Auto Service World
News   July 18, 2016   by David Leduchowski

COMMENT: Finding Flex Fuel Faults

Driveability issues are almost guaranteed when a PCM comes to the wrong conclusion at the end of a learn process.

Flex Fuel Filler capBy David Leduchowski

You wouldn’t think that the kind of fuel a vehicle uses could have a dramatic impact on its driveability. But with today’s highly complex engine-control strategies, even small variations in fuel characteristics can start a negative chain of events that leaves you stumped.

This was clearly illustrated to me recently when a 2009 Ford F150 pickup came in with an illuminated check-engine light. I didn’t think anything of the fact that it had a 5.4L flex fuel engine.

There were no associated driveability issues, so I hooked up the Ford Integrated Diagnostic System (IDS) and found codes P0174 and P0150 stored in the powertrain control module (PCM).

The P0150 indicated a problem with the Bank 2 upstream oxygen sensor circuit. The P0174 indicated that Bank 2 was running lean and that the PCM had reached its compensation fuel trim limit. A road test using the data scan feature of the IDS showed Bank 2 fuel trim at its upper limit (near 30%), and a slowly switching Bank 2 sensor. Bank 1 fuel trims were normal at less than 5% either side of zero.

2011 Ford F-Series Super Duty: Flex Fuel badge. (02/25/2010)There were some technical service bulletins that seemed relevant. TSB 13-3-5 warned that the PCM should be reprogrammed to the latest calibration for codes P0130 or P0150. And TSB 11-3-27 made reference to lean codes P2195 or P2197, and reminded techs to replace O2 sensors if the truck was built at the Kansas or Dearborn plants before a certain date. This particular truck was not displaying P2195 or P2197, but it did fall into the categories for plant and build date. Based on this information and the slow-switching state of the Bank 2 oxygen sensor, I contacted the customer and recommended replacement of both upstream O2 sensors and PCM reprogramming, as per the TSB.

The repairs were completed, the Keep Alive Memory (KAM) was reset, and the truck was road tested for 10 highway miles. Fuel trims on both banks stayed within the normal range at both highway cruise and idle conditions, so the truck was released to the customer. Another job well done, right?

Not exactly.

I was dismayed to see the truck back two days later. The customer was now complaining of a very poor cold start and rough idle with black smoke. The concern would last for about 50 seconds and would occur only after a cold start. After that, the truck would run perfectly. The check-engine light was not on but clearly there was over-fueling right after a cold start.

But… why?

Another IDS diagnostic revealed no codes stored, but I did see some interesting data. Fuel trims would go positive around 9% on the highway, and turn negative at idle. While those amounts are still within normal parameters, I suspected a mass air flow (MAF) issue. I watched the MAF data during wide-open throttle runs. The data appeared to be normal. Nevertheless, I pulled the MAF sensor, visually inspected it, and sprayed it with a cleaner. But that didn’t solve anything.

I decided to hook up a gauge and watch the fuel pressure. It showed 56 psi on initial start up. Then, after five seconds of running, it would drop to 45 psi. A quick tap on the accelerator pedal would immediately bring the pressure back up to 56 psi. After a few seconds, it would drop to 45 psi again. These drops and increases were instantaneous. The spec shows that the fuel pressure should be 55-60 psi. So did this engine have the proper pressure or not?

It was back to service info to do some research.

Screen Shot 2016-07-12 at 10.30.31 PMIt turns out this truck uses a mechanical returnless fuel system with a module-controlled two-speed pump. Under high fuel demand conditions, the PCM asks the fuel pump module (FPM) for high speed and the FPM delivers constant battery voltage to the pump. Under low fuel demand conditions, the PCM asks the FPM for low speed and the FPM delivers an on/off duty cycle to the pump to slow it down and save energy. The FPM is located under the truck on the frame rail at the rear. The wiring is easily accessible for testing, so I used an oscilloscope to verify that it had a nice clear duty cycle signal in low speed and a solid battery voltage (and ground) in high speed. Clearly the pump was receiving the proper supply… but was the pressure supposed to go lower than 55 psi in low speed?

