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Alan L's Avatar
 
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scary noise

I've been fiddling with fuel pressures, and cold start issues since my cold start injector is/was defunct. Took a while to get to that diagnosis with many other issues. While I had the engine labouring at very low revs, cold idle (too rich) a knocking noise could be heard from clutch end of engine. Best I can tell is in the #3/#6 area, and if I had to guess, #6. Putting the clutch in does nothing for it.
But when the engine is idling warm, and no load, ie running nice, or when you rev it at standstill, no noise (none that you can hear above the fan etc anyway). There was at times under these conditions a very faint noise from the same area - but you had to stick your head deep into the engine cavity while it is running, to pick it up. But when it was labouring at about 300-500 rpm (and running a bit roughly) you could hear a definite knock. Because it is not running smoothly at those revs, I wonder if it could be something trivial, or how would I determine if it is more serious?
One of the aspects I thought positive for the car pre purchase, was the oil pressure seems very good - around 3 bar at idle, and runs to 4+ under revs.
Alan
Old 12-11-2007, 02:06 PM
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You think there is oil starvation somewhere at lower than typical revs? If it clears up at standard idle I wouldn't worry about it.

Also - your injectors are in route. Shipping was only $8.65 so it's on me.
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Old 12-11-2007, 05:12 PM
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Hydrodynamic flow gets stronger with rpms- defenatly try to keep it from going that low in case it is the rod/crank bearings. Even if it clears up, repeating the knock at low Rpm could still damage the bearings.
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Old 12-12-2007, 06:14 AM
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The idle speed is way too low, but you probably know that.
Three bar at idle? What is the deal with that? Is that cold oil pressure or when the engine is up to temp?

If you hear a knock coming from the rear of the rear of the engine at idle....with that kind of oil pressure, I would take the valve covers off, top and bottom and inspect the camshaft lobes for any signs of wear/scoring.
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Old 12-12-2007, 06:28 AM
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Quote:
Originally Posted by Alan L View Post
I've been fiddling with fuel pressures, and cold start issues since my cold start injector is/was defunct. Took a while to get to that diagnosis with many other issues. While I had the engine labouring at very low revs, cold idle (too rich) a knocking noise could be heard from clutch end of engine. Best I can tell is in the #3/#6 area, and if I had to guess, #6. Putting the clutch in does nothing for it.
But when the engine is idling warm, and no load, ie running nice, or when you rev it at standstill, no noise (none that you can hear above the fan etc anyway). There was at times under these conditions a very faint noise from the same area - but you had to stick your head deep into the engine cavity while it is running, to pick it up. But when it was labouring at about 300-500 rpm (and running a bit roughly) you could hear a definite knock. Because it is not running smoothly at those revs, I wonder if it could be something trivial, or how would I determine if it is more serious?
One of the aspects I thought positive for the car pre purchase, was the oil pressure seems very good - around 3 bar at idle, and runs to 4+ under revs.
Alan
Do you think it could be piston slap on a cold engine?

Les
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Old 12-12-2007, 06:30 AM
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Quote:
Originally Posted by Alan L View Post
But when it was labouring at about 300-500 rpm (and running a bit roughly) you could hear a definite knock.
Errr. Just about any engine will do that running at half their idle speed.

Don't let it do that.

Hell, the only time I drop down to 1,000 RPM after starting the car and before the oil is 190 degrees is when I'm shifting into 1st or reverse. (and Grady Clay's tip of shifting into 2nd before 1st or 3rd before reverse is golden for preventing "clang" without waiting 30 seconds with the clutch in for the shafts to stop rotating for me).

I keep the revs up to about 1200-1400 at stop lights until the oil is hot - the difference is the difference between 25-30 PSI and 60 PSI oil pressure - and who knows how much more flow?

Folk lore says that most wear happens when the motor is cold and oil isn't flowing well. In aviation, the life of a piston engine is not only measured by hours run, but cold and hot starts are also logged...

3 bar at idle just seems weird, I think.

If cold, mine would read around 60 PSI (4 bar), if I ever let it idle (and the owners manual says not to do that as well). But this is meaningless - and depends on what oil is in it too.

At a 2,000 RPM fast-idle it reads 100 PSI (nearly 7 bar) when cold until the internal thermostat opens. And then it maxes out at the "protect the external oil cooler" pressure of 70 PSI (5 bar) once that happens.

If it's hot it generally reads just over 20-25 PSI at idle.

That darn well better be right, it's got about 5,000 miles on a complete rebuild...

I think the rule-of-thumb for all 70's and later Porsche is "1 bar at idle, 1 bar per 1,000 RPM and peg @ 5 bar". When hot. If you have those minimum figures, you're good.
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Old 12-12-2007, 10:18 AM
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Sorry about late replies guys - ended up in hospital unexpectedly. Out now.
Thanks for Injectors Brian.
I didn't set the idle that low - thats what you end up with on a cold start when the cold start injector keeps spraying fuel in and wont shut down. The 3 bar idle pressure is with the oil temp just above the first white band. My SC runs at this temp all day, but on the 930 it seems to be only partially warm. But I expect it has very good oil pressure. The car is/was not been driven at these revs - still on jack stands. Trying to work out whether I should pull the engine or not. Supposedly just done 500k on top end rebuild - missing gudgeon clip? It seems a number of possibilities that could only be checked by stripping the engine. Is it worth it at this stage?
Alan
Old 12-16-2007, 02:18 PM
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How do you know the cold start injector is staying open after the car starts?
There's no visual on that, and the only test I can imagine is to check for voltage going to it.

