erwalker

My understanding of the WOT switch is that at a certain throttle position, it applies a richer fuel mixture (Please correct me if I'm wrong). The switch may function, but if there is a wiring issue or an internal DME problem, the enriching won't take place. If there is a wiring or internal DME problem, whether the switch is connected or not(or working or not)won't matter.
Just trying to cover all the possible bases. Having a second car in good operating condition to switch parts with is a huge plus. Even if you just compare vacuum measurements, voltages, resistance, etc would make life so much easier.

Tom Rathjen

I verified the WOT wiring from the switch to the pins in the DME connector, and I sent my DME in for bench testing a couple months ago and it checked out. So I think the wiring and electronics associated with the WOT are good. Thanks for the suggestion, though.

Below is a visual of the repeated exhaust test that I did today and described a couple of threads up. One significant difference between this test and the test done a month ago (post #82) is that back then I could not get the engine to rev above 4K. Now, thanks (I assume) to improving things in the fuel delivery, it goes above that and I can see this knee in the curve.

Again, these data points are where the manifold vac readings stablized at each RPM....not where it first drops do when the throttle is moved move open.

SharkSkin

OK, that looks suspicious IMHO. Too low at the higher RPMs. Like so many other things you've found, this seems to be marginal -- not bad enough to show the classic symptoms, but not good either.

If you can cobble together a low pressure gauge(like the one Dwayne and Roger have for checking for intake vac leaks, IIRC it's a home depot natural gas gauge), say 10psi full range, connect it either to the O2 sensor bung(you will need to make an adapter) or remove the checkvalve from the air pipe to the cat and install your gauge there, and see what the exhaust back pressure is. You should see at most, 1 or 2 psi there -- more indicates something is plugged. Then as a final test, see how much the readings change when you drop the exhaust just aft of the cat -- this will help you figure out where in the exhaust the restriction is.

Landseer

You can, in addition, drop the exhaust and run a vacuum cleaner hose down there to suck debris out prior to cats.

Or, do a removal and turn the system on-end, tap it gently, and try to get stuff out or listen for rattles.

WallyP

Some comments that might or might not help:

If the fuel flow and pressure both meet specs, I would forget fuel filter, pump, bent lines, etc., etc.

If closing the plug gaps made no change, I would forget coil, plug wires, cap cracks, etc. By closing the gaps, you lowered the required voltage enough that you should have seen a change if secondary ignition is the problem.

One thing that I wonder about - the coil fires at the proper time, HV flows to the center of the distributor cap to the center of the rotor. The rotor points at the contact in the cap, and the HV jumps the small gap to that, and from there to the plug wire and plug. What if the rotor isn't pointing at the right place? The tip of the rotor must be wide enough to cover the cap contact with the centrifugal and vacuum advance anywhere from zero to full advance. If the advance is enough that the rotor tip no longer covers the cap contact, the gap would get too big to jump. This might happen at very similar RPM and advance levels every time...

Wrong rotor? Something else causing it to be just off a bit?

Tom Rathjen

Yup, I concur that the next thing to look at is the exhaust. Not sure if I will be able to rig up a back-pressure measurement or not. I might just drop it and take a look first.

Still not sure it isn't the distributor; and still saving that to last for now. I am pretty sure it is the right rotor, though.

I am Mr Mom this weekend, so probably won't get time to work on it. I will post updates when I can get back on it.

Thanks, all!!

Landseer

Can you show a picture of the installed distributor from a couple angles, with and without cap?

SharkSkin

I'll be interested to hear your results, Tom.

Tom Rathjen

Here are a few distributor pictures.

Landseer, I like the picture of the dogs in the snow! I take it that is from today's snow storm. Everyone in the family but me had a snow day. I had to plow through snow to the train and still go in to DC. Thank goodness for the Cayenne's 4 wheel drive. First time I've really used it!

As I predicted, I did not have time this weekend to work on the exhaust. And unfortunately, I will not likely have time this coming weekend either because of a gig and then work travel. So...I'm likely going to have to prolong the suspense on what's going on with the exhaust to weekend after next. (Maybe it will be warmer by then!)

Dennis Wilson

There appears to be too much carbon on the rotor. If the inside of the cap has the same carbon, that could cause your high rpm misfire. It is possible the ceal in the distributor is bad and you are getting crankcase vapors which could be causing your problem.

Dennis

SharkSkin

I agree with Dennis, that doesn't look good. You may be able to clean it off with denatured alcohol and see if anything changes. Also, it looks like you can get the distributor apart fairly easily with a phillips driver. You'll have to remove the green wire and the advance pot, then you may be able to unscrew that plate and get a look inside.

