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

All true. When the compression is bad from valve leakage I would be seriously remiss rising to the level of dishonest if I did not prepare the customer for the expense of a valve job. If it needs less, great.

Every single customer deserves to know the worst case when we embark on ripping their car apart. Not doing so is a simple case of bait and switch. Something I simply will not participate in.


I have been working on 355's professionally since their introduction almost day in and day out. In a very few cases I have seen carbon build up on the back of the valves sufficient to cause a running issue. They do not do that on their own. The engine management system is far too well developed to allow that. Oddly enough it is a symptom of a more serious problem and oil leaking past worn guides is generally it. These motors run very clean unless something is wrong.


Additionally I find it interesting that having been Shop Foreman of what was one of the very largest Ferrari service departments at the time that Ferrari accepted cylinder leak down results as sufficient evidence to approve valve guide replacement on the very large number of 355's we did that job on under warranty yet according to some "Enthusiasts" do not consider that a valid test. I guess I over estimated Ferraris expertise in the area of engine design and diagnosis.

Like Josh said earlier, some just do not want to hear the truth. The body of evidence is far too great to rewrite history now.

 

I have a related question - my daily driver is a 1996 Porsche 993. It began giving Manifold Air Pressure codes for a Check Engine light. Intermittently - about once every 2000 miles or so.

My technician - RAC Performance (a sponsor here) - immediately diagnosed it as valve guides.

We did a full top overhaul - and he was right...50,000 miles ago.

The question is this: How do valve guides mess up the MAP sense numbers enough to cause an onboard diagnostic code to appear? It was my impression that this engine still had good compression, but just the beginning of valve guide wear. Did RAC just know from experience that this usually meant valve guides, or is there a specific relation to MAP bad numbers from this condition?

 

Modern engines and modern engine management systems have gotten complex enough that they often have different symptomologies in many cases for the same problems. Especially when it comes to what sets a particular error causing a check engine light.


I suspect you are right. It was their specific product knowledge that led to the diagnosis.

Bad valve guides = bad compression = bad manifold vacuum = bad MAP reading.

Never seen a Ferrari do that so I suspect Porsche engine management is more sensitive to that particular issue.

Like someone said earlier, bad guides is a matter of interpretation. From the day the motor is first started they are no longer perfect. When wear becomes symptomatic is when something needs to be done. Evidently in your case symptomology came in the form of a CEL.

 

Thanks Rifledriver. Yes, that was what I suspected.

BTW - you were right about the Rolls-Royce. Magnificent car, but with magnificent maintenance issues and costs. I still have it against your advice.

 

I am amazed. Someone can read English without reading something into it that isn't there. Most just prefer to assume I am an ass hole.


Thats their problem. I can't correct widespread illiteracy.


I did say however design and diagnosis. No mention was made of willingness to buy good parts. If not for that we wouldn't be having this conversation.

 

I actually find this thread very interesting and (without sarcasm) there a plenty of you who have seen more 355 engines apart then I ever hope to. While we must agree to disagree on the need for a valve job and the validity of a leak-down test to advise it, In the spirit of information, I'd like to ask..what is the permanent FIX for the guide issues?
And by the way, I hope no one was aiming the comments about not facing reality or deferring maintenance at me, cause I will pick on just ONE of the service history books on any of my cars, Ferrari or others, and hit ya over the noggin with it!

 

Rick we may be the only two who seem agree on this topic, but here I must comment. Who would take the time to pull the heads off for "de-Carboning" and NOT do a valve job, including the guides if they hadn't already been replaced? The parts and labor here are pretty small once the head is on the bench.

When I DO finally need the heads done, I will do the full meal deal regardless of the condition revealed once its apart. But thanks for backing up my contention that the typical 355 valve is more likely to be carboned up from over rich running than burned from too lean running.

 

Good quality valve guides, not the crap Ferrari installed as OE until mid year 98.

I am not sure any valve guide is a permanant fix. Even good friends within Ferrari said they feel that with the size of the valve stems the lack of guide cooling due to the 5 valve head design and the exhaust temps in those motors that no guide is permanant.

 

Copy that, thanks! I agree with your contention about the cooling in the 5 valve design, just wondered if someone had come up with some sort of "unobtanium" material to make the guides out of and get more than 30k out of a set!

 

I'm not sure I know you well enough to to aim any judgemental comments in your direction. Unless there's some guilt laying dormant in your belly regarding this topic, we're cool.

It was more of a generalization regarding the wide spread behavior of people that prefer to argue the validity of one's experience, as opposed to footing the bill to fix a broken car.

This thread created a nice opening to point the behavior out and discourage those folks from wasting people's time by chiming in.

 

There have been some very significant posts made by Pro's. Thanks guys for posting.

 

Not shocked about that.


I will say this, whoever coined the term "Lap of luxury" was driving a RR at the time.

I really did like driving the late British RR's and Bentley's. Not so much the German one's.

 

+355

 

Yes, that is what I have - 1995 Silver Dawn with 36,000 miles now.

Complete climate control computer failure, and have had to rebuild the hydraulic system including both pumps. Oil pressure sender has failed now - will cost about $1000 to fix.

But, as you say, a really relaxing (and quiet!) driver both in town and on the road.