I decided to jumper the pump at the FPM so that it stayed in high-speed mode and delivered a constant 56 psi, which was in spec. I left it outside overnight and tried a restart in the morning. It still had a terrible idle with occasional stalling and thick black smoke. Again, after a poor start, the truck would run perfectly.

Since I had the IDS installed, I took the truck for an extended road test out of sheer frustration. I was looking through data readings and saw inferred flex fuel (IFF) showing 66% alcohol content. I know the customer does not use E85 fuel, so I’d expect to have 10% or so alcohol content in the fuel. I thought something was amiss with the 66% reading.

A Ford general service bulletin (GSB) states that on 2004 and newer Fords, the flex fuel sensor and module have been deleted and the ethanol percentage is now inferred. Resetting KAM will cause the PCM to initiate a relearn and infer the air-fuel ratio immediately after going into closed-loop operation.

Now everything was making sense! This truck learns alcohol content by watching fuel trim, MAF readings, and injector pulse width. Low fuel pressure will have the same effect on the learn process as high alcohol content in the fuel.

I performed a KAM reset to return the alcohol content to zero, and I left the fuel pump jumpered to high-speed mode. The next morning, the truck started perfectly and ran smoothly. I did a data scan road test with the pump still jumped to high. I drove long enough to allow the “IFF LEARNED” display to switch from “no” to “yes.” The learned alcohol content showed 12%. That’s about right for our fuel. I removed the jumper from the pump and let it function as designed. After another KAM reset and road test (until the “IFF LEARNED” display switched to “yes”), the alcohol content was showing over 60%.

Each molecule of ethanol contains less heat energy than gasoline. For E85 or other high alcohol fuels, this means a richer air-fuel ratio (9.8:1) is required. The inadequate fuel pressure/volume when the pump was in low-speed mode was causing longer injector pulse widths to keep the oxygen sensors happy, thus fooling the PCM into inferring a much higher alcohol content than actually existed. This learned alcohol content caused the PCM to command more fuel overall, causing the over-fueling condition on cold starts. It ran OK warm because as soon as the O2 sensors warmed up enough to force the system into closed loop, the oxygen sensor input gave the PCM feedback on controlling fuel supply.

The remedy was a new fuel pump and a tank cleaning. Now the gauge showed over 55 psi of fuel pressure under both operating parameters. A KAM reset and another completed “IFF LEARNED” road test showed a learned alcohol content of about 12%. The truck was running normally.

The final question is why did the truck not exhibit the problem until after the oxygen sensor concern was repaired initially? If the PCM senses an oxygen sensor issue, it will likely ignore the unreliable reading and not use it to learn alcohol content. The KAM reset done after the oxygen sensor repair forced an IFF relearn. The relearn was inaccurate due to low fuel pressure.

Essentially, by fixing one problem, we restored the PCM’s ability to learn and store flex fuel info, which then exposed an existing failing fuel-pump issue.Dave

As engine control systems continue to evolve, there will be more things to keep in mind when we diagnose drivability issues. Before you start poking around, make sure you know what factors could be complicating the situation.

David Leduchowski is the owner of Integrity Auto Inc. in Teulon, Man., and a member of the CARS Advisory Panel.


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11 Comments » for COMMENT: Finding Flex Fuel Faults
  1. Chad Gagnon says:

    This is some next level information, me and a fellow tech were just going over the same issue today. 2016 F-350 with rich codes for both banks, culprit weak fuel pump, low fuel pressure on the low side. Engineers sure do us favours with systems designed like this, low fuel pressure means less fuel, means LEAN not RICH. This article above is one of the best descriptions I’ve heard, this man is on point.

  2. Tim B says:

    Thank you for taking the time to post this issue, diagnosis and solution!

