Pre-ignition [Archive] - SaabCentral Forums

: Pre-ignition

11th March 2003, 07:03 AM
My 99GL has done about 94k miles. Most times (but not always) when I turn off the ignition, the engine carries on trying to catch and there's what seems to be some timing chain chatter. The guy I bought it off says it's done it for ten years (I know him so no problem there) and the engine runs very sweetly. Any idea what it could be? The title of this post, by teh way, is becasue my dad used this phrase when my mums Fiesta had a a similar problem but it never made much sense to me..should be post ignition really!

11th March 2003, 07:57 AM
This is actually called 'Run-on' or 'dieseling' and is caused by either compression ignition where the air-fuel mixture is auto-ignited by the general high temperature, compression of the combustion chamber and fuel quality or by surface ignition - usually due to a build up of carbon on the pistons which remains hot enough to provide a source of ignition after the spark ignition is terminated.

Compression ignition can be reduced or eradicated by increasing the octane quality (RON) of the fuel but you may need to de-coke your engine to eradicate surface ignition.. :(

11th March 2003, 08:20 AM
Interesting, cheers Munki. The car has, AFAIK, always been run on 4 star or LRP so fueling shouldn't be a prob but it's only done 94,000 miles in 20 years of faily short journeys so coking up of the pistons sounds like a likely cause. It's not really worth getting the head off just to do that, I assume the "dieseling" itself doesn't do the engine any harm?

11th March 2003, 08:37 AM
My car does this too and is a lot worse when running unleaded, so I guess what Munki says about the octane rating makes sense. :) ............Hang on let me think about that, Munki making sense :o

BTW Sparky, is your car a B-series engine?

11th March 2003, 08:53 AM
No, it's an "H" series. Has yours been doing it for long HeavyT? Any problems as a result of it?

11th March 2003, 10:22 AM
Mine tends to do it only if I use unleaded or if the engine is running quite hot when I switch it off. Ive done about 4000 miles since having the car and it hasn't caused any problems

11th March 2003, 03:42 PM
Mine diesels sometimes, probably due to low octane fuel. Cylinders are slightly coked too which wouldn't help.

Not suprising - the previous owner only averaged 1500-3000 miles per year..

12th March 2003, 02:31 AM
Mine averaged 400-500 since about 1983 so I think we're in the same boat! :o