Answer
Dec 30, 2025 - 09:34 AM
Ah, this is classic big-block GM TBI/MBI behavior when a few things start lying to the ECM. Based on what you’re describing (won’t hold idle, temp-sensitive, RPM hunting on the road, fuel acting erratic), here’s how I'd look at it.
What’s most likely going on:1. Idle Air Control (IAC) valve
This is high on the list. When the IAC gets carboned up or sticks, the truck will:
- Stall or won’t hold idle cold
- Hunt idle once warm
- Surge RPM while cruising
Pull it, clean the pintle and passage, or just replace it if it’s original. These old GM IACs don’t age gracefully.
2. Coolant Temperature Sensor (CTS)
Not the dash sender — the one feeding the ECM. If it’s reading cold when warm (or vice versa), the ECM keeps chasing fuel trims:
- Rich when it shouldn’t be
- Lean when it shouldn’t be
- Idle all over the place until it “sorts itself out”
Very common on 90s GM trucks and cheap to rule out.
3. Throttle Position Sensor (TPS)
If the TPS signal is jumpy or has dead spots:
- ECM thinks you’re moving the throttle when you’re not
- RPM rises and falls on its own
- Idle goes nuts
Backprobe it or scan it — you’re looking for a smooth voltage sweep.
4. Vacuum leaks (especially temp-dependent ones)
Big block + old hoses = cracks that open once warm. Check:
- MAP hose
- PCV hose
- Brake booster line
- Intake gaskets (especially if idle changes as it warms)
Spray test still works fine here.
5. Fuel pressure / regulator
You replaced the pump, but:
- TBI/MBI systems are very sensitive to pressure
- Weak regulator spring or leaking diaphragm = unstable fueling
- Oil pressure sender used to back up the pump — wiring issues here can still cause weird behavior
Gauge it hot and cold, not just once.
How I would usually handle this:- Scan live data (CTS temp, TPS %, IAC counts, MAP)
- Check fuel pressure while the issue is happening
- Smoke or spray test for vacuum leaks
- Clean throttle body + IAC passage
- Replace CTS if data looks even slightly off
No guessing, just ruling stuff out in order.
If you want step-by-step diagnostics, specs, sensor values, wiring, and test procedures, this 👉🏻 1992–1998 Chevrolet Silverado (C/K) Service & Repair Manual goes deep into:
- MBI/TBI fuel system operation
- Idle control diagnostics
- Sensor testing procedures
- Wiring diagrams and pinouts
- ECM logic and fault tracing
That manual is exactly how GM expected these trucks to be diagnosed back when they were new.
