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Gary, ASE Certified Technician
Category: Pontiac
Satisfied Customers: 743
Experience:  Senior Technician
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There is spark on cyl 3 the misfire counter goes to zero

Customer Question

There is spark on cyl 3 the misfire counter goes to zero about 1300 rpms
Submitted: 1 year ago.
Category: Pontiac
Expert:  SuperDave replied 1 year ago.

So at higher rpm's it does not misfire.At idle you should have a bright orange spark.

Customer: replied 1 year ago.
Correct missfire goes away higher rpms over about 1200 to 1500 which is where it would idle before i did the intake upper and lower gaskets
Expert:  Gary replied 1 year ago.

new expert here

i am sorry for the long wait

was it misfiring before at idle ?

Customer: replied 1 year ago.
Hard too say it idled about 1200 when she asked me to do the upper and lower intake gaskets. I did those put a tstat and coolant temp sensor cleaned the iac the throttle body put plugs wires new. Found missfire p0300 watched counter cyl 3 swapped coil packs injectors did compression test 210psi still stays on cyl 3
Expert:  Gary replied 1 year ago.

most of these that will misfire at idle then go away when you bring up the rpms off idle and stop missing end up being a vacuum leak from the lover injector o-ring or a vacuum leak in area of the # ***** cylinder usually the upper intake

can you be sure there are NO vacuum leaks testing with a can of carb cleaner ?

Customer: replied 1 year ago.
Iv sprayed two cans looking i replaced all injector orings when intake gaskets were changed. When it goes in to get a state inspection it fails from missfire
Expert:  Gary replied 1 year ago.

if you swap # ***** injector with # ***** injector will the skip move ?

Customer: replied 1 year ago.
Im not sure i have put diff injector in #3 from a good one i had in my tool box and nothing changed sythiscope they all nice loud click
Expert:  Gary replied 1 year ago.

I would be doing a leak down test on Cylinder 3, since swapping injectors, coils and spark plugs has not moved the fault. Clearly the fault is within the cylinder. If the engine wasn't misfiring prior to the intake coming off, did a piece of foreign material or gasket fall into the intake port of cylinder 3?

Seen gasket material under valve seats too many times to count now.

I had an engine recently, identical fault to yours, had a valve not seating due to one collet missing, can you believe that? Lucky it didn't drop into the cylinder, took a lot of effort, but did the whole repair without head removal.

Please do the leakdown test, compression test may not reveal your fault.


Customer: replied 1 year ago.
I will get to that soon as i get out to the shop today
Customer: replied 1 year ago.
Not the valves
Customer: replied 1 year ago.
If i unhook the 24x crank sensor the missfires goes away is that the 1 that tells computer about missfires i tested with volt meter switches from 9.8 to zero by hand
Expert:  Gary replied 1 year ago.

The 24x would be engine speed, the engine may still start and run from the cam sensors.

Does it start with the crank sensor disconnected?

Are all the injectors connected to the correct harness connector for each cylinder?

What pressure did you have on the first gauge with the second gauge at set?

100 psi is what I use.


Customer: replied 1 year ago.
it still starts with 24x crank sensor unhooked. Yes all injectors only hook to there correct injectors as been that way since new i guess. Second gauge wont move i turn that to the set not like my old set went bad it can set 30 mins it dont change its not mechanical
Expert:  Gary replied 1 year ago.

Ok, that's a good sign.

Can you get that the NO.3 injector without lifting the intake?

I want to know does it have injector pulse with a noid light while the engine is running?

Have you swapped the spark plug, injector and spark plug wire to other cylinders?

Have you checked the oil pressure? I have seen valves not seating from excessive oil pressure.


Customer: replied 1 year ago.
My brother has my noid lights he is bringing back tonight. I will test then and get back to you
Expert:  Gary replied 1 year ago.

Ok, I will await your reply.

Best of luck for today.


Customer: replied 1 year ago.
I think i got it i am going to keep this open till i drive it tomorrow. There was a bare wire on cyl 3 injector wire swapped with new connector and found the brake booster fitting on intake was loose right over cyl 3 intake runner
Customer: replied 1 year ago.
Sorry its back cyl 3 going nuts she had a new cat pit in bout 1.5 years ago its been awile since i really watched 02 sensor data seams funny within a few seconds cold start its in closed loop? And is it possible the starter is part of the issue? Every bout 4 starts it sounds terrible lots of grinding and engine noise?
Expert:  Gary replied 1 year ago.

Are the cats factory or aftermarket?

Is the the grinding noise from the starter motor or the flywheel ring gear?

Where is the crankshaft sensor located?

It is entirely possible you have a valve problem from the lean condition that existed with the poor injector harness connection and the vacuum leak.

What O2 sensor voltages do you have at idle, pre and post cat?


Customer: replied 1 year ago.
She is not sure if factory or aftermarket. Under 100mv sensor 1 700 to 800 mv second i can freeze frame tomorrow. Has 2 crank sensors one firewall side 1 3 5 side and the 24x. She had drove for almost two years aftef i first diag a no start from bcm security found the intake issues then. Starter noise sounds like the drive not engaging all the way when it does that there is no miss fire till almost like looses a ground
Expert:  Gary replied 1 year ago.

Ok, sensor 1 is certainly lean. Is the 700-800mv post cat for that bank?

Can you swap the front o2 sensors bank for bank?

What is the cranking voltage?

Are all the ground points clean and tight?

Did it have the noisy starter prior to the no start condition (BCM failure)?


Customer: replied 1 year ago.
the 700 to 800 is post cat only got 2 sensors 1 pre 1 post cat after y pipe. I will look in the am to see if they can b swapped. No idea if all the grounds even there. No good diagrams that i can find. Just started with the noisy starter her bf informed me. It sounds honestly like the thrust bearing is allowing the flexplate to move. Iv had many sbc sound that way after i been told engine is good bought to throw in the towel on this 1
Expert:  Gary replied 1 year ago.

Hi again,

I can see I would be starting to "throw in the towel" as well, LOL.

Can you check the crankshaft endfloat?

Had a customers car that had a complaint of noisy clutch, ended up being no thrusts left on mainbearing. New engine required. Lack of servicing.


Customer: replied 1 year ago.
No real good way with the flexplate still attached
Expert:  Gary replied 1 year ago.

How about at the crankshaft balancer?

I had one engine that I needed to verify crank endfloat. I was able to use a dial indicator and consistently get .005 as my reading, for which I was happy.

If you can do it at the balancer, and you end with .020+, I say that's a good place to start.

Then sump off for inspection.

How many miles has the engine done?


Customer: replied 1 year ago.
208k iv seen many more on these old v6 gms very weird
Expert:  Gary replied 1 year ago.

I'd say at 200k+ miles, it's probably getting a little "long in the tooth".

I remember spending 30 hours looking for a fault another young tech caused, it took me about 10 minutes to fix it once I found it. All ok while under warranty, but who pays when retail?

Difficult to quote a customer like that, and you are now in the same predicament.

I feel your pain with this, does your customer understand, and are they still willing to spend, or have you proposed to them "what do you want to do now etc?".

I would like to be with you, sometimes difficult to advise when I can't hear or see what's going on.

Your thoughts now?


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