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Discussion Starter · #1 ·
Trying to figure if my IAC valve is bad on my 97 w/ 4.6. Most symptoms seem to involve the "dashpot" function:

1. The car shakes a little while idling, when in gear. It doesn't feel like it's going to stall, you can just feel the car shake. The idle speed is stable. I need to check the idle speed with my torque app, but it appears to be about 550 while in gear. But it's normal when you shift into park. Absolutely no fluctuation of idle.

2. When you let off the throttle after it shifts into OD, the car revs up 200-300 RPM. I know the TC unlocks when you let off the throttle, it just feels like it has an extra boost, very pronounced. Does it every time.

3. When at hwy speeds with CC on, if you let off going down a big hill, after a couple seconds, the car sort of "surges", and it keeps it doing every few seconds until you apply the gas again.

All the tune up parts are good, vacuum lines replaced. MAF is 1 yr old. The most recent thing I changed was a new Ford PCV and grommet, the old valve fit loose in the grommet.

I just took a long trip with a lot of hills, and the problems are getting worse, much worse than recent trips I took on more level ground. Something is getting replaced, lol! I don't want a bad IAC valve messing up the transmission, which on some cars they can if left alone long enough.

Thanks for any tips on something else to check before I go to Autozone. Ford discontinued that part.
Al
 

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97 Thunderbird 4.6, 98 Mark VIII LSC
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Idle in a drive gear is 560 RPM, so that sounds normal.

Before dropping money on an IAC, measure the voltage of the TPS, just to be sure (since it's easy and doesn't cost anything). Under 1V at idle, usually .95-.98, is good.

You can also take off the IAC and shoot some carb cleaner through it while opening and closing the valve manually; sometimes if it's sticky that can help it.
 

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Like Brandon said, try cleaning it first. Use TB cleaner or maf cleaner, not brake parts cleaner.:) Brake cleaner will kill the O2 sensors.
Usually when they fail, they just quit working, and you have to keep it running with your foot.
 
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Discussion Starter · #4 ·
Well, I finally hooked up my torque app, and my idle is too high! It's 880-940 (fluctuates up and down a little), and 640 in gear. I capped off each vacuum source one by one at the TB to check for leaks, with no change in idle speed. Also disconnected and capped off the IAC. I understand it's supposed to be around 650?

One thing worth mentioning is I did install a PI intake last April. The rest of the car is stock. Would that make the idle higher? I didn't take note of the before and after idle speed, but I'm pretty sure it used to be lower.

I also wasn't able to check the voltage on the TPS. I am guessing I need a little device to pierce the wire, or is there an easier way? I couldn't find the voltage in my torque app.

Thanks,
Al
 

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Yes; you'll have to use a set of sharp leads to measure the tps. But, you can read it thru the odb port.
I'd check for a leak at the manifold; pi intakes can be hard to seal. The one on Lazarus is glued on with black rtv, due to corrosion pits on the head flange. I use my Xcal2 and laptop to set the TPS, and it works well.
 

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Discussion Starter · #6 ·
If the manifold was leaking, wouldn't that throw off the fuel trims? Both long and short term look good, 1.5 - 3.0%. Also, wouldn't there be a hesitation?

The car has great throttle tip-in and response. The idle is consistent at 880-920.

My torque app won't pull up the TPS voltage. I haven't been able to get the mult-meter probe to penetrate the wire.

Thanks,
Al
 

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97 Thunderbird 4.6, 98 Mark VIII LSC
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Are you looking at STFT and LTFT? If you're looking at only STFT, be sure adpative fueling is off in the tune.

The idle is high. Drive gear is 560 and neutral/park is 800.

If you can get TPS absolute, it should be under 20%. Usually 17-19%.
 

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Just so everyone is on the same page; the fuel trims go to 4096 at ~4V; idle is about 1024, or 1V
 

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Discussion Starter · #9 · (Edited)
Are you looking at STFT and LTFT? If you're looking at only STFT, be sure adpative fueling is off in the tune.

The idle is high. Drive gear is 560 and neutral/park is 800.
My torque app show both LTFT and STFT. This is a stock vehicle, no tune.

Stock idle is 800? I thought it was supposed to be much lower, like 700.

Al
 

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When I use the Torque app I pull the data into a spreadsheet and add LTFT1 to STFT1 to look at overall trim on each side also.
TPS in the Torque app is a percentage. You can graph TPS with engine RPM to get an idea of how the TPS is functioning. Higher TPS% = higher RPM.
 
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