'88 302 High Idle
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'88 302 High Idle
Hey all....Need help
1988 F150 4x4 Manual 302
The engine has an Idle of about 2000 rpm.....no change when engine warms up. KOEO test reveals a 12 and 13.....Throttle Air Bypass functioning properly.
With the hood open I can remove the air intake hoses, and cover the two air intakes (I mean totally sealed off) in the throttle body and the RPM does not change....AT ALL! I found this surprising, and I am assuming the engine is getting air from somewhere else(or the laws of thermodynamics are null in my driveway).
My question is where could the engine be getting the air? Could the plenum gasket be shot, enough to let in that much air in? or am I looking at a cracked plenum?
Any help would be appreciated.
Thanks
1988 F150 4x4 Manual 302
The engine has an Idle of about 2000 rpm.....no change when engine warms up. KOEO test reveals a 12 and 13.....Throttle Air Bypass functioning properly.
With the hood open I can remove the air intake hoses, and cover the two air intakes (I mean totally sealed off) in the throttle body and the RPM does not change....AT ALL! I found this surprising, and I am assuming the engine is getting air from somewhere else(or the laws of thermodynamics are null in my driveway).
My question is where could the engine be getting the air? Could the plenum gasket be shot, enough to let in that much air in? or am I looking at a cracked plenum?
Any help would be appreciated.
Thanks
Hotshot
Joined: Aug 2002
Posts: 12,679
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From: BC, Canada
'88 302 High Idle
The most common place for a vacuum leak on a 302 is at the back of the upper plenum where it attaches to the lower intake.
Take some Carb-Medic spray carb cleaner and spray it all around the intake with the engine idling. If the idle speed changes when you spray the cleaner at any particular spot you've found your leak.
Take some Carb-Medic spray carb cleaner and spray it all around the intake with the engine idling. If the idle speed changes when you spray the cleaner at any particular spot you've found your leak.
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Joined: May 2003
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From: San Diego
'88 302 High Idle
I just did this! Same engine, same year, same idle!!!
Take a look back about three days to a week for something like "88 F250 302 high idle; not for the timid", and read the whole scenario.
If you can't find it, the end result came out something like this.
It took me a month to finally find the problem, after learning about all the senders and the engine control system.
Take a vacuum hose with a crevice tool attachment and place the large end to your phone ear and the crevice tool between the throttle body and inside front side (right side of engine, passenger side) of the upper intake manifold. Move it up and down very slowly until you find a spot where it hisses like a snake. I found 4 inches of missing gasket there! If you can't find the hiss at this location, move it around the base of the intake manifold until you do find it.
It is the only place I could not spray with the WD-40. And besides, at 2000 rpm there isn't much increase to hear.
I hope this helps.
Regards, Robert.
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'88 302 High Idle
Hey! I want in on this chat...
An old timer I'm familiar with just went off on the big dirt road that stretches on forever. On the way out the door he threw me the keys to his '89 F-150, 302 EFI.
It ran like a washing machine with a bad bearing so with the basic skills I've developed for general repairs I found the a/c compressor bearing was shot - got a used one- no problem there. Then I changed the wires, distributor rotor and cap and plugs. I also squirted a bunch of carb cleaner through the intakes to clean off accumulated goo. When I restarted it Man! did it sound good! * BUT * (there's always a but on this site isn't there?) the idle is now real fast and I can't seem to make it slow down.
Any thoughts? Could I have the firing order out of whack? Can't find any vac leaks but then in reading this forum I see there are lots of sneaky places these things hide.
Time to learn a new machine I guess.
Thanks guys.
An old timer I'm familiar with just went off on the big dirt road that stretches on forever. On the way out the door he threw me the keys to his '89 F-150, 302 EFI.
It ran like a washing machine with a bad bearing so with the basic skills I've developed for general repairs I found the a/c compressor bearing was shot - got a used one- no problem there. Then I changed the wires, distributor rotor and cap and plugs. I also squirted a bunch of carb cleaner through the intakes to clean off accumulated goo. When I restarted it Man! did it sound good! * BUT * (there's always a but on this site isn't there?) the idle is now real fast and I can't seem to make it slow down.
Any thoughts? Could I have the firing order out of whack? Can't find any vac leaks but then in reading this forum I see there are lots of sneaky places these things hide.
