My truck is a 93 F150 4x4 with a 4.9 and a Mazda 5 speed manual. I have 105,000 miles on the clock. The engine idles pretty well. At just off idle it stumbles. The amount throttle it requires to go say 35 down a flat road causes it. With a little more throttle it steadies out again. Works fine accelerating and on the freeway.

I'm thinking of a few different things. First is a built up crud on the intake. Second is a leaky EGR valve. Third is a TPS with a bad spot right off idle. Fourth would be leaky cracked vacuum control lines. Are there other things to look for?

I would think that a leaking EGR would make the idle wacky too.

 

I'd start with vacuum lines; they're cheap and an easy thing to rule out fairly quickly. After 15 years on the road, they're probably pretty iffy anyway.

If that doesn't do the trick, then try the TPS.

Is the truck throwing any codes at all? Just from what you've given, I doubt it, but maybe it's got one stored that wouldn't necessarily turn on the CEL.

 

All I get is the 11 all in normal code. I haven't checked it for the last couple of months though. I should check it again just to b sure.

 

my truck does the same thing your do, but mine . 95 f 150 4.9 I:m going to do what jnewkirk77 said to do. this might sound stupid,what is a tps?

 

TPS is the throttle position sensor. It's mounted under the throttle bodies, not a big deal to change if you have to ... a flaky one can give you a good bit of grief. I had one in a stickshift Escort go south on me, but all I had to do was let the clutch out and off she'd go again. It did get old after a couple of weeks, so I fixed it.

 

vroom-9 please keep posting so other members will know how you fix the the problem with your truck I will do the same I don't want to take it to the ford dealer,every time I do it comes back with some else wrong with it.

 

Well I ended up attacking it in the order of what the replacement cost. I took off the throttle body and gave it a good cleaning. I put an ohm meter on the throttle position sensor (TPS)and it had some issues just off of idle. The reading jumped around a little. It's a closed unit so even though a cleaning would likely fix it, that was not possible. It was $25 for a replacement part.

Fifteen years of time and the heat of 100,000 miles of driving did a number on the vacuum lines. Those hard plastic ones were pretty much toast. Just about everyone that I touched broke. I replaced all the weathered runner hoses and those hard lines that had issues. It was the colored ones that became brittle and broke. The black ones were fine.

After putting it back together, the stumble problem is reduced, but not completely gone. It happening mostly when the engine is cold. When warm it is mostly gone. I went and got my smog check and passed with flying colors.

I do still have a problem though. I must still have a leaking vacuum line because my idle is up to 1100 RPM. I can't reduce it by turning the idle stop screw. I actually like it though. It was too low before. At idle a hard turn on the steering wheel could stall the engine. Not a problem now.

 

<i>I do still have a problem though. I must still have a leaking vacuum line because my idle is up to 1100 RPM. I can't reduce it by turning the idle stop screw.</i>

You didn't turn that screw did you?

It is not a idle adjustment, it is a stop screw to allow the butterfly to fully close but keep the butterfly from digging into the throttle body.

Check the IAC, try cleaning it and see if that helps. It won't always work, some are to far gone and or have motor problems and not just carbon build up issues on the piston/seat.