I have an 86 f150 302efi, I've been having sorted idle issues since I bought the truck recently , and have finally decided to buy a code reader rather than randomly throwing parts at her. My KOEO passed with 22 in CM my KOER codes are 12 low rpm at test 21 ect out of test range 41 hegos indicates lean 34 evp voltage above closed limit 13 rpm unable to reach lower test limit. Now I could very well begin changing parts and hope for the best, but I figure I would be much better off taking advantage of the wisdom available to me here. This may be unreasonably simple for some of you, not so much for me. Would my ect be able to create idle issues? When I start the truck, it starts without hesitation but idles I'm guessing around 2500rpm, I have no tach so I can't be certain. After a couple mins maybe 4-5 it lowers to an acceptable range, but begins dropping until it sounds as if it will stall then surges and repeats. And always idles kind of rough, I could smell it was running lean to begin with so I don't suspect my hego sensor is not sensing properly. I'm not even certain what evp voltage above closed limit would indicate, or if it could be the cause of the lean mixture or rpm variances. I unfortunately don't have the money to just replace suspected components so any help, direction, or means to accurately diagnose I could get from you guys would be greatly appreciated!

 

You will get a engine coolant temp sensor failure(21) if you do not warm the engine up before doing the test.

Code 12 is saying there is something wrong with the throttle body, the idle screw is out of adjustment, the idle air control motor is stuck or clogged, or the throttle body is just dirty.

41 is oxygen sensor reading lean like you mentioned

34 is the EGR sensor is not reading correctly.

The first thing I would do is take the throttle body apart and clean it up and it's passages(even though it may say "don't clean"). Then leave the idle air control motor unplugged, and start the engine and set the idle screw till it runs without stalling. Then plug the idle air control motor back in and see how it does.

 

Recently I've thoroughly cleaned the throttle body, replaced the iac, and tps the engine was warmed up prior to the test. When I first bought the truck I noticed the idle control screw turned all the way in and tried to set it as close to factory as possible just by listening to the idle, initially it idled ok just a little rough so I thought the iac was dirty and cleaned that along with the throttle body, with no change I replaced it and that's when the high idle on start up began. Is there a factory specific set point for the idle control screw or is that how they suggest to set it, I was trying to set it with the iac still connected.

 

You have to set it with the IAC disconnected, or it will fight you when you try to turn the screw.

I don't know where you are officially supposed to set it. Most people set them with the IAC disconnected, to where the engine barely runs. Then re-connect the IAC. I have set them that way though, and on a manual tranny truck I had trouble with it wanting to stall all the time, so I kept turning it up till it drove the way I wanted.

If the engine was warmed up, you may need to investigate the temp sensor. If the computer thinks the engine is cold when it really isn't, it will hold the idle high, and make the mixture richer than it should be.

 

Code 22 is the MAP sensor, which I understand will affect the rich/lean adjustments of the engine. I would unplug it, clean the electrical connections, and make sure the vacuum line is good and isn't leaking anywhere.

I once replaced the IAC in my DD without first disconnecting the battery. When I started it up, it idled at something like 2,000 RPM.

Point being, what I've learned... Anytime I now change the electromechanical devices (TPS, IAC) I'll first disconnect the battery, this lets it forget its settings and makes it go re-learn them (which has a code by itself but it's just an informational message).

Franklin is correct about everything, set the base idle speed with the IAC disconnected such that the engine barely stays running, might even die if it's cold.

Fuel Injection Technical Library is a great site to learn stuff from.

 

Classic symptom of a bad MAP sensor.

I have come across several trucks that ran just like that, replaced the MAP sensor and all was good.

Your other codes may be stemming from that.

 

I have had the battery disconnected multiple times since replacing iac and so on to correct electrical issues created by the PO installing an aftermarket stereo, which was hacked beyond belief. So it should definitely have had a chance to relearn its parameters. The code 22 is in continuous memory so its not an active failure, although its something I will continue to monitor, its not failing at time of the test although the symptoms are still there, so I chalk that up as an intermittent fault until it completely fails. As for the idle set I'll go try that now and see if it helps, and check electrical connections. I appreciate the feedback guys!

 

That's what I was curious about, whether the low/high idle codes or lean mix code can be caused by a failure that wasn't found or in active memory at the time

 

Ok so I just set the idle. Initially starting it without iac connected it stalled, I turned in the set screw 3/4 turn and started it again it sustained idle as soon as I connected the iac I heard it blast open and my idle went high. So I had adjusted it too low to compensate for whatever the actual cause is, visually checked and wiggled map, egr connectors. What vacuum should i be seeing at the map sensor on start up?

 

The MAP sensor has got an electrical plug and a vacuum line connected to it, make sure that line fights tightly and hasn't fallen onto a hot object and a hole melted in it.

Fuel Injection Technical Library » Manifold Absolute Pressure Sensor (MAP)

 

Sorry I haven't posted an update lately I've been busy with life. I got around to replacing the map sensor, the egr valve, and coolant temp sensor. I disconnected battery leads over 30 mins while replacing parts, seemed to start up with exact same symptoms. I drove the truck 12 miles I think to allow it to relearn and pulled codes again. Now the codes I'm getting are 12, 16, 13. KOEO and CM both 11 system pass. Truck still idles rough but idle is much more consistent and doesn't start with high rpms.I don't have a tach on this truck so its hard to say exactly what my rpms are but seems good to me, I know the 16 could possibly mean idle too low to perform hego self test still smells like its running lean tho. I don't know where to go from here. There's a loud tick from what seems to be an injector but is louder than I've heard an injector before, my exhaust also doesn't seem consistent seems to have momentary spurts. Not sure if any of that is relevant but I figure the more information I give you the easier it might be for you to help me out. Before I go buy a fuel pressure gauge, could this all be related to low fuel pressure? 16 in my chiltons book indicates no signal from IDM curious as to what that could mean.

 

I don't know what an IDM is... scratch that, Google to the rescue, seems to be Ignition Diagnostic Monitor circuit failure. Don't know anything more than that, don't know what such a device would look like or how to test it.

But yet I also find some stuff saying it can also be related to the idle speed perhaps being too low and not being able to be set correctly, at least for some small engines in Rangers:

OBD-I Diagnostic Trouble Codes

Let me ask... Did you goose the throttle at the start of the test (the code reader should have told you to do that early on)?

Have you ever cleaned/checked/replaced the TPS?

You might be able to borrow a fuel pressure gauge from a parts store for free.

You can use a length of rubber hose to act like a stethoscope and narrow down the source of the tick.

 

In the book for the code reader it lists the 16 codes as rpm too low to perform hego test, idle rpm high with isc retracted, or rpm above self test limit with isc off, but my chilton book says IDM circuit failure although the code reader book lists IDM circuit fault for cars only and I'm not sure I should trust the chilton book, there's a lot of misinformation in it. I did perform the wot test and the knock sensor sensed properly etc. And I have replaced the tps within the last few weeks or so. I have a stethoscope seems to be an injector on the drivers side of the fuel rail can't pinpoint specifically but I ran into a few write ups with similar problems suggesting low fuel pressure to be the culprit, not exactly the same symptoms as my own but similar. Which could give me the lean mix and stumbling and ticking even in theory that is not sure how the computer would try to correct that tho

 

I dunno... if it were me, I think I'd go down the injector road if I knew for certain it was an injector causing the noise, then pursue the idle speed stuff (if it reappears).

There is a lot of (mis)information not only in Chilton but on the 'net and manufacturer documentation, too. Hard to know who/what to trust sometimes....