Cferrel4

Hello everyone,

Normally I am quite good at trouble shooting misfires, but this one is getting me a bit.

The vehicle is a 99 Firebird Formula, it has 170k miles on the original motor, trans, rear end. There is no modifications to this engine. The only performance mods are C6 wheels and some tubular suspension things which would not affect this. This engine had been tuned up (spark plugs, wires, throttle body cleaning, injector cleaning regularly). Front oxygen sensors are new within 10k miles since the heater element failed. My wife noticed it had a "vibration/shake" at idle. The engine still feels great off idle and pulls great at higher rpm as well. This car keeps right up with my 02 formula M6 with 23k original miles when we go racing together.

I have access to a Snap on Versus and Matco Maximus. Both showed no history, pending or current codes, but you can feel a slight vibration at idle. I checked the misfire counter and lone and behold at idle cylinder 3 is misfiring. Can watch the misfire counter climb. Off idle the misfires go away, driving down the street/highway no misfires.

Long term and short term fuel trims are within + or - 5 at an idle. Always stay well within the factory + or - 10.

Calculated load gets as high as 96%, even swapped a couple spare MAF for the best result since its quick and easy.

I sprayed the intake with intake cleaner for a leak, no change in fuel trims or how it idles.

I swapped the spark plug and wire with spare new ones, no change. Spark plug showed no signs of carbon tracing, burning anti-freeze or fouling out.

This engine does not consume any anti-freeze. The cooling system is drained and flushed every 3-4 years to keep things from getting to acidic.

I did not swap the ignition coil because I know that they traditionally fail more when there is a load/higher rpm and this is not the case. I do have a spare and could switch it...

I did not swap the injector yet because I have thought that would be a consistent issue across the rpm range.

I am getting a compression tester this afternoon to see if it is low. I am thinking though that poor compression would be a consistent problem throughout the rpm range. Similar to that of a bad/broken valve spring.

I am trying to correct this problem because I do not want to destroy the oxygen sensor and converters over the long run. This is also our go to travel car in the summer if we do not take her Impala on long trips.

Has anyone run into this issue before with their LS1? A single cylinder misfire at low rpm that goes away?

a05c

How old are the plugs?

Cferrel4

4 years, 12k miles. I already switched that to new for the sake of trying, No difference.

pdxmotorhead

I had a 4.3 v6 do that it was a combination of a leak a the intake manifold (bottom side) and a intake valve stem that was dirty and not letting the valve fully close/seat right.. Compression test showed nothing found it with a leak down. (Air coming out the intake manifold.. )

a05c

I'd move the coil and see if the miss moves with it.

sbcgenII

iTrader: (1)

I would swap the injector and coil and see what happens.

Cferrel4

Switching the coil made no difference.

I am not sold on an injector fault since it tends to be an issue throughout the power band. I will switch it this weekend to see for the heck of it.

I will also try and get a leak down tester from a friend. May also try to get a fitting to put in to air up the cylinder.

sbcgenII

iTrader: (1)

You don't think a clogged injector could cause a Missfire throughout the rpm range?

Cferrel4

If an injector was clogged it will cause a misfire throughout the entire rpm range.

I switched it, no difference. Did a compression test, came up good.

Haven't done a leak down test yet, need to get the proper fitting.

Thinking it could possibly be a worn valve/guide that will actually work at higher rpms but not be happy at low idle rpms.

RockinWs6

Have you tried loosening all the intake bolts then torque them 89 inch lbs? Just because the rpm or fuel trims didn't change when you sprayed carb cleaner doesn't mean there are no intake leaks. If I were you I'd try a unlit propane torch around the back and under the intake AFTER loosening the bolts and torqueing them. Also its possible you have a bad or dirty lifter, this will cause misfires you can feel at a idle. Also have you checked if a little oil is getting on plug causing misfires? As far as having any real trouble from a few misfires at a idle I wouldn't worry too much about.

Cferrel4

Just wanted to let everyone know. The valve guide was indeed worn in the cylinder head. Heads are off to get cnc ported and fixed. Looking at a new cam as well.

Essentially the cylinder head was the problem for everyone to know.

sbcgenII

iTrader: (1)

Thank so for the update.