Ok Guys had my C5 Vette since 09 never missed a beat till the other day. Need some help?
Car seems to have a mechanical missfie on #2 cylinder. No codes and no missfire data when I plug in tech 2 and check missfire graph.
This is what i have checked so far just seeing if i have missed anything before I start pulling the head.
All plugs and wires and coils good. Has spark on all cylinders
Fuel injectors good has injector pulse and have measured injector resistance all the same.
Pull #2 injector plug no change to engine pull any other plug and there is a change.
#2 exhaust pipe temp cold. Definately problem with this cylinder
Switched injector 2 and 4 to see is problem moves cylinder however it does not. Same dead #2 no change
Switched coil packs and wires and plugs no Change
Checked fuel rail for blockages all good.
Pulled valve cover off to inspect valve springs rockers and push rods all look good
Checked valves for movement in valve seats all good
Ran compression test all cylinders same around 230
checked all the wiring for rub marks on injector harness all good
sprayed carby cleaner around injectors and manifold no apparant air leaks.
Checked fueses and grounds all seem good.
Car is a low milage car under 50k on it.

My next step is to pull intake and see if intake manifold seal may have failed then pull heads.

Anyone have any other ideas or previous similar issue which may be able to help before i start tearing down this engine?

I have a feeling its something stupid as I had this happen to me abouth 6 mths ago driving noticed a miss was about 2 miles from home got home and it dissapeared checked all the wires found one not seated properly so thought that must hav been the issue as it went away and only now reappeared and will not go away which makes me think it has to be something stupid and not mecanical.

 

So I take it that when you pulled the spark plug on #2 it was clean and dry?

Compression test confirms it's getting and holding air.
Electric pulse to the injector, so it's getting command from ECM.
Issue does not follow the injector, so injector functions.
Coil, wire and plug are getting spark command from ECM
Issue does not follow coil, wire and plug, so coil, plug and wire are good.

So, you have air, fuel, and spark but a no fire condition. Assuming the plug was clean and dry, not fouled by gas, oil or coolant.

Is the misfire present at all times, or just under certain condions? Open loop (cold start) Closed loop (operating temp) Idle, partial acceleration, w.o.t.?

 

If compression test is good on that hole, I’d try a leak down test on same hole. If leak down test is ok, which is likely, don’t pull the head. It will do you no good.
I’m suspecting intake seal here or something amiss in the harness, regarding either the injector or coil, for that hole…which can be a real pain to find. keep in mind that the ECM grounds the coil and injector, so harness wiring needs to be ohm’d out all the way to the ECM, for correct testing.
I’ve also seen spark plugs do strange stuff internally, yet still read as though it is firing fine. Will cause an intermittent issue…hard to figure out, because it fires 99% of the time.

 

No DTC’s or current or history misfires with the Tech 2 ??…what do your fuel trims look like right now ??…with an ignition misfire you’ll see your long terms elevated maybe around 10% and with a injector misfire (lots of oxygen in the cylinder maybe 15% or so…if you think you have a vacuum leak look at your long terms at idle then run the engine at 3000 RPM…if long terms slowly decrease…short terms will go negative that’s a vacuum leak. Was any prior maintenance done to the car ??…now there is only so much you can do with a Tech 2 regarding misfires..I rely heavily on scope testing and in cylinder pressure testing as a last resort…are the misfires felt at idle or also under load…are they constant or intermittent ??…yes, as was mentioned you can do a leak down on that cylinder but that won’t detect all cylinder defects…a wiped cam load is one of them…this is something I’d go incylinder (intake and exhaust too) for and video below !!..oh, and another thing…perform a crank variation relearn under “special functions” with the Tech 2.

 

You seemed to have check everything out. So it's odd that nothing seems to be wrong. How did you measure the injector pulse. The PCM supplies the ground for the injector. Did you connect the probe right to the injector wires at the injector? If you used a chassis ground for the return your results may be incorrect. Also if you did not measure it with a load, injector in the circuit, you might see a signal but if there's high resistance in the circuit you would have a signal, but it might not fire the.injector. It's also odd that there is no check engine light on or a misfire detected with the tech 2.

 

You could pull the injector and leave it connected and make absolutely sure that it is spraying fuel, place it into a bottle or container so there won't be fuel spraying all over the place.

 

As C5MSG2004Vert mentioned as far as injector testing if that’s
where your diagnosis is heading you can only do so much and without a scope you are really be going down a rabbit hole…before pulling the head I’d find a diagnostic shop in your area and at least pay for the diagnostic time and they will tell you what’s wrong and you can do the work…unfortunately I don’t know of anyone in your area…with a scope you can check the coil pack and injector current which can maybe narrow it down if it is that…fuel trims for me anyway is the first thing I’d be checking for misfires…in the pic below the left waveform is a coil pack which pulls a little over 4 amps…the right is the injector which pulls around 1 amp at idle…the arrow points to a little notch which is the injector opening !!

 

Pull the valve cover and look for any sign of valve train malfunction. You already checked the big three (fuel, air, and spark). Kinda swinging in the dark here though, so I'm really interested in seeing what you find

Edit: Disregard, I missed that you already did this. Now I'm really stumped

 

I checked plugs all clean and not fouled and firing also changed thre location into other cylinders. Also moved coil pack and spark plug wire so definately not spark related. Plulled wire off coil while car running strong spark on that cylinder. Put stethescope on injector can hear it pulse. Moved injector from position 2 to 4 still the same.

Issue is pressent all the time from cold start up to operating temp and all through rev range but no misfire data on tech 2 at all.

