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· Registered
1 Posts
Discussion Starter · #1 ·
Hi all,

Been lurking here for years and used many threads here to correctly fix the issues.
1991 NAS RRC w/ 3.9L 14CUX, ZF4 auto tranny.

New one popped up recently, and car is undrive-able at speeds over 20+ MPH.

The primary symptom is really terrible load following. Recent test drive w/ Roverguage data collection shows under full throttle RPM won't rise over ~2300, and oscillates very heavily (sputtering?)- therefore cannot maintain speed in drive

Blue below shows RPM, red (exaggerated to fit scale of RPM) throttle position.

Rectangle Slope Plot Font Line

NOTE: This RPM/throttle pos mismatch ONLY occurs under load (driving/hills) and does not appear parked.
Engine will rev up to 3K+ RPM no issue.

Code 48 also appearing. electrical tests, A/C actuation tests, Roverguage monitoring, and inspection of Idle bypass indicates it is functioning correctly. Throttle potentiometer also seems to work fine based off Roverguage results.

Verified fuel pressure and regulator diaphragm OK. Significant electrical testing of ignition systems (coil/plugs/leads) and it seems ok.

Current guesses-
Timing/dizzy related. Amplifier module replaced recently- tested timing +8, +12 +18 to no real effect.
Vac advance diaphragm also functional. Rotor arm/cap?

Really looking for anything, but we are pretty confident the EFI system components are OK and it's getting good signals.


· Registered
1995-2002 Range Rover Classic
1,771 Posts

two parts will possible cause your concern, throttle position sensor, check sweep with ohm meter, and speed sensor failure. either way review the link for an explanation on what component is responsible for what.

· Registered
1,241 Posts
Classically the TPS does this.

The base level mapping for fuel mixture runs optimal to burn the mix effectively, but absolutely needs enrichment to accelerate and very particularly under full load.

Its the same for any petrol combustion engine in reality, just the method of delivery that's different. The TPS should be providing the ECU with competent data that activates the proscribed fuel needed (and mapped for in ecu logic) without that it effectively goes full lean and stalls the combustion into "flat spot" at which point the motor won't accelerate.
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