Frequent but intermittent slight stumble/miss/power loss on light acceleration; pulls smoothly for all it's worth @WOT.

Wires & plugs are new; no codes; nothing shows when analyser is connected; idles down normally - no vacuum leaks (verified by acetylene); tried a different known good oxygen sensor; tried capped vac line to EGR; 25 year GM master mechanic stumped.

Suggestions welcome!

Bad spot on throttle position sensor?

Keep in mind, most wear occurs on these closer to the idle position.

Try blocking off the EGR completely by sliding a piece of aluminum can between it and the intake manifold.

As suggested check TPS, EGR, and re-check plug wires.

I've had plug wires arc even when new on my 2.5. check at night so you can see if they are arcing.

Someone on the board had these same issues after checking these things and thier fuel pump was starting to go bad. How new is the fuel pump?

Also make sure the throttle body screws are tight as described here.
https://www.fiero.nl/forum/A...060811-2-070607.html

Also good info here:
https://www.fiero.nl/forum/A...020825-2-018286.html
https://www.fiero.nl/forum/A...000604-2-001454.html

[This message has been edited by mike-ohio (edited 12-06-2013).]

The EGR valve on the 4-cyl uses both exhaust backpressure and manifold vacuum to operate the valve. If you just capped off the vacuum line, you only eliminated half of the input, so to speak.

Also, it's been my experience that if the valve seals are leaking bad enough, the puff of oil on takeoff can make the engine stumble.

I had something like this and I had a loud creaking sound as I let the clutch out. Especially going up hill

Turned out my dog bone mount wasnt actually connected, the bolt that went through it broke of at some point. Put that back in and it fixed the problem good as new

Thank you all for responding. Looks like Patrick might win the 'long distance diagnosis contest' this time!

Steve, (a mechanic - not the owner) from Steven's Auto Electric & Machine Ltd. went for a test drive with me.
He tested the TPS & showed me the voltage was erratic, which could cause the symptoms.
The root cause of the erratic readings isn't necessarily the TPS itself; the throttle shaft has too much play in it.
Voltage readings would vary from wiggling the shaft without rotating it. Hence no ECM codes.
We'll see if a new TPS will resolve the issue or mask the symptoms. If not, the TBI shaft play issue may have to be addressed.

05Apr2014 update:
Soo... I got a throttle body assembly from an '87 2.5L at Glendale Auto Parts complete with all the sensors for $40. The throttle shaft had significantly less play. I installed it with a new gasket & ... wait for it ... no change. Oh well, at least the throttle shaft issue was resolved. An analyzer showed all systems functioning normally. Hmmm... time to buy more stuff...

31May2014 update:
Soo... I got a DIS assembly including the crank sensor, ignition module and coils from the same car as noted above for $22.60.
I installed it & went for a test drive ... woohoo! Annoying intermittent missing resolved. And cheaply!

I wonder what component was malfunctioning?

Put the suspect coil packs back on and see if the trouble re-occurs.

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When there is the highest amount of air in the cylinder the voltage required to jump the gap is the highest. When is the cylinder filled the most efficiently? Basically it would be the same time you would get engine knock. Lower RPM and more open throttle. When the AT drops down to the lower gear, the RPMs increase and the efficiency of filling the cylinders drops.

Defective insulation in the secondary of the coils cause the misfire. It shows up more when the voltage required to jump the gap is highest.

High resistance plug wires will cause the secondary voltage to increase. So will worn out plugs with excessive gaps. And when one or both of these conditions occur the higher voltage required to jump the gap will damage the secondary insulation of the coil packs.

Thanks for the excellent info & video phonedawgz!

I'm reluctant to tinker with the system now that it's working properly...

Since the hesitation under easy acceleration was immediately alleviated by opening the throttle, it seems like the additional air/fuel mixture that should create greater pressure/more resistance wasn't causing the hesitation. Somehow the system was working well when working hard. Could the ignition module or crank shaft sensor cause the hesitation?