Engine misfiring and cutting out. - NOW FIXED!

[jimbow]

I've been having a problem with my 911 misfiring occasionally for a month now. To start with it was just an occasional cough, but it has now progressed to the engine cutting out for sometimes a second, sometimes ten seconds (it is usually ok for the first five minutes from cold).

I've replaced sparkplugs, ignition leads, rotor arm, cap, coil and cleaned all the connectors and earths . My main suspicion now is the infamous CDI box. (6 pin)

The car seems to be able to tick over all day with no problems, revving the engine while static never seems to show any problems at all. I have pulled and wiggled every wire and connection to try and make it go wrong, but I have to be driving down the road before problems occur.
When it is going, it runs fine on all six cylinders, when the problem occurs it always cuts completely. All the instruments including the rev counter still read correctly, so is not the ignition switch playing up.
When it fires again, it makes a huge bang as it ignites the unburnt fuel in the exhaust and runs fine again for a little while.

Can anybody tell me where to go next? Get the cdi box looked at? find another one? change to MSD? or have I missed something

HELP! The sun is shining and I want to be driving it, not fixing it!!

[johnM]

Try putting in a new set of points.

Regards

[jimbow]

Try putting in a new set of points.

Regards

Its a 3.0 so no points! (Hall sender)

After reading up on the Pelican forum I decided to have a look inside the CDI unit and found this.



The connection from the PCB to pin 31/1 on the socket (power ground) was corroded away! So I got the soldering iron out and repaired the connection, full of hope I had found the problem. I couldn't see any other bad joints or overheated components.

Put everything back together and...............still the same.

Could running with the poor earth done further damage to the unit?

[impmad2000]

I've taken a look at the circuits, and the 31/1 connect is the main GND for the 6 pin CDI. This means that the other one (31/d) is a screen for the Hall sensor pickup. I suspect if all GND had been lost, the unit would simply have stopped working, through lack of GND, rather than killed it.
The symptoms you describe are not uncommon with a failing thyristor. The main power switch in the device. Here lies a problem, the modern equivalent devices are not a direct replacement and I haven't sorted a suitable replacement yet. Someone may have an alternative, I've tried the ones recommended on Pelican and these don't seem to work !!
Hall sensors can be unreliable, (modern's use VR sensors now) it may be worth trying a replacement one of these first if they are cheap ??
I really need to resolve the thyristor issue !!

Cheers
Tim

[jimbow]

Hi Tim

Thanks for the info, several people on Pelican do seem to have repaired these, it seems a shame you can't find a thyristor that will work. I've been looking around on the internet to find a source of a Hall sender but I can't seem to find anyone who lists them at all. If the Hall sender wasn't sending a signal to the CDI unit, surely the rev counter won't work? My rev counter still works fine when the problem occurs.

[impmad2000]

My rev counter still works fine when the problem occurs.

I think it must be the Thyristor in the CDI then !
The tacho feed comes off the part of the circuit that triggers the thyristor.
Looks like you'll have to get the CDI fixed, or buy one of the alternatives. When my car goue back on the road it will fue a crank fire system.

Cheers
Tim

[jimbow]

A quick follow up for anybody with similar symptoms it was the CDI box.

I have fitted an MSD 6AL unit in its place. The car is running great and now feels much smoother than ever.