if you have misfire on cylinder #4 read this

[aki334]

Hi all I am just posting my experience hope it will help somebody some day.

 

When I had engine shaking problem due to the alternator I was getting misfire on cylinder #4 small count every driving cycle but not enough to illuminate check engine light for P0304.

 

So if you have a good scanner that can read live cylinder misfires or scanner capable of reading mode 6 and if you see misfire on cylinder 4 and you checked sparkplugs, coils and vacuum leaks it could be your alternator causing this. I hope this will help somebody some day. Note this is on 2.5 l engine.

Edited August 4, 2021 by aki334

[motorbreath]

Thanks for the information, would you be able to confirm it is the alternator by checking the voltage while the engine is running? Or was the voltage within a normal range (14.2-14.7V I think?) even with the bad alternator?

[aki334]

Thanks for the information, would you be able to confirm it is the alternator by checking the voltage while the engine is running? Or was the voltage within a normal range (14.2-14.7V I think?) even with the bad alternator?

 

alternator was charging normally. This once happened to me on GM car. I had p300. Everything was normal, replaced sparkplugs, no vacuum leaks, replaced ignition coils, replaced injectors and just gave up. I drove the car with slight misfire for a long time. One day alternator failed it was not charging. I put in the new alternator (Duralest Gold from AZ) and engine worked so smooth I could not believe. Since then I learned alternators can cause ghost misfires that most technicians will overlook.

[ehsnils]

Interesting. I wonder what you would have seen putting an oscilloscope on the alternator output. Could have been a failed or failing diode causing a wild ripple. That in turn could of course manifest it self as misfire on just any cylinder, not only #4.

 

 

Stranger things have happened.

[aki334]

Interesting. I wonder what you would have seen putting an oscilloscope on the alternator output. Could have been a failed or failing diode causing a wild ripple. That in turn could of course manifest it self as misfire on just any cylinder, not only #4.

 

 

Stranger things have happened.

 

 

I do not have scope, I am still looking for one (they are expensive). I did check diodes when alternator was cold never remembered to check when alternator gets hot. I did have clamp amp meter and amperage was jumping +/-3 Amps with same load at idle. Scope would show that much better. Checked for AC mV on battery posts with idle but of course DVOM is too slow to show any abnormal spikes.

Edited August 4, 2021 by aki334

[ehsnils]

Looks like this would be sufficient: https://www.amazon.com/Hantek-Automotive-Diagnostic-Oscilloscope-Programmable/dp/B00BSR98KW/ref=sr_1_6?dchild=1&keywords=cheap+oscilloscope&qid=1628132660&sr=8-6

 

 

But I'd pick this: https://www.amazon.com/Siglent-Technologies-SDS1202X-Oscilloscope-Channels/dp/B06XZML6RD/ref=sr_1_3?dchild=1&keywords=cheap+oscilloscope&qid=1628132660&sr=8-3