Tom Rathjen

I don't know, I think the pictures might be misleading. I have cleaned out the cap earlier, and it just didn't seem dirty. And the smudges on the rotor really don't seem like carbon but more like pulling it on and off with dirty hands or something. And the contact points are very clean. A new cap are rotor are cheap, though, so if I don't find anything in the exhaust that's probably the next thing to try (unless I can find a whole distributor to swap for troubleshooting).

I have taken the distributor apart down to what's under the plate with the phillips screws, back when I was replacing the vacuum advance. Nothing looked amiss in there, but that's where I was afraid to disassemble deeper without wrecking it. The problem could be down in there somewhere, and is where I will go once I've ruled out the exhaust more positively. I'm still especially wondering about some internal rev limitor mechanism. And if I do wreck it...well maybe I would have had to get a new distributor anyway.

Also....I'm not sure I'm describing the "stumbling" very well. The symptom when trying to accellerate through 3-4K under load (up a hill) is a bit like a misfire...but it seems more like it is just being robbed of power....like it just doesn't have any more "umph" and becomes sluggish. It is a little different when trying to rev past 4K in neutral...when it hesitates around 4200 for a couple seconds and then revs up smoothly beyond 5K. At the point it hesitates, it is hard to describe if it is "misfiring" or not. (And when I have had a timing light on it when it does this, I can't visually detect misfire. But it is really hard to tell.) It kind of reminds me of when I've run up against rev limiting on other cars. So maybe that is a misfire. Ahrggg. Sorry if I am just not doing well at articulating the behavior. The most obvious symptom is that it just has no power to accellerate under load at 3-4K. And it should! It's a 928, darnit!

Anyway....thanks for the feedback on the distributor. I'm definitely still considering it suspect.

Hey, is there supposed to be a seal between the cap and the distributor body? I just realized that there is not one. The cap fits on nice and tight, so I would be surprised if that is the problem if it's missing. But I should definitely get one if it is supposed to be there.

Thanks

SharkSkin

Nope, no seal there. This is a real head-scratcher, Tom. Something else you might try CAREFULLY is testing the rotor at high voltage. Sometimes a microscopic crack in the rotor will allow a short to ground at high voltage.

To do this test you will need a pair of insulated pliers, a small jumper wire, and a helper.

1. Remove the distributor cap and lay it aside so it won't interfere with the rotor while cranking.
2. Attach the jumper wire to the metal on the pliers, and the other end to chassis ground(this is a safety precaution).
3. Unplug the coil wire from the cap.
4. With the pliers, hold the coil wire 5mm-10mm above the center contact area on the rotor, and have your helper crank the engine. Grip the wire well back from the end, on the insulated portion.

If you get an arc to the rotor in step 4, the rotor needs replacing. What happens sometimes is a crack or a defect in the plastic breaks down over time and eventually a tunnel through the plastic is created which becomes a path to ground through the distributor shaft. It's an easy test, worth a try.

Tom Rathjen

Just got back from work travel. I pulled the cat converter and rest of the exhaust this morning. I was surprised to see absolutely nothing inside the cat. I was especting to see some sort of honeycomb substrate packing it or something. But there was nothing....completely empty. Just open space all the way from one end to the other. What is supposed to be in there?

Also, there did not seem to be any back pressure in the rest of the exhaust. I didn't do anything fancy with a pressure guage. Rather, I stuck the hose from my shop vac in at the forward end (where it attached to the cat) to both suck out any debris (found none) and also blow to see if flow was restricted. There did not seem to be any restriction at all. Sucking or blowing against a restriction causes a noticable change in the sound of the shop vac motor. And there was no change. Qualitatively, I seemed to get the same air flow out of the tail pipes as directly out of the shopvac hose.

So....something does not seem right with the hollow cat converter. And the non-stock exhaust has sounded terrible all along but seems to be very non restrictive. But....it would seem that a more "open" exhaust path that I appear to have would have the opposite effect on the low-power/stumbling problem I am having. Wouldn't it? That is, it would make sense that a clogged exhaust would have the kind of effect I am seeing. But not an exhaust the isn't "clogged enough".

While I am pondering this....before putting the existing exhaust back on or ording a replacement....I discovered that my coolant reservoir is leaking. So I need to replace that now before going forward much farther anyway.

Also....I didn't try the high voltage test of the rotor suggested by Dave before pulling the exhaust. I'm thinking I might just order a new one, since it is less than $20 I believe, rather than risk getting shocked. Especially since I need to order a couple other things anyway. I'm feeling a little chicken on that test.

Thanks!

PS--Last weekend, a friend was driving behind me. He said the smell coming from my car was unusual. He described it as a mixture of "water, rust, and a rich mixture". (To me it just smells like exhaust.) The water/rust smell could have been because of the coolant leak at the reservoir (and dripping on to the hot engine or exhaust mainfold or something) that I just found. If the mixture really is rich, though, perhaps that does point back to an weak ignition problem.

Dennis Wilson

Have you done a compression test lately?

Dennis