Everettb

Hey guys, I drive a 1990 xlt Lariat f150 with a 5.0 engine. 2wd.
Lately, when I start my truck up the RPMs while I idle, once it's ran for a minute or two, are around 1500 on average. Once I put it in drive it goes down to a more regular number, although still slightly higher than normal driving RPM.
Once I start braking and come down from say 35 mph, once I hit 15 mph the RPMs spike momentarily and if I put it in park or neutral my RPMs race up to around 1500 again and it sounds like the truck wants to race off.
When I let off the brake at a stop light the truck slightly takes off, and sometimes I hardly have to give it any gas and we get up into the 30s sometimes 40s.

Gas pedal isn't stuck, I just cleaned the IAC valve to try to fix it. 6 months or so ago I replaced the TPS also. Please help!

techrep

iTrader: (1)

Replace the IAC and clean the throttle body

Everettb

Thank you. I cleaned the IAC valve and the throttle body and still am experiencing high RPM's when I idle. Just idled at 1500, any other suggestions?

techrep

iTrader: (1)

I said to replace it.. 4 things in mind... you already replaced thr TPS and cleaned the Throttle body.. now replace the IAC .. if that does now solve your problem, then you have a vacuum leak somewhere..

FUfarmtruck

could be timing is off too. 10to 12 degrees over top dead the 302 is happy there.

Everettb

I went ahead and replaced the IAC. Still high idle. Found a few cracks in my vacuum hoses, although I sprayed some carb cleaner on them and the idle didn't shoot up I'm gonna replace them also

FUfarmtruck

have you tried unhooking the cruise control. long shot. but. the cable could be holding the throttle open if its screwy, also the return spring on the throttle body itself. if it lost tinsil it may be just not returning it back to closed. both are long shots, but lol

Toplait

Test for codes to see what sensor might be effected if no codes then continue on.

While it's running if you unplug the IAC does the Idle go to about 600 RPM? If idle drops then that is still your problem. Remove it and tape up the holes or build a block off plate and see it it runs normal. Make sure you have installed THE IAC in the correct direction....Sometimes tapping on the IAC will have an effect and lower the RPM.

I would also check the back of the intake manifold to see if the PCV line came off or is screwed up that goes into the passenger rear of the valve cover causing the intake manifold to suck more air then normal. You can test this by removing that vacuum line from the intake manifold and plugging the intake manifold.


Other then that I would check for other vacuum leaks, test the TPS output, and also check to make sure none of the cables or throttle body are sticking...Also, if some one screwed with the IDLE stop screw on the throttle body you may want to put it back were it is suppose to go.