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High idle


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#1 Josh280z

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Posted 05 January 2012 - 06:00 PM

Hi again,
Ive done my homework and checking... My idle sits at about 2300 rpm at normal idle speed. I've messed with the idle screw on top of the throttle body, when it is all the way down the idle goes down slightly. I've put a new air regulator on. I've replaced all the vaccuum hoses I could see and find. I've replaced the throttle position sensor. Checked out the inside of the AFM, it's clean there. I've replaced the fuel pressure regulator and adjusted it to the right pressure. What next is there to do? Should I try and retard the timing through the distributor. What else can I do?
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#2 NewZed

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Posted 05 January 2012 - 06:38 PM

Looks like a 280ZX of some kind, possibly stock, maybe not? Probably early since you have an idle screw on the throttle body.

Have you checked the BCDD? They get stuck sometimes. On the bottom of the throttle body. I have found a leak there before by spraying carb cleaner around where it mounts to the intake. The idle speed will change.

The idle speed is mainly controlled by how much air is allowed in to the intake manifold. If the air gets past the throttle body but still moves the AFM flapper, you'll get a high idle, more air plus more fuel. That's what the idle adjustment does. A small vacuum leak can raise your idle too, a big one leans everything out and the engine dies. If you get everything sealed up right, you should be able to kill the engine by closing the idle speed screw.

#3 rossman

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Posted 05 January 2012 - 07:29 PM

I'm with NewZed. You probably have a air leak. Search for the leak by spraying around the intake manifold with starter fluid or carb cleaner. When the fluid gets sucked into the engine the idle will increase and you found your leak. Have you checked that your throttle plate is completely seated and not cocked/loose for some reason? Did this condition just start one day or did you recently replace something, perform a fresh rebuild or what? A little background helps diagnosis.

#4 Josh280z

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Posted 05 January 2012 - 11:09 PM

I wanna say it just started one day. But it happened after I replaced the timing chain, fuel lines; filter, the fpr. As described I replaced all the vaccuum hoses. I have not checked the bcdd. It is a '79 280zx, pretty much stock. I had to retime the engine completely.
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#5 Josh280z

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Posted 05 January 2012 - 11:15 PM

Also, the throttle plate is seated, and the mix is running rich, though that may have been because of the high fuel pressure it was adjusted too. I just fixed that today though.
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#6 NewZed

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Posted 06 January 2012 - 09:51 AM

Check the PCV line under the intake, pinch it closed and see what happens. Check the brake booster line and check valve. The EGR system lets exhaust gas in to the intake, maybe it's leaking fresh air in at idle.

Engine speed is controlled by air allowed in. If you can't choke the engine speed down to nothing, you have uncontrolled air getting in. Intake leaks are hard to find sometimes.

Edited by NewZed, 06 January 2012 - 09:53 AM.


#7 Heroez

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Posted 06 January 2012 - 02:45 PM

If you measure your manifold vacuum you should have 20-21. If you have less you have leaks. Since you installed the distributor 180 degrees off and had to switch the wires around to get the car to start, its probably another newbie mistake manifesting as high idle. If you have good manifold vacuum at idle, the problem isnt leaks. New parts doesnt mean they work properly. Especially with parts bought from chain stores. Sorry you are learning the hard way. As the saying goes, fools rush in. You got it backwards kid. Research first, work on car second. Not work on car first, research failure second.
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#8 rossman

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Posted 06 January 2012 - 05:17 PM

Have you verified your timing? Advanced timing will cause high idle.

#9 Josh280z

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Posted 06 January 2012 - 06:08 PM

I haven't checked manifold pressure, how would I got about doing that?
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#10 Heroez

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Posted 07 January 2012 - 06:29 AM

Once you get your timing right, you just hook a vacuum guage to a hose on the intake. If you read some archives and do some web searches you can find all kinds of goodies to help you. There are how tos and common problems to read and think about.
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#11 Josh280z

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Posted 11 January 2012 - 09:50 AM

The timing is right, if it hadn't of been right, then the high idle would have happened immediately after the timing chain rebuild. So, it must be something else. I have not yet confirmed it, but I do believe it is the bcdd as stated above.
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#12 Josh280z

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Posted 27 January 2012 - 08:21 PM

Like I surmised, it wasn't the timing. It was the cold start injector just staying open. I removed the injector, plugged the line and the hole. Now, the engine idles like new.
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