Should I Pull the Trigger on an IACV on my my 2001 Grand Marquis

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2001 Mercury Grand Marquis. 125K miles. Only one powertrain - 4.6 with 4 speed automatic.

There's two issues here. The one I need to solve is the low idle in gear when warm.

When warmed up (after a highway drive, even sitting in traffic), it wants to idle at 500 RPM. This has been causing a cylinder 5 misfire. I do believe that cylinder is a bit weak and won't fire well at that low of an RPM. If I do a clear flood and crank over, every 8th cylinder sounds odd so it's not a flat spot on the flexplate's ring gear. I would assume it's a weak cylinder?

I've squirted around with starting fluid, some propane and have not found any vacuum leaks. At idle (900-1000 RPM) in neutral, it's pulling 18.5 in/hg of vacuum. That's well within spec. Idling in gear, it's 13.5 at 500 RPM which seems normal. I'll try to smoke it this weekend in case I missed something.

I cleaned the IACV (and did the relearn) and that seems to have made the issue worse. Now if I stop hard (instead of coasting where it will figure itself out), it will drop down to 400 RPM for a second. Same happens when I put it in gear warm.

Tested the voltage on the TPS and it appears to be working 100% , the readout on my basic scanner also agrees with it working right.

There is no pinging / hesitation during moderate throttle acceleration with torque converter locked at low RPM. If I get on it too much it kicks down. But no pinging / hesitation during low RPM acceleration , and no pinging / hesitation at part throttle cruise leads me to believe EGR and DFPE are fine.

I believe the IACV to be bad. If I'm in traffic or a drive thru where it wants to idle low and rough, if I two foot it and lightly blip the gas pedal a few times, it will eventually compensate and adjust and idles at 570-600 smoothly and won't set the misfire code. And it will work as long until I take off from the stoplight / drive thru and come to a stop again.

Someone else tried a very basic way to figure this out before. It still had all 8 original coils but the plug in cylinder 5 was an autolite, the other 7 were original motorcraft.

So ...
  • 8 new motorcraft plugs and coils - wasn't troubleshooting anything, just maintenance.
  • Idles at 500 RPM, drops to 400 sometimes
  • Cylinder 5 misfire
  • Can't find any major vacuum leaks with starting fluid or propane
  • If I tap the gas a few times idling in gear it'll figure itself out and idle normally
  • Eliminated TPS as problem
  • No pinging / hesitation ; don't believe EGR or DFPE
  • Cleaning and relearning the IACV made it worse
  • Adjusting idle screw and relearning didn't do anything
  • Cleaned MAF with CRC MAFS cleanr

Obviously if I replace the IACV it's only going to be with a Motorcraft one. Not interested in aftermarket junk electronics.

If NY didn't require no CEL to pass inspection I wouldn't care.
 
Last edited:
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When warmed up (after a highway drive, even sitting in traffic), it wants to idle at 500 RPM. This has been causing a cylinder 5 misfire.
Why would low idle speed cause a misfire in just one cylinder? Not familiar with IACV but if it flows air, it could also be clogged with dirt and carbon lowering air flow and dropping idle speed. Can it be cleaned? How cheap is a new one?
 
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Vote for EGR passages, Ford were known for Acidic Exhaust that caused Carbon build up and destroyed DPFE valves, try using an old speedometer cable on an Electric drill after souking the passages with carb cleaner
 
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If it is a bad cylinder you need to confirm that then go from there. Spending money on little peripheral parts isn't going to make it run right.

I wouldn't say it's a bad problem unless its stalling out. Dashboard tachs are not accurate, especially at idle speed.

You could devise some way to leak metered air around the IACV to increase idle speed and create the effect of a more properly working V8. The computer is going to fight any tendency for idle speed to increase above spec by closing down the IACV.
 

SwampSurvivor

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If it is a bad cylinder you need to confirm that then go from there. Spending money on little peripheral parts isn't going to make it run right.

I wouldn't say it's a bad problem unless its stalling out. Dashboard tachs are not accurate, especially at idle speed.

You could devise some way to leak metered air around the IACV to increase idle speed and create the effect of a more properly working V8. The computer is going to fight any tendency for idle speed to increase above spec by closing down the IACV.


