What the heck's the matter with it?

[Brian]

Any analysts out there can give clues to what's wrong with our Bravo 1.2SX? 15K miles. New plugs even though no change 5K ago.
Starts and runs fine from cold and when hot. After a stall (it's a driving school car so stalls happen!) then it will start fine but then "blips" ie irregularly misfires on acceleration for 5 minutes or even longer. Turn it off and back on again and it still does it but stop for lunch go back out after an hour and it starts and runs fine.
A learner stall can be a bit vicious and give the engine a good shake.
All help appreciated.

 

[Martin]

Could be the fuel switch

I think that underneath the passenger seat there is a fuel cutoff switch that is supposed to only activate when the car has been in a front on collision. It is meant to cut off the fuel so that if there is the possibility of a fire no more fuel is sent to the motor. Perhaps after a stall the switch is trying to turn off the fuel supply.
If I remember correctly this was a problem on some switches on Bravos and there was a recall on them. Take it to a dealer and ask for a new switch.

 

[Brian]

misfiring

Thanks for the input Martin but the fuel cut off switch should do just that ie totaly cut off the fuel supply in the event of severe g force, I'm not experiencing that thank heavens otherwise the car wouldn't start afterwards. It probably is electrical, but what?

 

[John]

Misfire

Brian, does the car misfire with a bang or a small pop that does not sound like the old fashioned bang of a misfire. What it could be is a blocked airfilter, if you do alot of town driving airfilter might restrict airflow. The reason I mention this is between the airfilter and inlet manifold is a airpressure . ration switch, this consists of a wire that heats up and the air passing by to the engines cools it down, the amount it is cooled will affect how much fuel is pumped into the ngine to keep the ratio mix correct. If airflow is hindered or in some cases increased I.e. with a KN571 filter then the air is arriving faster or in the case of a blocked filter slower than the ECU anticipates this then results ina misfire. If it only happens after a stall then check all the vacuum hoses from the braking system to the engine as one may be lose.Hope this helps, if not then you need to sort the misfire out otherwise damage may occur. It could also be one of the ignition leads, best way to check this out is remove 1 spark plug at a time to see which one is heavily oiled or dirty with unburnt fuel, if there is one then this is the lead that is knackered. Of course Fiat can do all this if you are not sure but they will charge charge charge.

Good Luck John

 

[Brian]

Some extra clues to go on

Hi John. Thanks for your input. No the air filter is as new. It's not a backfire but just like an intermittent plug misfire would be on a conventional ignition car on acceleration. It always happens after a stall but also happens on a near stall where the revs have been dropped so low (eg below 500) where perhaps the wrong gear has been selected and it was only luck the thing didn't stall. For the next 5 minutes you can look forward to hiccups on acceleration, a wandering tickover like the mixture is going rich to lean or you can just turn the engine off, wait two minutes and everything is fine again.
It could be mechanical but as everything is fine after a 2 minute shutdown I'm thinking more electrical.
For instance, certain components are out of the system when the car is first started eg lambda sensor isn't hot enough so it is ignored and the ECU just choses "get you home" settings it thinks will do until everything warms up. I'm thinking if the mixture is wandering all over the place (which is a bit like what it feels like) then that will give me the misfire and the symptom righting itself either with time (self adapting) or a reset with turning the engine off and back on again. The fly in the ointment is that with normal driving from an experienced driver it just won't happen at all.
Thanks for all advice though. It's all pieces of the jigsaw.

 

[AHC]

RUNNING PROBS

It could be so many different things .the inertia switch is a non runner they moved it on your car and it has solved the original problem .the first thing to have checked is that you are running the latest version of fuel program and that varys on model take it down to your local dealer and have him check its programed properly (alfa romeo dealers are the best bet as they have the right tool but alot of fiat dealer have it as well)the other thing might be the generator .it could be overcharging ....just a thought .anyway good luck

 

[Brian]

My latest thoughts

Thanks for the input. I had the car recently remapped for a different fault (no tickover) which made the car run much better throughout the range but the stalling/misfiring problem still remains. I'm interested as to how the alternator overcharging could cause misfiring problems but for the moment my thoughts are with the KNOCK SENSOR!! This is a device situated usually on the back of the engine which is meant to sense the vibration of a car pre detonating through either a wrong fuel or if the ECU has gone potty and is delivering damaging mixtures/igntion timing to the engine. It retards the ignition and goes on retarding it until the vibration or "knock" stops. Then the ECU advances the ignition until the knock happens again and eventually finds a happy medium. This is exactly what it feels like happening. The engine shake of the stall, but with retarded ignition the engine misfires again and retards still further to I suppose rock bottom. Then gradually advances until it's running smooth again which can take quite a few minutes. When starting from cold then the lambda (mixture) sensor is out of the system and so is probably the knock sensor and the ECU soon tunes to the optimum setup. It's purely theory but I shall carry out some tests and post the solution if I get a result.Again, most drivers wouldn't notice but might wonder why their car runs rough after a stall. It's nice to know how the system works though and might help others with their tuning problems.

 

[Brian]

The answer

OK. That proves it. It's the "knock" sensor! Any "knock" that affects the engine is sensed by this transducer which sends a small 1volt signal to the ECU which then retards the ignition. The ECU then gradually advances the ignition until pre detonation occurs again and gradually adapts to the optimum setting. It should only operate if pre detonation occurs either by poor fuel (not likely in UK) or if the ECU is putting out the wrong ignition timing or fuel mixture. However, a stall causing the engine to jolt can make it think there's something wrong, retards the ignition so that it misfires anyway which then retards it again and it keeps on going till it hits rock bottom. Switching the engine off and waiting a while before restarting allows the system to reset.
To test the theory I disconnected the knock sensor entirely ( no Fiat fault codes displaying surprise surprise! Which just goes to show that it isn't essential to have your car serviced or seen to by someone who has a Fiat fault code reader as quite alarming faults don't show up most of the time) and the car runs perfectly and restarts perfectly.
Food for thought is that I think if someone had a misfire that caused the engine to vibrate enough then the knock sensor would also probably make matters worse by retarding the ignition in chunks. Mind you if you have pre ignition or misfiring it's time to get fixing or throw your cheque book at the nearest Fiat dealers anyway.