Erratic Idle and Hesitation upon acceleration


Old 05-12-05, 08:35 AM
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Erratic Idle and Hesitation upon acceleration

I have a 1990 Ford F250 with a manual transmission and a 5.8L engine. The engine idles erratically after it gets good and warmed up, or oddly enough, any time it is above 70 degrees or so outside. It speeds up to about 1100rpms then comes back to 750. It also hesitates and stumbles on the low end when I accelerate just after shifting, but only after it's warmed up. Any ideas about what to chase first?
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Old 05-12-05, 09:58 AM
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Were the '90 truck engines, at least the 5.8s, still Speed Density SEFI?
I think they were
The wandering idle is common on those (SD) engines, and even if it's Mass Air, some of the the things to chase down are the same

Pull the codes
The EEC IV computer does store trouble codes and won't always throw a Check Engine Light when it does
Be sure to do this before unhooking the battery, as that will reset the computer
The codes may point to something right away, or you may have none
But even no codes tells us something
And it's the best place to start

Clean the throttle body with carb cleaner (unless there is a sticker on/in that says it's ceramic, but I doubt it)

Check the voltage on the Throttle Position Sensor
You'll need to find what the specs should be, I think it's .95-.99V, but don't quote me
If it's wandering while you check it then it's bad anyway (check it with the engine off)

The Air Idle Controls go bad alot, and get gunked up alot
You might want to pull it off and clean it

Look for a vacuum leak
They are not always obvious
The underside of curved plastic/rubber tubes is common

If it is Mass Air then check the sensor and see if it's gunked up
Clean it with carb cleaner
(I use sensor safe just to be careful)

Then there are some sensors, but you don't want to go there w/o some codes, or other evidence, and checking out this stuff first
Old 05-12-05, 11:12 AM
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Thanks for the guidance. I'll have to figure out what SEIF vs Mass Air is and which I have then go from there. I appreciate the leads.
Old 05-12-05, 11:41 AM
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Sequential Electronic Fuel Injection or often called EFI
I don't think even the trucks still had carbs in '90
Or Throttle Body Injection
I'm pretty sure you're SEFI
A metal plenum (I think plenum is greek for thingie) on top of the engine, looks like tubes could be in there
Might have a plate that says 5.8 on it

Early Ford EFI's had "guessed" or implied the amount of air going into the engine
It was called Speed Density management
It often has a wandering idle and burps, hiccups, loping, and other such annoyances
Later they added a Mass Air Sensor which actually measured the air going in
This helped with a lot of these problems
Trucks did not get the Mass Air Sensor till a few years after the cars did
I don't recall which your '90 has

Speed Density:
The (big black) tube from the Air Filter housing goes right to the Throttle Body on the engine

Mass Air:
The (big black) tube has a Mass Air Flow sensor in between the Air Filter box and the TB
The MAF sensor will be a box-like thing attached to the tube
It will have a wiring harness attached
You can detach the air tubes and remove the unit, but you'll want to get the actual sensor out...if you have one

SD=straight shot from filter to engine
MA=box and wires in between
Old 05-12-05, 04:06 PM
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Thanks for your help! I did a crash course on the internet about speed density vs mass air. My 90 5.8 is speed density. I am going to replace the idle air control valve/motor tonight and go from there. I'll probably try to pull as much of the air intake "thingie" off (looks like the coupler where the two air intake hoses connect comes off) and clean inside the best I can.
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