That depends on the vehicle. I’ve owned several vehicles where you can’t see the valve train action. There’s a little baffle plate covering it. If I removed the valve cover then I could. But on modern vehicles with all the plumbing it’s far easier to do a compression test
You’re NOT checking to see what the compression is. You’re checking to see if there is compression. No compression on several cylinders is a good indicator that the belt is broken.
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All of my modern cars have baffles in the way. I can’t see anything but a small slot at the bottom of the fill hole.
I was perplexed initially but came to the same conclusion as Mike. They simply put a compression tester in spark plug hole(s) and cranked it over. Little to no compression, timing belt no longer in commission. Easy peasy.
I personally might be reluctant to do this because one sure way to create a problem is to continue pressing your luck. I’ve heard of miracles before where even interference engines seemed to survive relatively unscathed when the belt broke. Think we may have even heard an example or two on this site before. But to crank over a suspected broken belt is almost a guarantee that the miracle is short lived…The testing guys had nothing to lose…
What I use to do was put my thumb over the hole (if it was easy to reach). You can feel if there’s compression. Not trying to get an exact reading…just check if there’s compression.
But the OP said the mechanic said that one cylinder was down to 30 psi and the timing belt was missing. I understand if you get no compression you know you have a problem, but getting a reading of 30 and no belt seem to be mutually exclusive. One or the other but not both.
I dunno, I had a shop tell me my Riv engine was shot when the chain went and offered to buy it, but $150 and a weekend and it was on the road again. I think I’d like to hear a second opinion and at least pull the cover to confirm the belt broke before junking the car.
So maybe those were the valves that were almost closed all the way but bent. You only need to check one cylinder so pick the easiest one to access…anything less than 50% of normal would be construed as an issue with the belt.
If you have a valve that’s partially stuck open it could show some compression. I wasn’t questioning the mechanics procedure…just the comment later on that that a compression test can’t reveal anything.
Checking compression is a valid way to see if a timing belt is bad.
The title of the post says timing belt broke. The question now is what damage has been done to any valves (bent?) and maybe pistons (broken tops?). Then decide what to do.
Jumping time can and often does cause a great deal more problems than the belt breaking and the cam(s) stopping. a few cogs skipped on the crank pulley will retard the timing and often bend all the exhaust valves. If the valve cover is removed and any valves are seized partly toward being open the the damage will likely be $ignificant. The price of new valves, incidental pieces and labor to replace them might be more than the price of a used engine installed.
30 PSI, as stated by the OP, is vastly different than 0 PSI. One also does not crank over an interference engine if a broken timing belt is suspected unless your intention is to ensure there IS engine damage.
The question is how do you check compression when the timing belt is broken and isn’t actuating the valvetrain. I get checking if there is compression, but if they’re checking compression and returning numbers across all the cylinders, that suggests they’re able to get the valve train to move when they crank the starter.
And how did they manage to do that if the timing belt is snapped and the valve train is disconnected from the crank?
There are two types of interference engines:
-Valve-to-valve interference engines have valves that need to get out of each other’s way.
-Valve-to-piston interference engines have valves that need to get out of the piston’s way.
On a V-6 you probably either have the first type or you might have an engine with both types. If you have both, I’d scrap the car rather than spend a lot of money to fix it. It’s 16 (almost 17) years old, so it can’t be worth that much that it would be worth fixing.
If it is just the first type (valve damage only), the repair might be worth paying for.
I once paid for a new engine on an old car, and it ended up being a nightmare. I’ll never waste that kind of money again.
Not to argue, but he based that on what the mechanic at Precision Tune said. He said he didn’t know anything about cars so is just going by what he was told. Not saying the guy was wrong but I’d just like a little more verification of whether the belt broke, slipped, or what before junking the car.
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As would I. I’m basically just trying to figure out a way to evaluate whether the car is trashed my self.
For that you’re going to need a second opinion from someone who has actually looked at the engine. You either need to tow it to another shop or ask to see for yourself, take pictures of the damage, and post them here.
If you really have a broken timing belt on an interference engine, that would mean spending money with no benefit other than peace of mind.
Thank you to everybody who helped out and gave advice.
I have the same question. I can also see that running the engine on the starter may cause or add to damage to valves and pistons, so the safer route is a leakdown test: remove enough of the timing belt cover so you can turn the camshaft. Turn the camshaft to the valves-closed position for a cylinder, then screw in the leakdown tool and pressurize that cylinder. If it holds pressure, good news. Then go to each cylinder, turn the camshaft to its valves-closed position, etc., etc.
Any cylinder that does not hold pressure needs more diagnosis.
that may be quite time consuming
what about removing camshaft sensor and visually checking if shaft is stale or still rotating?
All we know is compression value for one cylinder. I didn’t see any information indicating others were tested.
Where is that stated, I must have missed it.
It also says they indicated no belt on cam. Sounds like they looked after finding first cylinder dead.