Valve adjustment necessary prior to do leakdown & compression tests?

[kp.sfo]

I am considering to buy a 73.5T and have a PPI scheduled for next week.

The shop recommended to do a valve adjustment first in order to get meaningful results from compression and leakdown tests (they haven't seen the car yet but recommend this in general).

Does this sound like common practice, or is this over the top?

[proffighter]

As former motorcycle mechanic I can tell you that it is recommended to adjust valves first. Seems logical to me that it's valid for cars too

[80-911SC]

yes ... definitely .. if you have a loose valve you will lose compression in that cylinder rapidly ... they need to be right to get a proper test ....

[docrodg]

Adjust them. If they are not correct compression and leakdown will be off.

[Grady Clay]

kp.sfo,

No, don’t adjust anything yet.
The purpose of a PPI and the cylinder leak test is to determine the condition as it is, maintenance comes later.

The most accurate cylinder leak tests are just after extended (and ‘sporting’) driving.
Do not start the engine again, test while the engine is still warm.
Set the engine to the cylinder to be tested and only then remove that cylinder’s sparkplug (this prevents carbon from the end of the plug from falling on an open valve seating surface.)
If you have questions about the results, repeat the entire test - always 1-6-2-4-3-5.

If you find any valve(s) leaking, check for clearance on that valve.
Too tight valves can cause cylinder leak, too loose valve settings do not cause cylinder leak.

Remember, one of the most important parts of the cylinder leak test is to tell what is leaking.
Be prepared to listen for escaping air in the (open throttle and sensor) intake, exhaust and crankcase breather.
This is properly done without any extraneous noise.


There is an appropriate situation to check valve clearance.
This is when the valve covers are removed to inspect for (much more important) possible broken (or pulled thread) head studs.


A good PPI starts with observation and tests that will yield the most information with the least cost.
Many PPIs stop at the first sign that something serious is wrong (so as to not waste money on a car you won’t buy.)
Good, real-time communication is critical.

Most important is a ‘knowledgeable eye’ looking for signs of rust, rust repair and prior crash damage repair.
The term ‘surface rust’ can be a misnomer; the rust starts from an inner surface and what you see has come through the metal structure.

A 'knowledgeable eye' can also spot improper repair, a good indicator of PO attention to service.

As a PPI progresses, inspecting the magnetic drain plugs, inspecting the sump screen and cutting apart the oil filter become instructive.
If transmission issues are suspected, having a new spare 915 shift pivot cover gasket is useful.

There are some good PPI 'check-lists' posted on the Forum.
Do some searching.

Good luck on your quest.

Best,
Grady

[proffighter]

Maybe I am alone with my opinion, but for me, a leak test is to get an idea of valve sealness and piston/ring cylinder wear.

So why spending time with maladjusted valves and maybe make a test twice, just to know they were off?

To get accurate results you need to adjust anyway...

Just to get this right, leak-down test is reading the pressure produced when cranking.

...but if you talk about setting the combustion camber under pressure with air through plug hole, then I can understand your point of view

[Grady Clay]

Roland,

No, a cylinder leak test is performed with the engine static (not cranking).
You are referring to a ‘cranking compression test’, also useful diagnostic.

The cylinder leak test has the engine positioned at Top Dead Center (TDC) on the compression stroke (both valves closed), starting with #1 cylinder.
You then remove only #1 sparkplug (reason noted above) and screw-in the hose/fitting for the cylinder leak tester.

Proceed through the firing order (1-6-2-4-3-5), turning the engine CW 120º to the next mark on the crank pulley.
Never ‘back-up’ (CCW) the engine.
When you are at the next cylinder, then remove that cylinder’s sparkplug.
You can use the distributor rotor position to confirm you know what cylinder you are measuring.

The test quantitatively measures the leaking air flow into the cylinder.
Ideally this should be zero but these are automobile engines.
On an excellent 911 engine, the ‘leak’ is about 1-2%.
With a ‘usual’ street-driven 911 a typical leak might be 5%.
All of the leakage is past the rings.
When the ‘leak’ gets to about 10% or there is leakage past the valves, most start to express concern.

In addition to having a quantitative measurement, qualitative listening to the engine for escaping air in the (open throttle and sensor) intake, exhaust and crankcase breather gives an idea of there a problem may lie.

