'95 Park Ave... Herky Jerky problem

Be aware that I have posted on this ongoing problem before. And yes, I have read and followed up on most (I think) of the suggestions made. Taking
it from the top:
This is regarding our 1995 Park Avenue, 104,000. I call it the Herky Jerky problem. Here it is in a nutshell:
Symptoms
While driving at constant speeds there will be a slight hesitation, as if the engine had died for a couple seconds. I call it the Herky Jerky. At the present the symptoms are mild and intermittent. At its worst, it will occur much more frequently and with more severe hesitation, but still not all the time. When it's that bad the symptoms can be stimulated by accelerating on a grade, e.g., going up a freeway interchange fly-over.
History
This has been going on for several months.
Some reading on the Internet (alt.auto.gm) led me to believe it may be caused by a failing torque converter clutch (TCC). However, others said the same symptoms can be caused by the engine missing and recommend changing the spark plugs and wires. I did that and it seemed to help. However, the problem then reappeared so I took it to the independent garage that has been doing repairs and maintenance on the car for the last several years. They replaced the air mass flow sensor which seemed to help. However, the symptoms returned in a day or two, or more likely had never really gone away. In fact, it started exhibiting the hesitation so severely that the car was hard to drive, feeling like it might not make it through the intersection when accelerating from a stop. The garage then removed all the plugs and did a compression test and inspected the plugs. The compression test results were fine, but the #1 plug was fouled. That plug was replaced, with remarkable results. The car ran perfectly for several weeks. However, the old symptoms then began to reappear.
Today I took it to a Buick dealer that serviced the car when it was still under warranty. I was usually happy with their work, but they are too far away to be convenient. I did this because the independent was clearly floundering, and I had heard the dealers had better scanners to hook up that could read recent events, i.e., not depend entirely on the problem presenting itself while the tech was driving around with the scanner hooked up. Well, that turned out to be in the fond hope category. The tech, who said he had 34 years experience, drove it around for a while, but "could not get it to happen enough to get a good reading." The dealer wanted to keep it over and give it another go tomorrow. I declined, thinking it may be best to keep driving it till it gets bad enough that it's easier to diagnose. I'm also thinking maybe it's time to take it to the tranny shop. I had it there once early on, but it would not do it for him in his test drive. It's been about 70k miles since the tranny was serviced, so it might not be a bad idea in any case, and maybe it will fix the problem. OTOH, the fact that changing the plugs and wires had such a dramatic effect tells me that it probably an engine problem, perhaps stimulating the TCC to flip in and out.
So, there you have it. What would you recommend at this point?
Ed
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Ed wrote:

Has the fuel pressure been measured? What was the reading?
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Paul,
Thanks.
Not that I know of. I'll ask the techs to be sure. I have a fuel pressure gauge myself. Is there a test port on the Park Ave?
Ed
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Ed wrote:

There is a Shrader valve near the rail, on the input side. I'm suspecting low fuel pressure, at times, is not feeding the injectors under load. 40 psi is ok, 50 psi is better.
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Sounds like it could be the fuel pump. That "bucking" you mention - especially at higher speeds - is a good indication. It is a rather large job for DIYer, the fuel tank requiring removal and the pump/sending unit needing replacement. I hope that steers you in the right direction before you replace all the sensors on the car when they aren't broken anyway. Stephen
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Thanks, Stephan. How can I be sure it's the fuel pump, or at least raise the confidence level? You're right, though, I probably wouldn't take it on replacement of the pump myself. But I might put a fuel pressure gauge on the FI rail if that would help with the diagnostics.
Ed

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Ed wrote:

I still favor the TCC theory. It's very sensitive to engine vacuum/torque so perhaps the plus altered the operation slightly. I don't know why you can't disconnect it for a test. Maybe it's not as easy as just pulling the plug as on older models.
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Just to throw another theory on the floor, I had a '92 Park Ave that acted in a manner similar to the OP's description. Turned out to be the TPS. Replaced it, did a seat of the pants adjustment because I didn't have anything to properly adjust it with at the house, and all was well. I don't recommend the seat of the pants adjustment technique, but I was on the spot and had to do something - got lucky. I would look at the TPS though.
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Thanks, Mike.
I can see the TPS causing problems while accelerating, but can't see why it would cause missing while driving at constant speed. But, when I get a plan of attack I will be going back to the dealer or an independent garage and will have them check
Ed

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Could be Ed. I really don't remember back well enough to when we had that Park Ave, to remember if the problem was just under acceleration, or if it happened while cruising also.
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Al,
Thanks.
I had/have similar thoughts. That is, there are two things going on. One, the TCC is on the verge, and misfiring of the engine causes it to drop out, amplifying the effect of the miss.
I have been told it can be disconnected, but the tech was reluctant to do so as it causes the Check engine light to stay on.
Ed

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Ed wrote:

1. Fuel Pump 2. EGR Valve.
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CL Gilbert
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Thanks, Gilbert. I can see a fuel pressure issue, but how can the EGR valve cause this? Any way to check it?
Ed

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Ed wrote:

If EGR opens up then the fuel mixture will have less oxygen. If it does this at the wrong time, you will loose power. It acts pretty much just like a bad fuel pump if its sticking open. If it sticks closed then you may get an engine knock if your computer is not able to compensate.
Note: I have had this happen twice to my car. As well as adding fuel pumps about every other year for 10 years. The symptoms on my car were indistinguisable. Not enough fuel vs. not enough oxygen is pretty much the same thing. OBDII should diagnose the EGR problem though. However, my 95 Blazer had OBDI so I can not be sure what your 95 park Avenue has.
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CL Gilbert
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Gilbert, I did a Google on OBDII and found that it was installed on vehicles 1996 and later, so I would guess my '95 has OBDI. So, am I to understand that OBDI will not tell me anything about the EGR?
Ed
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Yes it will. Just not as much as OBDII. OBDI will tell you if its unplugged, and maybe some other stuff. But OBDII will tell you when its not doing what the ECU told it to do. So it would detect sticking. OBDI might, but I am not sure it must.
Your car may have OBDII or may not have either since in 95 I don't think it was required.
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I was wondering if it might help if I bought a scanner myself so I could leave it hooked up all the time. Is this going to tell me anything, or are the reasonably priced scanners unlikely to be up to the task?
BTW, what type of scanner is used on the Park Ave Ultra? It has the supercharged 3.8L engine.
TIA
Ed
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Ed wrote:

You could. It may be a waste of time though. What is the rail pressure?
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"« Paul »" <"=?x-user-defined?Q?«?= Paul

Thanks, Paul. I'll see if I can get a reading of the rail pressure today. But since the problem is intermittent am I likely to see a problem with the pressure when testing it in my garage?
Ed
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Ed wrote:

I don't know. I suspect the armature wobbles in the bearings at times. Or the brushes are at the end of their travel. Like you said, you may not get a low reading until the pump acts up. But then you might. If it's under 40 psi, I would probably replace it anyways, considering the year of car. Also, don't forget to check the regulator! It could be the problem. http://www.mightyautoparts.com/library/library_ontheline.html
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