Another IAC Question or 2!

edspringer

Registered User
My '94 SC auto starts OK and idles at about 1200 RPM for about 20-30 seconds. Then it slowly goes down to about 800 RPM and idles a little rough. I shift into drive with the brake on and the A/C on and it goes down to about 650 RPM. It damn near shakes the radar detector off the windshield. It accelerates fine and runs fine above idle. The in gear, A/C on and brake on idle just sucks! I have 94K on the SC and replaced the IAC at 73K. Also, when I pull the connector off the IAC it makes no difference in the idle. Anybody have any data on the average life of an IAC? How long do these things last? After reading some other posts I guess I screwed up buy trying to clean it with carb cleaner although it is no better or worse. This rough idle problem is a pain in the donkey. Any ideas or help would be appreciated!

Ed Springer

Black XR7 5-speed: 70mm TB, Fresh Air Intake, '94 Supercharger, 5% SC Pulley, Raised SC Top, I/C Fan, Underdrive Pulleys, 190 l/h Fuel Pump, 36# Injectors, Headers, Magnaflow Muffler (no cats) with stock resoantors, 3:27 Gears, Ripper Shifter, 73 CL MAF, Magnacore Wires, NGK Iridium Plugs and Rear Spring Air Bags.

94 Red SC Auto: 70mm TB, Fresh Air Intake, Stock 90 SC Pulley, Raised SC Top, I/C Fan, Underdrive Pulleys, 190 l/h Fuel Pump, 38# Injectors, Headers, Raven Muffler (no cats) with stock resoantors, 3:73 Gears, 73 CL MAF, Magnacore Wires, NGK Iridium Plugs, TRANSGO Shift Kit, ProTorque 2500 Stall TC, Haydon Auxiliary Transmission Cooler, American Racing 17” Sniper Wheels, Michelin Pilot Sport A/S Tires and 235 RWHP with 315 Lbs.Ft. Torque.
 
I'm having the same problems, I have an IAC on the way, if it fixes it I will let you know, if you discover some other fix for it let me know.
 
(Edited by Admin.)

My idle, in gear, at a stop light is 550 RPMS, like it says on the VACUUM routing sticker on the front end of the car, it doesnt shake like that. It idles fine, i do believe.

I say u just get a new IAC. I dont think IACs can be cleaned, some people have gotten away with cleaning them.
 
struggling with similar problem..

edspringer said:
My '94 SC auto starts OK and idles at about 1200 RPM for about 20-30 seconds. Then it slowly goes down to about 800 RPM and idles a little rough. I shift into drive with the brake on and the A/C on and it goes down to about 650 RPM. It damn near shakes the radar detector off the windshield. It accelerates fine and runs fine above idle. The in gear, A/C on and brake on idle just sucks! I have 94K on the SC and replaced the IAC at 73K. Also, when I pull the connector off the IAC it makes no difference in the idle. Anybody have any data on the average life of an IAC? How long do these things last? After reading some other posts I guess I screwed up buy trying to clean it with carb cleaner although it is no better or worse. This rough idle problem is a pain in the donkey. Any ideas or help would be appreciated!


Ed Springer

Black XR7 5-speed: 70mm TB, Fresh Air Intake, '94 Supercharger, 5% SC Pulley, Raised SC Top, I/C Fan, Underdrive Pulleys, 190 l/h Fuel Pump, 36# Injectors, Headers, Magnaflow Muffler (no cats) with stock resoantors, 3:27 Gears, Ripper Shifter, 73 CL MAF, Magnacore Wires, NGK Iridium Plugs and Rear Spring Air Bags.

94 Red SC Auto: 70mm TB, Fresh Air Intake, Stock 90 SC Pulley, Raised SC Top, I/C Fan, Underdrive Pulleys, 190 l/h Fuel Pump, 38# Injectors, Headers, Raven Muffler (no cats) with stock resoantors, 3:73 Gears, 73 CL MAF, Magnacore Wires, NGK Iridium Plugs, TRANSGO Shift Kit, ProTorque 2500 Stall TC, Haydon Auxiliary Transmission Cooler, American Racing 17” Sniper Wheels, Michelin Pilot Sport A/S Tires and 235 RWHP with 315 Lbs.Ft. Torque.

I am not an expert or my car would idle smooth, but I do know when you plug out IAC engine should stop running or close as mine does. You might check IAC connector for whatever voltage or signal should be on it, or break in wire, etc.. Maybe IAC is ok, but effectively disconnected.....good luck, I will post if my new one makes a difference.
 
