Engine needs valve job

Hypothetical question , A vehicle with fairly high mileage comes into your shop needing a valve job due to low compression in a couple cylinders . The engine runs quiet with no abnormal noises & doesn’t smoke . What do you consider critical in repairing this engine ?

You need to be specific on the vehicle…All “valve jobs” are not the same…Costs can vary over a very wide range depending on what you are working on…

It has been many years since I diagnosed an engine as needing a valve job. A proper leakdown test should be done before condemning the valves.

In my hypothetical question you have diagnosed improperly sealing valves as the cause of low compression & a poorly running engine . Cost isn’t being considered in this question . For the purpose of this question the pistons haven’t been damaged & the only problem that has presented is a poorly running engine because of leaking valves .

"What do you consider critical in repairing this engine ? "

A check that doesn’t bounce.

I’m not a mechanic and don’t like tests or hypothetical problems but I would never do a valve job on a high mileage engine. Especially if it runs fine and smooth, I wouldn’t believe it was a valve issue in the first place.

I said it ran quiet & didn’t smoke . I said it ran poorly due to leaking valves . I also insinuated the mechanic presented with the vehicle diagnosed the problem as a burnt / improperly sealing valve problem . I don’t know what else I can say . The lifters don’t rattle , the rods don’t knock , etc . As far as critical I just want to know what would need to be done as a matter of necessity other than getting the heads rebuilt or installing new heads .
I assume it’s likely that taking care of the valve problem would raise the compression in the cylinders back to near new pressure .
For example , are checking wrist pins , crankshaft bearings , measuring the crankshaft , etc. considered critical in the scenario I presented ?

If it’s only valves, then only the head needs to be repaired. OHC or OHV?

It’s OHC . No pushrods .

I smell a rat. Am I the only one?

Timing belt or chain?

I had a gut feeling people was going to view this as a got-ya type question , it’s not . I have the engine in question but I’m not going to reuse it is why I posed it as a hypothetical question . I’ve already purchased a replacement but inquiring minds want to know . Mainly mine . Originally I did consider repairing it & wondered about the questions I asked . Still wondering .
I have read that doing a valve job can & does lead to bearing failure & other problems & I’ve read that’s a myth . The short time I’ve been on this forum I’ve realized there’s some very smart people on here concerning vehicle repair & to satisfy my own curiosity I posed the question .
I would still like to know the answers . { Timing Chain }

Well, it’s VERY hard to give a meaningful answer to a very general question. OK, now we know it’s a timing chain OHC engine (that is necessary info to answer your question).

Aside from the actual valve job, truing up the head(s), and inspecting the cam(s) for unusual wear, I can’t think of other issues that are critical. I also don’t think that doing a valve job will cause other failures. Those failures either were on there way, regardless (a common issue with a worn engine), or are the result of faulty engine reassembly.

This isn’t something you can get a definitive answer to as a hypothetical. What make of engine? How much is the car ir is in worth? What are your goal for the car, get it running well so you can sell or trade it, keep it a couple of years, keep it 10 years ? How many miles are high miles? Is it a Pinto or a Dusenberg? Also, how happy are you going to be if the increased compression exposed worn crank or rod bearings. It is up to the car owner to let the mechanic know what he wants done. If you want the crank measured and the wrist pins that means a complete disassembly of the engine you are not going to get that with a valve job unless you indicate ahead of time that you are willing to pay for all that extra work.

Well there is top end and bottom end. The top end is the valves and the bottom end is the pistons and crank. If you go into the bottom end, you are talking a full over-haul. Just doing the top end would be the normal course for valve problems but many people report increase oil usage after doing valves on an old engine. So if it doesn’t smoke now, it would then. IMHO anyway.

Then you are into the whole over-haul issue of whether you do it yourself, have it done, or just swap. Others will disagree but for me as a consumer and not a mechanic, I would never open up a high mileage engine for that kind of work. And if I were going for a new engine, I’d be looking at a manufacturer crate engine instead of like a Jasper, etc. So to me, valve problems means swapping engines or cars to not run the risk of throwing good money after bad.

Hypothetical question-does someone have too much time on their hands?

At the moment , possibly . I am a like to know type person . It seems to me people who work as mechanics would get presented with the scenario I described at least occasionally . I just wondered if there was an accepted protocol for this type scenario among professional mechanics .

I would expect to have the head completely serviced. Head surface milled at least; valves renewed or ground depending on condition; guide clearances checked and repaired as necessary; springs checked for square, height, and installed height force; check the installed height; machine the valve stem length to allow valve clearance setting; inspect tappets and cam for damage and wear; check alignment of cam saddles; check cam(s) for runout; etc.

In addition I would check the chain guides for wear; the chain for stretch; and the tensioner for operation. I might check the cylinder taper(s) and see if the ring travel diameter is beyond service limits. If a timing belt is involved I would replace that along with idlers, tensioner, and water pump.

Otherwise I would reassemble the engine and return the car to the owner to do any future repairs that crop up. I will do additional work if it is personal but only bring the engine back up to serviceability for a customer. I will advise but not pressure.

researcher , thank you very much , that is exactly the type answer I was hoping for . Is there a special tool for measuring timing chain stretch or maybe I should ask how is that done ? Wear on the guides would be obvious I assume ?

how can it run quiet and not smoke, and yet still run poorly ??
how do you know there are “leaking valves” ??
you have to be more definitive.