How does a leak-down test confirm good/bad valve guides?

Everybody knows that it is a good idea to get a leak-down test before buying certain cars (355's come to mind), since one of the things it checks for is the condition of the valve guides. But does it really?

The leak-down test is pressurizing the cylinder with the valves closed. Thus, even if there were no valve guides in place (a little extreme, I know). You would still get a good seal with no leakage from the valves. The only way bad guides would show up is if the valves were rocking around enough that the valve and/or seat was damaged. I would think the wear would have to be very severe for this to be the case.

So is the leak-down test really checking the condition of the valve guides, or do you just think it is?

Rick

 

For a valve to seal it needs to land in exactly the right spot, and straight, with precision. A valve guide with the proper clearance is the only thing that makes that happen.


Either that or everyone since Nikolaus Otto has it all wrong.

 

You have a spring with a lot of force pulling it closed on a conical seat. When the engine isn't running (during the test) I don't think the guide needs to do any guiding. The conical valve seat will center itself.

 

Why can't it be somewhere inbetween and offer some insight in some cases?

Even though it isn't measuring guide wear directly, I'd agree that a good leakdown test result would be confirmation that an engine hasn't (yet) been operated for long periods with poor condition guides, or that, if you had a low-mileage engine that had mediocre/highish intake valve leakdown, that might be a warning sign that the intakes are starting to beat themselves up a bit. If I'm a Seller, a good leakdown test result is absolute proof; if I'm a Buyer, unless you have a video of the guide replacement + receipt + a warrantee from the Shop, I want a guide discount regardless of leakdown -- let the negotiation begin...

 

Mr No Profile:

You think that the seats will be perfect? The leakage test will show valve leakage, experience has shown that the more common cause is from worn valve guides. The 355 uses very small dia shaft and heads on the intakes, add in poor material choice, high rpm, it will and has failed.

 

To further add interest to this thread one could add in the reasons for and the variances of high pressure FAA leakdown tests vs. Low pressure leakdown tests.

 

With a worn out guide the valve will more than likely contact the seat on the side opposite of the pressure from the rocker or cam lobe. this will create extra wear to the seat and valve face. When the pressure is off the valve the spring may pull the valve back to center on the seat. The leakdown test will show the leakage at the valve to seat junction requiring attention.

A good tech will check the guides carefully for wear and overhaul the head as needed replacing guides, valves, seats, springs, keepers, seals ETC.

A hack will just swing his hammer and put it all back together. Unfortunately there seem to be more and more hacks out there with dealers preferring to pay them nothing verses hiring a well trained tech that takes pride in doing a job correctly.

 

What was the question again?

 

A worn valve guide will allow the valve to hit the valve seat differently each time. Over (not so much) time the seat will no longer be concentric enough to seal even if the valve hits it perfect. The face of the valve also suffers damage in these off center landings.

This is why when one puts in new guides, one puts a new face on the valve (or uses a new valve itself) and machines a new seat for the head.

 

There's some quite nice explanations ofboth typesof test and what they can tell you here

How to Check Engine Compression

Engine Cylinder Leakage Testing

 

And did you notice that there is nothing about worn valve guides when diagnosing engine problems with a leakage test?

 

A leakdown test shows the sealing ability of the combustion chamber IE Piston and rings to cylinder wall, Valve faces to the valve seats and head gasket seal.

If you have leakage into the breather the piston and rings are suspect.

If there is leakage into the cooling system or where the head meets the block the head gasket or head or block is suspect.

leakage into the throttle body or exhaust shows that the valves are not sealing. If the valves are not sealing then you can just freshen up the seat and face slap it back together and hope it holds or you can do it the correct way.

Measure the stem to guide clearance at top center and bottom of the guide as they wear in a bellmouth shape I.E. Larger at the top and bottoms and less in the center. If the guides are worn replace them.

Then get a new valve, recut the valve seats Do Not cut the seats until the worn guides have been replaced as the new guide might change the center line of the valve.

Going about this in a hap hazard manner will only cost you more in the long run.

Simply put if your leakage is less than 10% then run it 10-15% use this as your warning that you are going to need to go into the engine in the future. More than 15% = tear down time.

 

I am really stepping in it but I am going to side with Carbuilder here. I am not an engine builder but as a veteran racer and having been involved in many racing engine programs over the years I can say that most of you are jumping to conclusions when a Ferrari has a bad leakdown. The first conclusion jumped to is that the valve guides are bad. Because its a known issue with the 355 cars AND because the cost of doing the heads is so high, its a fair assumption, but by no means is it conclusive. Engines with poor leakdown don't necessarily have bad guides and those with good leakdown are not necessarily OK. Ferrari engines are more closely related to Motorcycle and F1 engines and these all are more prone to have leakdown issues than a big American 2 valve V8.

The 355 for example has 5 tiny valves and generally rich mapping, the 355 is more likely to have some carbon build up cause leakdown at the valves than bad guides. "Babied" cars, driven in town or for short distances, never getting out of the warm up maps will very often leak down at more than 15% and still run fine. In racing engines we have seen as much as 30% leakdown with only a very small reduction in measured horsepower. In high RPM engines this is because the combustion events are happening so fast that there is no time for the leakage past the valves to cause any issue. Its more concerning if you see idle problems or smoke.

Certainly bad guides can cause a poor leakdown result, but its my feeling that many of these cars have had valve jobs done when the engine was in fine shape and all parties ignored the perfect running engine and good power simple because some internet wives tale told them the engine must be junk. I'd be more concerned about the results of a compression test or leakdown past the rings than the top end.

In my own experience, when I bought it, my car had one cylinder leakdown at 20% with only 1500 miles on the valve job. After a "spirited" drive from Boston to Atlanta the re test showed 5%. Maybe the "Italian tune up" is the best option before jumping into a major surgery!

