I have a '98 F150, 5.4 L. Recently the throttle had been sticking, so I figured it was time for a cleaning. I got the tb gasket from the dealer, pulled it off and used an aerosol tb cleaner. I also disconnected the battery so that the computer could learn the new idle conditions.

Now that it's been a few weeks, I'm noticing some odd behavior. Every time I start it, the engine revs up to 1500rpm and then slowly drops back to curb idle. Also, it idles at 800 rpm in neutral/park, and 600 rpm in gear. Previously it would always idle constantly at 600 rpm. Otherwise the idle is very smooth and the throttle very responsive.

Is this normal, or have I messed something up? I didn't touch anything but the tb, didn't even see the idle air control. The only thing I can think of is that previously the small air bleed in the throttle plate was clogged solid, now it's free and clear.

You'll have to forgive my ignorance with this engine, I'm used to old GM products

Thanks for any suggestions,
-Michael

 

From my experience it's normal for a vehicle to jump up in rpm on idle, and slowly come down to the set idle rpm on start up. Also I never payed that close of attention, but now that I think about it I do usually notice a little lower rpm when shifting into gear because you are putting more of a load on the engine. Then again I know more about working on airplanes than I do the inner workings of a gas engine.

 

Do this, comp might be a little buggy - Reset your the KAM. Disconnecting the negative battery cable and touching it to the positive terminal.

Keep Alive Memory(KAM) It's your PCM's short term memory,a kind of capacitor that stores things like idle data, fuel trims, trans tables, etc. It's a good idea to reset the KAM whenever you fix something engine or trans related, so the PCM will go into adaptive strategy and relearn to your latest repairs.

Touching the negative battery cable to the positive post won't result in sparks or fire. It'll just clear the PCM's memory

But hell my 98 idles at 7 - 800 hundred , that's normal. and it idles up when it's first started - part of the testing procedure , it should do this every time.

 

Everytime I start my truck it revs to 1500, settles to 850-900 depending on outside temp. Drops down to 600-650 when in gear. So yes, this is normal.

 

my 97 4.6 may be getting old and weak but she has always fired up to 1500~ and dropped to 1100~ while warming up. then idled round 700-900 depending on temps and such. been so for 40k

 

Thanks! Sounds like I'm just not used to this new idle I will try clearing the KAM and see what happens. I did have the negative cable disconnected, when I did the cleaning, and it did idle high and rough for a short while when I restarted.

Previously, once it was hot it would idle at 600 rpm in neutral, and also in gear. When I put it in park, the idle would increase, then drop back to 600. Just the clogged tb, I guess (also my perception, because my Chevy stuff likes to keep the idle speed constant in and out of gear).

 

Your idle should be roughly the same in gear and out - if the TPS is set properly and the IAC is working properly.

There are 2 settings in the PCM for idle - in gear and out of gear. It is the IAC's job of keeping the idle where it is supposed to be.

I changed my TB just last week. I too had a screwy idle situation. Fought with it for 2 days...

I bought a new IAC from Ford ($110), re-adjusted the TPS correctly and the truck idles properly again.

I think the IAC for my truck is more than a "regular" F-150 - they had others for $60 or so, but not for mine...

Good luck.

 

Actually sounds normal, if anything it might slightly change as time goes on. I remember the first time I did mine, toothbrush, tbody cleaner, the whole bit. Did not disconnect battery that I remember and it idled funny for a while too. I think it revved high for a bit, then low for a bit. This was mainly in park. After a while it got it's head together and ran fine. You're idle in respective positions sounds right on the money, 600 is great in drive, around 750-800 in park/neutral.

 

The normal idle speed is set in PCM program at about 650 rpm +/- about 50 rpm or so.
Whenever any change is made to an existing operation, the PCM has to relearn.
The first thing to check assuming there are no air leaks, is that the throttle stop is not holding the plate open an excess amount. This will not allow the idle to come down to design value if it is adjusted to open to far. It must be set below the normal idle speed so the IAC has some dynamic range to operate over.
To check this on a fully warmed engine at idle, remove the IAC connector. This should make the engine idle go low into the 550 range. If not adjust the stop for this approx idle then reconnect the IAC. Now the idle should pick up to near design value.
If all is well, reboot the computer then start the engine with a full load of lighting and a/c or heater motor.
This begins the PCM leaning process under the highest loads normaly experieced by the engine. Seveal restarts and about 5 miles of driving with some restarts should nearly complete the PCM learning period.
If it dosn't work out then something is faulty or out of adjustment.

The idle normally will go a bit lower when the trans is put into gear from the loading the converter causes and may be even more on a cold trans when the fluid is a higher viscosity before heating up from driving..
The engine should not idle at 800 + when fully warmed. This causes to much creep at stop lights etc that should not have to experienced to excess.
This procedure applies all the way back to 1985 on all FI engines.

 

Wanted to update on this problem:

I finally disconnected the battery and discharged the Keep Alive Memory. Then I switched on the headlights, a/c, high blower, and started the engine. This time all my radio presets were gone, where last time they had remained. Now it idles correctly, the idle stays around 700 rpm once it's up to temp. Also no more hanging at 1500 rpm when I restart the engine.

Thanks again to everyone for helping me with this, I'm glad it was such an easy fix!

-Michael

 

Good to hear Mike.

A procedure has to be done to address the interactive learning that goes on with the PCM.
We will award you a Assoc. degree for this work.
Good luck.