Tell me what these spark plugs mean.....

[Samckitt]

They are from my 03 Silverado LQ4, i'm fighting a bad miss when it is cold, when hot it is fine. These are plugs from left to right: 7,5,3,1.
https://static1.pt-content.com/images/noimg.gif

2,4,& 8, look like 3,5&7. Plug #6 is about halfway between a clean & a carboned one.


So I take it 1 & 6 are the ones missing.

I also did a compression pressure check, it is a 6.0L with almost 170k miles.

Cylinder - Pressure (PSI)
1 - 150
3 - 120
5 - 150
7 - 150

2 - 120
4 - 150
6 - 120
8 - (couldn't get the gauge in)

[Jameson]

Are 1 and 6 wet with fuel after running it and pulling the plugs out?

[Samckitt]

Yep

[cornfedbill]

Here is a reference: http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp

I could be wrong, but 1 and 6 may be misfiring and causing the O2 sensor to see extra oxygen. That will make the ECM richen the mixture. The remaining firing cylinders are then running far too rich. The wetness on 1 and 6 makes me believe they are not firing at all. Otherwise they would be dry.

What do you plug wires look like?

[Samckitt]

Plug wires looked OK. I replaced them within 9 months ago.

[cornfedbill]

There must be something that is causing a faulty spark in those cylinders. Did you try new plugs?

[Samckitt]

I have been fighting a bad miss when cold for a while now. I changed the plugs maybe 6 months ago (i said 9 above), hoping to resolve the miss problem. Took them out last night & this is what they looked like.

[Jameson]

Here is a reference: http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp

I could be wrong, but 1 and 6 may be misfiring and causing the O2 sensor to see extra oxygen. That will make the ECM richen the mixture. The remaining firing cylinders are then running far too rich. The wetness on 1 and 6 makes me believe they are not firing at all. Otherwise they would be dry.

What do you plug wires look like?

X2 this is what I was getting at... Atleast with the good compression you know that it isnt a valve issue not opening or closing, not to mention the fact its wet with fuel and isnt a blown headgasket weeping in water.


I have been fighting a bad miss when cold for a while now. I changed the plugs maybe 6 months ago (i said 9 above), hoping to resolve the miss problem. Took them out last night & this is what they looked like.

It might be a good idea to test for spark on 1 and 6, Are you getting a CEL pointing towards a misfire? Perhaps your coil packs could be the issue.

[Samckitt]

P0300, general miss fire. Not limited to one cylinder. Saw the graph on a tech 2 & it was all over the place.

[Samckitt]

The other code was P0650(I think), something to do with crank position sensor. I replaced that, no difference.

[cornfedbill]

Is the motor stock and still in the '03 Silverado?

This may be a longshot, but I found others with the same misfire and traced it to plugged cats. http://www.performancetrucks.net/forums/tuning-diagnostics-electronics-wiring-161/lq4-misfire-help-441545/

I have seen this before on other cars. It is real easy to miss. With 170k miles, this is possible.

[Samckitt]

Yep motor is stock in 03 Silverado.

Someone else mentioned the cats. Would that go away when it's hot? After something gets heat saturated (after coolant is up to operating temperature) it runs fine, (fine enough that I don't feel the miss).

[Motorcitydak]

If you have a temp gun, use it on the exhaust manifolds checking the temp of each one. If not, do the old time trick. Get a plastic water bottle and poke a hole in the cap. Use that to squirt a little water onto each exhaust runner where they join the cyl head. This is better off being done on tube headers because there is a risk of cracking a cast iron manifold is you change its temperature quickly like that. On a running cylinder, the water will boil very quickly. On a bad one, it will probably just run off. The engine would have to run for a minute or two before checking this tho.

Also you can try with the truck running, pull each spark plug wire off, one at a time. If the engine gets worse, you know that cyl is working just fine. It is stays the same, you know that cyl is not firing. Also with it running like that, put the spark plug wire close to a grounded metal part of the engine. Anything works, exhaust, cyl head, valve cover, etc. If spark shoots out of that plug to the metal, the coil is firing and at least trying to fire the plug. If nothing comes out, that coil is not firing. You can try this same test if you disconnect the wire from the coil itself instead of at the plug. If the coil is mounted on the cyl head, that may be close enough for it to arc to that metal. I have had spark come out of my coil and go about 1-2 inches to reach ground. It is pretty impressive. Make sure you wear thick gloves so you do not get shocked either. That will hurt!

