Hijet van misfire

Hi,
I have a Hijet 1300 EFI, It misfires between 1000-2000 rpm,it only does it under load. I have replaced the spark plugs and done a visual on the vacuum hoses. Has anybody got any ideas
Thanks
C.

Misfires can be one of the hardest faults to pin down and solve.

First it is helpful if you can pin it down to a certain cylinder.

To do this I unplug an injector or a HT lead from a cylinder at a time to see if you can work out which cylinder is misfiring.

Low RPM misfires if they are cylinder specific are normally caused by an ignition fault so a coil pack, or an injector and its driving electronics. Never have I actually found one caused by the injector and electronics though, but its possible.

The other cause of low RPM misfires are faulty valves, at low engine speeds the valve will leaking, but as the explosions increase in fury they actually push a dodgy valve to seal better. Some kind of compression testing can sometimes help detect a cylinder with a lower compression valve.

If you can't find a cylinder at fault we need to look at more generic items, such as the TPS sensor, the map sensor. You need to test their resistance and see if the resistance of the TPS smoothly increases with throttle opening and a smooth resistance change with vacuum for the map sensor

I forgot to say:-

The 1.3 hijet runs what we call a wasted spark twin coil ignition system. Each of the coil packs spark for two cylinders - at the same time - as one will be sparked at the fire point, while the other get sparked at the end of the exhaust stroke. That is why it is called wasted spark.

With this setup you will find if one of the coil packs is faulty, then the fault will show on two cylinders.

This may help you diagnose a coil pack - but at the same time as the coil packs have a HT lead to connect to the other plug, that lead could be at fault.

Good luck

with wasted spark you can also get a mis fire if the efi is over fueling and then the spark igniting the left over fuel on the exhaust stroke.

Thanks for your replies,
The problem is that it only misfires when being driven, it does it on acceleration and de acceleration drifting down in second gear. .how do I isolate each cylinder from the coils when the coils are part of the spark plug cap on two cylinders, I will try disconnecting the ht lead to cylinders one and two, one at a time and drive a short distance.
Regards
C.

Little burp or a big hesitation?

Will misfire as long as I drive it in that rev band

I think, what logi is getting at, and what I was just thinking about while walking round the supermarket....

Describe the "miss fire"?

Are we getting back firing, total momentary power loss, just a feeling it not running on all four....

Try to be as descriptive as possible.

Loss of power and engine vibration, feels as if its about to stall, more so on tacking foot of throtle in lower gears within a low revs (2000-1000 rpm). seems to get worse when engine is run for about forty five minutes to an hour

What age is the van? Could be the TPS on the way out?

2001

Throttle Position Sensor on mine's on it's way out had vaguely similar symptoms, get a loan of or buy a OBD reader, plug it in & see what it says.

Early 1300s had wasted spark which is good cos it helps to ignite unburnt exhaust gases from the port am I correct in thinking. Later 1300s had four coil packs though which makes it easier but your very good suggestion of unplugging one injector at a time is a great idea.

Did not know there was a place to plug a Obd reader in my van, would have thought the on board engine managment system would have found the fault and lit up the diagnostic light on the dash board

Should light up the eml, but its worth checking anyway. May show something pending.
OBD socket on rhs below the dash.

Hi Logi
Thanks for your help, I cannot find the OBD socket, the van was first registered in July 2001 but could of sat in a depot and be older. Will try removing ht leads, then coils one at a time.If that doesn't work I will disconnect the injectors as Highlyjetted sugested. Should see some lights on the dashboard then.