96 Buick Roadmaster Wagon, pls help me...
Looking for help diagnosing my 1996 Buick Roadmaster wagon with the LT1.
Initial symptom:
While cruising around 70 mph on the highway, I felt a sudden brief loss of power with a small thunk. It went away, but then happened a few more times over a 2-hour drive. It started feeling less like transmission/drivetrain and more like intermittent engine power loss.
Then at idle:
I pulled over and let it idle. The RPM would randomly dip, then recover. No flashing CEL. The dips got more frequent and it almost died.
Parts replaced so far:
- Replaced the MAF sensor — it ran noticeably better at first.
- Replaced the fuel pressure regulator.
- I have not replaced Optispark, coil, ICM, plugs, wires, fuel pump, or fuel filter yet.
Fuel pressure:
- Before/around the regulator replacement, key-on/engine-off pressure read about 31 psi.
- Later it showed about 40 psi key-on/engine-off.
- I still need to verify pressure while cranking/running/bogging.
Current symptom progression:
After the MAF, it drove around the block and felt great at first. Then after sitting/running a few minutes, it started bogging at idle anytime I gave it gas.
Then it progressed to cranking but not firing.
If I held the pedal to the floor, it started, but ran very rough, misfired badly, bogged worse with throttle, and got smoky/sooty.
Unplugging the MAF made no noticeable difference.
Codes:
I scanned it and saw an EVAP code and an O2 sensor code, but I don’t have the exact code numbers in front of me yet. I’m wondering if the O2 code could just be from the rich/misfire condition.
Other observation:
There is an open hose near the aftermarket cold air intake. After about 10 minutes of running, it started hissing/sucking air. I believe it may be the secondary AIR/smog pump inlet hose. Capping it briefly made no change in performance.
Current theories:
- Fuel-fouled plugs from running rich/misfiring
- Weak/intermittent ignition: coil, ICM, coil wire, plug wires, Optispark
- Bad coolant temp sensor causing it to run extremely rich
- Stuck-open EGR
- EVAP purge issue
- Plugged cat/backpressure issue
- Fuel pump/filter only if pressure drops while cranking/running
Next tests I’m planning:
- Pull plugs and check if they’re wet/black/sooty.
- Check spark with an inline spark tester.
- Check fuel pressure while cranking.
- Check scanner live data for coolant temp/ECT reading compared to ambient temp.
- Get exact EVAP/O2 codes.
- Possibly pull upstream O2 sensor to test for clogged cat/backpressure if spark and fuel check out.
Does this progression sound more like ECT/rich flooding, ignition/Optispark, EGR, EVAP purge, or cat restriction? Any LT1-specific tests I should do before throwing parts at it?