So, about 2 months ago I picked up a 91' F150 ExtCab LongBox, it has the Windsor 302 and the M5OD-R2 Manual tranny. Looking at about 165,000miles. Runs like a beast when I want it to, and sips gas at 17mpg with the 5-speed when I'm easy on her.

It runs great as she sits now while driving, idling, and it does alright coasting in gear...without the IAC hooked up, but with the IAC hooked up, (a new one to be sure it wasn't specifically the issue) it jumps up and down between bogging down and 1,200rpm at idle. With the IAC disconnected, after the engine is warm it idles just fine at 650rpm...

Question #1: I'm under the impression that the Intake Air Temp sensor is bad, hense why when the IAC is connected it idles strange, could that cause it? It's also hard to start and smells like the 88 octane I fill it with when starting.

Also, the stranger problem of which I have no idea what could cause it is, after the engine is warm/as it's getting to operating temp, the check engine light comes on for a few seconds, then turns off... then every now and then it'll come back on when I'm up in the higher rpms right before shifting (like the 2500-3000 mark). It also occasionally comes on when I hit a bump...

Question #2: The CEL always resets when I restart the truck, and it usually turns itself off seconds after it coming on, and by the time I make it to the shop to check the codes, there aren't any stored... What could be causing this...? Because it runs just fine for the most part, chuggin along like he tank these things are known to be.

Any help would be quite appreciated, I've been doing work on the suspension and steering of this rig as well as some body/electrical (lighting), and next summer I plan on upgrading/updating/revamping the motor completely. She's my project, and I plan on keeping her around for as long as she'll keep on truckin.

 

The PCM stores codes in Continuous Memory (historical). Unless someone removes the battery lead, clears the codes or you have a bad PCM the codes will remain.

How is the shop trying to read the codes?

You can do it yourself with a piece of wire: Fuel Injection Technical Library » How To Run a Self-Test

The symptoms you describe lead me to believe the PCM is getting bad sensor data back therefore it is driving the IAC in an erratic manner. Retrieving the codes and properly diagnosing the problem is the key here without tossing parts at the issue.

 

I put a code reader on it and it doesn't show any stored codes after the light comes on and off.
And also turning the truck off and back on always turns the check engine light off if it is on. Which that doesn't really seem normal to me, with my 05'Taurus you have to erase the codes for the light to go off.

 

Is it "no stored codes" or "no stored problem codes"? At a minimum you should get either a 11 or 111 (depending on vintage) in both KOEO and CM, both codes indicating that there are no problems found during KOEO and there have been no problems noted during operation.

 

Many times the fancy code readers, yes I have one too, may not communicate to the PCM due to poor connection at the DLC connector. Try using some contact cleaner and/or a small fingernail file to clean the pins then retry. As noted previously there is a difference between getting no stored codes and the all clear code of 11 or 111.

 

So I ran it again and did the KOER test as well and got a few of the 2-digit DTCs, I don't have them on hand but wrote them down, had 6 of them, 4 of which had to do with EGR codes between 29 and 41. Which leads me to believe it needs a new EGR valve and valve pressure sensor. Had one code for air injection solenoid failure, could be due to the IAC being disconnected. And one for vehicle speed sensor malfunction, and seeing how my speedometer drive gear slipped down the output shaft in the T-case and isn't giving the driven gear a reading.

 

It would behoove you to note the codes by number and when they were displayed, i.e. during the Key On Engine Off (KOEO) tests ((HIGHLY suggested you start here)), Continuous Memory (CM) display ((occurs after the KOEO tests)) or during the Key On Engine Running (KOER) tests ((fix the KOEO/CM codes before running these)).


Code 29 is a CM only code: Insufficient input from vehicle speed sensor. You already know your VSS gear is fubarred.

Code 41 is a CM or KOER with relatively the same definition: No HEGO switching detected always lean. HEGO = O2 (Oxygen) sensor.

Neither of those codes have anything to do with EGR operation.

Keep in mind the codes are what the PCM thinks is wrong. Blindly replacing parts without further diagnosis will result in a seriously lighter wallet.

It's a process of checking codes then diagnosing what it causing them. Some basic tools such as your code reader, a cheap Digital Volt/Ohm Meter and some hand tools are a very wise investment before tossing parts on.

Another piece of advice: Warm up the truck before running any of the tests, otherwise you will have false sensor out of range errors.

 

Now that I'm home and was able to grab my results from my truck.
I hooked up a reader to the truck, one of the ones that an Oreilly's/Shucks has.
Truck was at operating temp because I had been driving it all afternoon, but also take into account the outside temps recently haven't broke 10 degrees. But this also was happening before winter kicked in.

As soon as I hooked it up and set the ignition on, the P0420-Catalyst System Efficiency Below Threshold (Bank 1), code came up and that was it for initial read.

I then ran the KOER test and it threw DTCs 82, 29, 31, 32, 34, and 41 in that order.
DTCs 31, 32, and 34 all point towards the EGR and the sensor on top of the EGR, and I've had 4 high mileage rigs of this vintage 89-91 and I've replaced EGR's on 2 of them...
And you're saying 41 would be aiming towards my O2's?
And is 82 likely due to the disconnection of my IAC?

 

Don't know about all of your egr stuff.

But,

41 seems to mean just that -- the O2 sensors think that the you're running lean --at least some of the time. Might be because of the disconnected IAC.

There's a site (hopefully still there) called oldfuelinjection.com that has some good info. It used to have more, but Ford made them clean up some copyrighted information.

Is it possible that you have a vacuum leak as well?

Good Luck,

hj

 

Thanks HJ, looked at the site you sent and it was definitely a good read.

I don't believe I have a vac leak, I'm sure it's possible but Id have to look it over, and I'm sure I would've stumbled across a leak by now, I cleaned my engine compartment with extreme scrutiny right before winter hit, cuz up here you have to worry about glacial silt anytime you hit the off-road, so cleaning everything afterwards is absolutely necessary.

I'm pretty sure my 41 and P0420 codes are because of the EGR and IAC not functioning correctly, and the 31, 32, 34 codes are saying the EGR is failing.

And with information gathered from the site you sent me, my IAC cold be malfunctioning due to the fact that my VSS is not functioning and not sending my computer data.

 

So with how it's looking with this information, this should hopefully be an easy fix.
I fix my VSS drive gear.
I replace the EGR Valve, clean out the EGR tubing, and replace the sensor.
And I reconnect the IAC, which with the VSS functioning correctly it should idle more consistently.