donnieknoxville

1987 Carrera Cab 3.2 w/218K miles. Engine rebuild at about 160K. New clutch at 118K.

About a week ago while driving at about 60 mph, I detected something that I first described as a shimmy, but not in the front end of the car. I applied the brakes and felt a little pulsing. Suspecting a bad rear caliper, that night I inspected both and thought the cuplret was the left rear. Purchased, installed and bled the brakes and really no improvement. Checked the left rear again and it was definately bad. So after several days of driving with a sticky right rear caliper and what I would descripbed as the faintest of slippage between engin rpm and power to the wheels, I wondered if I might have another problem.

Last night, I replaced the right rear caliper and the brake probem is definately 99% better. So that clearly needed to be done anyway.

I asked one of my local mechanic friends about the possibiliy of a worn clutch and he said before you go down that path, bleed the master/slave clutch. I'm embarrased to say I didn't even know this was something that should be done. While I've flushed/bled the brakes with new fluid every two years or so, I had not bled the clutch ever. I hope it was done when the engine was rebuild about 4 years ago. Anyway - Rennlist was very helpful, and like many others, I bled the system last night and got a ton of very dark (almost black fluid) out of the slave cylinder. Let gravity do its thing and bled a lot of fluid through, also pushing the clutch in and out several times until it ran nice and clear. Tightened the bleed screw and took it for a test drive and symptoms were greatly improved, but not fixed.

I sill have a what I would describe as either very minor slippage or a slight hesitation. I can accelerate very strong shifting through all the gears but its almost as if the engine is sucking in just a drop of water every so often. Its so faint that the tach shows NO change as its happening.

Pulled all six plugs and all were in great shape - having been replace about 6 months ago.

Over the summer, I had some truble with fuel delivery. I would need to crank the engine for 2-4 seconds before it would start. The problem only occurs one in a while now - but it still not totally resovlved.

About a month ago, we had a prolonged rain storm with the car outside and the distributor cap developed a ton of condensation. Of course as I was pulling into a parking lot to resolve, I got half way up a steep hill and stalled. With traffic backing up behind me, I was able to restart the car and rev the engine but it would quit as soon as I engaged the clutch. In an effort to bully my way through the problem, I tried revving the enging and popping the clutch a few times to get off the hill but all I did was succeed in heating up the exhaust to a nice reddish hue. Was amazed that just a few attempts heated things up that fast. Long story short - got off the hill, tried of the distributor and everything was fine.

I bring this up because I'm wondering if I may have damaged the o2 sensor in the process.

Before I fixed the brakes and bled the clutch, the problem was worse at low RMP and under load. Coasting down a hill in gear produced absoulutely no issues.

At this point, I'm thinking the folliowing:

Q2 sensor
Fuel pressure regulator / or return valve
One (or more) bad injectors
Combination of the above.

Would appreciate any thoughts or suggestions about how to narrow down the possibilities before I start yet another Porsche Air Cooled wild goose chase.

Thanks,

Don

dshepp806

"sounds" like a fuel issue (assuming you're certain you've put any brake issues to bed). I think your list is "on"...pump/system pressures, injector health....O2 can be metered..

Best of luck!

Doyle

rusnak

Did you take the distributor cap off, and clean out the carbon and dry out any water inside the cap?

I think you might have a vacuum leak somewhere, which causes a lean condition. Depending on where the leak is and how bad, it can cause the symptoms you describe. When you bled the brakes, did you try to turn the wheels to make sure the cv joints are turning freely and not clunky?

rusnak

On second thought, the cat turned glowing red? That might indicate a severe misfire. If you have a catalytic converter, it might be gone. O2 Sensor can be tested with an air fuel ratio meter.

Rick K

Don't rule out a combination of a few things versus just one issue.

Regarding the hesitation at low RPM - is that w/ light/mid throttle? Does it happen in every gear?

As suggested above, add a tired fuel pump to the potential list of culprits...

Ed Hughes

I find it hard to believe a bad O2 sensor would cause this. If one were inoperable, I'd think it would be overly rich.

You seem to have a lean condition, I'd go thru the entire fuel system, from tank to injectors.

rusnak

I'm thinking he dried the outside of the distributor, and the inside of the cap was still soaking wet. A new cap would seal better. This would be a good time to check for radial play in the distributor shaft, and put a drop of oil under the felt washer.

donnieknoxville

Re: distributor cap - took it off - dried it off, brushed all contacts clean inside and out.

This evening I checked the resistance on the speed sensor and the reference sensor. The speed sensor was just a couple of Ohms out of tolerance and the reference sensor was dead on spec. Switched the two and - don't worry - switched the connections in the engine bay - so were still apples to apples. Took a test drive and really no difference.

I also bled the clutch a second time - because the first time was soooo dirty. Now my hydraulic fluid is running really clear - even better than last time.

Tomorrow I'm going to put a screwdriver up against each injector to feel for vibrations and test the resistance at the injector (per Bentley).

I'm still thinking its eather an intermittant injector or the fuel pump, pressure regulator, pressure relief valve. Still not ruling out the clutch.

Uggggggg.

User 4221

One thing to consider is the AFM. If it gets worn, weird things can happen such as stumbling and erratic fueling.

