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What are your fuel pressure readings at idle with vacuum, idle without vacuum, and under load when it acts up? What kinds of voltages are you seeing at the O2 in the same scenarios? Have you tried running the car with the O2 disconnected?
Your compression pressures (10 BAR or about 145psi) is good and indicates, for the most part, the cylinders are sealing. Compression test does not tell us if you have burned up exhaust valves. You'd need to do a leakdown test to verify that. I doubt that you have valve spring issues as you'd have obvious misfires as well as mechanical noise from the valvetrain.
What are your fuel pressure readings at idle with vacuum, idle without vacuum, and under load when it acts up? What kinds of voltages are you seeing at the O2 in the same scenarios? Have you tried running the car with the O2 disconnected?
Your compression pressures (10 BAR or about 145psi) is good and indicates, for the most part, the cylinders are sealing. Compression test does not tell us if you have burned up exhaust valves. You'd need to do a leakdown test to verify that. I doubt that you have valve spring issues as you'd have obvious misfires as well as mechanical noise from the valvetrain.
No idea about the fuel pressure. It hasn't been measured as far as I know? However as when the car was ran on dyno the AFR readings were good - a bit even on the rich side. If I would have fuel pressure problems that would have shown in AFR readings in the dyno.
O2 voltages haven't been measured and the car hasn't been ran with O2 disconnected.
Doesn't compression test tell if a valve is burnt or not as the car would lose compression? Or under which scenario compression would be OK with a burnt valve and a leakdown test would only tell that? Just trying to understand the scenario.
edit: as per my understanding compression test would tell if there is a problem with the cylinder keeping pressure. Is that an intake or exhaust valve that doesn't tell - a leak down test will tell that - but compression test will indicate if a problem exists nevertheless.
If the compression test was done properly it should show a burnt valve as a low, slow-to-build cylinder. If the compression test was done with the throttle plate closed I am not sure what it might show. A leakdown would be more interesting.
Maybe a broken valvespring would get lazy under load and start to flow air/fuel out the exhaust?
Can you IR gun the header and see if all 4 are hot or if it's more one than the others?
Running with the O2 disconnected is a good thought. If the AFR was post-cat then it'd have burnt off some fuel in the cat no? Fuel pressure under load would be interesting. Cats can run red from a miss pushing fuel/air into them or maybe from leanness... I am not sure if you'd get a glowing red cat from that or just high EGT.
You could IR gun the exhaust also to see if the cat is heating the exhaust or if it's hot all the way down. i.e. is there fuel in the exhaust that the cat is burning to create more heat? Seems to me you must.
Myself I'd throw away all the assumptions (particularly the wrong ones) and start with looking at the plugs and doing compression/leakdown.
If the compression test was done properly it should show a burnt valve as a low, slow-to-build cylinder. If the compression test was done with the throttle plate closed I am not sure what it might show. A leakdown would be more interesting.
Maybe a broken valvespring would get lazy under load and start to flow air/fuel out the exhaust?
Can you IR gun the header and see if all 4 are hot or if it's more one than the others?
Running with the O2 disconnected is a good thought. If the AFR was post-cat then it'd have burnt off some fuel in the cat no? Fuel pressure under load would be interesting. Cats can run red from a miss pushing fuel/air into them or maybe from leanness... I am not sure if you'd get a glowing red cat from that or just high EGT.
You could IR gun the exhaust also to see if the cat is heating the exhaust or if it's hot all the way down. i.e. is there fuel in the exhaust that the cat is burning to create more heat? Seems to me you must.
Myself I'd throw away all the assumptions (particularly the wrong ones) and start with looking at the plugs and doing compression/leakdown.
I don't suppose anyone has done a plug chop.
Compression test was done properly i.e. throttle fully opened.
AFR measured post cat i.e. from the tailpipe as typically in a dyno its measured.
One other person also suggested that valve springs might be broken / which would case the exhaust valves not to seat properly under load.
I guess you're right with the assumptions and prior work done: everything out from the window and start from square one. Arghh...
if your engine wont do more than 0.2 bar, and cylinder test is ok.
Is there any leaks on the ic piping?
Can you do a video?
0.2 bar seems like a hug backpressure problem.
There is a video on page 4. Taken from the inside while the car was driven. Not much of info there apart from the sound and about 0.2 bar of boost.
We haven't checked the piping ourselves but the shop it was in earlier said they pressure tested the whole system and it didn't have any leaks.
Somehow the whizzing sound appears to come from under the car and the sound was the same when the car was driven w/o cat and muffler. Sounded like an old tractor
Again - as a background the car was driven on a track - not floored - and then on the way home there was no power = the problem started. Nothing out of the ordinary happened whilst driving in the track. Fixing the damn thing would be far easier if there would be a clear problem.
Even though the shop didn't find the problem they did try their very best and it is not a backyard mechanic but an independent shop who have done Porsche repairs for about 20-years or so.
Pardon if you already stated this or answered it. Is this header glowing uniform across all 4 cylinders? That could split this between an isolated failure (e.g. you have exhaust valve issues on 1 or 2 cyls) versus and across-the-board tune issue. You can get IR temp guns for really cheap these days. Hell, I got one just so I can check the temperature outside, using the window pane, without leaving my bed in the morning
Pardon if you already stated this or answered it. Is this header glowing uniform across all 4 cylinders? That could split this between an isolated failure (e.g. you have exhaust valve issues on 1 or 2 cyls) versus and across-the-board tune issue. You can get IR temp guns for really cheap these days. Hell, I got one just so I can check the temperature outside, using the window pane, without leaving my bed in the morning
If I recall correctly - I will dbl check this from my buddy though - the header was glowing uniformly when the car was on dyno. It really becomes glowing red.
I'll get back to this topic once I have checked this for sure.
If we would know what's wrong with the car it would have been fixed already. The guy who owns the car is mechanically really adept i.e. he's running a repair shop of a well known brand of outboard engines and motorcycles and his business partner handles the sales side of the business . So he really is pretty familiar with the world of engines. However this P-car had proven to be a tough nut to crack. Or actually impossible so far.
The car was driven on a track on a P-car club day and the track was full of cars so tracking was more or less driving on a line of cars. Occasionally there was enough space to drive hard but even at that time the driving instructor was screaming murder i.e. brake and don't drive that fast etc. so the car really wasn't driven "hard".
Below a few pics from yesterdays skull-breaking and mind-boggling session. I didn't take a pic of the mark on the flywheel but they do align.
Someone suggested that cam gear keyway would have gone bad and caused the cam gear to slip and thus timing would be out of whack. We tried to open the hex-nut but it is SO tight that we gave up. Hard to believe on the cam gear slipping theory. Anyways out of ideas and appears that anything is possible at this point.
Timing mark
Non-hairy cat on the floor.
Muffler on the floor
Last edited by WPO; 08-09-2017 at 04:31 PM.
Reason: typos and added info
08-08-2017, 07:12 PM