I know there are those who will disagree and I respect other views, just thought I'd express mine.

 

It is pretty simple. If the valve leaks, it is due to poor valve seat or valve face. In Ferrari terms, they don't get enough mileage to just plain wear out. If the guide fails to hold the valve square to the valve seat it hammers itself into a poor fit. Now you have valve to seat leakage, (air sound from the exhaust on 355s).
So, with most Ferrari cars, poor valve seat seal will be due to bad guides. Not worn out valves.
Leak down test can confirm bad valve guides.

 

Enjoy staring at a vacuum gauge whilst the engine is running, attempting to decipher a valve seat problem.

A leak down test in conjunction with a compression test helps. A little leakage and an elevated compression in the same cylinder helps give a more complete picture.

As the others have said, a flopping valve suffers uneven wear on the valve's seating surface as well as the seat's sealing surface. These imperfections are picked up during the leakage test.

 

I agree, the compression test and road test are all part of an engine's report card. But to try and minimize 355 owners losing sleep or rebuilding the engine for every PPI, it should be clear, as I illustrated above, that a bad leak-down does not PROVE the guides are bad. and a GOOD leak-down doesn't prove that there is not a developing issue. It is only one tool in the diagnosis process. If I subscribed to the "mob opinion" I my have bought my car, or I'd have driven it to the nearest dealer and been $10k poorer based on its leak-down only. Given the history of the early 355 guides this is an understandable conclusion to jump to, but its an expensive jump to make. One poster went so far as to say he would "expect" to negotiate a discount for bad guides, unless there was a video of the job being done. Its an amazing world this Ferrari-land

 

Not to be combative, but I feel that a lot people that argue against those of us that have done the test, tore down engines and rebuilt them are simply doing so because they can't handle the truth.

No one wants to hear that their engine needs top end or complete overhaul and no one wants to spend the money it costs to do so correctly either. People love to argue their way out of reality. Disagreeing with the matter, does not change the legitimacy of the facts.

As far as carbon build up goes, true race engines are built with different tolerances than a street engine and also tend to run on Lead based fuel which is known to create measurable deposits.

To compare those engines to a street driven pump gas engine is not apples to apples. Also, to claim a Ferrari engine, which runs quite tight and efficient, to suffer from substantial carbon building up in approximately 15,000mi is reaching. These engines run cleaner than that.

The last (2) 16,000mi 355's I pulled apart for top end rebuilds had guide failure in all 40 guides. The tops of the guides where measure to be .020-.030" out of spec. The accepted clearance is .0015 to .003.

On the second engine, which measured leakage of 26% through 2 of the exhaust valves, had burned the valves badly enough that we replaced them and didn't even bother to cut them.

In many cases, once things have got to the point where you're seeing major compression/leakage drops and smoking through the exhaust, the engine beyond a simple valve job. It's often referred to as deferred maintenance.

The best thing anyone can do for the 355, stop debating this topic, accept it and get the engines that need to be fixed, fixed right and running properly so they can be enjoyed for many more years.

 

It's a crap shoot at best. You can indeed detect bad guides with a leak down test, but it's not a single shot process. Sometimes you have to check a single cylinder 3 or 4 times, record each reading, and then average it out. At that point you can see the changes in valve sealing. That will confirm the valve is not sitting correctly on a seat, and is moving because of guide wear. You will not see this unless you perform the leaked down test at least 4 times, and each time you do it, you must crank the engine over in order to get the the valve opening and closing, to see if it lands in a different position on the seat. Check it, then do it again.

How many people have seen anybody do this? I do it if I suspect there is enough of an issue to be concerned about, but even then I try to narrow it down to the suspected worse cylinder of the 8. Usually on a 355, it's number 2 cylinder.

 

Very well said, Josh and true. Rarely do the numbers lie.

 

But as experienced techs what do we know? Well said Josh.

I don't recall any post purporting that bad number on a leakage test necessitated a MUST DO rebuild, just that it would from expert analysis and further testing, very often turn out to be that way. As also said, ignore at your peril, the electronics will try to compensate on an already lean set engine, which can often lead to much more serious damage. Hence those $20-25K 355s out there.

 

No. The 355 that I had a PPI done on a few weeks ago had the heads removed for "de-carboning" 3 years ago. I saw the pictures. It was full of hard carbon. Enough that it prevented an exhaust valve from closing properly, burning the valve. A Ferrari engine doesn't run any tighter or more efficient than a Honda engine. I know nobody wants to hear that, but the Honda S2000 engine (from roughly the same time period as the 355) made more power per liter than the 355.

My point in starting this thread was that the Ferrari guys seem to think a leak-down check will tell them the condition of the valve guides, and generally other car enthusiasts tend to disagree. I just found it interesting.

Rick

 

Any car driven occassionally and for short trips generally to profile rather than an adrenaline rush will carbon up. i.e. A typical 355.

 

To have so much guide wear .020-.030 at 16,000 miles there must be a lot of side load on the valve stem or a problem with the guide material.


Ago

 

Well documented material problem...

 

Rick, you're right. It isn't over until the fat lady sings. You don't know what's broken until the engine comes apart and everything is checked. The 355 in particular has proven that it's wise to be prepared to completely rebuild the heads. It's best to assume the worse and hope for the best. It's always easier to cross parts and labor off the list than it is to keep adding them.

Not every Ferrari suffers this fate, this is a model specific concern.

On your 355 PPI, I would want to see notes specifically stating that the machine shop measured the valve guides and confirmed that they were within spec., if they weren't replaced. An ommission of truth, isn't considered a lie...

Hell, I remember when some were claiming that only the exhaust valve guides suffered from this failure. I personally have never taken apart a 355 engine that only needed exhaust valve guides.