The target idle speed for this is 575 - 600 hot in gear. If it idles at the right speed, which I can do by blipping the throttle a bit, then it's fine and will hold 575-ish and idle smooth as silk. It's only when it drops to 500 that the weak cylinder starts to misfire.

I'm using an OBDII scanner to see the RPM. These cars didn't come with tacs until 05 or 06. If I didn't live in a state that requires the check engine light to be off to pass inspection, I wouldn't care.


I had an 02 F150 with same motor and a very common problem is the egr valve passage way gets clogged. It is at the back of the motor a couple bolts to remove. You can run a safety wire or coat hanger to clear it out.

This thread from a quick search is typical for that problem and how to test. https://www.f150forum.com/f11/4-6l-warm-weather-egr-problem-p0401-19483/
BTW we found that only Ford parts fix these issues


I'll take a look at the EGR passages - thanks for the tip! As for OEM parts - I try to go with OEM parts for anything like that if available. Not worth it to buy aftermarket stuff and "save" $10 just to have to keep replacing it.
 
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When warmed up (after a highway drive, even sitting in traffic), it wants to idle at 500 RPM. This has been causing a cylinder 5 misfire. I do believe that cylinder is a bit weak and won't fire well at that low of an RPM. If I do a clear flood and crank over, every 8th cylinder sounds odd so it's not a flat spot on the flexplate's ring gear. I would assume it's a weak cylinder?
Low idle isn't causing a misfire, the misfire is causing the low idle. More than likely if you're hearing an RPM jump while cranking in clear flood mode you have a compression issue requiring mechanical repair and replacing the IAC is going to do nothing except waste money.
 
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The 4.6L calibration files I've seen show a 600 rpm target idle speed in gear, Like mk378 said.....You can't trust the Tach. Use a Scan Tool. The IAC might be getting a bit slow in it's old age, So replace it before doing any modifications.

If you really want to raise Idle Speed above specifications, The Idle Stop Screw isn't the answer as it throws off the TPS resting voltage. As mk378 eluded to.....The ECM will keep closing down the IAC to achieve Target Idle Speeds, So you will have to bypass enough air to make the IAC seat all the way.....Then increase Idle Speed from there.
What we did back in the Gen II LT1 days when tuning for big/rowdy camshafts was too drill Air Bypass holes into the Throttle Blades, In fact some factory Throttle Bodies have Air Bypass Holes.

If your TB doesn't have a Air Bypass hole....Start with a .030" Hole, Most drill chucks will not hold anything smaller than a .060" drill bit, Starrett 162B Pin Vise works great for small holes with a cordless drill. Go slow as bits this small a very fragile!
*Increase the hole size .005-.007" 'til you reach your target Idle Speed.
*Test drive the vehicle thoroughly before increasing hole diameter.

If it does already have a Bypass Hole.....Don't go crazy as it's not easy to make the hole smaller if you get my drift? In fact keeping with the .005"-.007" increments would be wise.

Use this information at your own risk, And don't be surprised if you have higher than expected Idle Speeds at times.

Drill in this location if no Air Bypass Hole is present....
F6JW8CG.jpg
 

SwampSurvivor

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The 4.6L calibration files I've seen show a 600 rpm target idle speed in gear, Like mk378 said.....You can't trust the Tach. Use a Scan Tool. The IAC might be getting a bit slow in it's old age, So replace it before doing any modifications.

If you really want to raise Idle Speed above specifications, The Idle Stop Screw isn't the answer as it throws off the TPS resting voltage. As mk378 eluded to.....The ECM will keep closing down the IAC to achieve Target Idle Speeds, So you will have to bypass enough air to make the IAC seat all the way.....Then increase Idle Speed from there.
What we did back in the Gen II LT1 days when tuning for big/rowdy camshafts was too drill Air Bypass holes into the Throttle Blades, In fact some factory Throttle Bodies have Air Bypass Holes.

If your TB doesn't have a Air Bypass hole....Start with a .030" Hole, Most drill chucks will not hold anything smaller than a .060" drill bit, Starrett 162B Pin Vise works great for small holes with a cordless drill. Go slow as bits this small a very fragile!
*Increase the hole size .005-.007" 'til you reach your target Idle Speed.
*Test drive the vehicle thoroughly before increasing hole diameter.

