Engine Miss

[Harvs82]

Hi All,

Was idling at the lights this morning, and the engine momentarily faltered, shook for what felt like 3 sparks, revs dipped down, and then picked up again by themselves. It felt like a misfire basically. Everything is fine under acceleration, and normal driving. It used to happen when the engine was warming up, perhaps after 10 minutes of normal driving.

I had the Crank Angle Sensor replaced about 4 months ago, as the car stalled twice with no warning, and Code 47 came up when I did the diagnosis, and it has been fine since having it replaced. The car was displaying the same misfire symptoms before I had the sensor replaced, at times.

Does anyone know what else could be causing this problem? Just like the CAS, it is an intermittant problem. I replaced the leads and plugs 10,000kms ago.

 

[hako]

Have to be a lead shorting momentarily one would like to think. I had something similar, a intermittent one cylinder miss whilst idling and solved it by gapping the plugs to 1.0mm instead of 1.5mm. The reasoning is that the smaller gap makes it easier to jump the sparkplug gap than short out thru the plug lead. Good Luck.

 

[digisol]

Likely the plug was RS.

Gap normal good plugs to 1.0mm and it will run like a total dog if at all, I doubt it would idle, sure it will run but not much more than that.

While plugs are meant to come pregapped, (they are not exact or rarely even close) check the gap on each new plug so you get an easy sliding 1.5mm, the difference there is a tight 1.5 may well be 1.3/1.4 and when the feeler guage is inserted it simply flexes the electrode out to 1.5 and when removed the gap closes up and it will run like a dog.

I played with plugs and different gaps one day just out of curiosity and anything less than 1.5 simply will not work.

Gap it at 1.4 mm and your car will idle rough both hot and cold + it will miss under acceleration.

Gone are the days of setting the old red motor at .032", sure my old semi race engine would run best at .030" and at .028 you may get another 500 rpm but plugs lasted 3 months at best, that beast had a 50,000v Mallory coil and a twin point dizzy which would give ya a pretty good heart start if ya played with em while it was running, the commodore will also give a good boot of voltage.

 

[hako]

Gapping to 1.0mm worked for me and also has for others...but I wouldn't argue that it would always work and I also must admit that something must have been wrong for the plug to miss. More than one way to skin a cat.

 

[Harvs82]

Guys I should point out that it is a very intermittant issue, I've driven to work and back today, and yesterday afternoon, and everything has been fine.

I just hope it isn't the crank angle sensor playing up again, or the DFI module. The same symptoms were occuring prior to me replacing the CAS. It is definately not a fuel related problem, when I had the CAS replaced, the fuel pressure and regulator were both checked and were fine. With the leads going across the top of the block and behind the alternator, is there any way of making sure these leads don't short against each other? At the moment all three are just sitting in the wiring loom stuff for protection from the block.

 

[hako]

The only way I know to spot an electrical short like that is to run the motor in total darkness and watch for the tell-tale spark....though from what you say you may have to watch it for quite some time!
Electricity will always find the shortest path to earth which is why I suggested gapping the plugs to 1.0mm. Regards