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I had to redo my timing and I think I may have done it wrong because, on my left (passenger) engine lobe (V6), when each of the three cylinders are in TDC (top of orange tape on the rod in the picture), not all four valves are closed shut. I know because 1. the cam lobes are not symmetrical and 2. I blow in a hose used for cylinder leakdown test (which adapts to the spark plug socket) and I fill lots of blow-through:

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Now notice on the next picture, which is the right (correct) lobe, the cam lobes are symmetrical to one another and when that cylinder is in TDC, blowing in with the test hose does not blow through (creates compression like trying to inflate an already inflated chamber).

enter image description here

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  • @paulster2 I was under the impression that the oil fil cap was ALWAYS placed over the valves for cylinder #1, so you could visually check and verify the valve postions for Top Dead Center (TDC). Is that true, or just an old mechanic's tale?
    – zipzit
    Commented Dec 21, 2021 at 6:33
  • @zipzit in my case (V6 Tacoma), it is placed right over the left (drivers) side where the timing chain goes over camshaft sprockets, which would be near cylinder 2 (1 is on the right engine lobe) but still not over it -- can't be seen through the fill neck
    – amphibient
    Commented Dec 21, 2021 at 16:07

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Your V6 firing order is 1-2-3-4-5-6, where cylinder number 1 should be on the front right of the vehicle ("right" as you're sitting in the driver's seat). Cylinder 1 and 2, while not sharing the same crank journal (as seen with many V8s), are relatively close together (at a guess, I'd suggest they are about 20° of crankshaft rotation apart). Your V6 should have a 60° V, which means you'd need to add that to the crank rotation, along with the 20°-ish between crank journals ... so something like 80° between when 1 comes up on TDC and then 2 does the same. The same goes for 3 & 4, as well as 5 & 6. What this means is, when 1 is coming up on its compression stroke, so should cylinder 2, hitting TDC about 80° after cyl 1 does. The valves for both, while not being completely in phase together, should be fairly close. During the compression strokes for each, all valves should be closed for the entire time (through compression stroke all the way over onto the power stroke). If you're not seeing this happen, then I'd suggest there's an issue with your valve timing.

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  • But on the compression stroke the cylinder should also be in TDC, together with the valves closed so if those two are not synchronized, there could be an issue with timing -- is that correct?
    – amphibient
    Commented Dec 21, 2021 at 1:38
  • During the entire time of when cyls 1 & 2 are going through their compression stroke and over into the power stroke, all valves should be closed for both cylinders.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 21, 2021 at 1:52
  • yeah, that's not happening with #2...
    – amphibient
    Commented Dec 21, 2021 at 3:36
  • Timing is always about cylinder #1 at top dead center regardless of the other cylinders and valves because camshafts and crankshafts are machined, fixed relative to overall timing. Your concern is to ensure cylinder #1 is correctly timed with camshafts and crankshaft timing marks aligned to casting marks or whatever fixed marks are used. When cylinder #1 is correctly timed, you should be able to rotate the crankshaft 720 degrees (two full revolutions) without valves meeting pistons and see timing marks aligned again.
    – F Dryer
    Commented Dec 27, 2021 at 23:21
  • @FDryer - The engine in question is a V6 DOHC ... there's more to it then cylinder #1. You've got four cams to get timed correctly.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 28, 2021 at 1:16

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