I have just reassembled the top half of my engine (3.4 V6) after the machine shop rebuilt my valve heads. Now that it is back together, it’s running rough and giving me misfire error codes on cylinders 2, 4, and 6. Those are all on the driver’s side. The plugs and wires are all new, I was really careful reconnecting the engine grounds, and I can’t imagine that I put the camshafts together wrong. But obviously, I’ve made some kind of mistake somewhere. So what would cause only the cylinders on one side of a V6 engine to misfire?
Cam timing or valve clearances.
If it were valve clearances, that would indicate that the machine shop didn’t rebuild the head correctly, right? That seems unlikely to me, this is an extremely reputable shop. But I suppose I can pull off the intake manifold and check the clearances, I don’t know what else to do.
As for cam timing, the only two factors that I know of are that the exhaust and intake camshafts line up correctly according to the dots stamped onto the sprockets (I was very careful about that), and that the timing belt was installed correctly according to the TDC marking on the crank, and the TDC markings on the cam sprockets. I was really careful about that too. Is there any way, other than visually checking these two things, that I can verify cam timing?
Anything else? Does it make sense that I might have problems with the drivers side fuel rail, and not the passenger side?
Valve clearance is usually check after the cylinder heads are installed, the camshafts must be installed to check the valve clearance. Did the machine shop have the camshafts?
Yes they had the camshafts, and they assured me that they would do the valve adjustments too.
Why not tell them what’s up and ask them to have a look? They just did major work for you and there is problem that wasn’t there before, as I understand it. As competent as I am, I have sometimes made a mistrake.
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Thanks, I’ll do that, as soon as I’ve ruled out any boneheaded thing I did putting it all back together (which is the far more likely scenario). Right now the battery charger is on, I’ll do a compression test and see what that says.
That test can say a lot about this. Good luck and please let us know how it works out.
Good idea. Measure all 6 cylinders. Maybe the evens will show a considerably different compression than the odds, indicating a possible valve timing or clearance problem. On a V6 (which I presume this is) one has to consider a possible exhaust blockage too, affecting only one bank.
If this engine configuration has the timing belt going around two different camshaft sprockets, my guess is that’s where the problem lies. Getting the timing belt on properly with that configuration is a common complaint here, and that difficulty has been mentioned on the show as well. You might be able to remove the timing covers and check each side’s dynamic alignment with a timing light while the engine idles, to see if the evens are different than the odd. When I changed the timing belt on my Corolla, that’s a test I did.
OK so I got a compression tester, and as suspected the D.S. cylinder bank is not holding any compression (all three cylinders on the passenger side were around 200 lb/sq.in.).
Let me explain that on this V6 engine, each cylinder head has an exhaust camshaft and an intake camshaft - the intake camshaft is driven by the timing belt, and the exhaust camshaft is driven by the intake camshaft. That’s probably pretty standard for overhead cam V6 engines. Anyway, when I put the camshafts together after torquing the head bolts, I lined them up according to dots stamped onto their sprockets - kind of hard to mess up. And then when I put the timing belt on, the camshaft sprockets were at top dead center, and there are two lines on the timing belt to make sure the camshafts are lined up correctly - also hard to screw up.
So with no compression in any of the misfiring cylinders, it seems this is a valve timing issue, right? If that’s true, I see three possibilities, #1 would be that I put the camshafts together wrong (not likely), #2 that I put the timing belt on wrong (not likely), and #3 is that the machine shop didn’t rebuild the valve head correctly (not likely).
So how do I diagnose what’s going on from here? And are there any other options than the three that I listed, that would prevent compression on all three drivers side cylinders?
You may have missed a step where after putting on a belt with the markers in a specified position you then rotate the crankshaft backwards a specified amount before tightening it all up.
But if the crankshaft is off a tooth, wouldn’t the timing for both valve heads be off? I may have done something screwy here, but I know for 100% sure that both camshafts sprockets were lined up to their lines on the timing belt correctly.
And if it was the timing belt that got off by a tooth, the valves should still be opening and closing, and building up compression, right? It seems like in order to have no compression at all, the exhaust and intake valves would need to be out of sync. But I’m no expert here.
What does the repair shop say?
If all 3 cylinders have no compression, that sort of says the valve clearances were all set alike, and wrong. Incorrectly set timing should still allow some compression, not none. If the intakes or the exhausts are not closing completely because of valve clearances, no compression will be had!
If the exhaust camshaft were installed out of sync with the intake camshaft, I understand how that could end up with no compression. If the timing belt were merely off a tooth (either at the crankshaft or at the offending camshaft), it seems to me that I’d still get compression. No?
Well, I’m about to find out about the camshafts, anyway.
They’d have to be quite a ways out AND synced to always allow a valve to be open. Not easy to do by accident and definitely not off a tooth. One or 2 teeth off would still give you compression - not 200 psi, but something. Heck, moving the cams back and forth with respect to each other by as much as 50 degrees (way more than 1 or 2 teeth) is what my engine does to shift the powerband.
It is super easy to check valve clearance, or should be, do it while you have the valve cover off. When both the intake and the exhaust on one cylinder are on the base circle of the cam, try and slip a feeler gauge in. If you can’t Bingo, there is your problem.
If the camshafts are OK vis-à-vis the crankshaft, seems to me valve clearances are next place to look.
As a next step, suggest to pop the valve covers on each side & observe the camshaft lobes as you rotate the crankshaft by hand. In particular; do the intake valves get pushed down by the intake cam lobes one at a time in the correct sequence over two full crankshaft rotations, according to the engine’s specified firing order? If so, then rotate the crankshaft/camshaft to a position where one of the cylinders on the problem side should have both valves closed. Remove that spark plug, and figure out a way to pressurize that cylinder to see if it holds pressure or not. Similar to a cylinder leak down test. Could be done with a bicycle pump and some adapters.
If the intake and exhaust lobes sequence correctly, and the leak-down test passes muster, the problem would have to be either with the air flow to the cylinder, or an inability to exhaust the cylinder.
Is 200 psi a normal compression reading for that engine? No experience with that engine, but it seems like it might be a little on the high side. If so, might be a clue to what’s happening.
My L4 Corolla 4afe engine has two camshafts, one for the intake valves, and one for the exhausts. The timing belt drives only one of the cams; the other is gear driven. That’s good news, b/c I don’t have to deal with the difficulties of looping the timing belt over two camshafts.
OK, mystery solved. When I put together the camshafts (exhaust and intake), there are little dots stamped into the sprockets where they mesh together. Well, unbeknownst to me, there are dots, and then there are TWO dots stamped exactly 180º away. So I had matched up one dot on one shaft to two dots on the other shaft. These dots are stamped on the firewall side of the sprockets, so you have to look with a little spot mirror, so it was easy to miss the one dot/two dot thing. So yes, it was my boneheaded mistake (I figured that much), but it was only a semi-boneheaded mistake.
Thanks to all of you who pointed me in the right direction. So it’s all back together and running strong, no misfires and no check engine codes. Next question, however: is it normal to see some smoke coming from the engine for a little while after a valve job?
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