Question for IAN (other techs, professionals as well if you wish)

Might be a long post, as I will need to be descriptive and in depth.
I have a 1995 Pontiac Grand Am SE, four speed auto, 2.3 "Quad Four" engine.
The problem in a nutshell is that the engine will sputter and surge after a cold start up. It occurs anywhere from 30 seconds after to 1 minute. The episode will last 5 to 15 seconds. If I never engage it in gear for the first minute or so, it almost always does it, if I drive right away, sometimes it will, but not always. Here is a list of some things I've replaced through various problems and preventive maintance. (Some actually didn't need it but oh well....) Ignition housing, coils, spark boots, and spark plugs. oil/air seperator box, and hoses attached. Fuel pressure regulator vacuum hose, map sensor and vacuum hose, IAT sensor, TPS sensor. Timing chain and hardware, waterpump, flex plate, oh, oxygen sensor too. The engine is modified slightly. CAI, Cat back exhaust, and smaller diameter pully. Many that I've talked to like to blame it on those mods, but they have been in place for a loooong time, and although it's had this problem for quite a while, those mods were in place for quite a while before this issue. Oh, no codes ever show up also. The engine has just over 126,000 miles.
Now on a side note, many of those parts were replaced just as preventive maintance, as I have discovered with a few that the ECM won't always pick up on something being out of whack a bit, unless it's almost not functioning. From my experience anyway. Some of it can probably be blamed on me, because I haven't always been kind to it. But oil changes etc are done strictly on time. All in all I have to say I am very pleased with it. 9 years, and 126,000 miles, it hasn't given me too much trouble. And with an engine with one of the worst reputations ever! I've broken a few things on it, but live and learn I guess.....
Hopefully you can help with this, it drives and runs beautifully otherwise.
Tony
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Just a couple of things to add.
I've just about exhausted my knowledge of what it could be. Three things come to mind that have never been touched.
Fuel pump Evaporative Emissions Control System And of course, the infamous "head gasket problem" Although with that last one, there is no other symptoms indicating that. No overheating, no poor drivability, no coolant loss, no oil consumption.
But anyway, do any of these sound probable? Or more so than the others?
Thank you for any expertise you can offer. I'm stumped.
Tony
P.S. Sorry for the long post, I wanted to be as informative as I could, as I know that helps the most.
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"Tony V." wrote

Driveability issues are not really my strong point. Especially issues that only occur for 5-15 seconds. You would probably be better off taking the vehicle into a shop and get someone who knows what they are doing to have a look at it. But, be prepared to spend some diagnostic money...and if you don't want to spend the money...just live with it.
Ian
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If nothing appears awry after a visual inspection of plugs, filters, connections, yadda yadda, I might be tempted to attach a fuel pressure gauge and see if it fluctuates when the engine stumbles and go from there. Ian might be right; it might be time for a visit to Mr Goodwrench, as transient driveability stuff like that can be fun to track down, particularly over a NG when nobody can see or hear the car.
On Tue, 06 Jan 2004 20:08:18 -0700, Tony V. wrote:

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This is the kind of thing a scan tool would probably be quite helpful for - you could see if the oxygen sensor voltage is doing something strange, or the engine RPM reading is going crazy, or the MAP, TPS, CTS, etc. are going off somewhere.
ECM could be a possibility as well - my parents actually had an ECM go bad on their '95 Grand Am with Quad 4, it kept setting a TPS trouble code even though the signal was fine.
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On Tue, 6 Jan 2004 21:08:18 -0600 (CST), snipped-for-privacy@webtv.net (Tony V.) wrote:

I'm thinking that something is sealing (gasket/mating surface) when the engine heats up and until it does, you have a vacuum leak somewhere. For fun, I'd probably try a little propane around gaskets and where manifolds meet up.

I had an OBD-I Nissan with a bad fuel injector shaking violently, and it didn't throw a code either.
I'd recommend taking it in for a formal diagnosis.
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