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Idle problem CNG related? Help!

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  • Idle problem CNG related? Help!

    I have a dedicated cng Ford (2003 F150 5.4L V8 108k miles) that the idle rpm seem to vary or surge, more so when the engine is warm. Doesn't matter if its in gear or not. RPM seems to drop low/rough for 2 seconds and then up to "normal" for 5 seconds. When coming off a 65mph interstate drive, sometimes at the first stop the engine will almost die, but automatically rev up to catch itself, then continuing to do its variable surge idle as described above. I have no check engine light, so I assume it hasn't thrown a code (unless the light is burned out!!)

    Things I have tried:
    1. Checked for vacuum leaks, found a small cracked boot on pcv valve line. got a new boot, did not help. Possibly more leaks, but unlikely.
    2. New coalescent filter.
    3. Cleaned fuel injectors - using motorcraft PM5 injector cleaner. Placed inside the coalescent bowl as per ford tech service bulletin (TSB_03206.pdf) Seems to have a little more power now, but did not rough fix idle.

    Things I have not tried but could be the problem?
    1. Replace o2 sensor(s) I can only find 2 and they are pre catalytic converter. I thought there were 4. Anyone know how many their should be on a dedicated cng? Should I replace all of them? or is their a way to test for a faulty unit? Should they be replaced with stock or CNG specific sensor?
    2. Bad fuel regulator?
    3. Bad injector(s)?
    4. Bad TPS (throttle position sensor)?
    4. Any ideas of your own?

    Thanks in advance for anyones help

  • #2
    Re: Idle problem CNG related? Help!

    Replaced both o2 sensors. Didn't really fix the idle problem 100% like I had hoped, but it seems better. However I did find my over 70mph has really smoothed out! Before it felt a little rough and like I had to push the pedal a bit more at those speeds. (I used a Bosch 15717 aftermarket part for $48 ea @ autozone. The length of the cable was exactly factory length.)

    I did discover that the idle problem is pretty smooth with the A/C off. But once I turn it on, the resonant vibration / rough idle comes starts is surging again.

    any ideas?


    • #3
      Re: Idle problem CNG related? Help!

      Have you cleaned the Mass Air Flow Sensor?




      • #4
        Re: Idle problem CNG related? Help!

        I checked the connection, but did not clean it. How do you go about that? MAF is the one in the filter chamber? or past that on the way to engine intake?


        I found this site that explains it very well.
        Last edited by CONEBOY; 07-31-2008, 11:45 AM.


        • #5
          Re: Idle problem CNG related? Help!

          LOL I gave you the procedure in my post.


          • #6
            Re: Idle problem CNG related? Help!

            oops! thats funny!! Thanks.


            • #7
              Re: Idle problem CNG related? Help!

              OK so the mass air flow sensor has been cleaned. Really didn't look that dirty. I'm surprised at how clean it was.

              So what is next? I bought spark plugs but have not installed them. Suggestions?


              • #8
                Re: Idle problem CNG related? Help!

                The plugs could be causing the problem, but I would guess a plug would misfire more at high speed than idle, the same with the coils. These are some thoughts, and just that...
                The Ford CNG diagnostics service manual I have states that the CNG is harder to fire than unleaded, which could cause bifuels to run okay on regular gas, but miss on CNG if the system isn't up to snuff.
                On a lot of the newer computer controlled vehicles, there is provision to increase idle when accessories, like a/c and power steering cause a slight drag. If you think about it, not only does the a/c compressor put a drag on the engine, but when it's on, it also creates a bigger drag on the alternator. When your are idling, the engine is putting out a fraction of it's rated horsepower and torque. However, the load from the a/c system is pretty well the same whether 70 mph or idling. To me, this would mean the a/c system uses a higher percentage of the available power at idle than at speed.
                Next time you have the problem, turn your a/c on high, then turn your lights on bright, and turn the steering wheel. If it affects your idle, it is some sort of problem with response to loads at idle.
                If you put a little more pressure on the accelerator pedal at idle, does it seems to help to bump the idle a touch? Try taking the transmission out of gear (with your foot on the brake, of course). This will also lessen engine load. Then turn all the accessories off. What you are doing is to see if your system is responding to the various engine loads idle.
                Even if you can't repair this your self, it will give you a better description for the service tech if you take it somewhere.
                I don't see how much mileage you had, but my son's bifuel truck had a problem. He changed the plugs and the seals on the injectors. He said his seals were pretty bad and broke up when he took them out. I am thinking he has about 140,000 on his truck.
                These are just some thoughts, and might not be accurate.
                Good luck with figuring this out, and let us know what you find. There are probably several others with the same problems.


