Misfire cylinder 7 after it warms up, advice???

[Jeffgis12b]

I recently purchased a 2010 SS with 13,000 miles. It has a magnuson 2300 with 6lb pully, injectors, stall,long tube headers, Borla exhaust,ect...

I have drove it a total of maybe 300 miles and i am already having a major issue. When its cold after first start up it runs flawless but after it warms up for maybe 10-15 minutes it starts to misfire on the drivers side at Low/Mid rpm, #1 seems to be ok, #3 and #5 have minor misfire but mostly cylinder #7. It still seems to run great and pull hard at full throttle. I called the shop that tuned it and he said I prob blew the motor and to drop it off, but in my head it seems odd that it runs fine when cold and seem to run fine at full throttle so i was hoping for some advice before I break the news to the wife that the car I just wrote a check for $2000 more then Kbb valued it at needs an engine. (no need to tell her about the modifications ,right lol)

It is getting a poping sound in the header at low/mid RPM and I assume its from raw fuel hitting the headers and igniting. I did pull the plugs and #7 was wet with fuel so I replaced it with a new plug and seems to be doing the same thing. It had 16 codes but mostly to due with rear O2 sensors that it doesnt have so i assume its tuned out as the check engine light doest observe them at normal operating. I have parked it until i figure it out so i dont wash out the cylinders or do more damage.

If anypone has any advice feel free to chime in,I will try to answer any questions that might be of help.

[EM1/SS]

Have you talked to the person you bought it from to see if they were having any of these problems.

[Jeffgis12b]

The guy I bought it from knows very little about cars or performace mods and never had a problem but he bought it as a conversation piece and investment, he only put a couple hundred miles on it in 2 years and soon relized the $890 bucks a month payment wasnt fun anymore. I talked to the guy that had it built and he had zero issues and was somewhat suprised i was dealing with any issues. I put almost 300 miles before an issue surfaced.

FYI I did hit the rev limiter a couple times by mistake as I was trying to Learn the car.

[Unreal]

Pull the #7 plug and look for damage.

Seen cars with cracked cylinders only smoke when they warm up and crack opens up. Same with piston crown go. Very easily could be a popped motor.

Check spark plug wires too while you are checking the plugs.

[simon.w]

The only way to know for sure is to trailer it to a reputable tuner and get them to log it.

You could spend hundreds of $ and waste many hours of your time trying a process of elimination before you figure out what's wrong. By then you could have caused irreparable damage.

And even then it could be, for example, a broken ringland in #7 that is causing the misfire (worse case scenario) ... and the only way you are going to determine that is with a leak down test.

Trust me ... I had a misfire and went through the painful and expensive process of replacing my MAF sensor, 02 sensors, plugs (twice) HT leads, MAP sensor, spent countless hours looking for vacuum leaks and exhaust leaks, and was about to replace the coils ... in the end I decided to take it to CMS ... within 10 minutes he told me it was my injectors. Same thing happened with my broken ringlands ... I went through 3 catch cans and a metco breather cap before biting the bullet and getting a leak down test; which by the way, confirmed my fears ... but at least I knew for sure what was wrong.

[Jeffgis12b]

I plan on getting it to a shop to have it looked at, I was just thinking maybe there was something I can do in the mean time - when I inspected the plug is was wet from fuel but no oil and it doesn't smoke a bit , just weird that it runs flawless until it warms up for 15-20 min and when i am on the throttle but acts up under low/mid RPM - either way it's gonna get fixed just not sure wife is gonna like it ��

[steven2680]

First change all 8 plugs and see if it stop misfire if not the check coils also check spark plug wires

[simon.w]

Originally Posted by Jeffgis12b

I plan on getting it to a shop to have it looked at, I was just thinking maybe there was something I can do in the mean time - when I inspected the plug is was wet from fuel but no oil and it doesn't smoke a bit , just weird that it runs flawless until it warms up for 15-20 min and when i am on the throttle but acts up under low/mid RPM - either way it's gonna get fixed just not sure wife is gonna like it ��

Yep ... can sympathize with you and your wife predicament.

FYI a cracked ringland will not necessarily mean it smokes ... mine didn't.

