Cam deviation -I am confused

[BFeller]

I am thinking about a clutch replacement. I decided to check the cam deviations. At hot idle, the values are 3 and 6. Rock steady. Run the idle up to 5000and when it drops down I see a brief hit at 7 on the side the shows a 6 at idle. Then it drops back to 6. Is the deviation that I am looking for the number at idle? The reason being is that I am considering replacing the imsb. I run a Guardian system. I have had no alerts. No debris in the filter ever or sump (when the Guardian was installed). I have a 2.5engine with a 5speed in 1998 car.

I am still reading threads to try and understand what the numbers mean.

Thanks.

[since95]

I am pretty sure a main cause of the deviation comes from the cam tensioner pads that wear out for the timing chain between the camshafts. My deviations were 6 and 9 rock steady. When I pulled the camshafts during a rebuild (unrelated to cam dev) both tensioners had plastic ramps that looked like a dog chewed them up. They would not have lasted much longer. The rest of the chain ramps looked almost brand new. I believe Porsche updated the material the tensioner ramps are made of to reduce the wear.

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[since95]

And to better answer your question now, the tensioners are part off the variocam system which alters the cam timing at I believe 3500 rpm. Since the ramps are so chewed up and worn the cams have shifted position. Being called deviation, my assumption is the sensors reading position of the cams are telling you how far out of position the camshaft is relative to the correct position which should be zero.

When I read mine the deviation was 6 and 9. My variocam advance was still correct (I think 20 degrees of change) , but the starting point was off due to the worn plastic ramps.

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[BFeller]

Thanks. What I am gleaning is that the 6 degrees is how far it is out of deviation The number at hot idle is the one to monitor- to determine if the chain ramps should be replaced. It is not the number as it returns to idle. The model I have, when activated just changes the cam settings in one fell swoop. So, the change as it drops in rpm is not the number to monitor.

Knowing this makes the difference between doing the clutch install myself and dropping the engine/or finding an Indy with a good reputation.

[The Radium King]

5 chain engine

crankshaft is connected to the intermediate shaft by a chain. this chain is kept in tension by a chain ramp that is pressed against the chain by a tensioner. this tensioner is static (doesn't move).

the intermediate shaft drives two chains - one to a camshaft on each head. again, there is a chain ramp and tensioner for each chain. again, static.

at the heads, the intake and exhaust cams are joined by a chain. this is a dynamic system, designed such that varying tension on the chain adjusts cam position (the variocam system). a signal is sent to a solenoid that activates an actuator that moves the chain in and out. there are pads on these actuators. these pads wear with time. there is a sensor in the system that measures the deviation between the two cams, which is a sign of pad wear. when deviation exceeds 9 degrees at hot idle you get a cel. deviation should be a steady number and not fluctuating at idle.

failure points

intermediate shaft bearing can fail. this is an interference engine which means that if valves are extended when piston is at top dead center there will be contact and things break. failure of the bearing will cause loss of timing and piston/valve interference. i won't get into mechanisms of failure here (lots of opinions) but, without the guardian device, the only sign of failure is a pre-failure wobble in the bearing that can cause rapid fluctuations in deviation at hot idle.

ramps can fail. this will cause loss of timing and piston/valve interference. mechanism? presume lack of tension from tensioners causes chain to slap and work ramps. there is an updated ramp available from LNE, and updated tensioners from porsche.

sensor or sensor wiring can fail. repair/relace. note that a mis-aligned sensor can produce incorrect readings (ie, read a deviation of -4 when it is actually -2). this is not a big deal, as what you should be monitoring is the change in deviation over time (sign of your pads wearing) and not the actual value.

soleniod or wiring can fail. on later cars the soleniod can be serviced from outside of the engine; on ours you have to open it up. typically shows up as cams not changing position when asked to by the ecu. mechanison of failure? it's an electro-mechanical part that is probably not rated for as many movements as the salesman told porsche it was.

actuators can fail. this will show up as a sudden increase in deviation at hot idle - in my case it was -16. actuator failure will not cause enough timing change to create valve/piston interference, but you will get a cel and loss of power. actuator failure will also show up as rubbery green bits in the oil sump. mechanism of failure - presume mechanical work due to chain slap. also, failing pads may cause it to extend beyond it's normal range of operation, such that running with bad pads for a long time will eventually result in actuator failure as well.

pads can fail. these will show up as hard dark brown plastic bits in the sump, and a deteriorating deviation over time. there is an updated part for these that lasts longer. again, they fail becuase they get slapped by the chain.

chain slap due to lack of tension on the chains during start-up is a big cause of most of the failures. there are many proposed solutions - updated oil pressure relief piston, tensioners, and pads (porsche improved them for a reason) higher cold weight oil to reduce drain down, pre-oiling via accusump.

[BFeller]

Thanks to both of you for the information.

[Lew]

I did a quick scan with my Durametric this AM and the 04 S has no fault codes, well, other than the passenger side overhead sensor fault P034. No bother however. My main concern is cam deviation and at Spec idle 680 RPM's cam number one shows a steady 1.16 and number two shows a steady 0.23.....no change in higher RPM. On my MAF at spec RPM a reading at 15.6 to 16+kg's and at 3k RPM 60+kg's. I think it's looking good.