IAC Not the Problem

Thanks for the responses! However, there appears to be no joy in SC land. I bought a new IAC and installed it. That did not fix the rough idle problem. The thing I can't figure out is when the car comes to a complete stop it maintains about 750 RPM for about 1-2 seconds and appears to idle fine then drops to about 550 RPM and starts shaking like a mother. I don't appear to have a vacuum leak because at idle in park the vacuum/boost gage reads right around 16-17 inches of vacuum. I believe I have an ignition timing problem. That is the next place I'm going to look. Will let you know of progress.

Ed
 
This will sound a little odd, but bad motor mounts can cause the symptoms you describe.
 
I have the same problem, runs fine and idles well in park, but as soon as i throw her in drive or reverse(reverse is worse) it dumps idle down to 500rpms or so and shakes, but as soon as idle is up'd a bit she smooths right out. I made a custom little intake for now, im getting it remade with the MAF repositioned to in front of the bend, i thought that my idle problem was possibly from this? everyone says that with the bend in the intake the air to fuel ratio gets a-wall... but the car performs great all the rest of the time. I have only had the car for about a month now and have done intake and exhaust, it didnt have the idle problem before. Another thought was possibly the timing, but i thought the EEC took care of that. Also, what is the AIC that is being talked about? thanks for all the help...Brett
www.cardomain.com/id/scmakeiv
93 SC AOD
 
Low Idle... check the engine mounts

As george said. :cool: Do a forum search on engine mounts to find out how to do this. A very common problem.
 
Still idles rough with new soild mounts

George.........I put new solid mounts in 20,000 miles ago so that is not the problem. I checked them and they are still good. I'm still baffled why it changes from a fairly smooth 750 RPM idle when I stop then about 2 seconds later it goes to a 500 RPM idle and shakes bad. It is like a switch is activated to change the idle. It shakes so bad I have to put it in neutral at stop lights with the A/C running. But it will not stall. My 90 XR7 5 speed idles smooth and it has the factory liquid motor mounts in it and the same mods as the 94 SC. I thought I was pretty smart at fixing this stuff but this one has got me beat.......for now.

Brett........the IAC is the the idle air control valve. It is located on the throttle body right in front of the throttle position sensor. Supposedly, when the throttle shuts the IAC opens and allows the engine to idle. I believe it is not adjustable on the factory TB and if you have an after market TB like I do (BBK 70MM) it is adjustable.

Ed Springer
 
Any codes? You might run the cylinder balance test. May not tell you anything, but you might get lucky. Be sure the engine is warmed up before you try this. Oh, and if the engine is cold and you disconnect the IAC with it runinng, it should stumble and/or stall. This is a Ford type stock IAC, right? Actually, if it were ruptured or an aftermarket model, the idle would race, so you probably have the right one. Hopefully, you haven't adjusted the TB set screw ;) The throttle plate should be closed with no throttle. Its the IAC which allows air past the throttle plate to control the idle speed.
 
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Iac

When the IAC goes bad, the car will generally idle much faster than normal, not lower. For example when mine went bad I had a nice idle of about 2500 - 3200 RPM's. :rolleyes: Let alone the fuel washing out my cylinder walls. Sorry I just caught this thread, as I may have been able to save you the $70.
 
Again, thanks for the responses. I have no codes. The only time I get a CE light is when I climb a fairly steep hill at about 40 MPH in O/D. The CE light goes away when I get on flat road. The code will not latch so I don't have a clue what the CE light is about. I did a cylinder balance test about 2 weeks ago under the same rough idle conditions with OK results. The IAC is a Ford unit. I screwed with the throttle plate adjustment on the 70MM BBK TB last night just to see if a smooth idle is achieveable. At about 1500 RPM it was fine but that is hard on the drive train. However, I know that is not a good fix so I put it back. I believe the initial BBK adjustment is .020" between the closed plate and the adjust screw then turn the allen screw one full turn clockwise. I can't find my install instructions so I'm going from memory. I can move the secondary idle adjust screw on the BBK and affect engine RPM but the engine will not smooth out. I think my next move is to pull the TB and inspect and clean it. When I changed the IAC I noticed some crud in the openings. If that yeilds no results then, I may pull the 70MM BBK off my 90 XR7 that idles smooth and install on the 94 SC to see if that makes a difference. However, before I go there I want to eliminate the easy stuff.

Ed Springer
 
edspringer said:
The code will not latch so I don't have a clue what the CE light is about.

That, in and of itself, is a problem. The EEC is supposed to store ALL DTC's that cause CE lamp illumination until cleared. If you're getting a CE light and no codes are stored, even if the light went out, then the EEC itself is defective (assuming all other pinpoint tests for no DTC's are passed, that is).
 
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