I have an 88 F250 7.5. The last couple months I have been having an idle problem. When the check engine light is on the engine runs great. I have only had the truck for a few months.

Here is the story. The check engine light is on almost all the time. The PO made a mess of the engine, disconnected almost everything to make it run better. Yea. (all kinds of codes) I have almost all of it hooked back up except for the EGR circuit. vacume lines gone. No vac line to EGR or EVR. EGR position sensor is plugged in. The only code I am getting now is a code 31.

BTW Major tune up, filters, plugs, wires, Cap, rotor, O2

Now for the problem. When the check engine light goes OFF, very randomly, just for a few seconds, and after the engine is warmed up, I get no idle at all. It is kind of random. It did it a lot this morning and on the way back home not at all.

I do not think it is Idle air control related. It is very hard to get the engine to idle with the pedal. When it acts up, I can't even get it to run under 1500 rpms. It just falls right off. If I hit the key it fires right up, and then then dies again. I need maybe 1/4 or more pedal to get it to kind of idle, then it has really high rpms (with me trying to feather the pedal). Then when it feels better (check engine light comes on) it runs fine.

If I am driving down the street and see the cel turn off I know if I slow down the engine will die. Weird. I just ran the codes a little bit ago and 31 is still the only code. KOEO and Memory.

Now that I think about it, I wonder if it could be a TPS problem. But it is not showing any other codes. ???

 

Well... Just an update. At least checked the code 31 out. Some help from previous posts and from the net, found out the signal should have a voltage of about .4 volts. Checked it was about 0 then moved some wires, went to .4 or so, then to 5.0. Hmm EGR position sensor had a good crack in it. Replaced it with the one from the junk truck... better but voltage still erratic. Cut off the connector from the junk truck and replaced it. Works great solid voltage and check engine light off for now. AND engine still running.

BTW while going over the wiring I found the +5 ref wire had a factory splice in the harness. All the insulation was gone. Taped it up, wondering if there are more???

 

I just replacd the Idle Air Control valve on mine and it gave the perfect idle! Autozone $44.99 and 10 min. to install.