BDMcGrew

Good evening all, greetings from Lake Havasu City, AZ!

I have a '09 Suburban 1500 LTZ with the L76 6.0L/V8 engine that I'm having a great deal of problems with right now. It's running horrible and OBD is throwing a P0307 Cylinder 7 Misfire Detected and, did I mention it's running like crap! I'll preface my story by saying I'm not a mechanic I only play one on Google but I do consider myself an expert problem solver and usually grasp a good understanding of the things I'm working on!

So for starters the fuel pump was replaced a couple years ago. Just after a year on the new pump the repalcement failed, shop said this was common, warrantied it and life was good - for a while. Then I started having power and stalling problems. Took it to Chevy and they said bad cat's so the shop (not Chevy) replaced the two cats. I later heard a rumor that there are in fact 3 cats but I have not confirmed this yet. And, the check engine light is on. But, it did run alright.

Last week I got a warning to service the traction control and the car started running like crap. My first though was that it was a bad sensor and maybe it was trying to rapidly apply the breaks which would account for the shaking. Today I scanned it looking for a traction control code and instead got the cylinder 7 error.

First thing I did was new plugs and wires, cleared codes. That did not fix it.

Then I swapped the ignition coil from cylinder 1 to 7, cleared codes. That did not fix it, the error stayed on C7.

Next I tested the fuel pressure. Key on, engine off I have a solid 60PSI. Start the engine and have ~58PSI for 10 to 15 seconds and then it starts running real rough and pressure is jumping between ~57PSI and ~60PSI and the jumping pressure is in unison with the engine firing so rough.

I've read this can be indicative of a fuel pump or an injector but now I'm at the end of my limited knowledge as to what to try and and I'm hoping you experts can shed some light on it for me. Where do I go next?

Could this have any relation to the traction control error at all? And/or the cats?

Appreciate any and all you guys can offer!

Thanks in advance,

-brian

73shark

Welcome to the forum.

There are three cats, one on each side of the block and the main one downstream in the exhaust pipe.

Need to do a compression check. No. 7 is one of the cylinders affected by the AFM. That is an expensive fix.

BDMcGrew

Thank you so much for the reply. As I said I'm not a mechanic but a DIY guy that learns as necessary and so I'm sure your knowledge shadows mine on the order of 10 to 1. At first read your comment made no sense (ie. compression relating to a misfire) but after a bit of reading and a couple hours in YouTube University it's clear now why you made that point. Holy crap I'm glad I don't do this for a living but I'll be ordering a compression gauge and check out all the cylinders next weekend.

Do you know approximately what the compression should be on the 6.0? I can't seem to find an exact number just a ~120 to ~180 range and I'd think that's kind of a general number for combustion engines and there's likely a target pressure of sweet spot for this engine?

Thanks again for the help!

-brian

73shark

Not sure the exact number but probably in the 150-160 psi range. But the critical thing is that they should all be within about 10% of each other. Also the throttle should be held wide open and ignition disabled.

BDMcGrew

Again, thank you! I finally have a minute of less chaos and going to work this tomorrow. Can anyone point me to a procedure for doing a compression test? I remember on the old Nissan's you'd start the car and pull the fuel pump relay until it died. Is it still the same or is there a more detailed or strict procedure to follow?

Thanks agian!

73shark

You will need to remove all of the spark plugs.

intheburbs

Find a shop that can do a relative compression test. This is an easy way to see if you might have a problem. It measures the current usage of the starter (as it fights the compression of the engine) to check that you have even compression on all cylinders. Easier and faster than pulling plugs and attaching a compression tester to each cylinder.