My 1991 4.9L has had a high idle ever since I bought it from my brother-in-law. My recent manual trans conversion has just exacerbated the problem. It usually idles around 1200-1400 RPM, even with the IAC disconnected and the base idle screw all the way out.

So last night I decided to get serious. I have checked for vacuum leaks many times and in fact every gasket in the intake system has been replaced. Taking a tip from another post I found in a search, I plugged the PCV line and then put a 2x4 over the intake ports. The truck died immediately. So I don't have a vacuum leak. As a test, I tried it again with the PCV in place and the truck would run at about 400RPM so it doesn't take much air to keep it running. Anyway, I was satisfied I did not have a vacuum leak that was causing the problem.

Turning my attention to the IAC, I pulled it and tried a different one from my 302 Bronco. No change. I noticed, however, that when the solenoid was in place, the IAC could not close all the way. As an experiment, I seperated the solenoid from the valve using some tiny washers and an extra o-ring to make it seal. This did allow the valve to close completely. However, when I put the modified IAC on my truck, it made no difference. This confirmed that the idle speed was completely out of the IAC's control. Strike One.

Next I pulled the throttle body off so I could look at the seal of the plates to the body by putting a light behind them. I did not see any light around the plates, but I sure did thru the hole drilled in each one of them. This is nothing more, IMO, than a holdover from the carburetor days. This was a trick that manufacturers applied to get more air into the engine at idle without having to open the throttle so far it would take the carb off of the idle circuit (owing to very lean idle mixtures). But this has NO application in an EFI engine that I can see. If the IAC cannot supply sufficient air to maintain proper idle speed under certain high-load conditions, then you just jack up the base idle screw or make the IAC bigger. There is no idle port to be concerned with. I think Ford made a boo-boo here. BTW my 302 Bronco and my 351 bench engine do not have these holes. So I plugged 'em. I threaded the hole, then I forced a slightly larger screw into the hole until it snapped off flush. This truck is my “test mule” so I have to remind myself that I can do anything I want, even override Ford’s engineering, and it’s OK, it’s not some colossal breach of my Ford-love. Take a breath.

Homer! It worked. Now I can actually set the base idle with the stop screw (which used to have no effect at all). I set it to 500RPM. With the IAC plugged in, the truck idles at 700RPM, just perfect IMO. I pondered these holes in the throttle plates last night and this morning on the way to work, and I still cannot imagine what they were trying to accomplish that could not be accomplished via a larger base idle opening.

BTW, in a speed density vehicle, as opposed to a carb or MAF, extra air, from any source, is supplied with the needed fuel. So a high idle could effect MPG even if it is a vacuum leak. I hope this is so in my case, because I have been disappointed in this truck’s MPG (about 16).

So, there you go. I have seen a constant flow of high idle complaints on these forums, so I thought I better type this up so, if you also have this problem, maybe you can get some relief as I have when you cannot find a vacuum leak.

 

thanks for the imformation...........take care.

 

Umm, I'll beg to differ on the statement that extra air is supplied with the fuel it needs. If it comes through the throttle body or IAC, or through the side of the plenum, or through a leaking seal on the EGR, or ANY place, it may not get compensated for, and may in fact be overcompensated for due to the low vacuum condition indicating it needs enrichment. Up to a point, yes, it will enrich the mix for a smooth idle due to the knock sensor detecting the pinging, or the HEGO indicating there is a lean condition. But, there is a limit to the adjustment, which is usually what causes the O2 to indicate a lean condition.

That base idle screw you refer to is nothing more than a stop for the throttle plates. It should be adjusted so that the throttle plates are closed, but do not bind. The holes in the throttle plate are so that ALL of the idle air does not have to go through the IAC. This permits the IAC to respond to load changes under idle, and readjust. If you adjust the screw, you also adjust the position of the TPS. This will almost guarantee a TPS fault from the EEC. If the TPS output is not between 0.85 and 0.95 volts at closed throttle, the EEC may (and should) trigger a fault code. By opening the throttle plates with the screw, you're actually telling the engine that the throttle is not closed. This may also affect IAC operation. The EEC controls the IAC using MAP, TFI output, TPS position, and the VSS. At anything other than closed throttle, the IAC should open wide open. This is to prevent stalling when the throttle snaps shut. MAP and TFI are used to detect load changes during idle. The VSS will keep the engine idled up when coasting so that it is possible to down-shift as well as avoiding stalling from sudden deceleration.

Mine too has the holes, and is the OE throttle body. Everything except the plenum gasket and rear main seal are OE on my engine. I had fast and rough idle issues, and if you'll take a peek at my gallery, you will see the largest cause of the idling issues. After replacing the plenum gasket, the idle dropped back down to 572 RPM (OE Spec) and cleaning the IAC helped tremendously. If you have to open the throttle plates to idle at 500, you just found out what the holes are for. If the holes were the source of your high idle, then it would have idled high from the day it was cranked up at the assembly plant. I don't think it would have left the assembly plant idling high. There is obviously another issue that you've compensated for by tweaking Ford's design.

