The reason it is recommended for routine service is that as the engine runs, each camshaft lobe turns round and round and each time contacts the corresponding valve, pushes it down, which opens the valve port in the cylinder head at the right time, to either admit air in to the cylinder on the intake stroke, or let hot exhaust gas out on the exhaust stroke. The problem with all this is that over miles and miles of driving the two surfaces wear, the dimensions of each diminish, so if you can visualize it, the valve then isn’t pushed open as much. The port become slightly blocked. The air can’t get in and the exhaust can’t get out as smoothly. As you might imagine, a constriction like this will affect engine power, mpg, and emissions. As mentioned above by @uncleturbo, failure to vent the hot corrosive exhaust gas efficiently can also damage an exhaust valve to the point it won’t close and seal properly, leaking compression on the power stroke.
On 4-banger econoboxes, the measurement process where you indirectly measure how much surface has worn away, that part is pretty simple, only requires removal of the valve cover. The amount of time and difficulty varies from car to car. Everytime I’ve done the measurement on my econoboxes, the measurement (valve clearance is what they call it) was still in spec, not enough surface wear to worry about, so all I had to do was replace the valve cover and drive on. But if enough wear has occured, then a shim has to be inserted to take up the slack, which is more time consuming and results in a bigger bill.
One other thing. The cleaner the oil, the less wear on the parts. Avoiding the expense of shim replacement is another reason to keep the oil and filter changed out according to the routine service schedule suggested in the owner’s manual . Best of luck.
@Tester I’m glad to hear you’ve fared so well.
But some have not.
2000 v6 vtec has 260k miles and never a valve check. I believe in the car and the manufacturer
@oldtimer, in the link I posted that shop also states that is the first V-6 they inspected, all of the exhaust valves were out of spec, and there was likely going to be engine damage if this had not been caught.
The problem is that sometimes they tighten up way before 105k miles; especially if overheating is involved, pinging problems exist, , etc. The majority of solid lifter engines, if set correctly and on the loose side of the tolerance, may go a lifetime without a problem. It’s the minority who may suffer and an expensive gamble when they lose.
Waiting until a valve tightens up and causes a misfire is a mistake because when that happens any proper lash adjustment leading to a good running engine may be only a temporary fix as the valve face and seat are already damaged.
I am convinced to perform this service. My wife drives a Subaru and her mechanic is a guru with them(30yr+ Subaru master tech). Subie indy charges $60/hr vs $100+/hr the Honda or Acura dealer does. My guess is service price would drop to 60-70% of what dealer charges.
Is the service something I need Acura or Honda dealer to perform or would this independent work?
The independent should be fully capable of performing this service as most Subarus use the same valve adjustment methods. The principles behind it are identical no matter the make so the indy should be fine.
The reason I may seem a bit touchy over the valve lash issue is that I was basically raised on solid mechanical lifters used in motorcycles, aircraft, and foreign branded cars and more often than not they need adjustment.
I’ve mentioned this story before but since you bring up Subaru, we had a Subaru enter the shop one time in which all of the exhaust valves had tightened up and burned the valve faces and seats. The cylinder heads were damaged so badly they were not worth fixing and went to the scrap metal pile.
This car only had a measly 7k miles on it and while we never got the full story from the owner, it is believed that the engine had a valve tick from a loose valve and that he adjusted them down to a couple of thousandths of an inch in an attempt to remove any ticking noises.
When the engine was cold it was not as much of an issue but when the metal expanded from engine heat it became a serious problem as the few thousandths of an inch went away and caused the valves to remain slightly open. This in turn took out chunks of the valve faces, valve seats, and even pieces of the cylinder head around some of the seats in just a few hundred miles.
Due to the car owner dinking around with the lash, Subaru refused to warranty the heads and the customer had to dig deep to pay the replacement costs. He never even argued the point.
I had a 71 VW bus that recessed the valve seats into the head so quickly that I set the valve lash at every oil change. with the van in high gear and the valve covers popped off I would roll it back and forth with one hand from underneath while adjusting the valves without getting out from under.