I have been chasing down a cold start problem and have found a problem(?) with the IAC valve circuit. The cold start symptom is extended or repeated engine cranking with an occasional pop or starter hang-up. Once it goes, it runs rather well with the exception of a not quite steady idle. Starts up again without hesitation when hot and has good power and acceleration. Fuel pump pressure was 57lb key on. I now think my IAC valve is closed all of the time. Would an IAC like that be the reason a cold start problem? O2 sensors are new, MAP and inlet air sensor is working properly, ECT sensor tests OK and the TPS is in specs and turns smoothly.

I was checking previous posts here for ideas and came across this bit of information.

"Fuel injected engines can't use an idle screw, no Jets, so they all use an air valve controlled by the computer to set idle RPMs
Ford calls their air valve an IAC(idle air control) Valve
Its on the upper intake by the throttle linkage, looks like a "can" on its side, has a 2 wire connector
Held on by 2 bolts, take it off
look inside and you will see the valve
Plug in its 2 wires if you took them off
Turn on the key
Valve should be open all the way now
Unplug the 2 wires while watching the valve, valve should move to closed position, it only moves 3/8" so not much but should move
Plug wires back in and watch it open again
Repeat as much as you need to decide it is working or is NOT working
You can only replace these with Motorcraft(Ford) brand, 3rd party cause other issues."


I knew about pulling the plug while running and expecting the RPM to drop. Mine didn't and thought I was up against a vacuum leak. I cleaned it a few years ago and just again a few days ago. It's hard to get it cleaned around the valve and my truck has 231,000m on it, so I thought it a good idea to just replace it. I should have checked here first, I did not buy a Motorcraft replacement. I tried the new advice and the valve remains closed. However, I now had a new spare IAC to plug in and see if the valve would open or not, and it did not function either.

With the key on, I get battery voltage on one pin to ground, and read .022v with the other plug pin to ground. With the key off 0 and 0. I checked pin opposite the voltage side, pin to battery ground, with an ohm meter at 200k and read 15.5 key off, and 34.5 key on. Shouldn't I pretty much have full continuity?

Where do I start? Electronics is not one one my better skill sets, any assistance would be greatly appreciated.

 

some IAC bodys have what looks to be a idle adjustment with a screw. make sure its backed off and not touching. if you unplug your IAC with truck warmed up and the idle stays high its not the IAC.
it should stall or almost run.

 

Thanks for the response.

The IAC valve is just a metal can with a plug socket on an Aluminum casting. There is nothing on it that looks like an adjuster.

I am aware of the IAC unplug test, and did that. With the original part, the RPM did not change, and I though I now had a vacuum problem and maybe still do. If one is present, it must be small or I would think the idle would be worse

Before I knew about the unplug test, I bought a new IAC valve to replace the original. It's hard to clean around the little valve and I have 230,000m. on my truck. I thought it to be a good idea to just replace it. When I put it on the truck and ran it, the idle was a bit low and was surging. Unplugging it didn't really do anything except smooth out the surging.

I was frustrated and was searching here for ideas when I found, I think your post, describing testing the part off of the vehicle. Observe the plugged in valve, key on and key off. It should open with key on and close with key off. I did this with both my old one and the new one. Neither one opened with the key on.

I did some testing with a volt and ohm meter, which I described in the first post. Key on, from the battery negative post to one plug terminal I got 12.59v, battery voltage. I checked the opposite plug terminal the same way (20v scale) and read .022v. ??? It's a small number but why would there be any voltage there? Checking both sides again with the key off, I got 0v and 0v.

My IAC valve is not operating when plugged into the truck wiring harness. I checked both the old and the new IAC valves off of the engine, wiring directly to the battery. Both of them opened with battery voltage applied and then closed. The parts are good.

Checking the ground wire, plug to battery negative terminal, with the ohm meter set on 200k, it read 34.5 key on and 15.5 key off. Bad ground? I don't have a wiring diagram. Should I pull the ECU plug apart and inspect it? Any other ideas?

 

the adjuster i was talking about is on the cable side, it looks like a idle adjuster. ill check my ground #.s today

 

so how are you looking for vacuum leaks? i would work on that first.

