Miss fire?

[keatsie]

I thought I had found the source of an irritating missfire that seems to be getting worse, but after a run this morning it's back.

Any ideas would be appreciated.

Basically after about 30/40 minutes of driving I start to get a missfire which appears under acceleration and is quite bad, to the point where the engine coughs and splutters violently. Reduce acceleration and the missfire goes, apart from an occasional almost undetectable occasional missfire. But accelerate and it's bad?

TPS sensor is near new, the Hotwire airflow meter is clean etc... Any ideas?

[adam quantrill]

I've had this and the last time there was a loose trumpet or two in the plenum.

Could be the ignition amplifier too.

[keatsie]

Looks like I might need an ignition amplifier...

[njhucker]

Coil, or ht lead from coil to distributor breaking down when getting hot.

[jon haines]

I have had the same and it turned out to be ignition amplifier

[SLB]

Change it quick before it stops working completely, which happened to me and I had to be towed home.

[honestjohntoo]

Keatsie asks

'TPS sensor is near new, the Hotwire airflow meter is clean etc... Any ideas?'

adam quantrill says

'I've had this and the last time there was a loose trumpet or two in the plenum.'

'Could be the ignition amplifier too.'

keatsie presumes

'Looks like I might need an ignition amplifier...

njhucker suggests

'Coil, or ht lead from coil to distributor breaking down when getting hot.'

jon haines says

'I have had the same and it turned out to be ignition amplifier'

So, its either the coil, dizzy, ht lead, ignition amplifier or loose trumpets.

Intermittent faults? Dont ya just luv em?

I would add any or all of the following

'possibly rotor arm, ignition switch connections, dodgy spark plug, suppression condenser, Efi throttle pot and/or connections, temp sensor and/or connections, fuel pressure regulator, etc etc.'

All genuine, but intelligent guesses, of course:

Unfortunately you may have an expensive replacement program using the scatter-gun approach to fault finding on the Efi or the ignition system.

For ignition - you may wish to test any or all of the components before shelling out any moolah?

there is even the possibility of testing for the intermittent ignition problem - if that is what it is?

All the tests re relatively simple and with your occupation in Automotive Electronics you probably have an appreciation of how your electronic ignition works in the first place, as seen here:.

http://www.vintagemodelairplane.com/pages/Download...

and you will just love the test programs - here:

http://www.vintagemodelairplane.com/pages/Download...

Then there is the Efi system

I have a full 15 point health check, free at the point of use, which is a great aid in tracking down Efi system faults.

Your's being a Hotwire system changes things a wee bit but most of the Flapper program can be applied as you wish.

http://www.vintagemodelairplane.com/pages/Snippets...

[utternutter]

No one has mentioned the regular easy fault with the v8 is the oil breathers, once they clog up it doesn't matter what you do to the air mixture, timing a clogged or old breather will cause misfires which mimics spark plugs, plug leads misfiring etc etc, it's all to do with pressure build up in the engine. so worth checking and cleaning or replacing including cracked pipes etc,
it happened to me on my v8 vitesse many moons ago so always clean mine on my wedge yearly after winter :-))

[keatsie]

Thanks to all, just built an interface for the ecu so will have a look. Agreed, I haven't done the basic checks yet I was just getting other people's experience just to avoid any obvious areas to check.

[350zwelgje]

Had this situation twice. In both cases it was overfuelling:
- coolant temperature sensor
- ECU needed resoldering, idle was fine but in the end using the trottle killed the engine.

Will come up if you follow honestjohn2 checklist/analysis if it is the case.

Rob

[honestjohntoo]

keatsie said:

Basically after about 30/40 minutes of driving I start to get a missfire which appears under acceleration and is quite bad, to the point where the engine coughs and splutters violently. Reduce acceleration and the missfire goes, apart from an occasional almost undetectable occasional missfire. But accelerate and it's bad?

Analysing the few available symptoms as reported.

1. After 30 minutes its probable that the engine and engine bay have reached their maximum operating temperature, which might suggest this is a temperature related fault.

1. At the same time the fault only occurs when under acceleration which suggests over fuelling caused either by

• a faulty throttle pot or
• the ECU acceleration circuits are (strangely) over active or
• the Coolant Temperature Sensor has become faulty or
• the CTS circuit has become open circuit.

Of all the above, the latter is high on the probability list.

You first need to read about the CTS

http://www.vintagemodelairplane.com/pages/Rover_Te...

how to test it and in particular how to test its connections to the ECU and to the engine earth stud(s) behind and below the LH rocker cover.

Because my analysis suggests over fuelling this needs to be confirmed by observing the condition of the spark plugs (one from each bank) when the fault condition has occurred.

I would expect to see them sooty black as a sure sign of over fuelling.

The problem with remote analysis is not all the symptoms are being observed or reported.

If there is over fuelling one might also expect to see the black spark plugs as mentioned, black smoke, carbon deposits on the tail pipe and a fuel smelly exhaust/engine bay. You need to report back on these.

But (horror) what if the missing symptoms indicate that it is NOT over fuelling.

There is a more unlikely possibility that this could be due to fuel starvation when accelerating after 30 minutes driving.

Blocked fuel filter, crud in the fuel lines, crud in the fuel tank would contribute to that possibility.

[keatsie]

More bodging and poor workmanship found!

The wiring has been changed and completed poorly, bare wires twisted and taped on the ignition amplifier. The distributor vacume advance is loose and although the dissy is not using vacume advance I really don't think the vacume advance unit should be rattling loose.

Trust in the basics, always start at the beginning and look for the obvious.

Thanks to all for input, trust I will not be so last next time and will get my hands dirty first