Causes of misfiring?

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?

Try replacing the spark plugs and wires...

Hi tman2093,
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.

I dont care what the manufacturer says, imagine if you were a spark plug, firing 15 times per second for 100000 miles, You'd be pretty tired IMHO.

You may have a defective plug and or wire. Replace that first and make sure the insulators are not ground the spark. Plat plugs last long, but if water gets on some of them, they may crack, causing a problem.
Good Luck.

For the test, I swapped spark plug and wire from Cylinder 4 with ones from Cylinder 6. Tested with my OBDII armed laptop in idle 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 have that feeling that wires and plugs are OK but it might be good idea to replace them as tman2093 says.
Would you recomend platinums again fallowing manufecturers recomendations? Also, do you have any recomendations for plug wires found in local part stores.

Go with AC Delco Plugs and wires. As you said though, the swap of plugs disproves my theory of the plugs, now I'm going with the injector...

Hi tman2093,
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.

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.

I'm hoping you can provide me some assistance as I am having a similar problem with a '91 Century. It was Grandma's car & I got it when she quit driving. Only 55K on it now, but it has always had a miss at idle. I replaced spark plugs & wires, but the miss is still there.

Fuel pressure is OK. I was told to check the injectors & the coil packs with an ohm meter. I accessed the manuals on Autozone.com, but could not find the procedure to do the tests.

If its not too detailed could someone post it here? Thanks.

A good reference for misfires is under load usually points to the plugs or wires. Misfire at idle usually points to the fuel system such as a bad injector.

I have yet to see any plugs go to 100k miles. Sure some do but some don't make it. The plugs are designed to go up to 100K different factors play a role in the plugs not lasting to 100K. Temperature, heat cycles, vibration, gas and the driver all affect the plugs.

Most plug wires should be changed at 50K miles.

As for the misfire counters.
During start up the engine can record a misfire. On a rough road the engine could read a misfire. I don't get too excited about the misfire counter since it can lead you down the wrong path. The only time I used the misfire counter is when I am looking for a misfire and I use the current misfire counter as the base of searching for my problem. The history counter is nothing to base a problem on unless you have a SES light on and a code stored. Then use the history misfire counter to determine what cylinder is misfiring. After you determine the cylinder now you use the current misfire counter and do your testing. The misfire counter is more for testing than for finding your fault.

One of my fellow techs chased a misfire for 2 days because he used the counter and conviced himself there was a problem. The current counter would show 1 or 2 and the history counter would show 200 misfires. One or two misfires isn't a problem.

A good reference for misfires is under load usually points to the plugs or wires. Misfire at idle usually points to the fuel system such as a bad injector.

I have yet to see any plugs go to 100k miles. Sure some do but some don't make it. The plugs are designed to go up to 100K different factors play a role in the plugs not lasting to 100K. Temperature, heat cycles, vibration, gas and the driver all affect the plugs.

Most plug wires should be changed at 50K miles.

As for the misfire counters.
During start up the engine can record a misfire. On a rough road the engine could read a misfire. I don't get too excited about the misfire counter since it can lead you down the wrong path. The only time I used the misfire counter is when I am looking for a misfire and I use the current misfire counter as the base of searching for my problem. The history counter is nothing to base a problem on unless you have a SES light on and a code stored. Then use the history misfire counter to determine what cylinder is misfiring. After you determine the cylinder now you use the current misfire counter and do your testing. The misfire counter is more for testing than for finding your fault.

One of my fellow techs chased a misfire for 2 days because he used the counter and conviced himself there was a problem. The current counter would show 1 or 2 and the history counter would show 200 misfires. One or two misfires isn't a problem.

heh... i would normally agree with what you said about plugs not actually lasting 100k miles... but the fact that my 96 century v6 ran great with stock plugs (as in the ones that it came with wen it left the factory) in it until my dad and i just decided to change the plugs for hopes of increasing gas milage last year, at which time it had about 165,000 miles on it. not bad id have to say, lol, but definately not a common thing to come across im sure. in fact, my 91 honda prelude had about 90k on its last set of plugs, til i replaced them cuz it was hard to start a few weeks ago... and they just about desintigrated right in my hand when i touched the electrodes hehe. hope you guys are able to help him out with his problem as i dont really know what else could cause his problem except for plugs wires cap/rotor and coil pack. especially with such low milage. good luck ya'll!

mike

I had the same problem a little while back someoone suggested to check the wires for burnt or brown spots where the wires were getting to hot from another source, i checked the wires and found a wire in the back (93 V6) near firewall was laying on the exhaust manifold and had burnt the wire i replaced the wire and the misfireing and loss of power went away. these were brand new wires too. the wires i had bought were too long and the clips couldnt keep them from falling on to the exhaust manifold.
Just adding for someone else's reference.

A good reference for misfires is under load usually points to the plugs or wires. Misfire at idle usually points to the fuel system such as a bad injector.


I finally stopped by Autozone long enough to look up the Ohm specs for the injectors in the Hayes manual. It says that they should read between 11.8 - 12.6 ohms.

Only 2 of the 6 fall within that range on my Century. 2 of the 4 that were off were pretty close at 12.7 & 12.8 ohms - actually this is kind of an avg. as I have a digital meter & they always fluctuate a little bit so I don't think those 2 injectors are far enough off to cause a problem.

The other 2 read 10.4 & 10.9. Is that far enough out of spec to cause a miss at idle? What does the low resistance mean? Is the computer trying to give it too much gas or not enough? Is there anything I can do to correct the injectors (other than replace them)?
Thanks for any help.

I finally stopped by Autozone long enough to look up the Ohm specs for the injectors in the Hayes manual. It says that they should read between 11.8 - 12.6 ohms.

Only 2 of the 6 fall within that range on my Century. 2 of the 4 that were off were pretty close at 12.7 & 12.8 ohms - actually this is kind of an avg. as I have a digital meter & they always fluctuate a little bit so I don't think those 2 injectors are far enough off to cause a problem.

The other 2 read 10.4 & 10.9. Is that far enough out of spec to cause a miss at idle? What does the low resistance mean? Is the computer trying to give it too much gas or not enough? Is there anything I can do to correct the injectors (other than replace them)?
Thanks for any help.

I don't know if the resistance readings are off enough to replace the injectors in question, however, they are out of spec. Low resistance means your digital meter will read towards zero more. For example:
if the injector reads 5 ohms that is low resistance and I would replace it.
High resistance is the opposite.

However, a miss at idle is indicative of a dirty or defective injector.

Also check the points you measured at in reference to ground to see if they are shorted to ground. If shorted to ground you will get zero (0.00) ohms also known as a dead short. Hold the positive lead to one contact on the fuel injector and hold the black lead to a good metal ground on the engine or the battery negative.

Good luck!