If you think a coil is not firing, you could switch that coil out with another one just to be sure that the wiring to the coil is good but just the coil itself is bad. I assume that this engine has 8 individual coils mounted on the valve covers correct? It is possible that one or more of your coils may have cracks in the epoxy, letting moisture in. Once it gets warm enough, the moisture is gone and lets the coil work the way it should

[srh3trinity]

I was wondering if it could be bad coil pack or two.

[Z06vet]

I would get it put on a good scanner that gives you real time data from all the sensors. Let it sit overnight & see whats going on while its missing. I think this might help eliminate some things or even point right at the problem. I have seen wacky stuff like this from a poor or loose ground. Also, I have not had good reults from those plugs in the past. I found the NGK or ACdelco to work best. I doubt they are whats causing your problem though. I've noticed on my 03 2500HD that its sounds like its missing on first startup in the morning. It goes away after 2-3 minutes and idles fine all day after that. I did plugs & wires because of that, and the truck ran much better, but still has the cold start chug chug chug.. Let us know what you find out. Scott

[cornfedbill]

If the cats are a problem, normally you will feel some degradation at full throttle and high RPM.

It is normal for any engine to run a bit better when it is warm. The fuel vaporizes better and is easier to ignite regardless the problem.

I do agree that you should make some effort to check your coils too.

[Samckitt]

I have a new temp sensor on the way & a new set of plug wires/plugs. I will try a different set of coils i have when that arrives.

[GBNL]

check your coils you might have some corrosion preventing the spark to get to the plug.

[absintheisfun]

Two things I see...Scortch marks on the porcelin of 7 and 5... possibly 1 look like faulty plugs...and they are bosches...nothing against them, but I have found that non delco plugs can burn hotter than what the design perameters are for gm's.

I know that sounds like a ploy to buy delcos and I laughed when I heard it the first time, but there have been many times ive run into it since working at a GM dealership.

Ditch the high dollar name brand stuff and get some delcos.

[cornfedbill]

Two things I see...Scortch marks on the porcelin of 7 and 5... possibly 1 look like faulty plugs...and they are bosches...nothing against them, but I have found that non delco plugs can burn hotter than what the design perameters are for gm's.

I know that sounds like a ploy to buy delcos and I laughed when I heard it the first time, but there have been many times ive run into it since working at a GM dealership.

Ditch the high dollar name brand stuff and get some delcos.

I tried Bosch plugs several times. I worked for Bosch for 11 years. I found that GM engines like Delco plugs. No doubt about it.

I don't run Bosch in my GM cars anymore.

I did, however, run them in my BMW.

[Samckitt]

I put on new plug wires, plugs (Autolites, couldn't find AC Delco ones that weren't $8 each), put on a different set of coils & changed the engine temp sensor. No different, still misses terribly when cold. And again, runs great once warm.

[tooblue]

What about the intake Gaskets, have you changed them?

[Samckitt]

What about the intake Gaskets, have you changed them?

Yep.

[GBNL]

Have you checked your map sensor and its o ring type gasket?
I put on new plug wires, plugs (Autolites, couldn't find AC Delco ones that weren't $8 each), put on a different set of coils & changed the engine temp sensor. No different, still misses terribly when cold. And again, runs great once warm.

[MrQuick]

You can see that #1 is a dead hole. Any of you guys run into valve seat problems? If you can scope it see what happens if you swap injectors. You can also take a temp reading of each exhaust manifold runner. Cold temp = no fire or dead hole. Pull #1 again, should not be wet.

[Samckitt]

Have you checked your map sensor and its o ring type gasket?

Yep, swapped one from a TB SS intake I have. Same PN on it, no different.

[tooblue]

Long shot but do you have a "Spare ECM" you could swap in? Strange bug for sure..

[Samckitt]

Long shot but do you have a "Spare ECM" you could swap in? Strange bug for sure..


Yep thought of it, but haven't looked for one, and won't it have to be reprogrammed for my key/vin to work?

[GBNL]

looks like you might have injector problems. Do you have hp tunners or a scan tool?

[tooblue]

Yep thought of it, but haven't looked for one, and won't it have to be reprogrammed for my key/vin to work?

