balance unless Bubba has been mucking with the linkage which was common by those who do not understand how to balance the throttle bodies.
Crossfire19822010-05-21 03:23:28

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5/21/10 9:57am - Reply: 'Fast Idle'
tb2k82ce Lifetime Member
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Lemon Grove, CA - USA

Vette(s):
1982 C3 Collectors Edition 44000 miles, sat in the sun most of it's life, My wife purchased it for me for fathers day in 2007 from her girl friend that had it for 19 years. It is on the road again. I'm retired but it is now my daily driver.

Joined: 10/17/2007
Posts: 2007
Thanks I was hoping to get something to ask him prior to picking it up.  I figure that is a good question.  Does that apply to the O2 sensor also?  I cannot tell for sure if the temp sensor is original or not.  The wires insulation was bad and I patched that.  The temp gauge works.  That had me figuring that it had to be sending correct data to the computer.  So it was not on my short list of possibilities. 

Are you saying that the temp gauge could be working and it still not getting the correct data.  By the way the oil temp gage somewhat follows the water temp gauge.  I'm a gadget guy so I and a thermal temp reader and I can measure the block or hoses to radiator.  Of course none of these match nor should they.  But all indicate that the engine is warming and the temp sensors are sending somewhat correct data to the gauges.

What temperature is considered warmed up? 


Tb2k82CE2010-05-21 07:07:58

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5/21/10 11:59am - Reply: 'Fast Idle'
KDADDY79
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Dutchess County, NY - USA

Vette(s):
White '79 Corvette. It's a driver.

Joined: 8/27/2007
Posts: 2484

Gilbert,

Also check your thermostat. See if it is other than stock. If incorrect it could be the beginning of a "domino effect".  I had a 1989 Chevy pickup with a TBI setup. It always idled too high until I put in the correct thermostat.

I'd think you'd want the RPM's lower at idle. Your trans., u-joints and rear differential could experience some premature wear due to being repeatedly "clunked" into gear at those RPM's.

HTH

Kevin

KDADDY792010-05-21 09:04:01

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5/21/10 6:07pm - Reply: 'Fast Idle'
kstyer Lifetime Member
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CUYAHOGA FALLS, OH - USA

Vette(s):
1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

Joined: 12/2/2003
Posts: 6410
All excellent advice, and it should be checked first.   Fouled plugs, and just about everything else will affect idle on these.   THe o2 sensors won't cause this problem, but will cause the plugs to foul.
 
Dont overlook the PCV valve.   Aftermarket PCV vlaves can be "close" and not quite right, even if the parts book says it's right.  Very touchy.  The slightly out of calibration PCV can cause the problem.   In reality, a PCV valve is a calibrated vacuum leak.  A factory valve could be worn out causing the problem as well.
 
If these fail. check the ignition timing.  Advanced timing will create a fast idle that computer idle cannot overcome.   Be sure to disable computer timing control before adjusting the timing or even checking it with a light, or you will be off.
 
Next thing to check is the minimum air settings.  These should be correct, but often are not quite right from the factory.  The minimum air screws should be sealed, and a plug removed to access them in some cases.  It appears to be a idle set screw, and each throttle body has one.   First, plug the air intake to the Idle Air Control (IAC).  This will bring the idle down to a minimum,  if not you missed something listed above.   If the engine is now at spec, good.  If not back off the screws until they do not touch, then run them in to just barely contact.  Now turn both screws an equal amount until the correct base idle is reached.   Now you must balance them.   On the top of the throttle body there is a small port that you can put a vacuum line on the unit.  It must be able to read in inches of water.   If  you have an manometer even better.  Adjust the screws until the vacuum is EXACTLY even, while maintaining the minimum idle speed.
 
There, they are balanced.  Remove the plug from the IAC inlet, and allow the engine to idle normally.   If you still do not have a proper idle, you could have a bad IAC motor, or one clogged and dirty, with poor action and a restricted passage.   Dont forget to clean both throttle bodies.
 
IF your idle is still off, you missed something in the first group of checks, or you have a computer problem, but that is very unlikely for this issue.  Make sure all of the power supplies and grounds for the computer are good.
 
Fully warm is 195 degrees, but anything over 160 will work for this procedure.  Keep in mind that is what the computer is reading, not actually temp, though they should be the same thing.


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5/22/10 12:51am - Reply: 'Fast Idle'
tb2k82ce Lifetime Member
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Lemon Grove, CA - USA

Vette(s):
1982 C3 Collectors Edition 44000 miles, sat in the sun most of it's life, My wife purchased it for me for fathers day in 2007 from her girl friend that had it for 19 years. It is on the road again. I'm retired but it is now my daily driver.

Joined: 10/17/2007
Posts: 2007
I picked the car up and drove it around to see if the computer would adjust. Some interesting things happened like it tried to die upon acceleration from a stop.  It did in fact to that just down the street from my house and when I go home and stopped out front the idle was 700rpm.  Ruff idle but it is down.  I will drive it tomorrow and see how it acts.  I talked to the shop and will be taking it back in on Monday for the ruff idle the surging issue.  Was not given any static by the owner.  Thanks for the info on the temperature.  I'm thinking my car runs cool.  I will take my electronic temp gun with me and measure the temp at the upper radiator hose.


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5/22/10 10:20am - Reply: 'Fast Idle'
crossfire1982
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Cramerton, NC - USA

Vette(s):
black 1982 coupe slate gray interior, 350 crossfire, 1985 fuel pump, Steeroids R&P conversion

Joined: 8/22/2006
Posts: 1094
Now, since you have mentioned the stumble off the line and the rough idle I have another suggestions. Yes, I know the car has only 44K miles on it but I suggest that you pull the fuel pump (which is the first year for in-tank fuel pumps on corvettes)  and replace it or at the very least replace the short piece of rubber tubing that connects the pump to the hard piping on the sending unit assembly. Even with low miles that rubber hose has been in gasoline since 1982 unless its been changed in the past.  I pulled the fuel pump on a friends 82 a while back and the hose LITERALLY fell apart in my hands.  If you find the hose is deteriorated, in addition to replacing the hose and pump I would also replace the fuel filter mounted on the frame rail under the passengers foot well. The stumble can also be caused by out of balance throttle bodies but that is usually caused by linkage wear between the two throttle bodies and I think it unlikely with a low mileage car such as yours.  Ken gave a good description of the balancing procedure in his post, its not difficult, just a little time consuming and you do have to uncap the factory seals to set the initial air settings and break the spot weld on the linkage so that it can be adjusted.  The reason that balancing of the throttle bodies is so critical in a crossfire is that the left side TB feeds the right bank of cylinders and the right side TB feeds the left bank of cylinders, if not balanced one side of the engine is trying to run faster than the other side.  If you arent already a member of the CROSSFIRE INJECTION VAULT you need to join, those folks live, eat, and sleep crossfire.  When properly maintained, the crossfire is a good system, and offers excellent fuel mileage. Its just a unique setup that was in production for only a few years on corvette and camaro so few are familiar with its innerworkings. Ask your mechanic about how the throttle bodies can be put back in balance if he thinks that is the problem.  If he says anything about bending linkage.... crank up and drive away... FAST and find someone who knows how to do it properly.
Crossfire19822010-05-22 07:26:21

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