A few weeks ago I went to start my truck and it just kept stalling. At first I thought battery, then realized that all electrics were working fine, so not battery. Anyways, I had it towed and the mechanic could not find anything wrong with it. Brought it back to me running fine, and its been running fine since. They could find nothing wrong, and it did not show any codes, check engine light is not on.

Forgot about it for a few weeks, and then today went out at lunchtime and the same thing is happening. I start the truck as normal and the idle is very low, talking 500RPM (it usually starts around 1200 and drops to about 1000) and will stall out. The only way to keep the engine running is to keep giving it some extra gas, I managed to drive it around a little, brought the engine up to temperature, but the idle stayed low and it kept feeling like it was going to stall. There is no check engine light, and I would guess no codes again.

What is causing this? The truck has approx 140,000 miles on it and is a 2000 F-150 with the 5.4 V8. I read it could be the IAC? and that its a pretty easy fix, and also relatively inexpensive. If not the IAC, any other ideas?

Thanks.

 

I have a similar problem with my 2000. It could be the IAC, it could be a vacuum leak, it could be the MAF....there are a lot of things it could be. Start with the simple thing first and then work your way up to the difficult stuff. Start by checking your vacuum lines for cracks/holes, look carefully, especially the elbow at the back of the throttle body. Clean/replace your IAC. Clean your MAF with electronics cleaner(do not touch the wire!) Chances are if you get a code it will be 0171 & 0174. Just going off of what has happened to me. I plan on doing a lot of work on it tomorrow, including plugs and COP's. So I'll report back what I find and what I've done.

 

Vacuum leaks cause a high idle , your PCV valve could be shot , but I doubt it and your MAF would generate a check engine light if that where the case .

Your IAC is sticking shut..

 

Thanks, think I will head to Autozone later and pick up an IAC. From what I can tell its a quick and easy job to replace one.

 

Sounds good - A good way to tell if everythings right . After you replace the IAC, reboot the computer and complete a drive cycle - she should idle between 700-750 rpms - it's usually right at 700 rpms..

 

So I have not yet replaced the IAC, but spent some time yesterday with a friend trying to figure out what was wrong with the truck. Saturday morning I went out and tried to start it, it started OK, but was still idling low (400RPM), although it was running without me needing to give it extra gas.

Sunday morning, I went out to try again before heading into town to pick up an IAC, and it would not start at all. The engine would turnover, but just not start. So I called a relative with far more knowledge than me, who came over and checked a bunch of things on the truck (used a scan tool that showed no codes at all). Along with finding a burnt out fuse (tow lights) and a couple vacuum hoses that (while OK) will need replacing soon, we could not get the engine to start.

Used a fuel pressure gauge to check fuel pressure, checked voltages all over the place, but still could not get the engine to start. After a few hours in the cold we gave up. 15 minutes later I hear an engine running, I look outside and see my truck is running. So I head back out and find my relative and the truck running. I ask him what he did and he tells me that he got mad, and simply tried starting it with the throttle all the way open. The engine then sat idling at around 750RPM (normal). We shut it off and started it up a couple of times and it idled fine once (engine had warmed up a little at this point).

The only thing that looked abnormal was the coolant sensor temperature was reading 190F, after a few minutes of the engine being switched off it was still reading 180F and hovered around this temperature (despite it being not much above freezing outside). Is this normal? or could a bad coolant sensor be causing the problem? Would the bad coolant sensor cause the low idle? I am pretty sure it could cause the starting problem, but once we got the engine running the idle was normal.

So could it be the coolant sensor? or still the IAC causing the problem? Or even a combination of both? thanks again.

 

If you are referring to the temperature gauge on your dash, then it would have no impact; however, if the Engine Coolant Temperature (ECT) Sensor is not functioning correctly, it would mess up the engine's cold idle strategy and could cause the symptons you have. I may be wrong, but I do not think the ECT and the temperature gauge sender are the same (they may be - I don't have a shop manual for these OBD II vehicles). Holding the throttle to the floor to get the engine to fire, sounds more like it was flooded. You may have a sticking fuel injector. Good luck.

 

Not referring to temp gauge on the dash, I had an OBD II scanner hooked up that was reading 180 - 190F at the coolant sensor. Usually when this problem starts I do not need to hold the throttle way open, the truck will fire but I need to give it extra gas to keep it running (it sticks idling around 400RPM, but if I hold the RPM around 1000RPM it stays running).

 

Chances are, it's still the IAC valve. Next time, try cracking the throttle 1/4 way while cranking and see if it now starts. An IAC that is stuck closed will not allow any air into the intake past the closed throttle plate. Partially opening the throttle bypasses the closed IAC and allows air into the system.

I suspect you had flooded the engine during initial stating attempts. Opening the throttle ALL THE WAY is the prescribed procedure to clear a flooded engine (see your owner's manual). In your case, it cleared the flooding AND allowed intake air to flow.

Steve

 

Cool, previously when I had started it I could get it to fire and then just give it a little gas to keep the engine running. It was only yesterday after several tries to get it started, that we then tried with throttle wide open.

Is there much difference between a dealer supplied IAC and one from Autozone? And lastly could the IAC only cause the problem intermittently? Last time it did it in October, then was doing it Friday/Sunday and this morning started fine.

 

Temps are normal , these engines run cool .. You don't have a coolant temp sensor on that model. All the ciphering is done through the CHT sensor.

Yea , replace the IAC - Auto-Zone's should be good enough. If it only happens cold before the converters kick in , that's what I would swap out first.

What may be happening , your PCV tube plugs into the manifold behind your IAC . Moisture can compromise the IAC just as well as dirt can. If your PCV line does not have insulator foam wrapping it , your IAC can rust o freeze in cold climates due to the moisture sucked in from the pcv port. That's just a few things that may be going on.

BTW - The Fuel rail turns off after only four seconds on starting if the engine has not fired - it prevents unburned fuel being pumped into the exhaust.

Good Luck

 

Are there different IAC valves for this type of truck/engine? The reason I ask is I picked one up from Napa yesterday. The guy asked what the first letter of my VIN was and I said I did not know. So he said most are usually an L, so got me the part that matched this. I got home and checked, and my VIN begins with an I. So I then looked at the IAC and it appears different, there is about a 1.5" long metal cylinder shape coming out the top.

If you look at the pic of the IAC here the cylinder shape would be at the back (the part you can't see). When I looked at the IAC on my truck I don't see this cylinder shaped part (although it was dark/raining so I could not get right up and look).

I will most likely just return the one I bought to Napa and get the one that matches the VIN code beginning with an I. Just wanted to check here first.

 

Is this a better view? Kind of shows the top....


Can you take a picture of the one you have? I believe mine was part number Duralast TV238, I'll check when I get home. To answer your question I beleive the 4.6 has a different IAC from the 5.4.

 

Ok the picture you posted is what mine looks like. On the picture of the furthest left part of the IAC (it almost looks like a cap in the picture, darker colored small area) the one that I got from Napa yesterday has this extended out about 1.5"

Pretty sure I got the wrong one. Sucks.

 

Got mine from Autozone, have had better experience with the guys there.