 

No adjuster, just a plain can with a plug socket mounted to an Aluminum casting.

I found out about the unplug test on this forum, but not before I bought a new IAC valve. I had cleaned the old one a couple years ago and again this past week. It's hard to clean the little valve thoroughly and my truck has 230,000 miles on it, so I thought it a good idea to just bought a new one. When I pulled the plug on the old IAC, there was no change, and I thought I now also have a vacuum issue. I installed the new IAC and the idle was low and was surging. I pulled the plug and it stayed the same, low and surging. I drove it this way to let the ECU figure things out and got no improvement in idle. After digging around here for ideas I found the above mentioned post describing testing the IAC, plugged in but off of the engine, key on and key off, observing the valve. I did this with both the old and new part and neither one did anything. I tested then again, off the truck, unplugged, and used test leads to the battery terminals. They both functioned properly direct to battery. The IAC is just not opening/ operating when installed on the truck.

I also think that I do not have a vacuum issue. With the new IAC on the truck, that won't open, the idle was low and surging. My old IAC has no change when you pull the plug and I think it's because it doesn't seal shut completely from carbon deposits.

I unplugged and checked the plug terminals for voltage, terminal to battery ground. Ignition key on I read 12.59 volts on one. I did the same with the other terminal and read .022 volts on the 20V scale.??? Why would I have any volts on that side? With the key off, 0 and 0 volts.

I then checked with an ohm meter set at 200k, ground or return terminal to battery negative terminal. With key on read 34.5 and 15.5 with the key off.

How do I interpret the results? Bad ground or corroded plug connection? I don't have a wiring diagram, does the ground wire go to the ECU? Should I pull the large ECU plug apart on the firewall and have a look?

Electronics is not my best subject. Thanks in advance.

Ah, the adjuster you are referring to is the throttle stop. I have not messed with it and don't believe that it is adjustable.

 

I did not do a real comprehensive vacuum leak search, but I did some checking with a propane torch that I put a length of tubing on. I also did some checking with carb cleaner. Both methods make me a bit nervous about doing this check with flammables. I did not find anything.

 

ok WD40 will work to,, it will raise the RPMs its most likely not too big. just make sure it not touching, its not the throttle stop. the computer wlll set he idle.

 

Mine doesn't look like that. The throttle cable comes in on the left side. The throttle lever contacts the stop on the lower left, below it at rear of the throttle body. It looks like it was pressed in, there is no hex, slot or allen socket to rotate it.

 

Ok so you were testing the ground side of the Iac. Key on and key off? I’ll go try that now and see what I get

 

OK key on i get 12v on the hot side and 23.55ohm on the ground side.

 

OK, similar result, I'm confused. Why won't my IAC open the valve with the key on, valve removed from engine, visual inspection test?

 

well im guessing the motor needs to be running? i doubt it moves much? guessing the PCM gives fuel to start motor, when the motor gets to the right temp it cuts back the fuel to give it a good idle. id check around the PCV and the line to the brake canister. if i remember correctly there is a large vacuum line under the intake manifold it feeds lots of stuff. i dont think its your problem but i think it feeds lots of small lines back by the firewall. how high is the idle? does the motor temp change much on the guage?

 

I didn't try running the engine with the IAC off. It's mounted on the vacuum manifold and would leave two rather large holes in the manifold or plenum, whatever it's called. I don't think the truck would run like that.

Once again this morning, I had a hard cold start. I'm Central Illinois, temps where in the low/ mid 60's when I tried this morning. Not very cold. Once it fires up it takes off and goes to a seemingly normal fast idle around 850 and then drops to an average idle of right around 700 RPM, bobbling a bit from 680ish to 720ish plus or minus. I ran the engine until it warmed up and tried pulling the plug on the IAC again with my OBD set to live RPM, not much if any change. Later, when the engine had cooled, I tried again. Started the engine, it had a normal fast starting idle. I pulled the plug during the fast idle and got a rather substantial drop in RPM. Previously, I had only done this with a hot engine.

