Intermittent misfire/ flatspot - calling all diagnostic gurus

[flagg Junior Member Find all posts View Profile Close flagg]

Hi guys,

I have a really bad flat spot problem that appeared last Friday when my wife last drove it after putting some fuel in it. Symptoms are that the car won't rev over approx 2000 (whilst driving) but will rev fine when stopped. You literally cannot give it more than the slightest throttle and it just misses and goes backwards while under any load.

It sat for a day and then I did a visual check of everything and took it for a drive Sunday and it went like a jet. later i had to pick up a round bale of hay (300kg) with the box trailer and it went fine driving out to the farm. I left it idling while loading then headed home. halfway home it did it again. I struggled to the local servo (10klms away) and filled up (it took 60 litres, i only have one tank as it also has lpg - im running on petrol at the moment so lpg is not a factor). I took off from the servo and its going like a jet again!.

My thoughts initially were bad fuel but im leaning towards electrical as it is night and day when it has the problem however I have seen weird things in the past like plastic bags in the fuel tank that block pickups and loose baffles in exhausts that have done the same thing as they block and then unblock. Before i look at anything who knows the usual symptoms for faulty sensors that may match the on again off again nature of the fault particularly that when it is playing up it will start fine, idle smoothly, rev when stationary but not rev over approx 2000 under any load accelarating on flat road unladen.

Hope you can help, thanks guys.

Regards Brett.

[LargeInAPrado Member Find all posts View Profile Close LargeInAPrado]

Ive just remembered something .... does it do it on both petrol and LPG? Hows it perform on LPG compared to Petrol. Initially thought TPS or Air flow meter which i wouldnt rule out but ... They dont normally produce missfires

I had one come into the shop over last chistmas that had no power it had bad plugs , plugs hadnt been done for a while and it was on LPG also Plug gaps were large ...

Its an interesting one sounds like one i would just go through the steps and start checking the above but would also like to feel the issue to understand if its electrical or fuel related

[Black 150 Member Find all posts View Profile Close Black 150]

Fuel filter perhaps, my old Pajero had a dirty fuel filter and behaved in a similar way

[LargeInAPrado Member Find all posts View Profile Close LargeInAPrado]

you say LPG is not a factor but LPG runs with the same electricals so i would switch over to LPG and see if it happens at all, if not your ruling out all electrical and heading towards the fuel system which will make sense that it stated when your wife fills up.

[flagg Junior Member Find all posts View Profile Close flagg]

Thanks guys, I've replaced the fuel filter 10000 kms ago and the throttle body and MAF sensor were cleaned about 20000 ago (it's done 230000). Plugs are probably due but I can't see them working fine one moment and then completely failing the next then working fine again. They are Bosch LPG platinum plugs (basically a heat range colder) The symptom is more like a BIG flat spot rather than a misfire. If you put your foot into it it just does nothing and looses power. If you back right off to a very small throttle opening it will run and you can limp along and very slowly build speed up to about 80 on the flat.

I'm about to start searching for a good fuel injection workshop. Would the computer have registered a fault code or something which they can trace when they plug an analyser into the ECU?

The LPG has a leak at the moment so we haven't run on Lpg for a while until we take it back to the specialist on the other side of town. It don't think it will be related.

Cheers Brett

[AussieAndy Find all posts View Profile Close AussieAndy]

So you filled up a 90L tank with fuel, it ran like crap but not consistently for a while, you then put 60L in the tank and it hasn't had a problem since? I would almost guarantee crappy fuel, possibly some water in it or the like. Run this tank of fuel as low as you dare then fill up, if it doesn't have a problem in the next tank of fuel (the big fill up) you can bet your bottom dollar a fuel issue.

[flagg Junior Member Find all posts View Profile Close flagg]

Just checked with the wife and she said it was nearly empty when she put 40lt in it and then it played up not long after on the drive home. It sat for two days and I drove it and all was good. Drove it again to get hay and it played up on the drive home so I limped to servo and filled up (it took 60lt). Drove home and it was good again. Water in fuel was my first thought but then I thought why would it rev fine with no load? With water in the fuel wouldn't it misfire badly regardless?

[AussieAndy Find all posts View Profile Close AussieAndy]

Nope, the engine requires a lot less fuel to burn to free rev than under load. The higher the load the worse it will play up, the harder you put your foot down the worse it will miss. So it was ok on the way to get hey, you piled the trailer full of hey then it started missing. When it starts missing the natural reaction is to put the foot down and the miss gets worse. Typical water in fuel symptoms of a petrol vehicle. I have seen home mechanics replace coil packs, plugs, etc. then ring the professionals. I pumped the tank of fuel out, cleaned out the tank with rags until dry, fill with new fuel and job done. I have done this on more than one occasion, and the symptoms are usually very similar to what you are describing.

[Anth120playdo Banned Find all posts View Profile Close Anth120playdo]

In my experience water in fuel will make a misfire.
Regardless of how low you run it, it's on the bottom of the tank getting sucked up, even after you fill.

If you can get your gas fixed, you can narrow it Down to spark, fuel or gas.
Again hard from here, so you need to narrow it down.
If it did'nt have gas, I'd check fuel pressure to start.

[turtlewa Member Find all posts View Profile Close turtlewa]

Try a some metho into the tank it absorbs the water if any in the tank and burns it off in the combustion process

[AussieAndy Find all posts View Profile Close AussieAndy]

Yeah, I wouldn't be dumping more shit in my fuel tank to absorb other shit..........
I have heard of people doing this before, but do you know what metho does to the fuel pump and fuel system seals orings etc.
If it is a fuel problem the right way to fix it is drop the tank, clean the tank, replace the filter and refil the system.

[flagg Junior Member Find all posts View Profile Close flagg]

Ok since I filled it up Monday its been driven by my wife on some small trips and I took it to work today and did three trips totalling 130 klms - all good!

I'd say at this point it looks like a bIt of water in the fuel, fingers crossed. I have heard of putting metho in and given the small dose needed it won't damage anything on a full tank so I put 200ml in as insurance this morning in case there was any residual water still lurking in the bottomed the tank.

I'll keep you posted in a few days, thanks for all your input.

Regards Brett