Team-BHP - engine hesitation when turning OFF

hi guys

just noticed this weird bit of engine hesitation / fumble / misfire when i turn the ignition off. It will shudder prior to going off. This happens maybe 1 or 2 times in 3 days. It starts with a slight twitch of the key, but this erratic turning off has got me concerned. i do the regular oil change and the babe has been mainatianed well.
its a carb version Nissan Altima 92

the shudder is such that the entire car trembles nothing unusual noticed otherwise. its oil habits are also great i.e. level remains same over the period say a month and half or until next change.

can any tech guys have some clues / checks to why this is happening.

thanks

Had this problem on my old M800 many years ago. Though, cannot recollect how it was fixed. I think it had something to do with the exhaust assembly getting rusted and having holes here and there, resulting in exhaust gases leaving the chamber through these holes, much before reaching the tail pipe from which only it should actually go out.

The engine seems to have advanced slightly in its timing. Now don't ask how..!! Maybe the distributor housing was not tightened properly. get the timing checked. Did you discover any change in the oil consumption?

Mod Edit: Avoid quoting the entire post. Specific parts of a certain post which you wish to reply to may be quoted.

This phenomenon is called "dieseling"

Let me start explaining by clarifying how a diesel engine works.
Consider one cylinder of the diesel engine.
High compression (Sometimes 22:1) produces enough heat for the explosion when a few droplets of diesel fuel are injected from the injector nozzle.
That is why diesels are called Compression-Ignition (CI) engines.

Petrol engines don't have enough compression to explode a petrol-air mixture. That's why we need an electric spark to explode the contents of the cylinder.

However, in a badly maintained petrol engine, carbon build-up can remain glowing red-hot providing enough heat for exploding a fresh breath of unburnt petrol-air mixture. The glowing carbon embers do the job of the spark-plug.

When you let go the accelerator in a carbureted engine, the carb is supposed to stop the supply of air-petrol mixture.
A badly adjusted carb may continue to supply some, even when you take your foot off the throttle.

Combined with the red-hot carbon buildup, the engine may splutter and run for a few seconds after the electrical system has been shut off.

Ram

"Dieseling" is unintended diesel engine like behavior in a carbureted petrol engine.
The phenomenon is seen in irregularly decarbonized, carbureted petrol engine cars.

A combination of red-hot intra-cylinder carbon deposits and a leaky carburetor throttle-valve, cause the engine to splutter and continue to run for a few seconds even after the ignition is killed.

Ram

Dunno about all this mumbo jumbo, but when I had this problem in our M800 all the guy did was adjust the timing and it went away. This happened on the carb gypsy also after that, but in that case it kept on reccuring, and replacing the timing belt made it go away.

its a petrol carb version
i feel what Ram says makes some sense as I havent overhauled the engine since bought used car.
but i have some few doubts that it may not be carbon deposits
theres no smoke coming of the exhaust
the cars starts in first attempt
doesnt seem loss of power
doesnt heat up

and also not a timing issue
as it doesnt misfire during idle.

i am not saying u guys are wrong but do u think it could be the spark plugs as they have not been opened up since bought.
i was planning to get it changed myself but time didnt permit as the garage was ripping me off for the labour cost.

2fast4u, if the timing is okay then check the idle cut-off solenoid. In first batch of Tata indigo petrols, solenoid was a big factor among startup/shutdown related problems, but I dont think it was an idle cut-off solenoid.

Other problem could be feedback from alternator in case of electronic ignition. This is a common problem with hobbyists who install electronic ignition on their home made machines, esp in RC aircraft etc., I dont know wether this problem is timing related

Well....I seem to be having this problem in my M800(carb) too...
So, i should get the timing checked, and it will go away?

There is this other problem.... my vehicle starts to knock very badly while being driven in heavy rain...what could cause this?

In the old carburetor aspirated days, they used to say, a long drive at a steady 80 km/hr would burn off the excess carbon deposits and stop the dieseling.

Modern ECUs measure(sense) many parameters, convert them to digital form them and fit them to equations.
For example: Air quality (humidity and oxygen).
Some other sensors for important parameters are:
O2 sensor (critical for the ECU to command correct fuel mixtures), Coolant Temperature Sensor(CTS), and the Manifold Air Flow (MAF) sensor. All this, in order to correctly compute injection and spark timing.

An engine lacking the computer-adjusting mechanism
will behave somewhat differently in very humid weather as compared to very dry weather.

My Catalina Safari station wagon's V8 breathing through a 4-bbl would run silky smooth when it rained.

Engines that have slightly advanced timing do behave better when the weather is very humid.
So it logically follows that engines that have somewhat retarded timing
will misbehave in heavy rain and the resulting very humid weather.

Ram

My dad's '82 Caprice Classic (carburetted V8) had a severe case of this problem when we were in Kuwait... engine would keep spluttering and idling after turning off the ignition, sometimes for upto 5 seconds! Dont take this lightly, because the mis-timed ignition sometimes tries to turn the crank in the opposite direction... could very easily result in a broken crank / connecting rods!

Get your engine de-carbonized and carburettor overhauled ASAP.

- T u r b o C -