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kshacklett's Avatar
 
Join Date: Nov 2007
Location: Reno, NV
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fast rpm or searching at idle

Help! I've got to smog my 914 in NV soon and I can't get it to idle slow enough. It's a '73 2.0, stock fuel injection. Lately it's been idling around 2k. The points, condenser, rotor and plugs are fairly new. Today I discovered that somehow I'd gapped the plugs way to big (or never gapped them). I fixed that but it still idles way too fast. I don't seem to have any air leaks. When the car gets really warm it will slow down, but usually it starts searching, which guarantees that I fail the smog test.

I made another startling discovery today. I can put the plug into the pressure sensor either way. It doesn't seem to matter which way I put it, and neither seems to help the idle. I haven't adjusted the valves in forever, nor has it been timed for several years, but it only recently is giving me a really hard time with the idle. At high RPM it runs clean as a whistle but I've GOT to make it idle better.

I broke the mixture pot on the computer. I put a new one in today that seems to work fine. I'm not sure which way is which, but when I turn it all the way counter cw it slows down. When I turn it all the way cw it now idles over 2k RPM.

Any clues so far? By the way, it needs to idle at 1250 RPM or less.

Thanks for any input.
Old 11-08-2007, 11:22 PM
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Try the screw on the throttle body. You have to reach under the air cleaner to turn it, but you can twist it by hand. Screw it in to lower the idle. If it's all the way in, you still have an air leak somewhere.

Try turning the ECU pot clockwise may help with your idle hunting, as that will richen the idle mixture.

--DD
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Old 11-09-2007, 06:41 AM
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Thanks Dave

The screw is all the way in. Have any suggestions about the best place or way to look for an air leak? Now that I think about it, it's a dead giveaway that I could but may hand completely over the intake until it sucked hard but still ran at a fairly healthy rpm.

A couple years or so ago I noticed a couple holes in the whatchicallit (intake flap) that looked like screws belonged in them. So I rustled up some that fit and that slowed it down a bit. But something serious is going on now.

Thanks for your help. I posted a question last night right before bed, the moment I got up there's a reply. Isn't the internet wonderful?
Old 11-09-2007, 07:49 AM
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You most definitely have a vacuum leak below the butterfly in the throttle body. Follow that path and check for any place you could have a leak... Here are the things I can think of to check off hand:

* Make sure the butterfly closes squarely in throttle body.
* Vacuum retard hose from throttle body (below butterfly) to distributor (if yours has one).
* Gasket between the throttle body and plenum
* All vacuum hoses that connect to the plenum.
* Crack in plenum. Most commonly that would be around the holes where the tubes run vertically through the middle of the box. (I put some aluminum tape over those areas top and bottom when I was tracking mine down.)
* Intake runner seals (the short hose-like pieces between plenum and intake runners).
* Phenolic block between intake manifold and head.
* Injector seals. There are two per injector.

Advanced timing can also contribute to fast idle.

Last edited by RandyLok; 11-09-2007 at 10:32 AM..
Old 11-09-2007, 10:28 AM
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Randy, Thanks for all the clues. I don't understand how the car could idle just over 2k until it's good and warm, then eventually drop to 1k or thereabouts. None of the original cold start apparatus is functional.

1k would be fine for the smog check but it usually starts "hunting" pretty severely at that rpm. This is what causes it to fail.

There must be a leak because I know in the past I couldn't cover the intake with my hand without the car dying.

I'll get to checking.
Old 11-10-2007, 09:07 AM
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I found the leak. After removing the throttle body I noticed that a repair I made several years ago had failed. There is a manufactured hole in the top of the plenum where apparently a nipple or some sort of gizmo had been. There is also a crack near the hole that was probably caused by the thing breaking off. Don't ask me how it broke off.

I had filled the hole with a thick layer of shoegoo. Unfortunately shoegoo doesn't dry very hard and the strong vacuum had sucked a hole in it. Maybe shoegoo doesn't handle heat all that well either. Perhaps unwisely I just put another blob of a different brand off goo on it. We'll see how hard that dries.

I would have waited for other suggestions but the registration expires Mon. so I'll see what it looks like tomorrow. At worst it will probably hold another few years. By that time the car will have disintegrated into a pile of rust anyway.
Old 11-10-2007, 03:14 PM
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That tube is the most popular place for them to crack. It's a reinforcement that runs top-to-bottom inside the plenum; there was no nipple or anything that attached there. There is another reinforcement on the opposite side of the plenum as well.

--DD
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Old 11-10-2007, 04:41 PM
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Duh.

After I put goo on the hole I reached in the plenum and discovered the tubes.

Then I came in an re-read what Randy had written:

"Crack in plenum. Most commonly that would be around the holes where the tubes run vertically through the middle of the box. (I put some aluminum tape over those areas top and bottom when I was tracking mine down.)"

Just as well. Goo down in the tube a little ways will likely help to hold the gob in place.

Thanks again to Dave and Randy
Old 11-10-2007, 06:32 PM
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Cracked plenum

Shoe Goo?!!
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Old 11-19-2007, 04:56 PM
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