The most likely causes of misfiring.

[roaddirt]

Problem: Hesitation and poor acceleration above 35 MPH

Vehicle: 1999 Buick Century
Mileage: 40K
Engine: 3100 V6
Firing order: 1-2-3-4-5-6
Firewall side cylinders 1-3-5, front side 2-4-6

Replaced air and fuel filters, according my tests fuel pump and regulator works and provide pressure within specs. Removed air intake duct and cleaned throttle body, no impurities found.

I used my OBDII CarCare diagnostics software for laptop. Besides generic readings it gave me some kind of "enhanced" GM diagnostics readings from PCM.
The most interesting was that I had random misfiring on cylinder 4. Under higher load, cylinders 1, 2 and 3 would have some low misfiring counts as well, but cylinder 4 had highest count and frequency . I guess misfiring count was not too serious for computer to be alarmed and treat it as malfunction though.

Considering this and occasional spark retard undeer high load that I noticed in data log, I wonder if my problem is in ignition system or maybe dirty injectors.

What does it cause misfiring? What does it cause spark timing retard under higher load? Any good tests for home mechanic?

[Ripn12s]

Have u replaced the plugs and wires?

Do the misfires go away during normal driving or do they get worse?

PCM needs to see something like 200 misfires every cycle or something like that to set a SES light.

Your car is seeing knock at high load that is why its retarding the timing. You should try colder heat range plugs and tstat and see if that helps.

[roaddirt]

The manufacturer says that stock platinum spark plugs and wires should last 100 K miles and although I do not have to use them so long, I thought that with 40 K miles on the car they should not be cause of problem so I did not replace them trying to keep “guessing” costs low as possible.

I am not sure how to interpret misfiring counts that my laptop with CarCare OBDII software see in Cylinder 4 but 95% of misfiring is in that cylinder only so must be something wrong related to it.
Although engine in idle seems to run quiet and calm, when I place my hand close to tail pipe I can feel random misfiring. I do not really feel or hear much of misfiring so I am not sure if it getting worse during driving but according to my software NO, and strangely enough, it seems that I have more misfiring at idle than in driving condition.

Today, for the test, I swapped spark plug and wire from Cylinder 4 with ones from Cylinder 6. Tested with my OBDII armed laptop and misfiring stayed with Cylinder 4, which kind of says me that spark plug and wire were not problem here. I also removed the coil pack and measured primary and secondary resistance on it and it was as specified in Haynes Service Manual.

I started thinking that the injector in cylinder 4 is possibly bad and I maybe have more than one injector in bad shape. I poured injector cleaner in the fuel tank it did not help.
Maybe I should have find and buy one of those cleaning kits with a cleaner can that you can connect to Schrader port and run the car in idle while it cleans injectors.

[Ripn12s]

My car from the factory exhibited misfires at idle. I had a shop look at it and they told me that they have seen this on other GM products.

The misfires would read 0 on all cylinders while driving and only showed back up at idle.

Id say that if you see 0 misfires during normal driving conditions and WOT runs dont worry bout it.

[andyd00]

I have the idle prob too. Seems to only be under 1200 rpms. Had it scanned and its all cylinders.

[MoshimoGTP]

In my car, the SES Light come on because "Cylinder 3 Misfire". The PCM was scanned by a OTC Tool. I think is a bad fuel injector or coils. I've changed the wires, and colder spark plugs.

[andyd00]

Its still not good to missfire as you engine is rapidly and erratically changing speed when it missfires. Thats strain on the chrank bearings. I noticed upon my upgrades after the battery has been disconnected the missfires go away for the first 5 minutes. Try that. A mechanic said to reflash the computer. Where I'm at it cost 250 buck to flash. Screw that, I'm getting a DHP for 300 with a free reflash. If it don't fix it ill still have the upgrade.

[kilroypr]

I would agree with the Ripn12s. If the SES does not come on. The car under light and heavy load does not misses and the miles per gallon are acceptable then the car is ok. Do not fix what is not broken.

[andyd00]

It'l eat your coupler alive. Thats what it did to mine.

[kilroypr]

Andy what you mean by coupler?

[andyd00]

Originally Posted by kilroypr

Andy what you mean by coupler?

A coupler in the supercharger that goes bad and makes a real nasty noise at idle. Its a peice of plastic that joins the drive nose to the SC gears. Its a 20 dollar fix but it takes a few hours to fix and wait for the rtv to dry.

[MoshimoGTP]

In my car, i was changed the sparkplugs and wires, and the misfiring on cylinder 3 continues. I think is a bad fuel injector. What do you say?. The OTC Scan tool says is Cylinder 3 Misifire. How i can stop the misfiring of Cylinder 3 because that cause me a SES Light.

Thanks a lot! =)

[FLGTP]

Most likely cause of your misfires (90% certainty) are sticking/clogged fuel injectors

If you have access to a fuel injection flushing tool
OTC makes a nice one (if you have access to an air compressor)
Go to your/A dealer and purchase one can of Top Engine Cleaner (pour in not spray)
use a 50/50 mixture of cleaner/Fuel
and hook the flushing tool directly to the Service port located on the fuel rail
Will need to remove the fuel pump relay while flushing the injectors and run off the mixture

Best bet is go to the Dealership and get er flushed

There is a GM published bulletin (not a recall) about the 3100s sticking injectors

If the misfire is only Under load and flush does not fix most likely cause is secondary ignition / incorrect plug gap Faulty plug wire
switch out bad cylinder plug or wire with a known good and recheck

Edit: that pour in tank cleaner is good Only to assist in preventing the problem from reoccuring and THE ONLY chemical I would recommend using is the Texaco Additive ~WITH TECHRON~
GM dealers sell a GM additive but its made by texaco and 20dollars more

Ooh and get it fixed asap or you will burn up your catalytic convertor
GM cats are junk, at least one a day gets replaced here

[Ripn12s]

To check if its the injector, just move it to another cylinder and see if the misfires moves also.