If you can solve this you are a real pro – page 3 – ford truck enthusiasts forums gas after eating yogurt

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You are correct – the "EVAP Vapor Management Valve" (sometimes called the EVAP Purge Valve) on the firewall is N/C. The EVAP Vapor Vent Valve at the other end of the EVAP system (beyond the carbon Canister) is N/O. So, you are correct that a leak from through the EVAP purge valve would be laden with fuel vapors from the tank & charcoal filter and would drive Fuel Trims toward the negative side. (opposite effect of what we’re seeing).

Reading back over prior posts and your good diagnostic efforts still ‘points’ in the same direction. The PCM is ‘marginally’ able to satisfy the desired fuel/air ratio based on O2 sensor feedback – but in order to do so is requiring significantly lengthen injector pulse widths on both banks (at the brink of Lean codes P0171 and P01740, which HAVE occurred, but not often – and that occurred only after changing fuel injectors and MAF sensor.

NO vacuum leak. MAF readings are normal (~7 g/s idle up around ~ 60 65 g/s revving). O2s11 and O2s21 replaced and normal readings. Fuel rail pressure normal ~ 39 psi, throttle snap increases it normally. This STILL sounds like inadequate fuel delivery or under calibrated injectors.

Throttle limiting or power limiting logic in the PCM has been suggested (Ie possibly ‘traction control). For additional facts to apply to that theory, it would be "VERY INTERESTING" to see 1.) what short term fuel trims and 2.) Throttle position [OBDII PID 0011] are doing when the truck experiences the ‘bogging’ or lack of power (that you mention most often occurs after deceleration or gearing down for a stop). The PCM will SHUT FUEL OFF during deceleration, and monitoring them should show O2 readings go to near zero volts and short term fuel trims go VERY negative. It could be informative to know if fuel trims go back to positive when you accelerate from the stop after that – ESPECIALLY when the severe loss of power happens.

I’ve researched the theory of power limiting or torque limiting a little further. IF happening – I think it should show up in one of the two above OBDII signals. Although there can be several sources causing the PCM to do it. Traction control being only one. There are several ‘FLAGS’ available via OBDII but in some cases will require reading specific PIDs and ‘flag’ bits within, OR are ‘coded’. I don’t know if your reader has that feature.

I don’t know what the hell happened. I was getting email for every response to this thread and then they stopped. I assumed no one had answered and since I don’t drive the truck in the winter and I didn’t have anything to add, I never came back to it.

I ended up putting it in my house garage where I can concentrate on it now. I changed back to the old injectors and pulled the plugs, since they had over 40K on them and there is yet another updated plug SP546. I wanted to give it a compression test, but had to order the adapter for this style plug. When I gave it a compression test, I can’t get good repeatable readings on 7 of the 8 cylinders. I’ve done hundreds of compression tests and I’ve never had this problem before. The needle doesn’t come up steady and I get a different reading every time I do it. Most of the holes are around 120 psi, except one hole I get 190 psi good and steady. Every time I check the other 7 holes, I get different readings and the needle doesn’t come up like any compression test I’ve ever done. It’s hard to believe this motor would have anything other than great compression. It’s been meticulously cared for, religious oil changes, never over heated, doesn’t use a drop of oil and no leaks or blow-by. I only checked it for peace of mind before I went and put all the new phasers/chains/guides/oil pump on a screwed up motor.

I am hesitant to squirt oil in the cylinders to check for ring seal because the plugs are such a bear to change and would not want to put the brand new plugs in after doing it. Right now it’s in my garage with the plugs and coils on the counter and I’m contemplating what to do next. I have had motors with worse compression than this though, and never run anywher as close to as bad as this one. I’ve done some research on compression tests with this motor but nothing really grabbed me as being similar to my experience.

Anyway, I finally put it all back together with the original injectors and I cleaned up the spark plug shanks with a wire wheel and put them back in. I drove it today, about 50 miles. My CEL is now off and it actually ran decent. It did not bog one time off from idle. I averaged 13 1/2 mpg when I wasn’t even getting 9 mpg before. It seems fixed but I’ve thought that before. Only explanation i have is that when I replaced the MAF and injectors at the same time, I fixed a lean condition caused by the MAF and caused a lean condition with the wrong injectors. It still doesn’t seem burnt up with power, but I was never impressed with it to begin with and because the problems, I have become very used to driving a 2016 F150 with a 5.0L, so it’s hard to judge. My 5.4L 2V Expedition also runs better but it’s a 98 model so weight difference may be an issue. I used to average about 16 mpg with the Navi so I still think there may be more problems, but I’ll have to drive it more to get a better idea of the fuel economy.