I ran the running test on my 5.0 85 F150 (EFI) and I got 3 codes:

41, 34, 13

according to the book I have 41 indicates lean. OK, that's the one i Understand.

Code 34 is not in the book I have for the running test. I see a code 34 as "EVP voltage above closed limit", but I think that's a continuous code. Does anyone know what this is? I did a quick search but I didn't find any results for the running test.

Code 13 is listed as "Cannot control low idle check speed during self-test"

What exactly does that mean? Should I care about this?

 

Most codes are the same, engine running or engine off tests.

Code 34 is a problem with the Egr Valve Position sensor. This is a variable resistance sensor that is mounted on top of the EGR valve. Your EGR valve works on vacuum. If you follow the vacuum line from the EGR you will run into a electric vacuum valve. This valve is how the computer controls the opening of the EGR, by controlling the vacuum. Once it opens the electric vacuum valve it looks at the sensor on top of the valve to verify that it opened and how much it opened. Your code 34 means the computer looked at the sensor and it told the computer the valve is already open. It did not like that and so it gives out the code.

Code 13 means it can't control the idle. The idle is set by a valve mounted up near the throttle blade area. The throttle blade is supposed to open just a little bit, not hardly enough for the engine to run. The rest of the idle air comes from the idle valve in that area, and the computer controls this. During the test is closes the valve to see if the engine rpm drops. If it doesn't, it gives a code 13, which is basically saying "I can't control the idle speed like I am supposed to".

I am wondering if this is all caused by a faulty EGR valve, or a EGR valve that is partially open. A partially open valve would cause the 34 sensor error, and also a partially open valve would make the engine run terrible and want to stall. To prevent that someone turned the idle stop screw on the butterfly wide open, and then that is causing code 13 since the engine can't control the idle. And all that air coming in could also cause the lean code.

If you want to do a test, make a metal gasket out of a pop can and sandwich it underneath the EGR to block it off, and see if everything straightens out.

 

Wow. Thank you for the very thorough answer. I will certainly investigate further.

For what it's worth, most of the time the truck runs perfectly fine. I'm troubleshooting an intermittent issue. Also, during the running test the truck did idle up and down, but maybe that's a high idle test -- which I think it another code (12 I think).

I will check the EGR and the idle set screw. Maybe someone opened it all the way.

Thanks again.