I posted this same problem in the '97-'03 F150 forum about a week ago without so much as a courtesy "I don't know." I got 18 views on that post when I checked it earlier today.

The deal is my CEL comes on and stays on until I clear it (duh, read on), but when I go to scan the codes, I don't always get one. I know why it's coming on.... to me, that's a non-issue. I was just curious why the light would come on and not give a code. I clear the memory anyway, CEL goes off, and the second closed loop cycle after starting it, the CEL comes back on - without a code. I'm expecting the same code everytime, but only when I reset the PCM (pull fuse or disconnect battery) does it give it to me.

Anybody have a clue why this is?

-Kerry

 

Did you ever figure out anything, Kerry? Sounds like possibly a bad PCM or PCM ground to me.

 

It's not a bad PCM, Jimmy... it's that I'm experimentin' with a top secret project.

Now before I get a bunch of "don't do that 'cause..." I know, I know, I know. Stoichiomtery is all messed up, I may toast the engine internals because of it, and I really haven't reprogrammed the PCM for the modification. So don't try this at home, kids.

I wanted to see the capability of the PCM in "learning" about an engine modification as well as play with the engine programming. I went ahead and installed a Lightning 90mm MAF before I get a supercharger. I know the PCM isn't truly capable of learning about this, since the MAF curve is a part of the programming and actually must be changed within the flash-rom of the PCM. My understanding of what happens is that the O2 sensors read a lean condition and the injectors' pulse width and curves are then adjusted to match... or get them as close as possible with the installed program. A check engine light or CEL results since the PCM thinks it has an unmetered air introduction... I have the stock MAF curve adjusted as far as the predator will allow (+10%, just a band-aid), I have the WOT fuel tables adjusted to 4% rich, not knowing if they'll help any. That's where I'm at. I could get a book on the stuff, get the wide band A/F metering equipment, get the software and the interface and go into the programming business... but I'm just messing around.

Other mods to note, that maybe involved in this lean code, the aftermarket oil/gauze air filter, custom homemade CAI, and 75mm TB. None of these have ever lead to a CEL, in my experience, when installed seperately or simultaneously.

The Ford programming seems pretty flexible. I'm still down at 4% timing retard with the 91 octane Predator programming running on 87 octane, without a trace of knock. Still getting around 14mpg with the daily commuter duty.

Back to my original question, then -
The expected lean codes come in, I clear the codes. Then after two closed loop cycles the CEL comes on without the lean codes coming back... I pull the PCM fuse, resetting the adaptive behavior, and the CEL comes back with the lean codes. I remove the codes, then again, two closed loops later, a CEL with no code.... I'm just wondering why. Could the codes be stored somewhere else, and the PCM recognizes the codes so it doesn't show them?

-Kerry

 

Can't help you with that one. Maybe Dave (Racerguy) can. I'm not sure why the codes would come and go.



I am kinda curious why the even put MAF on the 05-06 Mustangs, when almost every mod that you make, even a good CAI, requires reprogamming to correct a lean condition. I was always under the impression that that's what the MAF did, recognized the added airflow and compensated for it. That's how it works on my 88 5.0. They could save buyers money and go back to speed density since you have to reprogram the PCM anyway.

 

I would have figured Dave would have mentioned something... I know he's busy, but generally, if a thread doesn't get an answer he steps in. I'll probably have to PM him.

As far as the 05-06 and your 88, right away we're talking apples and oranges. You even mention it, MAF vs. speed density. I think it has to do with emissions, myself. They've got such a close tolerance to put out all they can when stock that they didn't bother to plan on the aftermarket. Why save the buyers money? Many of us just go out and reprogram the PCM anyway...

The MAF doesn't compensate for anything... it just puts out a voltage for the PCM to see, then the PCM adjusts the fuel flow to match... when the O2 sensors read what exhaust gases are left over they also send signals back to the PCM as a second check... when the O2s and the MAF don't agree, the PCM gives you the code. In my case I'm not operating in the window given. The MAF voltage to air flow range is set way higher than a stock 150s with the stock injectors I'm operating with. The O2 sensors see that the amount of air and the fuel mix aren't jiving.. but the PCM adjusts just to limp by and get as close to the 14:1 ratio as it can. I'm just not sure why I get the light and no code.

I do have the curves, just no understandable interface to make them work together. It'll get fixed when I get the supercharger sometime next year, and I get the new program.

Thanks anyway, Jimmy.

-Kerry

 

That's what I meant on my 88 (It has an 89 ECM and MAF) on it. It will allow the car to run and idle properly with opened exhaust, larger cams, better heads, intake etc, without having to get a chip for the ECM. The ECM would adjust itself for the added airflow. I know that the OBD-II is a bit different, and I guess Ford must have really narrowed the window on built in tunability for Emission reasons like you mentioned above.

 

I kinda figured as much, but I would have thought you went with a newer MAF/PCM... but not everyone reading this would know what you have going.
That's the only thing I can come up with... if it wasn't a narrow window then the PCM would have adjusted for something as simple as a CAI.

After I plugged the MAF in or reset the PCM, the idle stumbles at startup for a few seconds... then it'll idle just fine. If I adjust the flowrate for the injectors beyond 23lbs in the fuel tables (I think the stock units are 21lbs, iirc), it acts all funky - go figure. Anything more than a +4% rich in the fuel tables makes the exhaust smell like gas at idle.

Ahh, but the light still comes on, and no one's home.

-Kerry