Hey guys, I drive a 89 f150 with a 302. When I come to stop, the idling fluctuates, ocasionally making the motor die. I have replaced the throttle positioning sensor but that did not fix the problem. What should I do next?

 

does your check engine light come one???

 

sometimes, but not really during this problem

 

The Idle air valves in these things get clogged up with carbon and tend to surge. Also check to see if the idle valve has a plate between it and the intake. It will have two screws on it to adjust the basic idle. Ford had alot of trouble with surging and added these on to help. Don't ask me about adjusting them, mine works, so I just leave it alone. A good cleaning of both these will probably fix your idle.

 

X2 on the IAC valve. I would just replace it, when i cleaned mine it only worked for a couple weeks. I ended up just replacing it and alls well.

 

Thanks kaug, I'll look into the valves and let ya know how it turns out. Thanks

 

Is the IAC valve located on the intake?

 

Here's a photo.

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I've only owned the truck for about 7 months and that valve looks fairly new. I do understand that the valve could still fail. Should I replace if it looks shiny and new.

 

Take the valve off and look at the 2 ports and see how dirty they are first.

 

Rough unsteady ideling is often caused by an EGR valve that isn't closing all the way.

An easy way to test for this is to install a blocking gasket, (one with no center hole) between the EGR and the intake manifold.

 

as dumb as this sounds, on 1990 (and I guess some other models ) burned out taillight bulbs can cause poor idle issues. the BOO ( brake on/off switch ) reads a value, and bad bulbs skews the reading. this is relative to the emission controls. this may not be the answer for you, but check to be sure

 

BOO isn't used if it's a manual transmission. Doesn't need to know about the brakes because the clutch disconnects the drive-line completely. But, if it's an auto, yes, BOO counts.

But, FYI, ANY unmetered air (EGR, broken vacuum line, leaking plenum gasket) will cause a surging idle, and probably a lean-miss stutter when driving at low RPM and/or light load. Any time the vacuum is high, and the fuel is cut back, you'll notice the miss. The fact the idle is surging says the IAC is probably working (albeit incorrectly), and trying to compensate for high engine RPM caused by a leak from somewhere. When it closes off to reduce RPM, the EEC is also cutting the injectors back, which will cause the stumbling. Then, when the EEC sees the low RPM, it'll pop the IAC open and turn the fuel back on again.

FIRST, and foremost, pull the codes and see if the truck's EEC is complaining about anything. If the EEC isn't complaining, count on a vacuum leak SOMEWHERE. Finding it is the biggest task, but identifying it is relatively simple. Don't go buying expensive code readers. If you can count from 1 to 9, and your CEL isn't blown (you said it was coming on sometimes), all you need is a paper clip.

Have a look at my gallery and you'll find some photos of a leaking plenum gasket. That was my idle surge, but I won't say for sure that you have the same problem without the same amount of testing I did to confirm the problem. A vacuum gauge is your friend at this point. Get a good vacuum gauge, and start with vacuum checks. This will require the better part of a day to go through it 100% to identify if you have a leak somewhere, and what that leak is. The fact your CEL light has come on will mean that you have some codes stored. Get back to us with those codes, and we should be able to lead you in the right direction.

 

I'm having the exact same problem. Pulled codes 32 and 33. Replaced the EGR sensor, code 32 disappeared. Bought the new EGR valve, but can't get the old one off! Tried PB and heat, lots of leverage, but it won't budge. Had a couple buddies over to give me a hand, we quit after two hours. I'm not sure what to do next, I guess I'll have to take it in to the shop...

 

Just for grins--when the truck is idling at normal engine temperature and when it is idling badly--pull the IAC electric connector apart and see what happens. If the engine dies--the IAC is closing and there arent any apparent large vacuum leaks.

Do you have a tach--how much is the idle fluctuating?? And how often does this occur?? Do you have the defrost on when this occurs?? The AC will come on when the defrost is turned on and the compressor will cycle on and off.

And my 302 has a switch on the power steering hose that changes the idle speed when the steering is moved.