                • #9
                  Re: Idle problem CNG related? Help!

                  CNG Fords are known for burning up coils. If you notice the problem on CNG but not on gas, I'm betting it's the coils. $75 each Ford, $50 each Autozone, or $10 each on EBAY. While you are in there change out the plugs so you don't have to do it later. You may want to buy extra if you are buying from EBAY, because they will burn up again.

                  2003 Hummer H2 bi-fuel
                  2000 GMC Yukon XL bi-fuel
                  1999 International 4700 dual-fuel
                  2007 Chevy Avalanche bi-fuel

                  FMQ2-36 Fuelmaker w/ 24 GGE cascade


                  • #10
                    Re: Idle problem CNG related? Help!

                    If you are concerned about the TPS, use a Fluke 88 in record mode, KOEO, move the throttle slowly to WOT. If there is anything but a slow steady rise in voltage, you might have a TPS problem. The same process using a lab scope would nail the problem if it exists. I've changed a couple of TPS's on my 460.

                    You could also do pressure drops on injectors. OTC makes an injector pulsing unit. The pressure drops should all be vary close. this would get you started on a regulator pressures checks. This will pick up a leaking or partically plug injector. You can use the lab scop on the injectors check both voltate a current ramp.

                    The ignition coils have been a big problem as mentioned earlier

                    I don't what equipment you have access to, but I hope this helps.



                    • #11
                      Re: Idle problem CNG related? Help!

                      Well I had to take my dedicated F150 into Larry Miller Ford for another problem. I had the tech look at my idle problem while there. Here are the results:

                      Computer hooked up to engine and logged the following on a drive. No trouble found. No codes thrown. Running a little lean at idle, nothing to worry about.

                      Hrm... I would say then all fords have a weird surging at idle then. And we will all have to live with it...


                      • #12
                        Re: Idle problem CNG related? Help!

                        Have you checked the idle air control (IAC). You can pick up some oil residue from the PCV system. The dirt can cause some weird idle problems. Try cleaning it (don't soke the solenoid) and replace the base gasket.



                        • #13
                          Re: Idle problem CNG related? Help!

                          Hi I,m new to this sight. A few minutes ago in fact. I have a 2000 ford 5.4 bi-fuel and I have a ruff idle also on cng.
                          I changed the plugs and it helped a little. So I did some research and found out about the coil pack and not hot enough spark on cng thing, so I ordered some off ebay. I installed them at my parents house an hour and a half away, and on the drive home that is the best it has ran. They worked well for about 200 miles. And now its back to the same thing. If you do the C.O.P's I would recommend just spending the money. I would go with the MSD or ACCEL packs on ebay for 300 dollars or go to autozone for 100+ each. They are alot hotter than factory for spark. This is my next thing.


                          • #14
                            Re: Idle problem CNG related? Help!

                            I have a 2001 Ford E-350 5.4l CNG. The engine idle speed is very inconsonantly erratic. Sometimes driving around is fine but others the engine speed is high (maybe 2000 rpm) when stopping and putting the van into park. Just wondering if you found anything out?


                            • #15
                              I'm having a problem with my Transit idling too fast