The warm-up regulator (WUR) on K-Jetronic R107s controls control pressure during warm-up. When it fails, cold and warm drivability both suffer — often blamed on “injectors” or “the computer” the car does not have.
Related: K-Jetronic overview, hot start diagnosis.
What the WUR does
It varies control pressure with temperature (and related inputs depending on version). Wrong control pressure makes mixture too rich or too lean during warm-up and can upset idle and tip-in.
Common symptoms
- Hard cold start or prolonged rich smell after start
- Hunting idle while warming
- Poor running until fully warm — or the opposite pattern if stuck
- Fuel smell / black smoke when stuck rich
How to test (high level)
Use a proper CIS fuel pressure gauge setup. Compare system and control pressures to spec for your engine as it warms. Measure, do not guess. A WUR that fails electrically (open heater element where equipped) shows up in warm-up behavior and resistance checks.
Related parts to check in the same sitting
Fuel pressures also involve the accumulator, pumps, and fuel distributor. Confirm supply pressure before condemning the WUR alone. Vacuum lines on certain WURs must be intact.
Replacement notes
Match the correct WUR for the engine. Used units can be worn; reman or known-good tested parts beat mystery boxes. After install, recheck pressures and CO where required by your market practice.
Tools
A CIS / K-Jetronic fuel pressure tester is the right instrument for this diagnosis. Guesswork is how people replace five parts and still idle badly.
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Topic checklist
- What it does
- cold vs warm control pressure
- resistance 18-21 ohms
- rebuild/adjust
- cost
If any line is still unclear on your chassis, verify against the factory manual for your engine code or ask an R107-literate shop before you buy parts.
Warm-up regulator vs cold-start valve confusion
Owners mix these up. The cold-start valve adds fuel briefly when cold; the WUR manages control pressure across warm-up. Wrong pressures from a bad WUR can look like cold-start problems. Measure both systems’ behaviors instead of replacing parts in alphabetical order.
Electrical heater element checks
Many WURs include a heater. An open element changes warm-up enrichment timing. Resistance checks and observing control pressure vs temperature catch this. Wiring and grounds still matter.
When the WUR is not the villain
Low system pressure from pumps, a failed accumulator, vacuum leaks, or a tired fuel distributor can mimic WUR symptoms. Confirm system pressure and residual pressure first. The gauge kit earns its keep here.
After replacement
Recheck control pressure cold and hot. Road-test idle and tip-in. If mixture is still wrong, continue the CIS diagnosis path — do not stop at the first replaced part.
Owner prep for the shop
Write down: cold start behavior, time-to-smooth-idle, hot restart, and any fuel smell. That short note saves diagnostic time.
Keep pressure readings with temperatures noted in your service log. Future diagnosis goes faster when you are not starting from zero.
