JPBNY

Today's Mystery question is what will cause a power loss? Just installed balanced fuel injectors,new o2 sensor,fuel filter and new spark plugs.Head was off car for valve job and part of the reinstallation was the above parts car starts and idels fine but when you give it gas it gets to about 3500 rpm and starts to sound like its missing or starving for gas or getting too much gas.Car does the same thing when being driven ok at part throtle but when you floor it it does not kick you in the pants and it sounds like the above.I did the old check engine light code trick and it does not show any error codes!I have checked the vacumn lines ,cap rotor plug wires temp sensors and tried 3 different DME relys all with the same results.I also tried going back to the first set of fuel injectors and still the same problem.If you press on the gas with the car sitting still it will reve fine but if you rev it fast a few times in a row it starts to break up and now that I have driven it I find it does the same thing under load.All ideas guess's or clues welcome. Thanks John 1994 968 80k miles

PorscheG96

If the head was off for a valve job and the cams were removed it sounds like one of your cams is misaligned.

JPBNY

The cams were taken out with the variocam unit as one big part and the cam gear was locked in with the three small bolts.All of the marks were checked when it was put back in and it was at TDC .

User 41221

It sounds like a cam alignment problem to me. Even if everything is relatively in the same place, you'd be amazed at how the variocam can mess with you if you don't use the proper tools to index it.

Regards,

sayporsha

Amen to the cam alignment theory. I just went thru a similar problem with my 95 968 Cab. The car would surge from about 1800-2300 RPM, and was generally low on power.

My tech theorized the head may have been removed before and the cams were not properly indexed on installation. He said on the 968 you MUST index the camshafts with a dial indicator on the #1 piston EVERY time you r&r the head.

It turns out the head was never off the car. What he found was unthinkable: a Porsche dealer installed the timing belt off by 1 tooth. He r&r'd the belt with the correct timing and life is good.

The fact that your car acts up under load tells me you should check your fuel pressure, though . . .

JPBNY

Checked the fuel pressure and it was well within factory specs.Pulled the plugs and they also look great.Going to open it up and check the cams.The mystery continues!

JPBNY

Checked the timming it is right on the money but the car is still fluttering above about 4000rpm under load.

User 41221

You might want to also check the valve springs. A broken one will cause a power loss/fluttering at higher rpms. Did you use the dial indicaters to check the cams, or did you just look at the markings on the cam gear?

Regards,

JPBNY

The cylinder head was just done so I know the valve springs are OK. The cams were done as per the factory manual. I used 2 dial indicators and supplied the variocam unit with 45 psi of air pressure thru the correct fitting.After the last test drive it seams to pull better but I am still getting some flutter in the higher RPM range.I did on the last test run get a code from the check engine light it was for a knock sensor #1 but it was a 1-2-31 so now we will check the connection clear the code and drive it again.

TurboCab

JPBNY:
I have some doubts about the cam indexing procedure in the factory workshop manuals. I used all the correct tools and supplied the tensioner with compressed air, but still have doubts because even with the compress air applied the tensioner moves when you turn the engine. In my opinion the tensioner must be static in the variocam off position. Otherwise you will have an incorrect adjustment. I try to find the answer or to contact someone that has done this adjustment succesfully in the past with no results. Even the authorized factory mechanics in my area do not know how to do it correctly. If someone has done this in the past or know someone willing to share the knowlwdge please step up.

JPBNY

Just my 2cents on this is that what you really are doing is timing the exhaust cam to the crank.If you read the factory manual they go out of their way to make sure you are turning the engine in the "sense of rotation"this is because the cam chain is in tension on the bottom of the cam gears and that dimension does not change as long as you rotate the engine in the correct direction.When you put the cams in the first time you time them to each other with the colored links on the chain(7 outside links)I think the only reason for the slots in the cam gear are to make up for change in the overall length of the chain with time(stretch)The variocam unit only changes the intake cam in relation to the exhaust cam by altering the tension on the top and bottom of the chain.I also suspect from playing with the cams that you could move the cam gear around within the slots and it will still run. Thanks John

TurboCab

I agree that you adjust the exhaust cam, but the micrometer is positioned on the intake valve, inlet camshaft. And you move the complete assy through the exhaust camshaft. Any movement in the variocam tensioner must yield a potentially wrong adjustment, since the adjustment of the intake cam is just milimeters. This is probaly the reason for the factory to apply compress air to the tensioner. But the tensioner is a hydraulic unit, not pneumatic.

nedkelly

I think I need medication now!