Checked injecter pulse while running on that cylinder with injecter pulse light probe and was working

Valve cover has been pulled off no broken valve springs or abnormal movement when i put sideways pressuer on the valve springs which would indicate valve seats appear to be fine but would only be able to 100% confirm if i pulled head off.

I went to see a buddy of mine that specialises in LS engines and tuning cars for the track he has a very good reputation and we re checked everything I said above at his shop and he pluged in his HP tuners software and was turning off each cylinder at a time and definately problem with #2 but nothing specific came up o2 sensors all working no codes. He reccomended i pull the intake first look for a bad seal and if dont find anything he is saying its in the valve train either a bad lifter not pumping up or worn valve guide or cam issue.

I just have a feeling its something else as I had this problem once before for a few miles about 6 mths ago and went away which is why i am questioning everything before i pull it down.

 

So again I ask what do your fuel trims look like ??…can you feel the misfire ??..it’s all well and good that you can hear the injector working but that tells me nothing..you have to go a little deeper to diagnosis this…your tuner may be good at tuning LS engines but diagnosing is a whole different skill set !!…don’t know what else to tell you…the crank sensor is the misfire “monitor” so to speak so it’s strange it’s not picking #2 up…if I can’t see misfires on the scan tool on the enhanced side I’ll go into Mode 6 and sometimes you’ll see it there !!..good luck !!..which “seal” is your tuner referring to ??

 

It just seems strange. The only thing that would seem to explain it to me would be that the injector is getting pulse from the ecm but for whatever reason is not actually delivering fuel. The ecm is sending pulse, sending spark so it thinks everything is fine. Although I would think the crank sensor and knock sensors would pick that something wasn't right.

Maybe something blocking the fuel at the top of the injector where the rail feeds it? That would be the only explanation for no fuel delivery that doesn't actually follow the injector?

 

Maybe this youtube video will help???Misfire strategies for today’s vehicles with Dave DeCourcey

 

I have pulled the fuel rail and checked it for rust. Its clean with no obstructions.

 

Get yourself a fuel pressure gauge and do an injector balance test with the Tech 2…you might have a clogged injector (not rust and have never seen that) and that’s why I asked you what your fuel trims were.

 

Wouldn't a clogged injector follow the injector when OP swapped 2 and 4? Unless there's some sort of obstruction in the fuel bell leading into the injector that OP missed when he inspected the rail

 

Thanks young man !!…I’ll have to reread his post again !!

 

I made a similar mistake when I made my suggestion of checking for valve train issues. OP did a great job laying out a very extensive list of checking for things, and I missed the valve train inspection he already did. I'm following this thread because I'm absolutely stumped as well. Obviously, there's a problem, but I'm completely stumped as to what it could be and I'm really interested in seeing what the eventual solution is.

OP-- maybe pull the fuel rail completely off and run water through it. Look to see that the water flows freely out of all the fuel bells? Obviously get it good and dry before installing it back in, but maybe there's some kind of obstruction inside the rail that wasn't apparently when you did your initial inspection?

 

I’m still waiting for him to say what his fuel trims look like ??…this could be something like a cam lobe issue which you won’t see on a compression test…I’d say he should take it do a diagnostic shop…there is only so much a DIY’er can’t check without specialized equipment !!

 

Whatever is going on, what we know for sure, with the information supplied, is that A) the cylinder is not firing (cold exhaust manifold at #2) and B) the cylinder is not getting fuel (dry spark plug) which also rules out coolant in cylinder, and excessive oil in cylinder.

So, my best guess is one of two things. 1) There is a ghost in the machine, or 2) This thread is actually just one of those "try and solve the impossible riddle" things. If it's 1, good luck. If it's 2 good job.

 

Hi Guys here is the update of what ended up being the cause of the problem.
After all the diognostics and pulling my hair out checking for mechanical failures and vaccume leaks injector problems wiring etc etc I also checked the fuel trims and checked for codes using tech 2 and snap on scan tools which nothing showed any misfire faults on the graphs or any fault codes what so ever of any kind. I then pulled the wiring and checked continuity between the injector plugs to the ecu which were all fine and inspected the wiring which was also fine with no damage and pulled all the grounds and checked them which were also clean and tightly secured and had no corosion.
I then decided on replacing the crank position sensor after speaking to a mechanic buddy of mine that said he went thorugh similar issues as me on a customers car and found this sensor to be causing his problems with no codes. I also had noticed the dead cylinder had moved from #2 to #4 cylinder which had me really puzzled. ( This made sence later as I pulled those two plugs to do a leakdown test and must of switched them when I reinstalled them)
So I had to remove the longtubes and the starter installed new crank position sensor and reinstalled everything and the spark plugs but ended up not putting them back in the same order and upon restarting the issue was now cylinder 6 as I was measuring the temperature of the cylinder with a heat gun on cold start and problem got more noticlbe as engine warmed up and now #6 was cold and misfiring.
Well thats when the penny dropped and I said to myself this must be a faullty spark plug mind you they only had 5000miles on them and they were GM oem denso plugs so I pulled it and switched it with #1 and sure enough the problem moved now to #1. BAM we have finally foud our culprit.
So to clarify up until this point the plugs were all the same colour and not fouled and when you pulled the spark plug wire off the coil and held it close it was throwing a strong spark hence why I did not think it was a spark related issue.
So now I am thinking the spark plug was not causing issues until it got some heat into and started breaking down as on cold start I was very hard to tell the car had a miss on idle until it started warming up as the car has a cam in it but at the point when I finally figured it out and I pulled it out after moving it to # 1 for varification it was now completely dead and the plug was very wet with fuel.

So guys thanks to all those who chimmed in to try and help and I hope my experience may be able to help someone else as this is the first time in all my years I have seen a spark plug cause an issue and not show up as a missfire on the graph or throw a code.