I ain't sayin you're a dummy, just agreeing to disagree.

 

I understand that the holes were there from the factory. But how again is slightly opened throttle plates a bad thing? My TPS voltage is 0.89 volts. Something has changed on this truck since it left Kansas City to where the holes are too much. But it ain't a vac leak, and I already replaced the plenum gasket along with every other gasket in the intake system, as I stated. I've even replaced the entire intake manifold, top and bottom.

So has the motor loosened up? Have the bores in the TB worn? Heck, I don't know, but I do know that the holes left this truck idling at over 1000 rpm all the time and out of the IAC's control. If the holes were strictly necessary and the throttle plates MUST be closed, then I would expect my Bronco and my 351 bench engine and my '00 Expedition to have them, which none of them do.

I used to have a 1986 T-bird too, but it was a Turbo Coupe. Under the hood, there was a sticker that gave instructions on setting the "base idle" which was 400 rpm. You had to disconnect the the IAC to do it. So the stop screw is not always used just to keep the throttle plates from bottoming out. Thats the principle I applied here.

Not everything the engineers do is perfect. A lot of it is patch work. Ever heard of a "transfer port" or an "anti-dieseling solenoid". Those were quick fixes because of problems they ran into due to other changes, but they both had to do with throttle plates that were open too far. It was easier to do that than to fix it right, and consequently those things didn't last very long in the design. I think the holes were used to (which was another quick-fix on the carbs as I mentioned) fix an idle problem somewhere along the way. Maybe my truck has had something disabled or something worn out that makes that not necessary. I didn't buy it new.

Hey, I tried very hard to find a vacuum leak that apparantly didn't exist. I replaced everything from the throttle body to the head with parts from another truck, using new gaskets of course. Nothing changed. And I STILL thought it had to be a leak, but the 2x4 test convinced me it was not. So I had to go looking for other issues.

Like I said, this truck is my test mule. I found a pretty easy solution that has fixed the problem and I just wanted to pass it along. If I believed any longer that I had a vacuum leak, I wouldn't have gone this way, but I don't. I am very confident of that.

 

Jass88, you seem to be on top of things with the the high idle issue, mine has had chronic high idle for years now, and when i dissconnect the sensor on top of the iac, the idle goes way down as well, more normal if anything. Would a messed up iac valve affect driveability, creating a lean or rich condition or play any role in detonation? Good to see you solved the problem, theres nothing more satisfying than solving a problem, saving money, and learning something new all at the same time.

 

I should probably clarify, when I say I think Ford made a boo-boo, I didn't mean they sent the truck out idling high. What I meant was their solution at the time, i.e. the holes in the throttle plates, took away an adjustment I need now. Unnessesarily, IMO, and that's why I called it a boo-boo. Maybe an unforced error would be better.

Now on to your problem. When you say the sensor on top, I assume you mean the IAC? That's not actually a sensor, it's a valve. IAC stands for Idle Air Control. But since your idle drops way down when you unplug it, your problem is different than mine. Mine wouldn't change at all which meant my idle problem was not within the IAC's ability to control.

The IAC controls the idle by allowing extra air to enter the intake without moving the throttle plates. Some companies call it an Idle Air Bypass valve, because it just creates a variable bypass around the throttle plates, just like a vacuum leak, only the air is filtered.

So your IAC is the thing that is jacking your idle up to high? How high is it? My old '90 302 idled at 900 rpm, which I thought was high, but my Ford manual said it was normal.

Either you have a bad IAC, or the ECU is calling for an idle that high. The first thing that comes to mind is maybe it thinks the engine is cold? Have you checked codes by grounding the test terminal under the hood? I am wondering if you have a bad coolant temp sensor?

The IAC can't cause a lean condition AFAIK because it goes wide open after you open the throttle. It does this so that it can gradually bring the engine back to idle speed by closing when you close the throttle.

Could you possibly be missing the o-ring between the IAC valve and the solenoid? This almost never happens, but it would cause the IAC to lose control of idle speed when it is actuated since it will open a vac leak between the valve and solenoid when it is actuated. Kind of a longshot, but easy to check. Just a thought.

 

The holes in the throttle plates are more than likely there to soften the throttle response when the plates are first opened.

It sounds to me like you've come up with a band aid fix for a problem caused by a vacuum leak that you can't find. There are more sources of vacuum leaks on these than you can think of adn the 2x4 test won't help you find many of them.

 

If I were sitting where you are, I would say the same thing.

However, ponder this: According to you, a vac leak that adds some 500-700 RPM to my idle speed is not big enough to keep the engine running when I block off air to the TB, but one that adds 250 RPM (the PCV valve) is. How can that be?

Hey I am here to learn. Please explain what sort of vac leak would not keep the engine running when air is blocked to the TB?

