Exhaust valve guides leaking.

Hi there,

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I would like to offer a correction by saying that guide failure is not common throughout the Ferrari models. It does have a tendency to occur on the 355, but it is not "common" on the 355.

The 348 is NOT known to have valve guide issues. Something else is causing, or did cause, the problem. You should be fine with the stock guides so long as the other problem is addressed.

Please tell us about the car. Leakdown results? Compression results? Mileage?

 

Hi Steve. I had the same issue, bad leakdown due to worn valve guides on a 348. My shop told me the original material used was prone to this failure and the valve guides were replaced with a new material, but I don't know exactly which. If you want you can PM me and I can send you the contact information for the shop in question so you can call them to ask.

 

Your leak down test will identify leaks, but not necessarily the source (you listen to hear where the compressed air escapes). You could get bad leakdown numbers from bent/worn valve guides, valves that aren't seating properly, burned valves, burned/worn rings, etc.


Are you burning oil (lots of smoke)? When I bent my 16 exhaust valve guides (you can bend valve guides and you can bend actual valves - different beasts, though!) with a jammed cam pulley my engine would run, but pour smoke everywhere...and it consumed copious amounts of oil.

If I read right you said that you had good compression, but a poor leakdown on 3 cylinders, yes?

Have you looked into your oil reservoir with a flashlight to see if the oil there looks milky white? Have you looked into your coolant reservoir to see if there is a rainbow film on top?


Anyway, it's unusual to get a good compression test on a cylinder with a bad leakdown number.

Before replacing parts, I'd suggest having someone else perform new compression and leakdown tests (with you there) on all cylinders, and then perform a wet compression test on cylinders #3, 5, and 6 per your leakdown numbers. That data would go a long way toward telling you what's up (with the engine still in the car).

 

You want to first run the engine to get it hot because metal/objects expand (only accurate way for a leakdown) and initally put no additives in the cylinders to alter the leakdown test (does your Ferrari normally run with a tablespoon of oil in the cylinders?).

With engine off, next take out all spark plugs for the test. Remove your oil cap, coolant cap, and crack open your throttle.

At Top Dead Center (TDC), apply compressed air pressure via a differential pressure tester through a spark plug hole and note the reading. You should be maintaining somewhere in the area of 90% of what you put in for a nice "tight" cylinder. More importantly, all cylinders should be within 5% of each other.

When rotating a cylinder to TDC, don't go past TDC and then back. That might appear to "fix" what would otherwise show to be a leaking cylinder.

If you have excessive leakage, use your ears to locate the cause. That compressed air has to go somewhere!

Pop the oil cap off and listen for a hiss (blowby from rings or worse), pop off the coolant cap and look for bubbles (headgasket or block problem), listen at the exhaust pipe (exhaust valves/seating/guides), and finally listen to the intake with the throttle cracked/butterfly open (intake valves). This will help isolate your problem and direct you to the proper corrective action.


That's why it's important for you to be there when the leakdown test is performed!


FYI: most leakdown testers have two gauges, a pressure regulator, manifold, & hose. One gauge shows the pressure being applied, and the other shows how much pressure the cylinder is holding. The difference between the two gauges is your leakdown. So if you have 100 psi on the first and 80 psi on the 2nd gauge, then you have 20% leakdown on that cylinder.

 

I have heard that it can be as simple as carbon build up from lack of use (or use that really gets the motor up to temperature). So would also suggest you follow the above directions before spending chunks of money.

 

Interesting. My car definitely suffered from these symptons, the carbon build up was pretty bad.

 

Yup. Carbon accumulating on the valve and/or on the valve seat will prevent a valve from fully sealing...which lets some gasses escape (hence, a low compression # and a high leakdown number).

Hey, it's why I use Techron every other gas tank!

 

I am not pretending to be an expert, but I believe that Tom J. at Sport Auto reported seeing more than one 355 fail leakdown/compression purely due to carbon from lack of use. How many cars do you think have received new valve guides that maybe didn't need it? Let's face it, poor LD/C on a 355 and the knee jerk is the big dollar fix.

When I owned my 355 ( <sniff> ) I made a point every time I started it to drive it until it was warm and then give it several full throttle runs through the entire rev range over varied driving. It was a minimum 30 minute ritual, even if I just wanted to pull it out of the garage and wash it! ("Honey, I'll be back in an hour... I need to wash the car... )

 

...most of the ones that had a bad leakdown test but that mysteriously *weren't* burning oil, I'd guess.

 

But that particular F355, which I know too, actually did run hard lots of times.

(Not hard enough to keep pace with Eddie and me however. )

 

Well, the F355 did use the same guides as the 348; and then in middle 1998, Ferrari switched to steel-sintered guides.

Maybe the upgraded F355 guides will work on the 348? I don't know. Let's hear from someone who can give us the answer.

Also, you should consider upgrading your timing belt tensioner bearing. Daniel at Ricambi sells the Hill Engineering tensioner bearing, which is a much stronger and better bearing than the Ferrari part. I have used it for my car.

 

Nope. Different alloys and different sizes and different shapes in those valve guides.

 

Am I correct to say that the 355 at first used the same guides as the 348?

 

No. Let me see if I can find a picture of them...

 

Here we go, UCONN HUsky did a metalurgy analysis on valve guides: http://www.ferrarichat.com/forum/showthread.php?t=153589&highlight=355+valve+guide&page=6
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I agree – I’m new to the Ferrari world, I am no stranger to valve guide issues.
Porsche's (at least 928's, 944's, and air cooled 911's) have valve guides that I have seen worn out by 50,000 miles. I never saw a Porsche with bad valve guides that did not also show signs of oil in the cylinder.

With all of my cars I check the spark plugs a few times over the driving season to keep an eye on these issues. I’m hoping that pulling a spark plug or two on my fathers recently purchased 355 isn’t very difficult since I plan to do the same with his car. Better safe then sorry.