OK, does the IAC function to just provide a higher RPM at start up? In my case it reacts with a cold engine, but not hot. I may try the off engine, key on, key off test again when the engine is cold. The IAC valve range of travel I saw when wiring direct to battery is about 1/8",

There are no vacuum lines below the intake manifold. Vacuum is sourced from the Black plastic manifold or plenum in the image. I tried detecting vacuum leaks again with carb cleaner and again, found no leaks. PVC is old and on my hit list to replace. Engine temp is steady when hot. I have checked that with the OBD set to live while driving.

This is what my throttle body and stop looks like.

This looks like a pressed in pin to me, no adjustment.
Front

Side

Rear.

 

yes your motor is different than mine. one thing i did was get a bluetooth OBD 2 transponder and put some apps on my Ipad. now i can see all the data from all the sensors. O2 sensors were my issue. ill pull my IAC off today and ck what happens key on and key off.
even cold it should start right up. id try with cold motor squirt a little starting fluid in the throttle body. if it fires right up you know theres a fuel issue.

 

I did try starter fluid last Monday I think, no go. During cranking, I get an occasional pop or a starter hang up. I had also measured fuel pump pressure a that time, key on and read 57lbs. I wanted to check it again after running low on possible reasons for the cold start problem. I put on my fuel pressure tester and the Schrader adapter started leaking. The rubber seal in it is trashed. I have ordered a new one and will try again then.

 

theres also the fuel regulator that goes bad and there is a vacuum line to it? should be on the fuel rail. pull the vacuum line and check for the smell of gas. if so its leaking.

 

It's part of the fuel pump assembly, in the tank. It's been replaced once, probably 5 - 6 years or so ago. It's a super PITA to get to it. Take the bed off or drop the fuel tank.

Once the truck is running, it runs strongly. The truck is a manual 5 speed. I can gas it up through the gears, and when you hit the power band each shift, it wants to go with no hesitation. I did have an issue for a while last year with hesitation and cutting out. I replaced the camshaft synchronizer and it cleaned it right up.

It had the dreaded valve seat problem when I bought it. I got new heads put on it around 80 - 90,000 miles. It really perked it up after that and has always been a strong running truck. Still is, when it decides to start.

I defiantly plan on rechecking the fuel pressure key on and while driving, when my Schrader adapter comes in. I only got a pressure ready with key on, but the Schrader adapter crapped out when it put it back on later to check with the engine running.

I'm getting out of Dodge for a week, it will have to wait until I get back. Thanks for all the feedback you have offered.

 

The only thing I can think of is that if it won’t start right up on starting fluid, is it possible for some kind of spark issue? Have a good weekend or week.

 

I had an intermittent miss coming home on an 8 hour trip this past July. It turned out to be corroded coil terminals, so I replaced the coil pack. Last week I installed new SP500 Motorcraft plugs and spent the money for good plug wires. I had roughly 50,000 on the old ones. I did notice some performance improvement afterwards. Like I said, when it's running, it runs pretty good. I put 25 or so miles on it today with a couple of stops shopping, no issues. Are there any relays that could be involved with a hard cold start?

 

heres the way i think i works no relays. turn the key to start the PCM can tell if the motor is cold from the temp sensor, so i increases fuel delivery. same as a choke. the RPMs will stay high until reaches operating temp sent from the sensor to PCM. then it will decrease fuel and set the idle. i did have a new thermostat go bad, it stuck open. hard for the motor to warm up. but i could see it on the gauge. its possible the your temp sensor is bad? but it runs good after its warm? also the O2 sensors dont start to work until the motor is up to temp the it goes from short term to long term. so its all on the PCM until the motor is running temp. did you do the K on K off test to both IACs?

 

i just looked around


Common Causes

• Engine Coolant Temperature (ECT) Sensor:
  If the ECT sensor sends incorrect data about engine temperature to the computer, the fuel mixture will be wrong for a cold start.
• Fuel Delivery Issues:
  • Fuel Pump: A weak or failing fuel pump can lead to low fuel pressure, which is crucial for starting the engine.
  • Fuel Injectors: Clogged or weak fuel injectors can cause a poor fuel spray pattern, affecting cold starts.
  • Fuel Filter: A clogged fuel filter can restrict fuel flow, leading to low fuel pressure and hard starts

• Idle Air Control (IAC) Valve:
  A dirty or malfunctioning IAC valve can struggle to regulate the air going to the engine during a cold start, leading to a poor idle.
• Mass Airflow (MAF) Sensor:
  A dirty MAF sensor can provide incorrect air mass readings to the engine computer, impacting the air/fuel mixture needed for a cold start.
• Battery & Ignition System:
  • Battery: A weak battery or corroded terminals can cause slow cranking or insufficient voltage to the fuel pump.
  • Spark Plugs & Wires: Worn or fouled spark plugs can hinder the ignition process

 

Thanks for putting that up.

