The classic K-Jetronic complaint on a 450SL (and many other R107s): starts fine cold, you run an errand, park for fifteen minutes, and it cranks forever hot. That is usually rest pressure bleeding down — not mystical vapor lock. Diagnose with a gauge.
System overview: K-Jetronic on the R107. Pump: fuel pump.
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What rest pressure is
After shutdown, the system should hold residual pressure so the next hot start has fuel ready at the injectors. If pressure falls to zero in a minute or two, hot starts suffer.
Common failure points (in the order I chase)
- Fuel accumulator — diaphragm fails; cannot hold residual pressure
- Fuel pump check valve — pressure drains back to the tank
- Leaking injectors — drip into the manifold after shutdown
- Cold-start valve leaking when it should be shut
- Internal fuel distributor leaks — less common, more expensive
Test steps
- Reproduce with a timed heat soak (10–20 minutes)
- Confirm pump prime and relay/OVP power
- Tee in a CIS / K-Jetronic fuel pressure tester and record system pressure
- Shut down and watch rest pressure vs time
- Use isolation steps (clamp/test per CIS procedure) to see which side leaks
- Replace the failed part; retest the same heat-soak
Vapor lock — when it is real
Extreme heat, poor hose routing near exhaust, or fuel that percolates can contribute. Most “vapor lock” posts on R107 forums are rest-pressure leaks. Fix hold pressure first.
After the repair
Replace brittle fuel hose sections you disturbed. Clear any fault habits (long cranks with open throttle) that wash cylinders. Confirm cold starts still behave — a new accumulator should not ruin cold manners.
Details that finish this topic
Rest pressure loss
On an R107 this means verifying the condition on your year/engine before ordering parts. When in doubt, match the data card and engine number, then confirm with a specialist who works on 107s regularly.
accumulator
pump check valve
Work methodically: reproduce the symptom, check power/fuses/relays, measure before replacing, then retest the same conditions (cold vs hot, loaded vs idle) after the repair.
