Arnoud

Thanks Fred, for your follow up reply which continues to help me a lot with hopefully coming towards the root cause of my problem. And as I want to absolutely understand and fix the root cause of my cars stutters/buckling at idle and when giving quick full throttle (especially when quickly going over 3K RPM), and another few days more work is not really going to make a difference: I will re-time the cams for exact -2 PS and 0 DS degree and then redo the cold compression test.

Thanks SwayBar for your feedback, and as per above answer to Fred: I will do so and report back here.

Arnoud

This evening I went to do the timing changes, and retesting the compression.

Pictures or it didn't happen...so started first with putting the crank on 0 degree TDC:


Then I measured what the current PS was, i.e. as per previous done tests, 0 degree:


And the DS was +1 degree:


Changed them as requested and agreed, PS on -2 degree:


And DS on 0 degree:


And here how it all looked, just before I started the compression tests with these settings:

Arnoud

After I had measured everything, I did a control check where everything was now timing wise.
PS side still on -2 degree:


And DS still on 0 degree:

I was happy with that.

The outcome of this evening compression testing, with PS -2 degree and DS 0 degree, and with still a stone cold engine while having the throttle wide open (in order of my testing):

5: 207 PSI
6: 202 PSI
7: 207 PSI
8: 202 PSI

4: 197 PSI
3: 187 PSI
2: 190 PSI
1: 197 PSI

PS bank remains having lower values than the DS bank...I'm not able to get them all within 5 PSI of each other...

What is the verdict of this all: as good as it gets (so: finish it, and start the engine and see what happens), or still something wrong here - and if so what could it be?

Mrmerlin

if your cam timing is spot on and it appears to be,
then maybe the left head was removed for servicing and it was fly cut so the compression is a bit higher,
do you notice any differences in the head gaskets you can see them where they stick out from the heads

GregBBRD

I'd expect the compression to vary, side to side, with a stone cold engine.

The cam timing is set differently, cold, to compensate for the expansion of the block and the stretch of the belt, once the engine is to operating temperature.

Porsche did a bunch of testing to arrive at their recommended cam timing settings....they set belts, warmed up engines, and re-checked the cam timing (warm) to achieve the correct cam timing, once the engine was warm.

Therefore, once the engine is hot, the cam timing should be the same from side to side....and the compression numbers should be equal.

You have introduced some "unknown variables" that could render this to not be true....

1. All of the calculations, from Porsche (and thus from Porken), regarding the initial cam timing were made from a factory belt. As soon as a non-factory belt is introduced into the equation (which is bound to have different stretching from block expansion and belt heat than the original belt), the "suggested" cam timing settings become invalid and need to be recalculated. I'm not aware that this has been done with any of the aftermarket belt offerings.

2. Ken's cam timing tool is a pretty accurate method of setting the cam timing. (I use this tool, myself.) However, I've set enough engines using the factory method and then using Ken's tool to compare to know that it's not perfect.....and I've used enough different tools to know that it varies some some from tool to tool. The camshafts/cam drive pieces/and even Ken's tool all have a tolerance....and using this tool can have cam timing anywhere from exactly correct to +/-2 degrees (per cam.) (Don't take this wrong and start another "war"....a couple degrees of incorrect cam timing is absolutely undetectable by driving a car.....and I've yet to see a difference on a dyno.) However, a couple of degrees of cam timing will affect the compression readings, cold.

Schocki

All is in the green FMPOV. Fire it up and take it for a (long) drive and let us know how it behaves now...

FredR

Arnoud,

Compression test numbers to some extent have to be taken with a pinch of salt. Confidence levels for the results are not going to be particularly high and if you were to do a standard deviation calculation at around 60% confidence factor then the spread of values would be around 5 psi with that data set -surprise surprise!

Greg has eloquently summed up the potential for train to train variances and as I stated in an earlier post, with those numbers you are not going to be able to discern any difference performance wise.

I have often wondered about the wisdom of using the Gates racing belt given it is so resilient and thus has a different stretch dynamic to the stock belt at high rpms. Even if by playing around some more with the bank to bank cam timing, even if you were able to get them balanced in a compression test there is no guarantee that would optimise real world performance- that can only be determined by dyno testing or so I would think.

Time to box it up and enjoy- fingers crossed your original issue will be resolved and even if it is not, then you will have eliminated other potential issues so nothing lost as it were.

Arnoud

First of all: thank you all for your very quick follow up replies, as this is what makes the Rennlist 928 community as great as it is!
Where a relative "noob" like myself, as I am not by any means a car mechanic nor do I hardly see any other 928 than mostly only my own (apart from the occasional Finnish 928 car friends that I have), get helped by others who have a much deeper and greater and/or actual professional experiences with these 928 cars of ours. Hope we can keep it all going, as this was - and still is very much so today - key for me getting and staying into 928 ownership from the year 2011 onward.

Mrmerlin / Stan: very good point as potential possibility!
I'm lucky, as in: knowing the whole history of my car from more or less day 1 as in use. All previous German owners kept all and every records and bills, and had it serviced in official Porsche centers, apart from the last one. The last owner before me had it serviced with a Porsche 928 fully qualified trained "older" car mechanic, who had started business for himself later on in life. I actually visited this person before purchasing the car, and also had the pre-inspection done by him while I was there in person - as he had most of the Porsche factory 928 tools, and he knew the 928 in-side and out. I paid him by the hour on that day of the pre-inspection, and every hour he told me that I'm wasting my time & money - and he was right because every test turned out 100.0% correct. After 5 hours of factual testing I believed him...
Due to all this, I know either head has never come off thus far and the 2R heads and head gaskets are as they came from the factory (and yes: sooner or later I will need to replace those head gaskets, getting 25+ years old now...). The coolant fluid has been changed every 2 years.