I would consider this a last resort item as an error in cleaning/adjusting can be costly.

Here's a good writeup on the AFM and its associated characteristics: http://www.efinnegan.com/Porsche/Mis...FMRebuild.html

rusnak

That is THE article on the afm. It doesn't require disassembly of the afm to test it though.

And it only takes a few minutes to verify that it's working properly.

I still say he might have a vacuum leak. Might remove the oil cap with the engine running to verify that the rpms fall initially, then rise back up.

Steve Weiner-Rennsport Systems

Hi Don,

You didn't mention anything about a recent tuneup so thats where I start. One needs a solid baseline before diagnosing any running issues. This means new plugs, ignition wires, cap & rotor, fuel filter, and O2 sensor. After that, I check for vacuum leaks (quite common) where the manifolds join the heads. Only then, do I check the AFM and throttle switches.

The dead reference sensor should be replaced during the tune-up. If your DME relay is old or you have no record of replacing it, nows the time to do this.

Regarding your clutch hydraulics, since you flushed the system by pumping the pedal, don't be surprised if you need a new slave cylinder soon. This is due to the piston seals being pushed over the corroded surfaces on the ID of the slave cylinder. Like brakes, these should be pressure-bled only.

rusnak

I thought he had a dead reference sensor too, but then if he did....the dme wouln't pulse the injectors or coil, and the car would not run.

I think he meant it was "dead on" (poor choice of words to describe a sensor), meaning "within spec".

donnieknoxville

Purchased a fuel pressure tester and after some issues with having the correct adapter, tested the fuel pressure with just the pump running and again with the engine running. Both pressures were exactly what the Bently manual indicated.

I also removed and tested and reassembled the air intake and the 90 degree rubber boot. The rubber was in excellent shape with no signs of cracking, holes etc. Many of the hoses were replace a few years ago during an engine rebuild and all looked good.

When testing the air intake according to the Bently manual, it says to test the voltage between the third connector prong and the ground with the key on. Since i didn't know whether they meant position 2 or running, I did both. I get 1 volt in key position 2 (engine not running) and I get 10.1 volts with the engine running (and man does it run rough with that sensor disconnected - so I did it quick - but accurately). The Bently manual indicated 3-5 volts is the norm.

I also tested the resistance as instructed the Bently manual, while moving the gate through its full range of motion. The resistance does indeed varry - starting at about 800ohms in the fully close poisiton, then rising slowly and little sporatically to approximatelty 1250ohms at open about 1.5cm. It then drops very slowly back to 800ohms until nearly fully open when it falls off a cliff right at the end of its full open travel when it goes to about 250ohms. It was a little hard to understand the spec in the Bently manual but it sounded like it was according to spec.

I like the suggestion of starting with the sparks, wires, fuel filter and 02 sensor - then figure it out from there it that doesn't solve the problem.

I finally waived the white flag last night - scheduling an appointement with my local dealer for a couple of hours of diagnostics. Feels good to let go but also hate to admit defeat.

Thank to all those that read my lenghty posts and offered their input. I'll post the final solution once determined.

rusnak

many of the dealer guys don't remember or are not so current on these older cars.

Indy shops are your friend.

oldskewel

EDIT - based on the symptoms, I doubt the AFM is your only problem, if it is a problem. I agree with the others' suggestions to do the normal "tune up" type things first. But regarding the AFM ...

Regarding that link on the AFM, I agree it looks excellent. I actually did that fix on my '85 several years ago based on knowledge of a similar problem with VW Vanagons. I did not do the bridge wire modification, but it sounds like a good idea for next time I open one up. Question on that article - it refers to early and late 944 AFMs, discussing a 1984 (early) and 1986 (late) specifically. Are our 3.2 and 964 Carrera's all the same as the "late" one discussed there? I believe it's the same part # from 1984-1994 for the 911s. I bought a used one a few months ago to keep as a spare and tested it as thoroughly as I could, and it seems to be perfect, and the linear angle-voltage relationship matches the "late" ones.

Regarding your resistance measurements ... It is mentioned in many places that the resistance test is not conclusive, but you need to do the voltage test. But I know it says in the Bentley to measure resistance and make sure it's smooth (or something like that). From my recollection, mine (both on the car and the spare I've got) both vary similarly to how yours does - increases, peaks, then decreases, but mine do it SMOOTHLY. I assume R must be measured with all power off. So that matches OK.

The "rising slowly and little sporatically" does sound like a problem - the "sporadically" part. It should be completely smooth, with no jumps or open circuits. That actually is the main failure mode as far as I can see - the tracks get worn in the pot surface and once worn completely away, you get spots where both wipers have an open circuit. The dual redundancy seems to get you past the cases where just one spot has worn away.

So, I'd recommend doing the resistance test again very carefully. Power should be off. Analog gauge or good digital gauge set to hold range, move the flap through full range. Look for jumps (towards infinity) in resistance.

That was the problem I found before I did the bend-the-wipers fix on my '85 about 3 years ago. I had been suffering rough running and what felt like fuel starvation at higher RPMs - it just did not have the power up there. Doing that fix made it drive like a new car, and it is still doing so. I just bought the used as a spare since I'm paranoid and noticed how expensive the new ones are.