Idles rough and surges when started

CJT

MEMBER
1991 FORD F-250
5.8L
V8
4WD
MANUAL
136,000 MILES

I recently bought this truck (four speed with OD) with 136,000 miles. It came with a folder full of maintenance records, and has had quite a bit of work done, i.E ball joints, shocks, cap rotor, plugs and intake gasket etc. When started, it idles very rough, but smooths out if left to sit a minute. However, when taking off, it starts idling rough, and wants to stall. If stalled it takes several times of cranking to restart. When driving slowly in first or second it surges and idles up. At highway speeds it drives fine, but when slowed down or in neutral it will idle around 2000 rpm. If its shut off hot it idles normally until the throttle is used and then up comes the idle. When I first bought it once it was warm if it high idled I could tap the throttle and the idle would come down, but that seems to have stopped working. The IAC valve has been replaced, but I tested it anyway and it read 9.4 ohms which is within spec according to my Chilton manual. The vacuum lines to it look good but I do not have a vacuum gauge yet so I cannot verify that. I am looking for suggestions as to what I should test/look into next.

1 Reply

STEVE W.

MECHANIC
13,414 POSTS

From the description it needs a throttle body cleaning and has a vacuum leak. The leak is probably between the upper and lower intake manifold. The gasket can leak and as the engine warms up it masks the leak.
Get a can of carburetor cleaner and spray around the intake manifold gaskets when the engine is first started. If you hear a change in engine speed you found the leak. Do not go crazy with the spray just use enough to test. Make sure you are out in open air and have an extinguisher handy just in case (never needed one but).