First, make sure it's vapor lock
The pattern is specific. Starts fine cold. Needs long cranking after sitting 15–30 minutes hot.
If it cranks forever hot but fires instantly cold, it's vapor lock. If it's hard to start cold, that's a different fault. Look at the cold start valve and the thermo-time switch instead.
Reproduce it on purpose. Drive it for 20 minutes on the highway. Shut it off for 15 minutes. Restart. Any long crank is vapor lock.
Why it happens
The KE-Jetronic system is supposed to hold fuel pressure in the lines after you shut the engine off. That's rest pressure. A healthy system holds it for 20-plus minutes.
When something lets that pressure go, the fuel boils in the lines. That's your long crank.
Fuel leaves the pump, goes through the accumulator, then the filter, then the fuel distributor, then the EHA valve, then the injectors. Rest pressure gets lost in one of three places: the pump check valve, the accumulator diaphragm, or a leaking fuel line or injector seal.
Step 1: Sniff the tailpipe
Do this before you tear into the car.
Raw gas smell: it's flooded, not starved. Look at leaking injectors or the cold start valve. That's not vapor lock.
No smell: it's starved. Vapor lock or pump. Keep going.
One sniff saves an hour.
Step 2: Check the fuel accumulator
This is the cheap fix. Check it first.
There's a small rubber vent line on the back of the accumulator. Pull it off. If fuel drips out, the diaphragm is ruptured. Replace the accumulator.
A ruptured diaphragm means the accumulator can't hold rest pressure. No fuel at the vent line? Move on to the gauge.
Always depressurize the fuel system before you open any fuel line. That goes for every step on this page.
Step 3: Put a gauge on it
This tells you whether the system holds rest pressure. You need a fuel pressure gauge with a 0–100 psi range and a Schrader fitting for the KE-Jetronic test port.
1. Connect the gauge
It goes on the test port.
2. Start the engine
Read the system pressure with it running. On every 190E 2.3, Mercedes specifies 5.3–5.5 bar (76.9–79.8 psi) at idle (Mercedes-Benz 190E 2.3 CIS-E service manual, 1998 W201 CD-ROM, job 07.3-120, PDF p.58).
3. Shut it off and watch the gauge
It should hold rest pressure for 20-plus minutes. That's a healthy system.
What the needle tells you:
Holds pressure for 20 minutes or more: rest pressure is fine.
Drops to 0 psi in under 5 minutes: you're in the vapor lock zone.
Step 4: Pressure drops, and the vent line was dry
Now look at the fuel pump check valve.
It's a cheap valve at the fuel pump outlet. When it fails, pressure bleeds back to the tank overnight. Long hot crank, same symptom as a bad accumulator.
The pump is mounted under the car at the rear, forward of the fuel tank on the driver side. You get to it from below. No need to drop the tank.
Step 5: Look at the lines
A leaking fuel line or injector seal loses pressure overnight too. Before you condemn the pump, walk the lines.
Start with the short rubber sections over the rear subframe rail. Check them for cracks, chafing and leaks, and replace any hose that shows them.
Then the rubber-to-metal transition fittings, which harden and weep. Then the injector feed lines, for weeping at the crimp ends. Then the filter inlet and outlet, especially the downstream rubber section.
Won't start at all after a hot soak?
That's not a long crank. That's no start. Different branch.
The fuel pump relay sits in the additional relay holder, just in front of the central electrics unit (the main fuse and relay box) at the rear of the engine compartment (Haynes Mercedes-Benz 190 manual, illustration 12.4c). When it fails you get no start and no fuel pressure.
The test: jump Pin 30 to 87 on the fuel pump relay. That runs the pump directly. If it starts, the pump relay is dead. Ten seconds with a jumper wire. Do it before you condemn the pump.
Stalling at hot idle is something else again. That's the OVP relay. Check the 10A fuse on top of it first.
What not to do
Don't condemn the pump first. Test the relay, check the accumulator, check the rubber lines. Then think about the pump.
If the pump does go, don't do half the job. Replace the fuel filter and accumulator at the same time. Labor is identical. Parts are cheap.
Don't open a fuel line on a pressurized system. Depressurize first. Every time.
The short version
| Check | What you see | What it means |
|---|---|---|
| Sniff the tailpipe | Raw gas smell | Flooded — injectors or cold start valve, not vapor lock |
| Accumulator vent line | Fuel drips out | Diaphragm ruptured — replace |
| Gauge, after shutdown | Holds pressure for 20+ min | Normal |
| Gauge, after shutdown | 0 psi in under 5 min, vent dry | Pump check valve, then lines and injector seals |
| No start after hot soak | Starts with Pin 30 jumped to 87 | Fuel pump relay dead |
Work top to bottom. Smell first, accumulator second, gauge third. Cheapest first.
The heat was never the fault.
The rest of the car
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