There are occasions where the escaping air can be from between the head and the top of the cylinder.
This indicates a leaking head gasket with a broken cylinder head stud or the threads have pulled in the crankcase.

If there is a ‘leaky cylinder’ or few, after several repeat tests you can squirt some oil in the cylinder, turn the engine 720º and re-measure.
This should help seal rings but not necessarily a leaking valve.

Tapping (with a brass hammer) on the valve adjustment screw (or better the valve stem end with the rocker removed) can re-seat a valve with something lodged between the valve and seat.
Bore-scopes have become an invaluable tool for inspecting valve sealing surfaces.


The cranking compression test is a worthwhile test also.
It gives quantitative results and is a ‘dynamic’ test of engine condition.
This includes the functioning of the cams and valves as well as ring-to-cylinder sealing during the compression stroke.
It is not as informative as to where a problem may lie.

The two tests are valuable in conjunction with each other; representing both static and dynamic sealing of the combustion chamber.


A cautionary note:
The compressed air in the cylinder can cause the engine to suddenly rotate off TDC –either way (CW or CCW).
If it goes CCW, rotate the engine a complete 720º CW before you test again.

This can be very dangerous if you leave the wrench on the crankshaft bolt or have a part of your body in the way.
Be very precise coming to exactly TDC on the cylinder you are measuring.
Remove the wrench.
Keep hands, fingers, etc. clear.
Shop air can produce a lot of power to turn the engine.
Lower air pressures reduce (but not eliminate) the risk.


Best,
Grady

[docrodg]

Grady -

I think I see your point of view on this.

To me the natural way to conduct a leakdown on your car is to ensure valves are all adjusted and car has been well driven to ensure that you evaluate the actual condition of the internals and what work may be needed. Adjusting the valves first is so you do not have to repeat the test, as well as being periodic maintenance anyway. But that is on a car you own and are maintaining.

Your point of view on a PPI is to not adjust valves prior to leakdown in order to judge whether the car has been properly maintained - if there is leakage it could be improper valve clearance that an owner would want to eliminate before testing but as a buyer it is a warning as well as a bargaining position.

[kp.sfo]

I am impressed! So many qualitative responses within just a couple of hours, that's awesome. THANK YOU ALL!

Now I feel much more comfortable going into the PPI discussion with my shop next week. Using a few sound bites from Grady will allow me to talk to them at equal terms.

I'll keep you posted!

[Joe Bob]

I agree with Grady, test as-is and then adjust to see if there is any change.

[gshase]

If I am selling a car I would be Pissed if you had your guy adjust anything with out first talking to me.

[Grady Clay]

Originally Posted by gshase

If I am selling a car I would be Pissed if you had your guy adjust anything with out first talking to me.

I agree.
Even discuss the type of oil to be used if you inspect the sump screen, etc.


docrodg,

Yes, there are two different approaches: One for maintaining your own 911 and another for (economically) assessing a prospective purchase.
Yes, finding ‘negotiating points’ is a consideration.
No used 911 is ‘perfect’. You want to know the flaws before paying and driving the new toy home.

Many of us have paid for a dozen PPIs before we found the ‘right’ car.
I like it when the mechanic/shop takes a close look and says “Pass on this one.” not charging anything.
If the first mechanical test has cylinder leak: 2,3,2,4,2,28; I don’t need to inspect the oil sump screen.


For maintaining your own 911, you should do a cylinder leak test at every opportunity.
First, you become familiar with the procedure and the ‘normal’ range of results.
Most important, you have established a ‘baseline’ for comparison when you suspect something is wrong.
If you are diagnosing a ‘running problem’ and your first-ever cylinder leak test results are: 9,2,7,3,11, 4; how do you interpret this? Normal for your 911 or something wrong?

Your best ‘tune-up’ will be just after a 100 mile drive to a 3-day DE event and the return trip.
Then is the time to adjust the valves, measure the cylinder leak & cranking compression and perform all the ‘regular’ maintenance, not just prior to a weekend event or trip.
Here we are focusing on how to do the best ‘tune-up’.
Another important consideration is to never just do maintenance just before leaving on a trip.
Check your oil level – yes.
Check your tire pressures – yes.
Have everything newly assembled – no.



kp.sfo,
Here is Pete Z.’s excellent discussion:
The Pre-Purchase Inspection by Peter Zimmermann - Rennlist Discussion Forums

Here was some discussion about questions to think about:
Compiling Questions for Used 911SC Buying

There is a LOT more about PPI here on Pelican.

