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Surging / Lunging on Idle (petrol only)

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  • Surging / Lunging on Idle (petrol only)

    Hey gang, just joined the club and looking for some epic expeditions throughout the country!!! Already I've found heaps of great info on this site, thank you all!

    Need some advice here. I just bought a 2000 Pajero Exceed Dual Fuel. I checked it out before purchase, test drove it, everything sounded and felt great.

    Needed a blue slip in NSW, so insisted on one before purchase. Owner took it in, shop A noticed a small gas leak, but shop A didn't service gas leaks. So owner took it to shop B, which fixed the gas leak (by replacing the gas lock, $250 part and service), and ultimately I got my blue slip and bought the Pajero!

    The first time I drove it after having purchased, I noticed something. When running on petrol, the Pajero has trouble on idle - it will surge forward briefly, as if thirsty for more petrol. RPMs occasionally drop below 500, and car will die on idle. Once moving, it runs smoothly on petrol and I don't hear/feel it coughing, but the initial movement (forward or reverse) can sometimes cause it to surge forward/back and die. This is concerning, as I can't have it surging/dying when I'm in sand. It is dual fuel and, importantly, it runs great on LPG - idle steady at 1k RPM.

    I did not notice this issue when test driving the vehicle (twice). And the owner swears that he'd never noticed the issue (which I believe).

    I've searched around this forum (and others) and it looks like there's no clear culprit, but the most likely would be:
    a) idle control motor (ICM)
    b) idle speed control (ISC) needs cleaning
    c) mass airflow sensor (MAF) on the intake pipe could be loose or dirty
    d) hose from throttle body to purge solenoid is loose/disconnected
    e) cabling for cam shaft position sensor exposed/loose/disconnected

    Less likely candidates:
    f) faulty fuel pump
    g) faulty fuel regulator (just before the fuel rail to the injectors)
    h) blocked fuel filter
    i) injectors clogged

    So, my question, before I go back to shop B (which fixed the leak): what is reasonable to ask of the shop? I went in a week ago, they took a quick peak, and suggested that it could be so many things that it's not worth trying to get to the bottom of it, and to live with it. And I tried to for a week. But it's irritating, and I don't want to deal with this on sand. There seems to be a pretty clear cause and effect (issue did not appear until they fixed the gas leak), but I can appreciate that sometimes fixing one problem results in a new issue.

    Suggestions? Go back to mechanic and ask them to check their work? Cut my losses and try a new mechanic? Live with it and use LPG on sand (or other times when I can't risk the surge)?


  • #2
    I assume by the age of the vehicle that your gas system is not a direct injection. Accordingly, the LPG system will run virtually as an independant system, and if you are lucky will use the Oxygen sensor in the exhaust to control the stepper motor in the gas control. That would explain why it runs so well on gas, but not on petrol.

    First thing I would check is for an air leak. If air leaks into the inlet manifold, the Oxygen sensor will be frantically trying to adjust mixtures and you will get surging. This will be worse at low revs and idle. At higher revs, the greater airflow coming in thru the normal channels will drown out any minor air leaks, and the engine will run reasonably well.

    Check the airflow meter - it is located at the exit of the air cleaner box. It should have a clear central area with an aluminium honeycomb mesh in the outer sections of the meter. When the engine backfires on LPG, the first thing to go is the air cleaner box. The lid lifts up. The aluminium mesh is put there (I think) to locally increase the airflow past the main sensor in the middle, and a backfire soon distorts the mesh, even to the point of it blowing totally out of the air cleaner box. I panel beated the mesh several times in my old NL and eventually I lost a couple of pieces of it, so I made up an orifice plate to simulate the friction losses of the mesh. I tried to buy some more mesh, but the guy I spoke to at Injectronics in Melbourne said that they were very scarce, and a lot of Pajeros were running around with no mesh at all. So I left it with my mod and it ran OK - not that I used petrol all that often. LPG Pajeros are very sensitive to ignition lead problems, causing backfires.

    I would at this stage tend to discount petrol components as the problem. If you have a blocked fuel filter, it will probably idle OK, but as soon as you ask it for more power, it will starve of fuel.


    • #3
      Thinking about it further, if you have a minor gas leak into the intake manifold, that will increase the fuel misture (make it rich) and could cause surging as well. Since someone fiddled with the gas system, maybe that is a problem?


      • #4
        Gday Mike

        I see you are in Sydney, as I am.

        The forum members are very fond of Artarmon Automotive in Artarmon.

        I havent used them, but do know that they have their own Pajero, and do lots of work for various forum members fitting long range tanks and the like.

        Maybe worth a trip and have a chat to them.