Not totally sure but I believe those are controlled by BCM (Body Control Module)..

Check / Clean your MAF sensor and check for a good connection.

Sounds like while in Open Loop it is having a problem(s), from what I remember the PCM will use the Maf & Temp sensor for calibration (along with open loop settings in the ECM) until it gets up to temp.. Roger

[Denvervet]

Could be plugged injectors causing lean in those cylinders. I'd run a can of BG44k or techron thru it before throwing too many parts at it.swapping coils not a bad idea too.

[Samckitt]

looks like you might have injector problems. Do you have hp tunners or a scan tool?

A friend not far from me has HP tuners.


Not totally sure but I believe those are controlled by BCM (Body Control Module)..

Check / Clean your MAF sensor and check for a good connection.

Sounds like while in Open Loop it is having a problem(s), from what I remember the PCM will use the Maf & Temp sensor for calibration (along with open loop settings in the ECM) until it gets up to temp.. Roger

I have another MAF to try. I agree it sounds like something in open loop throws it off until it gets hot. Then runs fine.


Could be plugged injectors causing lean in those cylinders. I'd run a can of BG44k or techron thru it before throwing too many parts at it.swapping coils not a bad idea too.

I changed plugs/wires & coils Saturday with no difference, other than it runs good when hot. What is BG44k or techron?





I think it is getting plenty of fuel, definitely smells rich when it is cold & missing.


OK Dumb question now. Would an exhaust leak (assume it is an exhaust leak) around the exhaust manifold cause enough of a O2 bad reading to do this? Like I said I am assuming an exhaust leak, I hear a ticking sound until it gets hot. Ticking sound not heard when it is hot & running fine.

[jlcustomz]

Techron is Chevron techron fuel treatment . A gm specialist I used to know said dropping 2 cans in a low tank of fuel & running was pretty close to the same effect as a can of professional cleaner at the fuel rails. Wouldn't be a total surprise if it was that simple.

Also Bill earlier asked about oxygen sensors, A good scan tool can show how their response is in real time.

[Madspeed]

Disconnect the o2 sensor on that bank. if it goes away you need an o2 sensor. OOH yea forgot exhaust leak too. when it warms up the manifolds and heads swell and close up the leak

[Samckitt]

Techron is Chevron techron fuel treatment . A gm specialist I used to know said dropping 2 cans in a low tank of fuel & running was pretty close to the same effect as a can of professional cleaner at the fuel rails. Wouldn't be a total surprise if it was that simple.

Also Bill earlier asked about oxygen sensors, A good scan tool can show how their response is in real time.

Had it on HP Tuners scan & he said O2 sensors looked OK.

[Samckitt]

Disconnect the o2 sensor on that bank. if it goes away you need an o2 sensor. OOH yea forgot exhaust leak too. when it warms up the manifolds and heads swell and close up the leak

Can't remember if I have 1 or 2 on each side. If 2 I assume unplug both? What does unplugging it prove? Won't it just keep it from going into closed loop regardless of which on is unplugged?

Could the exhaust leak be throwing off the O2 sensors?
Thanks

[Red67Mustang]

Quick thought... Check your temp sending unit. These can fail and when they do the computer no longer knows the engine temp to properly apply the modification tables (enrichment) which can make the vehicle miss-fire / run terribly until warm then run fine.

[Samckitt]

Quick thought... Check your temp sending unit. These can fail and when they do the computer no longer knows the engine temp to properly apply the modification tables (enrichment) which can make the vehicle miss-fire / run terribly until warm then run fine.


Post #21:
I put on new plug wires, plugs (Autolites, couldn't find AC Delco ones that weren't $8 each), put on a different set of coils & changed the engine temp sensor. No different, still misses terribly when cold. And again, runs great once warm.

Already did, but thanks for the suggestion.

[Denvervet]

Exhaust leak upstream would cause O2 sensor to read lean and then compensate by enrichening ALL the cylinders to compensate. A good scan tool should have seen if it was reading a general lean or rich condition though as well as whether the O2 sensors were ok.
BG44k is another type of injector cleaner....most auto guys including euro mechanics think it's the bomb when it comes to cleaning fuel systems short of actual power cleaning.
Need to run that scan tool when motor is cool as well as hot to check for lean/rich...easy check these days.

[Samckitt]

Tried a different MAF sensor this weekend with no difference.