Warm-up regulator W123 280E (M110) defective? Testing & replacement
Short answer: If your Mercedes 280E, 280CE, 280TE, 280SE, 280SL or 280GE with the M110 six-cylinder starts poorly when cold, runs rich, smokes, or runs roughly, and is fine again once at operating temperature, it's usually the Bosch warm-up regulator 0438140057 that's worn. It controls the K-Jetronic's control pressure based on temperature. Solution: replace it with a tested genuine Bosch part with the same suffix number – our price 499,00 €.
Does your W123 280E only start reluctantly when cold, belch black smoke when you give it gas, nearly stall during the warm-up phase, and drink far too much fuel – then run normally again once at operating temperature? We see this pattern regularly in practice. On the 280 models with the M110 engine and Bosch K-Jetronic, the warm-up regulator is almost always behind it. This guide shows you how to narrow down the component, test it yourself, and – if necessary – replace it with the matching genuine part with suffix number 0438140057.
Symptoms
The warm-up regulator (also called the control-pressure regulator) only affects the mixture while the engine has not yet reached operating temperature. That's exactly why the faults typically occur during cold running and disappear once the engine is warm:
| Symptom | Likely cause |
|---|---|
| Poor cold start, stalls when you give it gas | Control pressure too low → mixture too rich during warm-up |
| Black smoke and high consumption while cold | Over-enrichment due to a defective warm-up regulator (heating element/diaphragm) |
| Rough, surging idle when cold – clean when warm | Wrong control pressure during the warm-up phase |
| Engine responds poorly to throttle when cold, "hangs" | Control pressure too high → mixture too lean if the heating element is defective |
| Only the very first cold start is rough, warm-up is clean after that | More likely the cold start valve/thermo-time switch – see Checking & replacing the cold start valve |
Causes
The warm-up regulator deliberately keeps the K-Jetronic's control pressure low while cold and raises it to the operating-warm value as temperature rises. This is done by a bimetal spring that's heated by an electric heating coil and by engine heat. After four decades, there are mainly three things that disrupt this process: the electric heating coil develops a loose contact or fails completely – then the regulator stays too long in the "cold" map and the engine runs permanently too rich. The internal diaphragm becomes hard or starts leaking due to aging and today's E10 fuel, so the control pressure is no longer regulated cleanly. And corrosion at the plug contacts or the ground connection interrupts the power supply. In all cases, the mixture enrichment during cold running is no longer correct. You'll find the general background on how it works in our overview guide Warm-up regulator defective? Symptoms, testing & solution.
Testing it yourself
Work your way from the cheapest to the most involved check – the fault is often in the periphery, not in the regulator itself:
- Visual inspection: Is the warm-up regulator damp with fuel, are the two pressure lines tight, and is the electrical connector seated firmly? Leaks and porous lines are immediately noticeable here.
- Connector and ground: Check the connector contacts for corrosion, clean the ground point on the engine. A poor ground contact readily mimics a defective regulator.
- Measure the heating coil: With the connector disconnected, measure the heating coil's resistance with a multimeter. No continuity (infinite) means: heating element defective, the regulator stays in cold-running mode.
- Check the voltage: With the engine running, check whether the vehicle's supply voltage is present at the connector. If it's missing, the fault is in the supply line, not the regulator.
- Measure the control pressure: The definitive test is done with a K-Jetronic pressure gauge in the control-pressure circuit – cold and at operating temperature. If the value stays too low when cold, or doesn't rise when warm, the warm-up regulator needs replacing.
Solution
If the measurements reveal an incorrect control pressure or a dead heating coil, the warm-up regulator gets replaced. What matters is the correct suffix number: for the 280 models with the M110 six-cylinder, it's the Bosch number 0438140057. Fit a matching genuine Bosch part – not a similar-looking regulator with a different suffix number, because the control-pressure map is matched to the engine and model. In our specialist workshop with our own test benches, we test the warm-up regulators on the test bench before selling them. The price for the 0438140057 is 499,00 €.
Important for distinguishing it: If you drive a 230E or 230CE with the M102four-cylinder, you don't need this suffix number but the matching variant – you can read which one that is in Warm-up regulator W123 230E: which suffix number fits?. You'll find all the regulators collected in our category Warm-up regulators.
Compatible vehicles
Suffix number 0438140057 is intended for the Mercedes models with the M110 six-cylinder. According to parts data, the following models are covered:
| Model | Model series | Engine |
|---|---|---|
| 280E | W123 sedan | M110 |
| 280CE | C123 Coupe | M110 |
| 280TE | S123 T-model | M110 |
| 280SE | W116 S-Class | M110 |
| 280SL | R107 | M110 |
| 280GE | G-Class | M110 |
If in doubt, we'll check fitment for free using your chassis number – just give us a quick call.
FAQ
What does the warm-up regulator do on the W123 280E?
It controls the Bosch K-Jetronic's control pressure depending on engine temperature, ensuring the correct mixture enrichment during cold running. When cold, it keeps the control pressure low (richer mixture); at operating temperature, it raises it.
How do I tell that the warm-up regulator is defective?
Typical signs are a poor cold start, over-enrichment with black smoke, high consumption and a rough idle – each only while the engine is cold. Once at operating temperature, the engine usually runs normally again.
Does the warm-up regulator 0438140057 also fit the 230E?
No. The 230E and 230CE have the M102 four-cylinder and need a different suffix number. You can find out which one in our guide on the Warm-up regulator W123 230E.
Can I replace the warm-up regulator myself?
With a bit of experience, yes: it's bolted to the outside of the engine and connected via two pressure lines and a connector. Cleanliness in the fuel system and a subsequent check of the control pressure are important.
How much does a warm-up regulator for the M110 engine cost?
The tested genuine Bosch warm-up regulator 0438140057 costs 499,00 € with us.
KFZTeile-Thot is a specialist workshop for tested and refurbished Mercedes parts with our own test benches, specializing in spare parts for Mercedes classic cars. Over 1,000 eBay reviews (almost all positive) speak for our parts. Not sure whether the warm-up regulator is really the cause, or which suffix number your vehicle needs? Call us at 0179 8991069 (Mon–Fri 8 am–6 pm) or use our contact form – we'll check fitment using your chassis number.