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Unread 08-02-2019, 08:32 PM   #211
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Default Leak down percentages

Got the leak down numbers which are paired with the compression number:

Cyl PSI LD %
1 140 10
2 140 12
3 130 16
4 150 10
5 145 11
6 145 8
7 140 12
8 130 18

The correlation on the graph is really clear. The leak down percentages don't mean anything as far as leakage except for relative amounts of leaking.

I had a craptastic "Pittsburgh" tester kit on the shelf and it was a mess as many Harbor Freight tools are. I couldn't make sense of the reading which would snap to new values on each test of a single cylinder. It took a while to get something repeatable out of it. The first step was to open the "percentage" gauge and re-adjust the scale.

Inside the gauge, which is pretty standard, there's a Bourdon tube that's connected by an arm and gear to the needle. The calibration is by bending a little arm to read higher/lower. (There's no zero and span, just zero.) I twisted it so the gauge read 90% at atmospheric pressure and that pulled the bottom up, too.

The real problem is the pressure regulator. On each use it'd pick a new applied pressure and wouldn't be repeatable even under identical conditions. It also did a craptastic job of regulating the pressure at all. The more flow; the lower the pressure even with a much higher input pressure than output.

So... What I'd do is to readjust the regulator on each cylinder until the measured percentage was stable and the applied pressure was 1 Bar. And tap on the gauges so they weren't stuck. This gave repeatable results going back to prior holes for a check.

If you haven't checked one out, a leak down tester is pretty simple. There's a regulator with its output measured by a gauge and then the air goes through a tiny orifice. After the orifice is the "percentage gauge." Then the air runs through a hose into the cylinder. What you're supposed to do is run the pressure with the outlet capped and adjust the regulator to make that read zero percent leakage. (Makes sense.) Then the second gauge shows the lower pressure proportionate to how much air is flowing through the orifice. But! The regulator is crap so each run would have a different zero and a different reading. Sometimes by 10 to 20 percent. Useless!

1 bar was often close to zero and seemed usable so I adjusted the applied pressure to at least be consistent. While hard numbers are appealing, relative numbers and consistency are good enough.

The final analysis is that #3 and #8 are worse but not terrible. I'm not convinced the rings are broken although that's a likely problem.

I really wish I'd have checked the gaps of the top rings after installation. I sized the rings and bores and put rings into specific holes to compensate but didn't measure the gaps on each hole. It'd be interesting to see if that top ring gap correlated at all to the leakage and compression numbers.
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Last edited by GlenL; 08-03-2019 at 09:48 AM..
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Unread 08-28-2019, 08:27 AM   #212
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Default Pressure brake-bleeding

No progress on the engine problems. Pull it, take it apart and see what's what? No rush to take on that work right now.

I did resolve a mushy brake pedal. My assistant (kids) are grown (mostly) and at least less available for brake bleeding. To get it done, I grabbed a vacuum pump/bleeder and used that for some bleeding and flushing. That moves a fair amount of fluid but I'd notice at some times it'd be foamy. I chalked that up to air leaking in around the bleed screw. Nope. It was sucking in air through the seals.

That turns out to have been a big mistake. An ex-Porsche tech finally told me that the vacuum bleed will suck air in through the caliper piston seals and seems he was right. After first doing it, I'd try some pedal-pumping bleeding when I had help which would make it better and then I'd vacuum bleed it which would be no change, or worse. (Duh!)

At the track, the ex-mechanic told me about problems with vacuum bleeding and a guy offered to use his pressure bleeder on the car. That was a clear improvement.

Back home, I got a Motive Products Power Brake Bleeder model 0100 for European brake reservoirs. It's the cap to the reservoir that makes the difference between kits. I ran a quart through, drove it, and then did another quart. Now the brakes are fairly decent.

So... Don't use the vacuum bleeder. I got it to test vacuum components and help tune vacuum-advance distributors. That's all it's good for.
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Last edited by GlenL; 08-28-2019 at 12:26 PM..
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Unread 09-21-2019, 10:26 PM   #213
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Default Bad ignition switch

I've been working on electrical gremlins lately. Warm evenings and baseball on the radio lend themselves to mild puttering.

Learning a lot about the wiring system on the 928. Some people have these things memorized but I'm getting there that bus 30 is unswitched battery, 31 is ground, X is accessory and 15 is run.

A few things have eluded resolution along the way. The turn signal indicator light and engine temperature are intermittent. The speedometer doesn't work. The volt meter bounces a lot with turn signals and brakes.

I had cleaned the grounds previously and revisited that. No difference. So I started to check voltages and resistances in the system. I won't list them all but some things don't seem to follow what they should be. What really was interesting was that bus 15 was about a half volt less than bus 30 with the car running and no other lights on. Hmmm... That should be a direct connection with a negligible difference

So it was time to pull the pod and check the wiring to the pod and the ignition switch. And, look, the ignition switch is busted and the halves are separating. Got a used out of the spares box and Ohm'd it out. Should work fine.

Hopefully this'll be the start of getting it all working. I'll be fixing that tang for the horn contact that broke off. Two steps this way, one step that way, it's inevitable.
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Unread 10-13-2019, 08:56 PM   #214
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Default

I've put the electrics to right...at least mostly.

The problem with the speedometer was the speedometer itself. With the instrument cluster out, I could compare the resistance into the speedo to anther unit. Swapped in another speedo and now it works.

The connector for the coolant temperature is still a little fussy. Needs a wiggle to make it work, sometimes.

The voltage measurement seems better but the gauge still drips too much when the turn signal goes on. What helped with that was cleaning the power feed from the jump post. There are three heavy gauge wires that run from the post and plug into the front of the fuse panel.

Something I found was that the wiring diagram doesn't completely match the wiring diagram. Something that really stymied me is below. Schematic Part IX shows that the "right side" connector to the pod has a wire between 7R and 11R. 11R connects to the ground for most of gauges and the wire from it runs, eventually, to the speedo pick-up. Measuring the speedo pick-up wire in the fuse panel to ground showed like 7 Ohms when according to the diagram it should be zero.

With the pod removed, I could measure that 7R was not connected to 11R. I expected a wire crimped into both of the pins. Looking at a spare harness (save everything!) it wasn't connected either. Eventually, what I found was that 7R and 11R were connected as a clean short in the pod. Argh...

But, now the speedo works. The turn signal light works all the time. The horn tang is re-attached. The intermittent wiper works. All the pod lights work.

But, that gear for the odometer was broken on the old speedo and just crumbled on its replacement. Next time...

I was at the track, BIR, last weekend for one final time this year. Had some issues with oil ejection and brakes but still had some good runs. With the speedo working, I could see the car posting 150 mph going into turn 1. I've often thought The Zombie had more power than my other 928 and, it seems, that's true. The other car will typically post nearer 140.

Now to get the engine fully sorted and see what the lap times are.
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