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Discussion Starter · #1 ·
I have a problem and dont know whats wrong. When i drive, my tack never reads the correct rpm's. That throughs me all off cause now i dont know if my speed is reading correct. You start the car and the Rpm's will bounce back and forth from 1000 to 3000 and it only gets worse as you rev the engine. Is there an easy when to fix the tack or do i have to take it to the dealer and have them look at it. I really dont want to take it to the dealer because all they do is tell you how much it well be and multiply it by 3. Any advice you can give me is a great help. Thanks
 

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Heu posted this a while back on the Resource:

dohcrxl writes: I noticed that the surging idle question gets asked from time to time and thought we could use my lil write up as a service to the community.

Three things I know and have seen to cause a surging idle:

1) air in the cooling system - when hot coolant comes in contact with the AICV (idle air control valve) it gives the ECU a reference as to what to set for an idle. The ECU then lowers the idle because it senses that the engine is hot. When it does that the waterpump of course slows down with the rest of the engine and is not spinning fast enough to pump a sufficient and constant flow of coolant because there are air pockets in the system preventing it. When that happens, the coolant that used to be in contact with the AICV now retriets, the ECU then senses that the engine is cold, raises the rpms, the waterpump surges and speeds up, pushing coolant back onto the AICV, and the cycle becomes endless.

2) bad Idle Air Control Valve - on the back of the IM and should have a blue w/yellow and a black w/yellow wire going to it. Cleaning the filter screen inside the AICV will help restore a more steady idle. To do so, you must remove the valve assembly from the throttle body and clean out with brake cleaner.

3) A leak in the throttle body or intake manifold gasket - with the engine running, carefully spray brake cleaner on the mating surface edge by the gaskets. If the engine bogs when you spray, there is a leak where you sprayed.

There is of course the possibility of leaking vacuum lines. If you don't hear or see any obvious leaks, unplug all the vacuum hoses one at a time. Each time after you unplug one, plug the exposed nipple at the manifold or throttle body with your finger. If the surging goes away you know it involves that line and or any component that line goes to. After you do this, put that hose back on and proceed to repeat that for each and every vacuum hose and it may uncover the problem. Another way to help isolate a pinhole leak in a vacuum line is to put your hand over the throttle body with the engine running. A leak will give itself away by the whistling it will make. Note* cover up only as much of the throttle body opening as needed to allow the engine to stay running yet force the leak to reveal itself. _________________
- Heu
 

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Well, "Surging Idle" is different from "Bouncing Tach"

The bouncing tach issue is electrical, and from what I've sen is usually fixed by replacing the distributer. King distributer on Ebay is really good to go with... do not buy Autozone.
 

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Discussion Starter · #5 ·
Would a MSD distributer work?
 

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Yes, but unless you have a highly boosted highly modified engine, an aftermarket ignition system does you no good. Most racers run stock distributers.
 

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Problem with tac could be caused by a corroded connection at the ignitor inside the distributor or a bad ignitor. The tac lead is the blue wire on the ignitor, try unplugging and replugging it a few times to clean the spade connector. To do this you will have to take the dist cap off, rotor off, side shield off, etc to get at the ignitor plugs.
 
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