Reciently my ford 302 EFI with 273,000 miles on it has devoloped some strange symptoms that seem to happen randomly. It all started after the truck sat for about 5 days due to my being sick. It started up fine but as i was driving down the road and let off the gas and was coasting the engine would die and then keep running. So it would slow down from the resistance of then engine and then the engine would speed up again and this would happen a few times in a sort of cycle. When I hit the gas it would seem to go away. Then it started dying down at a stop light. It sounded like it would if I shut off the key then turned it back on before the engine would stop turning. Then it stalled completely out. So I let this go on for a few days thinking it would fix itself but of course it didnt. Oh yeah and during these episodes the check engine light would flash or just stay on. So i read the codes and come out with some egr codes and throttle position sensor codes. So i decided to replace so things. First the ignition module then the coil. to make a long story short $114 later it still does it. The guy at the parts store said it could be the idle air control motor but thats $92 so i thought i'd get some other opinions before I go spending anymore money.
Thanks in advance

 

Disconnect your idle air control and drive it and see if it does it. You might have to adjust your idle after you disconnect it eith up or down so that it is normal around 700 rpm. If the problem stops this will probly be the culprit. But it could be a combination of things. Keep us posted

 

Wait, so did you replace the EGR valve and the TPS? This really sounds like a TPS issue.

 

The idle air control will have this effect on it as well.

 

I didnt replace the egr unless by replace you mean just put it back on which i did. And for some reason the problem hasn't happened in the past couple days. Oh and where is the TPS? Thanks for your help

 

OK...couple things.

There are two tests that are done to check computer codes...

KOEO/CM (Key on, engine off/continuous memory)
KOER (key on, engine running)

You'll get codes, and you should post those code #'s here for us as each code has a specific procedure to follow to fix it.

The EGR is probably not your problem - it's probably a malfunction with the TPS (throttle position sensor). The TPS is located on the bottom of the throttle body, it has three wires - one is a ground, one is a 5v reference, and one is the signal wire (.98 or so volts to about 4.5 or so volts).

You never just buy a part and hope it fixes the problem. Reason being because you can test the part to see if it's actually the problem before spending money on it. You can hook up a voltmeter to the sensor and read it's voltage to see if it's working. The sensor could be perfectly fine and you might have a bad wire between it and the computer - replacing the sensor won't fix that but will only burn a hole in your wallet.

I don't know why you tried to replace the coil and ignition module, ain't gonna do anything for ya in this situation.

Next, not to be a jerk but there is no such thing as an idle air control motor - it's idle air control valve. This is mounted to the side of the throttle body held on by two 8mm bolts. The engine shouldn't really run with this item disconnected. You can unbolt it and use some carb cleaner to clean the gunk out of it - that may or may not fix the problem.

I'd be checking the TPS voltages to find out if the sensor is bad. At closed throttle the voltage should be around .96 or so volts, not over 1v and not under .90 for proper operation. You can open the throttle by hand and observe the voltage - it should peg out around 4.5v at full throttle - and watch the voltages during movement to see if you can find any voltage spikes. If the voltage suddenly changes and isn't a fluent gradual increase/decrease with throttle position - it's junk and needs to be replaced. If it checks out - then I'd be looking into checking what voltage you're getting at the computer to find out if there is a bad wire between the sensor and the computer. That would mean the sensor is working correctly - but the computer still isn't seeing the appropriate voltage.

I highly suggest you pick up a Haynes manual as the correct instructions for fixing this are in that manual - as well as how to fix just about anything else.

 

Well i thought the idle air control motor was the servo that opened and closed the valve and tomorrow ill check out the TPS and when I get back to school after christmas break I'll read the codes and let you guys know what they are

 

You'll be able to look inside the IAC valve to see what it does - it divers air in the throttle body when the computer is trying to control the smooth idle for the engine. I believe it is considered an actuator.

 

well I finally got a chance to work on the truck today and it was mainly because it started acting up again so i checked the voltage readings on the TPS and then seemed a little iffy so first i unplugged the IAC and the truck still ran like crap. Then I unplugged the TPS and other than the truck idleing a little low it ran great. So then i took it on a test drive and it ran as good as could have been expected with the TPS being unplugged so. I went out and bought a new one for $50 and had to pull off the throttle body to put it on which i should have known i would have to do but anyway its on now and the truck runs great for having over a quarter million milles on it.
Thanks everyone for your help

 

When the IAC is unplugged, the engine will idle crappy and probably die out. The IAC is what the computer uses to smooth out and fiddle with the idle.

As to why the engine ran better w/ the TPS unplugged...

The EEC has a memory of drive cycles. I think it's 40 or more drive cycles at a time. If a sensor fails or starts showing unusual readings compaired to the last 40 or so cycles - it will switch into limp mode and reject the current sensor reading and replace it with what it normally saw during the last 40 cycles. So when you unplugged it, it lost that sensor input and over-rided it with previous memory. It can also rely on pre-programmed parameters for it's sensors and how to run the engine.

That's most likely what happened...

 

Well when I did unplug the IAC I adjusted the idle so the engine would run at a normal rpm

 

What do you mean by adjusted...

 

well i put a screw in the throttle body in place of that threaded stud so i could turn the idle up and down

 

The idle set screw shouldn't need any adjustment. Did you set the TPS voltage when you installed the new sensor or did you just screw it on and go? The TPS needs to be turned/set so that the voltage output at closed throttle is about .96 volts. If this isn't set right, it could cause the low idle.

 

The 88 computer has an idle normalizing procedure above the base idle set with the screw on the throttle linkage. Once you get the components operating properly and if you need further work on the low idle I can post the steps out of the 88 manual for the 5.0. i have only done it on a 300 six.