        2011 NW Pajero, White, Ex NSW Police vehicle, Smartbar with 9000lb Warn winch, Lightforce spotties, Britelights light bars, Tinted windows, Tekonsha brake controller, GME TX3400 UHF, Uniden AM CB, Scanguage, Garmin GPS, LEDs throughout, Milford cargo barrier, Bushskins bash plates and sliders, Tigerz 11 awning, Wetseat seat covers, Tigerz11 roof rack customized,Cooper STTs, Ultimate 2" lift, Airtech snorkel, 90l Flexitank, 3" custom exhaust, DP Chip, Pioneer Head unit/DVD player.


        • #5
          Has the battery been removed? sometimes the re learn is done quickly on gas/petrol but the ISC didn't get a chance to finish
          Old, NP Exceed 3.5 petrol, ARB Bull bar, Warn winch, Bush Skinz Bash plates ,2inch lift, Safari Snorkel. ***Now Sold***
          New, RG Colorado Dual Cab Tray


          • #6
            Thanks for the replies, all!

            To my knowledge, the batter has not been removed. Although it does have dual batteries and, once I get this figured out, I'll be keen to pick everyone's brains on how to test these out.

            I took at back to the mechanic, an LPG specialist. He said that a leak coming into the intake manifold would effect both petrol or gas, and quickly ruled that out. He took out and cleaned the mass airflow sensor. That improved things 75%! He bemoaned that anything further would take patience and money, trying several fixes before we got it right.

            He said that often with dual fuel, people run the gas too much and it dirties / dries out the fuel injectors. And that's what he suspects has happened. So as a (cheap) next step, he suggested using a petrol injector cleaner next time I fill up and hope that the fuel injectors get cleaned up. If that doesn't work, we can try having the fuel injectors cleaned, but that will take time and won't necessarily fix the problem.

            75% improvement is a step in the right direction. The Paj has surged a couple times since, but hasn't died. And that's definite progress!

            Will keep y'all posted on how the injector cleaner goes.


            • #7
              Hi everyone,
              I am experiencing same problem on my 2000 nm exceed 3.5L auto V75w fitted with impco kit.

              Initially, I had cold start issue. Took it to a battery shop and they changed my battery. Was running smooth for a while. I always run on LPG. While on freeway, I had to switch petrol when gas reached empty.

              After 15-20mins drive I heard a huge pop sound, lunged for 5-10seconds and ran smoothly. Switched to LPG and drove after.

              Next day, I switched to petrol and it's idling and lunging for couple of minutes and killing itself.

              ​​​​​​Checked airbox and found honey comb stuck in filter like a golf ball. Interestingly my previous owner put a pop-off valve type adjustment into air intake hose past the Mass Airflow Sensor. Still honey comb popped out, airbox clips broken.

              Gave a bit care:
              Changed diff, engine and transfer case oil.
              ​​​​​​Sprayed on MAF sensor with CRC cleaner.
              cleaned manifold flap where the air intake pipe connects.
              added fuel injector cleaner additive.
              Disconnected battery, touched both terminals after disconnect and put them bath to reset.

              This time I put in petrol and car started fine. Lunged for couple of minutes and it idle smooth. Ran it for 15 mins. Took for a spin.

              Now when I stopped and put gear in Park rmode. Same issue Agail, car lungs and killing.

              Is my MAF sensor and airbox are the issues?? How to test MAF. Airbox looks tight and can't feel any air leaks.

              Should I check my leads, sparks and coil?
              If I had to change leads and sparks; what people using? As I can see Repco silicone spiral leads on mine
              Should I check Fuel filters?
              Should I check my car intank fuel filter?
              You do not have permission to view this gallery.
              This gallery has 3 photos.


              • #8
                Welcome to the LPG problems club. I used to have a NL Pajero which ran almost exclusively on gas. I went through many iterations, and I found the best arrangement was as follows:

                High Tension Leads: NGK leads were the best - anything else failed for various reasons. I used some Aussie made leads (cannot remember the name now - SuperRoo??) and they were absolute crap. They caused a massive backfire which blew the air filter box apart, including the honeycomb. I panel-beater most of the honeycomb out with a 2 inch nail, but one bit was lost. I mused as to what the honeycomb was there for, and concluded that it was there to induce turbulence at low airflows, so the airflow meter could work properly.

                I also tried Bosch, but they were too long, and they drooped onto the rocker covers and shorted out to earth, also they touched each other and caused cross firing. Cross firing when you have an intake manifold full of gas makes a big BANG. They also glued the plug ends onto the plugs, making it difficult to remove them without destroying them.

                Ultimately, I went back to NGK leads. They are the correct length, and more importantly, they are numbered which makes it easier to get the correct lead to the correct plug when assembling the manifold again. It is critical with LPG to route the leads such that they do not touch each other or touch any point of earthing. To test the HT leads, put the car in a very dark place, open the bonnet and start the engine. If the leads are breaking down, you will readily see them.

