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Premium Member
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Discussion Starter #1
Don't ask my how or why, but our newly rebuilt '85 spider runs (in the shop) better with the AFM disconnected? Since we got her running a few weeks ago, I could not located the cause of a "surging idle". Finally I pressurized the intake system with 5# of air pressure and went around with a spray bottle of windex. Wouldn't you know the "hard" plastic intake pipe over the camshaft cover was deformed? Who would have thunk? Luckly I had a spare (greenhoosier parts car) and switched it out. I quickly got it together and she fired up and idled smooth. While the engine was warming up I noticed the electrical connector for the AFM was not hooked up. I plugged it in and the surging idle returned.

The engine seems to run fine (in the shop) with the AFM disconnected, what am I missing?

thanks
 

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But Mad North-Northwest
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10,524 Posts
Well, the car won't drive without the AFM, so that's not much of a fix :D

I suppose it could be the AFM. The temperature sensor on the AFM is pretty easy to test with an ohmmeter, but it's pretty hard to test for a worn carbon track so the easiest test is to swap in another one.

Can you describe the surging idle a bit? Under normal conditions L-jet idle will noticeably oscillate slightly due to the mixture control, and this can get more noticeable if the O2 sensor starts reading slow. This isn't what you're experiencing, is it?
 

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Premium Member
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Discussion Starter #4
From what I can tell there is no "backyard" test for the AFM. I have two and we have the same issue with both. Engine starts good cold or hot but has a very irregular idle. The tach is not working, but I figure a difference about 300 rpm at idle. New O2 sensor, new injectors, new, new, new.

Yep, I'm gettin my a** kicked
 

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But Mad North-Northwest
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10,524 Posts
Well, there's a test, but it involves rigging up a small circuit to power the AFM and then to measure current instead of voltage. Anyway, if it does it with two then that's probably not your problem.

I'm assuming you've checked the throttle position switch with an ohmmeter, correct?
 

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Administrator
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Do check the throttle position switch (TPS). It is a switch, not a sensor. It tells the computer if the throttle is at idle, not at idle (cruising) or wide open. At the idle position the computer will ignore the O2 sensor and follow its preprogrammed fuel map. If the TPS is faulty or misadjusted the computer doesn't know the throttle is at idle so it tries to use the O2 sensor inputs to adjust fuel mixture. But it can't do that very well at idle so it tends to surge as the mixture swings between too rich & too lean.
 

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Premium Member
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Discussion Starter #7
Throttle position ok, we checked all the sensors prior to assembly.

We are also doing a new exhaust system, the Magnaflow stainless steel front pipe with cat is the only one that fits correctly. The center muffler section required refitting as did the rear "stinger pipe" we are using.

I did notice the interior surfaces of the pipes are sooty black indicating a rich mixture. Where to go from here?

thanks
 
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