Time to learn a new machine I guess.
Thanks guys.
Post Fiend
Joined: Nov 2002
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From: Memphis, TN
'88 302 High Idle
Pull the codes and test the IAC by the Haynes procedure using a multimeter.
RancheroMK
There is no spec idle speed - the computer controls it thru the IAC, assuming you have no vacuum leaks & the IAC is working properly. Read the tune-up procedure on the emissions sticker in the engine compartment.
RancheroMK
There is no spec idle speed - the computer controls it thru the IAC, assuming you have no vacuum leaks & the IAC is working properly. Read the tune-up procedure on the emissions sticker in the engine compartment.
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'88 302 High Idle
Steve- Thanks for the quick response! Was just down in the shop trying to find anything out of whack. When the engine surges there is a very weak sucking sound at the low rpm before it surges but I can't locate it.
Anyway you refer to the IAC, being a newbie I assume this is an ignition control device? Where is is located?
I am in process of ordering a Haynes manual as everyone giving good advice on this forum tends to refer to it.
Anything else to look for?
Anyway you refer to the IAC, being a newbie I assume this is an ignition control device? Where is is located?
I am in process of ordering a Haynes manual as everyone giving good advice on this forum tends to refer to it.
Anything else to look for?
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'88 302 High Idle
Originally posted by steve83
Pull the codes and test the IAC by the Haynes procedure using a multimeter.
Pull the codes and test the IAC by the Haynes procedure using a multimeter.
Have already pulled codes...KOEO returns nothing, KOER returns 12 and 13.....IAC already tested, OK....
Post Fiend
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From: Memphis, TN
'88 302 High Idle
Dogguy
Until your Haynes arrives, use this site. The IAC (Idle Air Control) has 2 small (8mm=5/16") screws holding it onto the side of the throttle body, and a 2-wire connector on the back of the motor that operates it.
RancheroMK
Until you've positively identified your vacuum leak, you shouldn't buy any parts or do any major work. But you should concentrate on finding it - it's not good for the engine to run lean, or to suck in dirty air.
Until your Haynes arrives, use this site. The IAC (Idle Air Control) has 2 small (8mm=5/16") screws holding it onto the side of the throttle body, and a 2-wire connector on the back of the motor that operates it.
RancheroMK
Until you've positively identified your vacuum leak, you shouldn't buy any parts or do any major work. But you should concentrate on finding it - it's not good for the engine to run lean, or to suck in dirty air.
Last edited by steve83; Jun 5, 2003 at 12:29 PM.
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Joined: Jul 2002
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From: Vancouver WA
'88 302 High Idle
Dogguy, the IAC is the Idle Air Control. It is a small module that is screwed onto the side of the throttle body. It allows the right amount of air to bypass the closed throttle plates when the engine is at idle.
Have not done it myself, but have read here several times that spraying carb cleaner into the intake can damage the gaskets and cause vacuum leaks. How did you check for leaks? Did you do the "spray the carb cleaner around the engine and listen for idle changes" test or the "listen through a hose" test?
Have not done it myself, but have read here several times that spraying carb cleaner into the intake can damage the gaskets and cause vacuum leaks. How did you check for leaks? Did you do the "spray the carb cleaner around the engine and listen for idle changes" test or the "listen through a hose" test?
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'88 302 High Idle
Maybe we're on to something here...
I sprayed carb cleaner into the throat(s) with "relative abandon". I can see where an older gasket might get damaged. I checked for leaks all around the plenum housing and valve covers (where I could get at them) with a piece of 3/4" pvc hose. I also went around to the places I could reach with a can of WD-Squirty. Nada.
As I wrote earlier I think I hear a very small gasp when the idle oscillation is at minimum. I was unable to find it but will look a bit closer at the IAC/throttle interface.
What edition Haynes is "The Word"?
Thanks guys
I sprayed carb cleaner into the throat(s) with "relative abandon". I can see where an older gasket might get damaged. I checked for leaks all around the plenum housing and valve covers (where I could get at them) with a piece of 3/4" pvc hose. I also went around to the places I could reach with a can of WD-Squirty. Nada.
As I wrote earlier I think I hear a very small gasp when the idle oscillation is at minimum. I was unable to find it but will look a bit closer at the IAC/throttle interface.
What edition Haynes is "The Word"?
Thanks guys