If it does already have a Bypass Hole.....Don't go crazy as it's not easy to make the hole smaller if you get my drift? In fact keeping with the .005"-.007" increments would be wise.

Use this information at your own risk, And don't be surprised if you have higher than expected Idle Speeds at times.

Drill in this location if no Air Bypass Hole is present....
F6JW8CG.jpg

I do believe I'm dealing with a lazy IAC. It will eventually idle at 575-600 and smooth. I've been using an OBDII adapter to see the RPM, this doesn't have a Tac. Just the weird digital dash. It will eventually catch up and be fine. That's why I really think that it's an issue with the IAC that's just making another issue (weak cylinder) show up. It is quite common for these cars with bad / lazy IACs to idle at 500 RPM warm in gear.

Once it figured out to idle at the right speed, it's fine until I'm on the throttle a bit - which should cause it to back off the IAC a bit, then figure it out again.

I'm not interested in making it idle above where it's supposed to, just getting it to 600 is all I care. Right now I have the idle screw backed all the way out
 
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While you're checking the EGR circuit also check for leaky gasket. A leaky gasket can lead to a lean condition in the cylinder nearest the leak. To set idle speed the procedure is to adjust the TPS with a meter attached until target duty cycle % is reached. Mechanical wear can throw it off.
 
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EGR and an Italian Tuneup might be your answer. This car probably never gets above 1500-2000 rpms when it’s driven. Use the loud pedal with some gusto.
 

SwampSurvivor

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EGR and an Italian Tuneup might be your answer. This car probably never gets above 1500-2000 rpms when it’s driven. Use the loud pedal with some gusto.
I've thought about seafoam and driving it hard to see if that would do anything. Not sure how I feel about sucking a liquid down the engine, though.
 
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The 4.6L calibration files I've seen show a 600 rpm target idle speed in gear, Like mk378 said.....You can't trust the Tach. Use a Scan Tool. The IAC might be getting a bit slow in it's old age, So replace it before doing any modifications.

If you really want to raise Idle Speed above specifications, The Idle Stop Screw isn't the answer as it throws off the TPS resting voltage. As mk378 eluded to.....The ECM will keep closing down the IAC to achieve Target Idle Speeds, So you will have to bypass enough air to make the IAC seat all the way.....Then increase Idle Speed from there.
What we did back in the Gen II LT1 days when tuning for big/rowdy camshafts was too drill Air Bypass holes into the Throttle Blades, In fact some factory Throttle Bodies have Air Bypass Holes.

If your TB doesn't have a Air Bypass hole....Start with a .030" Hole, Most drill chucks will not hold anything smaller than a .060" drill bit, Starrett 162B Pin Vise works great for small holes with a cordless drill. Go slow as bits this small a very fragile!
*Increase the hole size .005-.007" 'til you reach your target Idle Speed.
*Test drive the vehicle thoroughly before increasing hole diameter.

If it does already have a Bypass Hole.....Don't go crazy as it's not easy to make the hole smaller if you get my drift? In fact keeping with the .005"-.007" increments would be wise.

Use this information at your own risk, And don't be surprised if you have higher than expected Idle Speeds at times.

Drill in this location if no Air Bypass Hole is present....
F6JW8CG.jpg


Everything you say is true (as always) - I still own an LT1 in my 94 C4. I feel old because people call me the LT1 guru lol. Everybody knows LSx but few understand and LT1/4
 

SwampSurvivor

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If anyone is still watching this ...

I smoked the intake this morning for a while. Surprisngly the 45000 miles of vacuum line weren't leaking. But I did find a large and small leak.

The PCV valve didn't fit at all in the gromet. I started it up and just wiggling the pcv valve would change idle. The valve itself is good (and a motorcraft part) so I just wrapped some electrical tape around and stuck it back in the valve cover.

The second one was the IAC . I took the tube off and put a plug over the end of the iacv where the tube goes, still leaking. Looked like the gasket so I bought and replaced that. Still was leaking. Upon closer inspection I found one of these two spots to be the leak. Either the crimp itself has corroded of whatever that tube / plastic cover thing is has some sort of leak? I don't believe this is a big enough leak to do too much.

Big thing is: The intake manifold isn't leaking.
 

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