Code recovery, ECM, MPG. Oh my

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May 24, 2026 | 08:11 AM

b00mhower

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Code recovery, ECM, MPG. Oh my

Hello all, recently started driving my 91 F150 (300, E4OD, 3.08 rear, 2WD) again, until gas hit $5/gallon and I did the math on my fuel mileage...9 mpg, went back to the cobalt for a bit after that math.

That led me to see if I had any codes stored, my dash is inoperable at the moment so I can't jumper the self test and count flashes. This hasn't bothered me since I still have a speedo, and standalone oil press and temp gauges.

I didn't have an analog meter handy so I bought the Ford obd1 scan tool aaaand nothing ... It will initiate the self test, relays clicking and what not but no codes. Same story with the analog meter.

I have 5v at the self test connector but something isn't happy enough to flash my codes

Question being could this all be related? The dash issue and the no codes? ECM issue? Seems unlikely seeing as how the truck runs and drives seemingly fine except for my fuel mileage. Any ideas appreciated, thank you

May 24, 2026 | 08:30 AM

90project5.0

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If the scanner isn’t even giving the 11 code, then you need to inspect the computer itself. That’s a sign of it being bad.

Remove it and open the case. Look for leaking capacitors.

May 24, 2026 | 08:34 AM

b00mhower

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Quote:

Originally Posted by 90project5.0

If the scanner isn’t even giving the 11 code, then you need to inspect the computer itself. That’s a sign of it being bad.

Remove it and open the case. Look for leaking capacitors.

I figured a failed computer would present with more symptoms than just not being able to pull codes but I'll have to look into it

May 24, 2026 | 09:00 AM

90project5.0

Fleet Mechanic

Joined: Oct 2025

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Quote:

Originally Posted by b00mhower

I figured a failed computer would present with more symptoms than just not being able to pull codes but I'll have to look into it

Usually, not necessarily though. It will be the computer or wiring from diagnostic port if you can’t get codes.

May 25, 2026 | 09:19 AM

manicmechanic007

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Joined: Jul 2020

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From: Near Salt Lake City

If it runs, it hasn't failed
There is a procedure for "no codes outputted" in the service manual
Could be as simple as a fuse out for the DCL of VIP self test connector
1991 PCED will have some info and the service manual too
I'd start with A EVTM and look over the circuit
I have a 1990 but it's probably different

May 26, 2026 | 03:12 AM

My4Fordtrucks

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Joined: Jun 2015

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Quote:

Originally Posted by b00mhower

Hello all, recently started driving my 91 F150 (300, E4OD, 3.08 rear, 2WD)…my fuel mileage...9 mpg

Question being could this all be related? The dash issue and the no codes? ECM issue? Seems unlikely seeing as how the truck runs and drives seemingly fine except for my fuel mileage.

Something is wrong with the EEC system. My ‘87 Bronco (302/C6) would get 14-15 mpg. Your truck should get fuel mileage in the mid to maybe upper teens.

May 26, 2026 | 03:38 PM

GoinBoarding

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Joined: Mar 2010

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From: Laramie, WY

Certainly different as my truck is a 1996 with obd-II, but it had a failed internal circuit in the PCM that made it lose Keep Alive Memory. So every start it was in relearn mode, with poor fuel economy and low power. New PCM and issues were fixed. It cleared codes every restart as well. It ran well enough that I just thought it was getting tired, not so bad I immediately knew there was a problem.

Last edited by GoinBoarding; May 26, 2026 at 03:40 PM.