                Spark Plugs: I found that Iridium plugs were the best. When an electrical discharge (a spark) occurs, you get a small amount of metal removed from the electrode. With a spark plug, the spark prefers to jump from a sharp edge - ideally a pin point. This is why they used to recommend that you filed the plug electrode such that it had square edges. Later plugs had multiple tips so that you always had a sharp edge somewhere. Modern plugs like the NGK Platinum plugs have a smaller electrode (0.6 mm diameter) which comes nearer a pin point. Iridium plugs have electrodes 0.4 mm diameter, which is even better. Modern plugs also are plated with more durable materials which resist erosion a lot better than the copper or steel electrodes of old. Obviously the smaller the diameter of the electrode, the more susceptible to wear it will become with time, but I found that after about 50000 km in my Pajero, the plug gap was essentially the same as when I installed them, so I expect that they would be OK for a100000 km life.

                Champion recommended that for LPG use, the gap should be reduced by 0.1 to 0.2 mm because the LPG/Air mixture has a higher resistance than the Petrol/Air mixture. Yes - the engine will work with the standard (1.1 mm ) gap, but it makes it easier for the spark to jump the smaller gap and thus is reduces the voltage required to make the jump. This reduces the stress on your HT leads.

                Heat range is another issue with spark plugs. If your Pajero is not burning oil excessively, you should run as cold a plug as possible. Hotter plug ranges are for when you have a lot of oil being burnt and the hotter plug tends to stay cleaner and foul less. If you have a higher heat range plug, the probe tends to bend away from the centre electrode, thus opening the gap and requiring higher voltages for the spark to jump at the plug. I ended up buying my plugs from RockAuto in the USA because they were cheaper but more importantly, NGK didn't list the heat range I wanted in Australia.

                Honeycomb: Looking at your photos, I cannot see the honeycomb, and I suspect that is the reason for your engine stalling at idle etc. At higher airflows there will be sufficient airflow for the AFM to get a decent reading. The designers put these things there for a purpose, and I think that is the reason for the honeycomb. I bashed mine back into shape and for the missing piece, I made up an aluminium plate with a lot of small hole, designed to try to emulate the losses and turbulence created by the honeycomb. I never had idling problems so maybe that worked?

                I think that your idling problem on petrol is caused by the lack of the honeycomb. The engine will ruin OK at higher revs because there will be enough turbulence for the AFM to get a more reliable reading. The engine ECU takes readings from the AFM, the engine speed, barometric pressure and the oxygen sensor in the exhaust to determine how much petrol it should inject. If your AFM is giving funny readings, it will throw the mixtures out of whack at idle, but when there is enough airflow, all will be good again. Have a look for a new AFM, but make sure it has the honeycomb because it is important. Alternatively, you could do what I did and try to repair the honeycomb and make sure that the mesh is in front of the AFM element. I put the aluminium plate on the side where I had lost the mesh just to try to avoid the airflow bypassing the AFM element. it seemed to work for me.

                Regarding fuel filters, it is unlikely to be your problem, although not totally out of the question. On my wife's TF Magna (it was also gas/petrol), I did have a problem with the in-tank filter. However, this problem arose when I was out overtaking a B Double and the engine died on me. I switched it back to gas and got past the truck OK, but it scared me somewhat. I did have to get the injectors cleaned on the Magna because it was running very rough on petrol. The difference was amazing after that. With the Pajero, I fitted an isolating relay in the petrol pump 12 V line so that when the beast was running on gas, the petrol pump was not running. This allowed me to run the petrol tank dry when outback, while keeping a full tank of gas as a reserve. If you run the petrol tank dry, you run the risk of ruining the petrol pump because it wills till run, but have no cooling (the petrol cools the motor). That is why they recommend that you keep at least a quarter of a tank of petrol at all times.
                Last edited by erad; 4 weeks ago.


                • #9
                  My understanding of the honeycomb is that it's there to ensure airflow is laminar before hitting the obstruction, causing vortex which is what the airflow meter reads, being a karman vortex airflow sensor. Vortex changes with airflow. There is minimal info around about this though, I think I read it in workshop manual.

                  One thing I've found with our Camry on mixer lpg is that going from lpg back to petrol it won't run real well. Whatever it does on lpg changes trim for petrol so if I'm running it for a while on petrol I have to reset ecu.

                  I think it would be worth trying to reset the ecu, then idle for 10 min on petrol and drive on petrol to see how it goes. Could help you narrow things down a bit


                  • #10
                    Stu: At low throttle openings, the airflow would automatically be laminar, and this is why they induce turbulence with the honeycomb so that at least the AFM can read consistently. At high throttle openings the airflow will for sure be turbulent.


                    • #11
                      Not that I'm trying to argue the point.....but given the length of each honeycomb it wouldn't induce turbulence. Also, being Karman vortex airflow sensor it requires laminar flow before the protrusion that induces the vortices