Greg: thanks for taking the time to chime in here, much appreciated! Fully understand and follow your above provided insights.

Schocki: I guess that is next then, I will do so and report back here.

Fred: I really hope the -9 degree retard on the PS cam side turned out to be THE reason, with as underlying root cause then a tooth being jumped during the year 2015... I will box it all up, and report back here.

GregBBRD

Looking through this thread and seeing how nice the work you are doing is, I'm optimistic that your original issue will be solved. (If not, I've got some ideas, for you.)

Great work!

Arnoud

Thanks for the compliment, Greg, as well as having some ideas in case my root problem will still not be solved (because I have none left at this stage)! I will let you and all others know what the outcome is "here", hoping for the best.

GregBBRD

Trust me, we all are hoping the same thing!

merchauser

great thread with wonderful information being conveyed. I will agree with greg, and wish you the very best. good luck!
looking forward to you next post

Arnoud

Thanks Greg, merchauser.

Just a very quick update: due to limited time thus far (including also today), I should have it fully "buttoned up" and being able to test drive it either tomorrow Tuesday evening or latest this weeks Wedneday evening. So a bit of cliffhanger for now, next update from my side will bring the answer...

Arnoud

As part of my final "buttoning up", I topped up the tensioner with motor oil (always used that, no problems ever thus far) by using a big syringe (while using very, very little pressure, so to not increasing the change of "popping" the tensioner gasket). I have used this method several times before, always works fine for me.


Oh, and: another WYAIT was for me to install some Greg Brown heavy duty skid plates. Even though I do have my car set on the factory height so to minimize hitting something underneath (many years ago a drive over a local country road full with boulders though me to do that), I like to have this extra protection in place there too:


Passenger side skid plate installed (given my front spoiler shape, it fitted best on the "inside"):


Driver side skid plate installed (and same here: given my front spoiler shape, it fitted best on the "inside"):


Both belly pans put in place as well, skid place position can be clearly seen from here:


Lowered the car, and went for a test drive...

Arnoud

Had the car warming up for about 15 minutes or so, mostly driving slow speeds (not more than up to 40 km / hour), all in automatic gear "D" i.e. low revs (by the way: never before have I seen my battery gauge indicating the 14V, as it did now. Really nice new alternator you made available for us, Greg, thank you!).

Then I put it into 2nd gear from standstill, i.e. having it leaving from 1st gear, and drove away while giving full throttle up to 5K rpm. It worked beautifully: with a very nice extra "shove" of torque with the flappy opening. I repeated such about 7-8 times, as well as driving it for about 1km only in 1st gear - so far so very good.

Then I decided to test it another time while standing still parked in "N", and giving quick/fast throttle: unfortunately for me, the exact same symptoms as before came back - i.e. sputtering / buckling from the engine. Then I gave another few quick/fast throttle test: sometimes it is fine, other times not.... What on earth can this problem be?!?

Full disclosure: I was under the impression that many months ago, I had completely emptied my patrol tank (with a super simple hand pump, and at least 1+ meter long plastic tube inside the patrol tank). A few days ago I put 20+ liters of brand new 98-octane super patrol (this is what I always use here in Finland) into the tank. To my surprise when I started to test drive it now, I showed 40+ liters on the patrol gauge... Therefore I must now have a mixture of 20+ liters of nice brand new 98-octane with 20+ liters of almost 2 years old (also 98-octane when bought new in June 2017) in my patrol tank. Surely that does not help much at all, but to me: this is logically still not the root cause of my problem.

For a quick recap of what has been tested during last year 2018 by me:
- Swapped in a good working MAF: no difference, problem remained.
- Swapped in a good working EZK computer: no difference, problem remained.
- Swapped in a good working LH computer: no difference, problem remained..
- Swapped in both good working EZK + LH computers: no difference, problem remained.
- Swapped in brand new second stages ignition modules: no difference, problem remained.
- Checked all distributors and Beru wiring: all in really good condition (was fully renewed by me in the year 2014) , and all measured in spec.
- Have done an intake refresh whereby I put in all new: all rubbers + all rubber hoses, CPS, TPS, ISV, both intake needle barrings, fuel regulators + dampers, both temp sensors I and II, and all relevant gaskets. All 8 injectors were ultrasonic cleaned and flow tested. No difference, problem remained.
- When putting in brand new spark plugs, initially the problem resolved for the first 7 km or so, After that: no difference, problem remained.

And during this year 2019:
- Found the PS timing to be retarded at -9 degree, as several Rennlister here told me to check after the compression test showed lower numbers for the PS side. Rectified that in the past weeks, with installing a brand new Gates racing belt (as the timing belt was last replaced by me in 2014 which was a normal Gates belt, so this was due soon anyway). Timing set with cold engine: PS -2 degree, DS 0 degree.
-Replaced the hall sensor, just to be sure.
- Problem is still there...

Any and all help of what to logically do / test next: super much appreciated!