Yes, both valves key on key off. I have not tried this on a cold engine though and will try that in the morning to see what happens.

I had replaced the ECT sensor maybe two years or more ago and used an OEM replacement part. When the cold start issue started, the ECT and IAT sensors are where I started looking. I was able to see on my OBD the temps that the ECU reported. I have one of those laser guns that measure temperature. I checked the ECT cold on two different days, comparing the ECU value with my temp gun aimed right where the ECT is mounted. First day the ECU reported 65 degrees, the laser read 70. I did the temp experiment again the next day, 52 with the laser and the ECU reported 50 degrees. I can see the temp values that the ECU reports and it all seems to be operating properly. I also ohm checked it in the vehicle and got 40.9 on 200k scale. The laser gave a temp of 63 degrees where the ECT screws into the manifold. A chart I found online has the 40.9 ohm value as being in specs at 63 degrees.

I did a similar check with the IAT sensor and found the ECU having values close to the laser gun.

The IAT is combined with the MAF sensor in my truck and I have cleaned it periodically. When watching the MAF with the OBD set to live mode, the OBD would show the value in Red if it is out of specs and it would often be in the Red with a low end value while driving. I have a clean K & N air filter in it, no problem there. I'm wasn't sure how to interpret the OBD results. Frustrated, I went and bought a new MAF/IAT and it gave similar results, no change.

 

OK, I tried the key on key off with the the old and new IAC valves with a cold engine and got zero response with either valve. After I got the engine started, it went to a normal sounding high idle. With the cold engine now running, I pulled the plug on the IAC and got a very definite RPM drop, lower than a hot idle. Plugged back in, and it went back to the high idle. After the engine had warmed up, I tried again but got zero response, no change in the idle. I think it's telling me that it works, and it's only function is to control cold engine idle. I think it also makes a case for no vacuum leaks.

With my OBD set to live, I rechecked the ECT, key on while cold, and the ECU reported a value very close to what my laser temp gun read. I got the engine running after several cranking attempts and let the engine get to full temp. The ECU reported values consistent with a functioning ECT. I could see it peak out a bit over 200 degrees and then drop back to around 180 or so when the thermostat opened. I watched the OBD ECT temps and it rose smoothly as the engine heated up. I think it works as it should.

I had also monitored the IAT, which is packaged with the MAF. With key on only, engine cold, it started out close to actual air temperature, but the value was climbing slowly. I had replaced it with a new one, so I unplugged the one installed in the intake tube, and plugged it into the old one in my hand. It also started at a value close to the ECT, but the temp also began to climb and the sensor felt like it was heating up. With key on only and the engine cold, the ECU was reporting climbing IAT temperature, which resulted in a higher higher value than the ECT. Is it normal for one of these MAF/IAT sensors to heat up?

From your hit list,

I think I can now eliminate the ECT, wich was replaced on 6/23

I replaced the fuel filter 20,400 miles ago on 7/23
I replaced the injectors with OEM parts along with new manifold gaskets 11,200 miles ago on 3/24
I wasn't able to get more than a key on reading with fuel pressure, 57lbs., before the Schrader adapter for my fuel pressure gauge crapped out. It is supposed to be here by the time I return from my trip.

The IAC functions when the engine is running and cold.

The MAF/IAT sensor is a bit mysterious to me and I need to find out more about it. I'm also unsure if it is normal for it to heat up.

Battery is new, terminals are clean, no cranking issues.

I installed a new coil pack which had caused random misfires from two badly corroded terminals in July. I changed out the plugs with OEM Motorcraft SP500 (AGSF 22FM) and new wires 4 days ago.

 

i hope its the Schrader valve does it. im at a loss good job