Folks, you all have been a big (and fun) help - lots of good ideas!

At this point I'm going with the hall sensor replacement as the next step. If the new hall sensor doesn't fix the problem then it'll probably go to the dyno as the 24th and the CT tech/dyno day will roll around soon enough.

I probably won't be done with the hall sensor until the end of this coming weekend. As far as I can see I have to take the passenger side cam cover off in order to get enough working space to get to the sensor. And to get the cam cover off you've got mess with the fuel lines, harness tie down, air pump valve, etc.

I won't get much more wrench time this week, so, look for the next report in several days.

 

Dave,

What kind of scope are you getting? How many channels? If it's two channels or more it might be interesting to superimpose the signals from the ignition systems to see if they are "in phase". Er... come to think of it, that's why you got it, duh. If you need any tips on setup of general-purpose 'scopes (e.g. Tektronix, etc.) I'll be glad to help. A healthy shark is a happy shark.

 

I picked out a 1-channel "mini" scope that's about the size of a brick. My old HP scope was the size of a small house. I thought about a two-channel 'scope, but I figured for $50 the easily-portable scope was a good investment even if later I wanted a fancier scope.

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If you need any tips on setup of general-purpose 'scopes (e.g. Tektronix, etc.) I'll be glad to help.

Thanks man. You know - a long time ago in a galaxy far, far away, I used to know all the ins and outs of using a scope and a digital logic analyzer and actually making things (that worked) out of physical entities. But, since I moved to the virtual universe of software all that know-how has leaked out of my head.

Quote:

A healthy shark is a happy shark.

And a shark driven a lot is an even happier shark.

 

A $50 scope that's the size of a brick? I'm interested; where do I get one?

 

What's that song... "da, da, daa, you can find it on Eeeee-baaaay..."

The one I got was a Tektronix 211 MiniScope.

But, given my luck with stuff from e-bay I'll probably have to put a new clutch and timing belt in it right away.

 

OK, one for the "now he tells me" department... You're going to have an interesting time detecting phase variations with a single-channel scope. One way to do it would be to get the coil wires close enough together that you can wrap some thin wire around BOTH coil wires at the same time... then see what your scope tells you. If the pulses are evenly spaced, phase is correct. If the pulses seem to be in pairs that are closer together than the distance between pairs, then your ignition subsystems are out of phase. Something like that could affect engine output...

Anyway, having at least one extra channel could allow you to check things like relationships between timing of hall sensor output, spark, injector activation, etc.

But maybe I'm just over-engineering the whole concept of "scope as automotive diagnostic tool"... I DO have a tendency to do that sometimes...

 

Dave,

Let me know if you want me to bring my scope (house size) on the 24th.
I should be going.

 

Hah! Cool idea. You, sir, are undoubtedly An Analog Man.

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But maybe I'm just over-engineering the whole concept of "scope as automotive diagnostic tool"... I DO have a tendency to do that sometimes...

When you don't have access to an entire automotive shop's worth of equipment you do what you must.

 

You'll have it on your trailer right?

I'll put mine in the glove box.

Will do. If the hall sensor doesn't do the trick then I'll let you know. We can hook-up the scope and see what we can see. I'll bring the laptop with all of the dox and a binder of the working set of printouts too.

If it is still illin' on the 24th then you should drive it too. Another butt-o-meter test may result in new data.

 

Not taking the track car, just driving up in the GT. No dyno for me.


So does that offer stand even if you fix it before the 24th?

 

Yup.

 

Analog, digital, software, clockwork, string & pulley, rubber bands & duct tape... you name it, I can probably either fix it or finish breaking it...

Don't I know it... I've used the freq counter on my Fluke DMM many times on different cars as a tach, using the "few wraps of wire" method to create a pickup on plug wires and just doing the math in my head to get true RPM. Done a few other weird things too... hmmm... maybe calls for a new thread...

 

Just to toss a couple things out there:

The symptoms sound similar to two problems I have had before.
One was a clogged fuel filter. It would give plenty of fuel at idle,
but as you would accelerate, it could not keep up with the demand,
and power would drop off. So I would check the fuel pressures
at higher rpms just to be sure.

The other problem, that someone else mentioned, was a clogged or pinched exhaust.
If you have a failing cat, or muffler, or pinched pipe, the car will have enough room to
get the gasses out, under normal acceleration. But as you get on the car harder, it keeps
getting more and more restrictive for the exhaust evacuation needs, and power drops off significantly.

 

Hey David, on Chuck Z's site I read a description of a '91 GT that the owner had the same symptoms as you, and he has a brief description of what was wrong and how it was fixed.

http://www.928registry.org/1991-928GT-0126.htm

 

Hey Carl, interesting catch. Here's the relevant quote:

"...IT JUST LACKED THE FULL POWER KICK IN AT 4000 RPM. LATER, A LEAK CHECK FOUND VAC LEAK IN THE THERMOSTAT CONTROL LOCATED UNDER THE AIR CLEANER/INTAKE COVER, AND APPARENTLY ONE FUEL PUMP WAS NOT WORKING AT FULL PRESSURE..."

Do you suppose that the problem above was a dead flappy and the description was lost in translation?

Or perhaps a vacuum leak in the heater valve?

(AFAIK all the thermostat stuff (thermo, temp switch, temp sensor) is at the front of the engine and none of them are hooked to vacuum - but, you learn something new everyday. Does anyone have any other ideas as to what the "thermostat control located under the..." could be?)

Now, as to the dead fuel pump, that is on my mind as a possible culprit. Specifically, a dead in-tank pump. The main pump is less than a two or three years old - depending upon when Porsche went from the blue-bodied to silver-bodied Bosch pumps - but I think it was three years ago that the part number changed so I don't expect it to be toast yet. (New fuel filter in the '89 btw.)

But, the in-tank pump on my '91 was dead for a while before the main pump seized (on track at Beaver Run) and the '91 exhibited no noticeable lack of performance.

A couple of years ago Ed Ruiz had a dead in-tank pump in his '90GT which led to overheating due to a lean mixture - but only on track - and he was, at the time, not complaining about the power level in his GT.

I have good fuel pressure up to at leat 4-5k and the '89 runs fine temperature wise. (But, I have not beaten it in a track-like fashion - so who knows.)

So, the symptoms don't all line up neatly and point clearly to a pump.

But, you can bet that if the hall sensor doesn't do the trick and the EGA on the dyno says that I'm running lean that I will be under the tank pronto.