 

Because it takes more air to make an engine run than it does to raise the idle. On most trucks, pulling the PCV hose off the intake will cause them to idle about 1400-1500 rpm with the IAC doing everything it can to bring it down. That's a 3/8 hose that has 9 times the area of a 1/8 hose. Two or three 1/8 hoses leaking could easily cause your idle to raise uncontrollably to 1100 rpm, but would not allow it to run. The evap purge solenoid could stick open and cause that much of a leak. Same for the throttle shaft seals. If you ever get to watch a vacuum integrity test done with a smoke machine, you will be amazed at what all leaks and is considered acceptable. EGR valves leak a lot, even when new, and are probably bad enough to get you 25% of the way to your high idle. Those sorts of leaks add up and it is nearly impossible to find them all even with the right equipment.

 

I have pulled every vacuum line off of the truck (except the MAF and fuel pressure regulator) and plugged them and NONE of them had any effect on the idle except the PCV, as noted. The MAF and regulator hoses I removed and tested. There are no hoses that are leaking. I have replaced the entire intake manifold assy. using new gaskets. Your story that I have multiple leaks all of which are combining to make the idle high, but cannot keep the truck running when air to the TB is blocked, is just a shot in the dark from someone who has never seen my truck. I don't know what the explanation is, but yours is reaching.

Oh, and BTW, there is no vacuum applied to the EGR at idle.

 

My explanation is no more reaching than yours is about the holes are causing your problem. The holes are not the problem, they've just returned it to a state that it thinks is normal.

The EVR solenoid doesn't send any vacuum to the EGR valve at idle, but there is still vacuum at the EGR valve in the passage that leads from the plenum to the valve. Vacuum leaks from that passage through the valve on almost every EGR valve I've ever tested.

I won't offer you any more "shots in the dark". You are under the illusion that you have fixed something, so I'll let you continue.

 

I don't want to get into an argument over whether or not a vacuum leak is causing my high idle, but you seem convinced that it is and that is the patent answer when somebody says they have that symptom. It's a vac leak, it is, it truly is, and any other answers will be ignored.

However, what I said was that Ford took away an adjustment that I need now and it is my opinion that they did it unnessesarily. I still haven't heard anyone tell my why adding air by opening the throttle plates versus drilling holes in them is so bad. Y'all do understand that the TPS is adjustable should the opening put it out of range, right?

The fact is that I did fix the problem. I just did it in a way that you don't approve of. I have sat exactly where you are sitting now, and you think I am a big buffoon who has fixed the symptom and not the problem. You may still be right. But I have put a lot of hours into trying to fix the problem the "right way" by finding and repairing a vac leak and I have exhausted my patience. If I took the easy way out (which I don't feel I did considering all the labor I have put into this issue) then so be it, it still fixed the issue, the CEL is not on, the trucks idles perfectly.

I just wanted to pass that along to others who might be in the same boat. Hey, here's a shortcut that might address your problem. That is all.

 

87 302 straight drive high idle fixed! i did the usual things. cleaned EGR, cleaned throttle body, cleaned idle air control, etc. and still it kept revving up first occasionally then all the time so i finally checked the air filter. i dont drive it much and knew i had replaced like 5 years ago. it looked like new on top...then when i pulled it out to look at the dirty side and found a mouse had pulled insulation out of cab and packed tight a wonderful warm nest! he or she had the air ports plugged and chewed up the filter but not all the way thru...so even if you have a newish filter check under it. there were leaves and acorns and the insulation was the real plugger. now it idles great. around here mechanics would have thrown parts at it and usually charge at least their mortgage payment. the problem had been unfixed like 2 years...going down the road and idling at 45 mph (my rpm gauge dont work) heck i was considering scrapping the truck. anyways i got some joy finally.

 

Thank You! jas88... I do not care if it is a bandaid or not it finally fixed my high idle problem. I like you changed parts ( tps, ,EGR, EGR vacuum control valve, fpr, map iac, pcv, gaskets, Hoses. I'll stop here you name it I replaced it. ) , plugged lines , sprayed, ( water, brake cleaner, dam near drowned it but it wouldn't slow down. ) looked, and eliminated all possible leaking points but still no joy. The fix you gave was the fix that finally worked....!!

 

So I’ve been on this board for months. Replaced throttle position sensor, IAC valve, cleaned throttle body, replaced throttle body (sent it back it was worse), check for vacuum leaks. Nothing worked.

Finally I built a homemade smoke machine and saw smoke coming from under the upper intake manifold/plenum. Pulled it off and the gasket was a crispy, broken and brittle. I could see the gasket sticking out after I had the throttle body off and starting to remove intake bolts.

I replace the gasket with a fel-pro kit and it was an immediate fix. I’m no pro but it only took a couple of hours to get it done after months of throwing every part I could at it. The smoke machine was a life saver.

Tip: I didn’t have help sitting the manifold on the new gasket. Take 3 or 4 sharpened pencils cut to about 2”-3” and put sharpened ends into the bolt holes to keep the gasket aligned.