My 2005 tahoe, which has been in the family since new, with synthetic oil changes no more than every 5000 miles, all of a sudden died while idling to charge a dead battery. When I tried to start it back up, it wont idle without a lot of throttle. The dead battery cleared the codes except for P0300, multiple random misfire. I changed plugs and wires because it was due even though I knew that wasnt going to fix it, and then had it towed to a buddy's shop. He called me and said its got 0 compression in cylinders 6 and 8.

A few weeks ago, I had an airbag light and I went to pull the codes and saw the P0300, but dismissed it as I needed plugs and wires, and bank 2 catalyst below efficiency, which I dismissed again because I thought downstream O2 sensor. Truck ran and drove fine. I left the GPS on for a few days and the battery was dead, jumped it and let it idle for a while, and when I came out half hour later it was off. When I did get it started again the temp was at 210 (completely normal) and it had 40lbs oil pressure (completely normal).

My thought is that since its 6 and 8, and I believe those 2 are opposite in stroke, that the head gasket between those 2 popped. The mechanic doesnt think so because he said normally a head gasket you get SOMETHING for compression, but my thought is that 6s piston going down is sucking from 8's cylinder while its trying to go up, and vice versa, causing the 0. He said he also only got 90 on cylinder 4, and then he just stopped testing, but it just doesnt make sense that all of a sudden, while charging the battery idling in the driveway, it has this catastrophic failure.

I told him to drain the oil, if theres any metal in it, Im towing it home and selling it on marketplace for some kid to swap the motor and squat it on 28s. If no oil in it, pull the head and lets see whats going on.

Anyone else have thoughts? I was originally thinking it was a crank position sensor, upstream O2 sensor, or clogged cat because it smells like fuel coming out the exhaust, not oil. I thought it was just overfueling from a bad sensor or corroded PCM connection, but this completely caught me off guard.

Thanks

 

I like to pull off all of the rocker arms and then hook up an air hose to each cylinder. Then all you have to do is listen to where the air is going. Out the TB you have a bad intake valve, out the exhaust bad exhaust valve, out the valve covers a broken piston, and out the radiator a blowen head gasket. I would check all of the cylinders it could have also jumped time.

 

I do not know how well you know this mechanic but cylinder misfires can be contributed to poor ignition, fueling or cylinder issues. A compression test is not hard to do for even the novice and the tool isn't all that expensive but I'd confirm that I have zero compression on those cylinders first and foremost before doing anything else as I find that hard to believe. You mentioned that you ran the battery dead well the engine has to run long enough for the ECM to clear all its readiness monitors and if you do have bad plugs, coils, wires, dirty MAF, air filter, throttle body, clogged catalyst and worn out O2's your going to have running issues.

 

The more I’ve thought about it the more I want more tests. Im so convinced it’s something dumb like PCM terminal corrosion. At worst head gasket between 6 and 8, or I didn’t ever check if it had 706 heads. I need to call them first thing tomorrow and have them pull the rocker covers, pull the intake and exhaust rockers on 6 and 8, pull the plug on 8 and put compressed air to 6. I know my friend was trying to not have the shop racking up hours on it, but I need to find out what’s going on, I just don’t have time with 3 little kids and traveling every other week to do it myself.

thanks for the replies. Anyone have other ideas on what dumb electronic issue it could be?

 

Update: guy at the front desk was realizing the sequence of events vs the diagnosis didn’t match so he pulled it into the stop, unbolted the Y pipe and it fired right up and idled fine. New diagnosis is what I brought it in for of clogged cat. New y pipe and cats on order, fingers crossed!

 

Always good to find someone who KNOWS what he's doing....

 

See post #3

 

Yup, those are all the things I wrote on the sequence of events when I dropped it off, I’ve just been traveling a lot and havnt had time to do it myself. I am glad the shop did take a second look at it, without me asking, because I had it up for sale as is for $2500 thinking I was about to get butt raped buying a new truck. I’ll be happy if I can limp this along until “let’s go Brandon!” crashes the economy enough to have car prices come back to reality.

 

Don't hold your breath!

Thank goodness I only have two horses to feed but I remember a time when a bale of coastal hay was $5. That was over a decade ago and it hasn't came back since.

 

Update: new cat and it still wasn’t running right, so I had it towed home to do further investigation on my own. Pulled codes and saw a P0106 for MAP sensor which was weird. After running it for a minute, took a temp gun to the manifold and 8 and 4 were hot, 6 was cold, and random cylinders on the driver side were also cold. Did a compression test and had 130-150psi on all cylinders except 6, which had 0. Pulled rocker cover and found the exhaust valve spring was broke and pushrod slightly bent. Picking up a new spring and pushrod today at 3. My theory is the exhaust valve hung open 1/4 inch was pushing spent gasses back into 6 and back into the intake manifold every time 6 went to intake, causing the random misfires on other cylinders. Fingers crossed it’s fixed in a couple hours.

 

Good catch, report back when she's all buttoned up and running! That's a clean engine for sure, definitely because of your oil change habits.

 

Springs were my first thought when you said it had 0 compression.

Compression is mechanical. Have to look at all the things related to makeing cylinder pressure.

 

Wood woop! All fixed!

 

Good to hear!

 

Such a good feeling when you get it all fixed and running right!

 

Glad to see you figured it out but haveing a broken valve spring would concern me that others could follow. Another thing I noticed was that your rocker arm pedestal is upside down. I have a 2005 Tahoe with a 5.3 and 706 heads which just so happens to be off of the motor and I can assure you that the GM logo should be upside down and not the way it's shown in your picture.

This is how it should look when installed correctly and this does have an affect on the rocker arm geometry.

 

I have 862s, and this is the first time the valve covers have ever been opened. That is interesting.

 

I have some 862's which are essentially the same cylinder head. If you don't believe me pull the rocker arm stand off and take a good look at the pedestals and you'll notice they are higher on one side than the other

 

Wow. You would know, having the truck in the family since new.
How much difference does it make in the geometry to have them oriented wrong, or "legible"?

 

Confirming this now!