Misfire Cylinder#8 in my Tahoe

[AkursedX]

Well I'm going to post this here since I haven't received much help on my Tahoe forum.

I'm just going to copy and paste my posts:

Well I will first start off with some background.

Over the weekend, I replaced my passenger side exhaust manifold gasket and fixed and extracted a broken header bolt by cylinder #8. Although a bit of a pain, everything seemed to go well and I got the bolt out and everything reassembled.

I then took the car out for a drive to the grocery store and back with no issues.

Fast forward to this morning. I had to take my kid to the doctor and I fired the Tahoe up and noticed right away that it was not firing on all cylinders. After I got back, I pulled up my codes and did a quick scan via HPTuners. I got a misfire code for Cylinder#8. My scan showed misfire counts around 180 at idle and the O2 sensor and fuel trims indicated to me that fuel was being injected and then unburnt. That seems to indicate that I have no spark in that cylinder.

I swapped a spare wire I had and also replaced the plug and there was no difference. My next step is to switch the coil-packs around.

I should also add that the Tahoe spent the night sitting out in a fairly heavy rainstorm and got very very humid, things that normally don't get along with electrical systems.

So my next step is to put in a new coil-pack and see what happens. I am hoping this will remedy the issue.

If this doesn't rememdy the issue, where should my next step be?? Compression check? Fuel injector? I don't know and could use some direction.

So I just thought I'd update.

I went to the parts store and picked up a new coilpack. I changed it out in the pouring rain and voila! she's back up and running again.

I'm glad it was just the coilpack and I didn't have to go any further....

Dammit......


I drove my Tahoe into work today. For most of the ride it was running fine. I was almost to work and was sitting at a light. I started noticing the same sputter again.

I stepped on the gas and the truck started shimming again and the SES popped back on.

I can't check the codes yet, but I'm willing to bet it's the same cylinder misfire. If that's the case, I'm leaning torwards a leaking injector. Considering that it was running fine with the new coilpack tells me that it had a strong enough spark to ignite the extra fuel, but as time went on, the extra fuel probably fouled out the plug.

Next in line, a fuel pressure test and probably a new injector.....

So do you think I am on the right path? I'll know more tomorrow when I can get it on the scanner and tear into it some more.

Oh, the specs are an '06 Tahoe with the 5.3 flex fuel v8

[CincinnatiFiero]

What does the plug look like? I assume you've already looked at it, but if you haven't I've seen plugs crack the porcelain and cause a misfire.

[AkursedX]

plug looks normal. Light brown. I am on plug#2 right now so I'm pretty confident it's not the plug.

I really hope it's the injector. Considering that the truck was misfiring, then recovered, then started misfiring again makes me doubt that it's an internal problem. If it was something internal, I wouldn't think it would be intermittent.

I will update tomorrow one way or another.

[CincinnatiFiero]

Does it smoke or burn a shitton of oil? If not you're making compression.

Be aware you may need a specific injector since its flexfuel. Keep us updated.

[AkursedX]

Today's update:


Well I got out this morning and the first thing I did was check fuel pressure. It would fire up to 55psi when the pump primed and would drop to around 48psi and held there after 15 minutes or so. I also popped the fuel rail off and did a visual for any leaking injectors. Everything looked good. I guess this spoiled my leaking injector theory.....


So I once again throw the scanner on it and check the codes. To my surprise, I did not have the cylinder misfire in #8 code anymore! But I did have the multiple cylinder misfire code along with a low voltage circuit coil#6 code. I have had this code pop before and not had any issues.

I reconfigured my hptuners scanner to scan for each individual cylinder misfire, O2's for each bank and fuel trims for each bank along with some other stuff. I then went out on a 45 mile drive to see what the scan would tell me. About 30 miles into my drive, I was sitting at a light and when I took off, I could feel the misfires coming on. It lasted about 5 seconds then went away. It wasn't enough to set off the SES.

I got back home and discovered that my misfires were now coming from cylinder #6 along with a few in cylinder# 5. So I have discovered a new problem.

I decided that I was going to go out and buy a new coilpack for #6. Before I went out, I played around with the coilpack connection to the harness. When I started it up, the misfire and SES came back right away. I turned it off and played with the connection again, and it was gone. I went to the store and bought a new coilpack and installed it and have had no problems in the 35-miles since. If I get the same low-voltage code, my next step will probably be to buy a new connector for the coilpack but hopefully the new coilpack will remedy the issue.

[CincinnatiFiero]

What pressure does the FSM specify the fuel pressure should be?

[Atilla the Fun]

Too long, did not read. A.D.D. but LSx-family engines are known for overheating around #8. And since that's where your symptoms are, it seems reasonable that this could be a contributing factor.
Are you active on www.ls1truck.com?