Feel free to ask more questions here.
Good luck next week.

Best,
Grady

[kp.sfo]

I received the results of the initial PPI. We didn't get to the compression/leakdown tests yet. This was more of a general walkthrough to rule out any potential showstoppers.

I'd be interested in your take (such as "not too bad", "fixable", "RUN AWAY!"). This is supposed to become a decent weekend driver, not a Concourse type show car:

FRONT BRAKES: 40% REMAINING
REAR BRAKES: 50% REMAINING
* FRONT BRAKE CALIPERS SEIZED
* BRAKE FLEX HOSES ARE VERY OLD, DRY. RECOMMEND REPLACMENT.
* TIRES VERY, VERY OLD, DRY-CRACKED REQUIRES REPLACEMENT.
* ALL SUSPENSION BUSHINGS ARE ORIGINAL, RECOMMEND REPLACE.
* ENGINE OIL LEAKS REQUIRE DEGREASE FOR DIAGNOSIS, BUT ARE EXTENSIVE, LIKELY REQUIRES EXTENSIVE REPAIR.
* FRONT TURN SIGNAL LENSES CRACKED.
* BODY WORK IS POOR, OBVIOUS FILLER USED IN R/R FENDER, OVERSPRAY ON MOST RUBBER GASKETS AND BEADING, FINISH IS POOR. ALL PANELS HAVE BLEMISHES, SCRATCHES, PAINT CHIPS.
* OLD SYLE VALVE COVERS INSTALLED, RECOMMENDED UPGRADE.
* ORIGINAL STYLE TIE RODS INSTALLED
* FRONT LEFT BUMPERETTE TORN OPEN.
* REAR BUMPER VERY LOOSE ATTACHMENT TO BODY. REQUIRES DIAGNOSIS / REPAIR.
* HEAT EXCHANGERS HAVE HOLES RUSTED THROUGH HEATER SECTION.
* FRONT FLOOR PAN / SUSPENSION PAN HAS SOME CORROSION FROM BATTERY LEAKING ON LEFT CORNER.

Haven't decided yet whether to do a full-fledged PPI incl engine diagnosis, but I probably will. In fact, it's always a learning experience.

[plumb4u2]

While a lot of that list is relatively easy to fix and somewhat inexpensive there some $$$$$ repairs such as body work, rust repair (especially if you cant weld), heat exchangers

depending on where the oil is leaking it may require engine drop

IMO this car doesn't seem to have been taken care of that well

[plumb4u2]

Are you experienced with these cars or are you going to learn to repair things yourself?

That list will be VERY expensive if your paying for the repairs to be done

[Grady Clay]

I don’t see anything mechanical (yet) that is unusual.

I agree, it doesn’t appear to have had recent TLC.

I would next take it by a body shop familiar with repairing 911s and rust.
This may be the most important part of the PPI.
Critical is it may have already had one (or more) poor quality rust repair(s) and crash damage repair(s).

If the body/chassis is relatively rust free and no serious crash repair only then spend the $$$ for more mechanical inspection.

Posting detailed photos will help.

Best,
Grady

[kp.sfo]

Originally Posted by Grady Clay

Posting detailed photos will help.

Here are some pictures

[Peter Zimmermann]

I'd love to do a PPI on that car; the pretty ones can get very interesting! Is the floor greasy/wet around the base of the gas pedal? If yes, it appears that the brake m/cylinder might be leaking, and dripping fluid into the pedal cluster area.

[clarkd]

* FRONT FLOOR PAN / SUSPENSION PAN HAS SOME CORROSION FROM BATTERY LEAKING ON LEFT CORNER.
The channel in the suspension pan (beneath gas tank support) is prone to rusting from the inside out if sulfuric acid finds it way there.

[kp.sfo]

Originally Posted by Peter Zimmermann

I'd love to do a PPI on that car; the pretty ones can get very interesting!

Pete,
I know. The car looks actually pretty decent on the pictures. Seems to be clean and well taken care of. But then... the car hasn't been driven a lot in the last couple of years. I'd say that's exactly the reason for a few of the issues the car has (such as seized brakes, etc)

Originally Posted by Peter Zimmermann

Is the floor greasy/wet around the base of the gas pedal?

the seller says no - I haven't seen the car in person, it would be a long distance purchase where I'd have to trust the PPI.