Totally stumped. Replaced the head gasket in Nissan 03 Sentra 3 months ago and car has been running great since with good power. Car suddenly completely dies after coming to a stop without any warning or any loud noises.
No compression in all cylinders. Seemed to be lots of oil in all four cylinders according to the bottom of the spark plugs. Pulled timing cover, timing marks still line up perfectly. Leak down test suggests valve problem.
Pulled the head and see all the exhaust valves are bent and the intakes are bent on pistons #1 and #3. (which is also odd as pistons #1 and #4 run in the same positioning)
Question is how can all the exhaust valves suddenly get bent simultaneously with the timing chain still registering perfectly on the marks it’s supposed to?
Things I’ve checked:
I checked to see if the camshafts have sheared through the key, but they seem fine. No deep grooves found to indicate they slipped or anything.
Are the valve springs and keepers still intact? If the cam/crank timing is correct, about the only thing that could do that (besides major problems on the lower end, like the crankshaft bearings failing) is the valves sticking open or coming loose from their moorings and rattling around, not following the camshaft lobes like they should in other words.
“Lots of oil in the cylinders…” could also be a problem. Maybe there was a hydrolock issue and the timing chain tensioner was bottomed out temporarily which then led to excessive slop in the chain and valves whacking each other or the pistons or possibly loss of oil pressure which affected the tensioner and caused the same thing.
It would be unusual for exhaust valves to contact the pistons so I tend to think the valves tapped each other.
QUESTION: “Did the valves nick the pistons or did the exhaust and intakes nick each other?
Camshafts rotate freely in their saddles; or as freely as a cam can rotate against spring pressure? ?”
It actually looks like the exhaust valves came into contact with the pistons as you could see where it smacked off the carbon residue on a couple of them so it had to take a pretty good blow.
The camshafts rotated freely and seemed to function perfectly fine when cranked by hand.
QUESTION: “Did the lock pins on the camshafts for the gears shear off?”
Nope, they were still intact and still in place on the cams.
QUESTION: “Are the valve springs and keepers still intact? If the cam/crank timing is correct, about the only thing that could do that (besides major problems on the lower end, like the crankshaft bearings failing) is the valves sticking open or coming loose from their moorings and rattling around, not following the camshaft lobes like they should in other words.”
They seem to be fine. Not likely what happened here as all valves went in an instant across the board and not just a few here and there over time.
QUOTE: “Lots of oil in the cylinders…” could also be a problem. Maybe there was a hydrolock issue and the timing chain tensioner was bottomed out temporarily which then led to excessive slop in the chain and valves whacking each other or the pistons or possibly loss of oil pressure which affected the tensioner and caused the same thing."
That seems to be making the most sense so far actually. I’ve been trying to figure out if just the tensioner failing would be enough to do it, but in most causes, it fails, which causes the chain to slip somehow and the chain was still on marks. Though with the hydrolock issue and the tensioner running on oil pressure…might have been enough to do the trick to slop the chain to stop the cams just enough to whack the exhaust valves…
It would be nice to be able to find a decidedly broken piece. I can’t do a valve job and put it all back together just to have it repeat the process 2 miles down the road.
Lots of good ideas to check here but let me ask a basic question. Did you over-rev the engine somehow? Shift to 2nd when you meant to get 4th when running hard? That alone will get you bent valves in a heartbeat.
I have 3 trackday friends that did this. Corvette, BMW and Nissan. Guess which one cost the most to fix?
No, nothing like that happened at all, in fact it was the exact opposite. I had come to a complete stop on a slight decline when a UPS truck pulled out in front of me. I was only going 20 mph before I had to stop anyways. But the moment the truck moved and I hit the gas, the car was already dead and all the valves were bent at that time I’m assuming. When I went to restart, it turned over really fast like there was no compression in any cylinder.
I didn’t hear any loud clanking or bangings going on either. It all happened pretty silently.
@Suni, That sounds like a broken camshaft key (or pin) that keeps the cam gear from spinning on the cam. That’s the only way this will still appear to be “in time” to the crank and bend all the valves. (looks like @Tester called it) You should be able to get away with a rebuilt head and a new cam and gear but check the piston tops carefully for damage.
@Mustangman, I just went out to double and triple check to be sure, but the keys are still fully intact on both cams. I checked the cam gears themselves to see if there was and deep groove or anything to show that the key slipped through and there’s nothing there either.
This was kind of the only theory I had when I found out it was a valve issue myself, but when I pulled the cams to find nothing wrong with them is when I just got utterly and completely stumped as to how the valves got bent to begin with.
@GeorgeSanJose I’ll be sure to. I scoured the internet for days trying to find my answer and couldn’t come up with one. If and when I do figure it out, I’ll be sure to post it here to give some soul a fighting chance at having a chance at someone already figuring it out.