My money is on:

1. Coil pack that fails when hot, so plug doesn't fire
2. HT lead that has a break and resistance increases when hot, so plug doesn't fire
3. Faulty injector
4. Broken ringland (I highly doubt this ... wouldn't just do it when hot ... check inside your throttle body for lots of oil)

If you want to try anything yourself without costing you money (and eliminating the top 3) ... do the following one at a time to see if the misfire shifts to a different cylinder:

Swap the following between cylinders 1 and 7

- plug leads
- coil packs
- injectors (Do this last ... as it's the least likely IMO and hardest to do)

FYI - I'm pretty sure you can remove the fuel rail and access the injectors on a Maggie without removing the blower ... you may just have to remove the vacuum actuator to gain access (someone else may be able to confirm).

Be VERY careful removing fuel rail off injectors and injectors from plenum. They have tiny o-rings that are easily torn. Same when you re-fit ... I use a smear of light oil on the o-ring before re-assembly. Also make sure they are kept ultra-clean ... so clean down the surrounding surface of the blower, fuel rail and plenum before removal. Keep the injectors in a zip-lock bag or wrapped up in a clean shop towel. I know it's overkill ... but that's just the way I am.

Make sure you bleed the fuel line at the quick connector down next to the passenger side strut tower first - It's much easier to collect fuel run off there in a tin and rag than at the end of the fuel rails. DO NOT DO THIS WHILE THE ENGINE IS HOT OR EVEN WARM!!!!

Let us know how you go.

[stevieturbo]

It could be as simple as a plug or lead, or even a coil.

It's a bit drastic thinking it could be the motor just yet

[Jeffgis12b]

I swapped coil and wire from #7 with #2 and new plugs -after warm up it showed misfire #7 - did compression and all 4 on left bank was 145-150psi , no guarantee my gauge is perfectly in but in my mind the main thing was all 4 on that side were the same. I also unplugged O2 sensors to see if that changed anything and it actually did it worst - I am thinking maybe swapping injectors around but not sure why it only does when warm, that's the strangest part for me -

[stevieturbo]

Could be an injector as you say, could be a valve sealing issue when warm, lifter/rocker etc Although the latter less likely.

[simon.w]

Originally Posted by Jeffgis12b

I swapped coil and wire from #7 with #2 and new plugs -after warm up it showed misfire #7 - did compression and all 4 on left bank was 145-150psi , no guarantee my gauge is perfectly in but in my mind the main thing was all 4 on that side were the same. I also unplugged O2 sensors to see if that changed anything and it actually did it worst - I am thinking maybe swapping injectors around but not sure why it only does when warm, that's the strangest part for me -

Hmmm ... OK, so at least we've eliminate those two things.

It is possible that there is a break or bad joint in the wiring loom that connects to the coil pack. Heat causes things to expand and also increases resistance. Run it until it has warmed up and the misfire is happening, stop the engine then pull the HT lead off the #7 plug and connect it to a lose plug ... just leave it resting on the headers for ground ... restart the car and do a visual inspection to check if the plug is sparking (or erratic).

If it has strong and consistent spark, I'd try the injectors next.

BTW ...a compression test will not necessarily tell you if you have a broken ringland ... you need to do a leak down test to determine that.

[Jeffgis12b]

Tonight I had a friend come over that is a mechanic by trade and brought is computer with the EFI Live tuner to get better monitoring. We pulled compressions cold and hot and all were really solid , with his gauge they all read 185-200PSI. We pulled the valve cover and everything looks great, ,took out the oil filter and looked for and signs of metal and couldn't find anything that didn't look great. We swapped injectors, coil and wires around. We didn't have the stuff to do an actuall leak down test tonight. It takes driving the car 15-20 minutes to warm up before it misses at all then started misfire on #7 but it comes and goes , very strange that if it was internal that it wouldn't make noises and it comes and goes- we was wondering if there was a chance the ECM was acting up and maybe going out ? The O2 sensors was appearing to work fine from his computer data but I swapped them just to make sure and see if it moved to the other bank and it did not. Isn't losing any coolant, no blowby whatsoever, it's like a damn Gremlin took over - he is at a loss as much as myself.

I honestly didn't see this being a problem with a car with 13k miles that spent most of its life driving and being babied to car shows(personally I don't do car shows but the previous owner was really into them)

[UNRIVALED SS]

I've seen this with a leaking intake gasket seal. In that instance the car would really surge once warmed up idling at slow speeds. Food for thought .