  3. Phil Groome says:

    Thank you for the post David, this is by far the most informative article i have read regarding this issue, and i see this problem alot at my shop!!! I live at 10,000 ft above sea level, and our thin air seems to cause this problem alot!!!! We usually find a dirty MAF to be the cause, but have also replaced a few fuel pumps, and many upstream 02 sensors….

  4. Chris kratt says:

    David. Dealing with a 2011 f150 5.0
    Flex fuel reading 80% we don’t have ethanol where I live. Low fuel pressure ok. Rich codes, fouling plugs. Black smoke. No vacuum leaks. Maf ok, purge ok. Inferred never switches to learned after keep alive memory reset and road test over 15 miles. Help please. Email preferably

  5. Chris Dykhouse says:

    Thank you. Your article led my independent mechanic to test the fuel pump on my 2013 5.0 f-150. The scan tool always showed a 60% alcohol fuel reading even though there was no E-85 in the tank. The fuel pump replacement and KAM reset caused the inferred fuel reading to be accurate. Gas mileage improved a lot. No more hard starts either. My mechanic has been in business for 8 years, works on every make and model, but has never come across this. You did a very thorough investigation.

  6. rod vanzeller says:

    Great post , thank you

  7. Cole Rempel says:

    Amazing article, and to think I found this on the world wide web written by someone from my home town. Thank you Dave!

  8. Jack says:

    David, EXCELLENT article! I came across this article while looking for information regarding the same Alcohol Content reading for a 2007 Chevy Silverado. Although this was not the problem with the Chevy, I kept this article for future reference for my Flex Fuel Ford Fusion. Thank you for your very precise and informative article!!


  9. 1st of all GOOD ARTICLE.! I just put it in rocker arms and timing chain and everything I just did 15,000 mi ago the mechanic over torque the tensioners and they were leaking and I just had all kinds of issues and so anyway I fixed it and when I go to start it since I replaced the throttle body too I did with the throttle body setup said which is wrong I found out later so now he’s based it and said it again and it turned out my timing’s negative my timing advances negative 24 by map sensors 59 but air flow rate is 1152 my engine coolant temperature was 219 my intake air differs 190 absolute throttle position percent was 50.2 at 4,128 RPMs I put in throttle body plugs coils phasers O2 sensors on the driver side not the passenger side I just did those like a year and a half ago .
    I don’t know if my smoke is black but it looks kind of gray to me but it’s pretty toxic it’s got some flavor to it I have no faults no misfires and it’s burning rich on one side lean on the other , let’s see what else I sure hope enough to keep fixing it I’m a carpenter not really a mechanic but I can’t afford to be taking this to the shop every 20,000 miles and getting a whole new rebuild that’s crazy but anyway so I’m going to try it again like you said I’ve been driving it around they bogs out at first but then it kicks in and runs good then it’ll bog out again especially it stop signs.
    I’m not really sure with those numbers mean but I don’t think they’re running right order if you got any more pointers I’d love to hear them thank you : or 737-500-6772 is my phone whatever is easiest for you

  10. Jade wilson says:

    I have 2 2000 f150 flex fuel fleet (cal trans) trucks I have replaced fuel pump and am having a heck of a time finding the fuel pump driver location. Only one truck is having. The crank no fuel to start issue . I’ve tested the injectors they have pulse , when cranking the pump is off . Jumped the pump and then there is no spark. The inertia switch is not tripped and the pats light is not triggered . I think I need to test the driver as I said … I can’t find it . Ideas?

  11. john cramer says:

    holy cow,
    since we got our exp in 2013, we always had it smell rich on cold startup and a couple seconds of sputter, and it burns the eyes, at 65k we had a cat melt down and clog, covered under warranty, with new cat the car still had the rich smell only on cold startup, since a gas station opened up with e85 at almost a buck a gallon cheaper we switched and never had the smell again until we took a trip and used regular gas. I bet we have the same problem, of course we didnt have the rough idle or smoke to speak of.

    thank you for the artical

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