Ok, I have an '88 F-150 with a 4.9, 4-spd. when the truck is at idle, it's running at 2500-2900 rpm's, and it doesn't matter when it's hot or cold, a buddy of mine said if my upper plenum gasket is bad, it could run at a high idle, cause he had an 86 with a 302 and 4 spd, and his did the same thing until he got the gasket replaced, but I don't think that's the problem I dont see anywhere that it might be leaking, but there are a couple things broken in the engine compartment, so please tell me if you think this might be the problem, top lid to the air box is broken, and some vacuum lines, but i don't know where they go, they're at the very back of the engine going under the plenum, I think there's only 2 lines that are busted but not sure. but that's about it, either that or I'm thinking the IAC may need to be cleaned, but I can't find it, anyone know where the location is?

macius

 

Has it always done this? If so, I'd look into repairing those vacuum lines.

The Throttle Position Sensor could be bad too.

 

which sensor is right on the firewall side of the throttle body, it's got 2 screws holding it in place, and it has a hole in the center, the engine is out of an 87 f-150, i'll try to get a pic and upload it on here, cause it's kinda hard to describe. I had to repair the orange wire on that sensor though, before i repaired the wire, it idled fine, but ran very rich, and now it doesn't run rich, it runs at a high idle.

 

ok, the sensor that i re-did some of the wiring on is the air intake sensor, which is why it had cost me damn near half a tank going 14 miles, but I still dont know much at all about the 300-6's. Also anyone know where the TPS is on these?, and where can i find a vacuum line diagram? Any help would be appreciated.

Thx,
macius

 

ok, this morning I replaced the IAC and now it idles at 2000, I checked the temp and the temp sensor wasn't working, so I replaced it, still doesn't work, truck is idling at 2400 rpm now.Need help fast before engine blows up.

 

if you have an open vacuum going to the intake manifold the idle will go up. you should fix any vacuum lines that are broken. trace them to where they go and ask about them here. another thing you can do is buy either a haynes or chilton manual for your truck. they will have just about everything you need to know about checking and repairing problems. your truck also should have a computer and will give trouble codes when something is wrong like that. a code reader is about $40. this board is plauged with trucks with idle problems and some linger on and on and on. it certainly isnt like it use to be with carburators where one turn of a screw fixes your idle problem. post back about exactly where the ends of the vacuum lines go to.

 

ok I found out where the vacuum lines come from they come from the solenoid controlled vacuum port underneath the egr valve and EVP. Anyone know where these go? One line is about the thickness of a pen or pencil, and goes around the back of the engine underneath the upper plenum but I can't find out where they go. Anyone have a diagram? Any help would be appreciated. Thx

Macius

Also I replaced the TPS.

 

Can anyone help me with this problem?

 

try disconnecting the IAC. the motor should idle around 650. if it does the idle is being directed by the computer. if the idle stays about the same i would say you looking at a vacuum leak.

 

Very true. Also, just to see, disconnect the vaccum line to the brake booster and plug it, and see if it calms down. If it does, then you might have a bad booster. Also, to verify it's a vaccum leak, disconnect the air inlets from the throdttle body and put your hands over the inlets. With the inlets blocked off, it should stall. If it does, then the problem is not a gross vaccum leak. If it doesn't stall, look into a major vaccum leak, such as the booster. the plenum gaskets, or even the TB gasket. I know that Chevys had this problem with TB gaskets on the older, TB injected V8s. Also, a destructed PCV valve might cause this too, or a leak in it's line.

 

You don't need a code reader. You need a haynes manual, paperclip, multimeter, and vacuum testor. $18,$0, $20, and $29.99 respectively. The haynes manual is optional because you can get better advice here. But, for most people it is hard to get the PC down next to the truck while working, and the replies don't come as fast as they could sometimes.
So, for 70 dollars you could probably sit on the fender wall and troubleshoot the problem with your idle. Don't start tearing the entire Electronic engine control system apart until you have a fair idea how it works, the sensors are all too expensive, and your problem may only be a 20 cent hose or gasket.

Are the vac lines made of a stiff plastic pipe? They should be color coded also, if your lines are dirty wipe them down, the lines can also be spliced together with basic rubber tubing.

 

Check your IAT, and ECT sensors.


If they have shorted out this will make your idle very high.


See if the idle settles down when you just disconnect their connectors.

 

Another dead thread, whatever happened?

 

Say 924X2150, You don't have an F150 with the 4.9 in it by chance do you?

I'm trying to find out what the vacuum outputs are for the AIRD/AIRB valve on a healthy engine (at hot idle). The signal to my bypass valve is off at high idle and the checkpoint tests in the Ford Shop Manual say it should be on. I'm getting the "surge" at part throttle problem and it goes away when I force the bypass on by applying manifold vacuum. All vac solenoids, short to gnd, pwr checpoint tests are OK, it's the PCM sending this signal.

 

Steve1428, I have a 92 4.9L built 9/91 E4OD 2WD completely stock, new smog pump(underneath alternator), and original air bypass valve(ABPV), and original AIRB. My only concern is that the Haynes Book always told me there were two solenoids (AIRB/AIRD) and an air bypass valve(ABPV)and an air divertor valve. I do not have an AIRD solenoid or divertor valve.

I have one solenoid, bracket-mounted on the drivers side engine valve cover to the right of (if you are standing by the drivers side fender) the EVR solenoid. I believe it is AIRB because the vacuum line goes right to the ABPV (over the top of my alternator).The truck was at hot idle and I measured the voltage(reference was chassis) at this solenoid, I had B+on the red wire, and the light colored wire is the return which was also at B+.

I measured the vacuum going to the ABPV at this time and it was zero, the smog pump was operating, I pulled the input hose off the ABPV and felt air coming out. The output of the ABPV was closed(it is normally closed on mine)
To make the ABPV open, I only needed to manually goose the throttle lever on the throttle body. The vacuum at the air bypass valve immediately went to 22inches Hg and opened the ABPV, the AIRB light-colored wire goes to near zero volts when the engine is revved.

Also my ABPV will begin to open with as little as 1 or 2"HG vacuum, it is fully open at 5/6"Hg vacuum and holds this for as long as 5 minutes, I tested it today.