The cold start injector is powered/opened by the current going to the starter solenoid low amperage 12volt primary wire on the starter motor, and then only when the thermal time switch thats mounted in the driver side cam chain box is sensing it's below a certain ambient temperature.

Because of this the cold start valve is open only while you are cranking the starter motor.
So once the car starts and you release the starter the cold start valve no longer has a source of 12 volts to open it's solenoid valve and let fuel spray into the intake manifold underneath the throttle body.
The thermal time switch also controls how long the cold start valve is energized or open depending on ambient temp should the car not start and you keep on cranking it so too much fuel does not get sprayed in flooding the motor...
The colder it is out, the longer it will spray fuel into the intake manifold until the car starts.

Also, if you have knocking at low idle speeds like you describe there is excessive mechanical clearence inside your motor somewhere... I've listened to my '87 930 motor when the aux air valve sticks shut once in a while and the motor starts really slowly but I don't hear any weird noises, and I have a good ear for that.
Or you may have forged pistons which expand more than cast pistons so they have to be installed with a larger cylinder to piston clearance at the piston skirt around 3/4 inch up from the bottom of the skirt and 90 degrees to the wrist pin than cast pistons.
Once they get up to operating temp they have expanded and taken up that larger clearence... but they can make some piston slap noises while cold.
In some engines you can actually hear forged pistons ringing a little bit as they rattle and rock against the cylinder walls when lightly reving the engine cold, if you know what to listen for.
After a couple minutes the noise goes away as the pistons heat up and expand.

The ONLY engine I've ever heard make ALOT of rattle and knocking noise while idleing and it was considered normal is the old 4 cylinder turbocharged Offenhauser engine last used by car and driver editor Patrick Bedard at the Indy 500 in 1981... I was there.
Those things sounded like they were about to come apart till they got really hot, and were running fast.
Old 12-17-2007, 06:31 PM
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[QUOTE=JFairman;3652032]How do you know the cold start injector is staying open after the car starts?
There's no visual on that, and the only test I can imagine is to check for voltage going to it.

Because intially it was not working at all. The car has had all sorts of issues. As I gradually eliminate one, I discover it was clouding another. The thing was a pig to start. So bad, I could not get it started initially when it came off the boat. Silly optimistic me left my self stranded at the port thinking I was going to drive it home. Had to come back next day and haul it on to trailer. Finally it would start after much cranking, on one cyl, then 2 then 3 .... until finally it fired up properly. It could take about a minute or two for this.
Eventually after fixing a bunch of other problems, I was still left with the pig starting process, and a very unhappy sounding starter motor (another job).
I pulled the injector out and put a meter across the coil. Had a continuous circuit. Put 12 v across the terminals and felt/heard nothing, except the coil heat up. Put some fuel in the top and pressurised it. Nothing came out. Put 12 v on it and gave it a wack, several in fact. It opened. Sticky piston. Worked it this way with various cleaning fluids until it generally freed up. Then back in car. Then engine loading with fuel. Pulled it again, put pressure on it with no voltage - injector stuck open. Since cut it open, found the sticky bit, and now it works freely.
The car may have forged pistons. Until I open the motor I really don't know what is inside. It has obviously had a lot of things done to it externally - all in the name of speed - EBC, plus some other electronic control I have not fathomed - a wire from a removed sensor at the exhaust to an elecronic box (japanese) plus a digital air temp monitor near the EBC, headers, heaps stripped off the engine - weight, non- standard distributor (more aggressive curve apparently), clock boost guage, after market shift light/memory recall tacho. So, what cams/pistons I have, I don't know. That may explain it. The noise is imperceptible when idling proper speed warm, but is there just as the faintest sound - when you stick your head over it. Stand next to it and you would not hear it. But the other day you would have heard it 20 feet away.
Alan
Old 12-17-2007, 08:54 PM
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The other electronic control and oxygen sensor sounds like an aftermarket lambda box installed with a pulse valve or inline injector to lean out the motor according to what the o2 sensor reads and lower emissions.
You might find a little anodized aluminum resistor hooked to it mounted somewhere too.
Old 12-17-2007, 09:16 PM
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The electronic box is a bit bigger than a match box. Thats about all I can determine. It had a Greedy EBC, which I have removed. But I can only conclude that it has had some form of further engine control. There was a toggle switch mounted in the door where the mirror control would be, and a bunch of burnt out wires from there to under the dash that took a bit of sorting out.
Pic of cold start injector posted in case people interested.
Had a pin head sized rust spot on the piston.
Alan
Old 12-17-2007, 09:38 PM
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