Start here: look at your dashboard, not your engine
Is the ABS light on?
If your 190E has a rough idle and an ABS warning light at the same time, stop reading the vacuum leak threads. You almost certainly have one fault, not two.
The Over-Voltage Protection relay — the OVP — feeds several systems on this car, including idle control and the ABS. When it fails it takes both down at once. Owners report this as two separate problems because that's how it looks from the driver's seat. It isn't.
Where it lives: right of the battery. Small aluminium housing with a red fuse standing up on top of it.
What to check first: that fuse. Not the relay. The fuse.
Pull it and look at it. A blown 10A fuse on top of the OVP is the single most common cause of this exact symptom pair, and it's a one-dollar part. In my experience this alone resolves roughly nine out of ten of these cases.
If the fuse is good, replace the relay. Forty to eighty dollars depending on where you buy, and they are a wear item on a car this age — not a mystery, not a defect, just something that eventually gives up.
Buy a spare OVP relay and put it in the glovebox. It will fail again someday, probably somewhere inconvenient, and you'll fix it in thirty seconds instead of getting towed.
No ABS light? Now you go hunting for air
If the ABS light is off, the OVP is probably fine and you have an unmetered air problem — air getting into the engine after the sensor that's supposed to measure it. The ECU doesn't know it's there, so the mixture goes lean and the idle wanders.
On a W201 there are four places this happens, in rough order of likelihood:
1. The idle control hoses
Small-diameter rubber lines around the idle valve. Thirty-plus years of heat cycling turns them hard and they crack at the ends where they slip over the fittings. Pull each one off and flex it. If it's stiff or crazed, replace it. Cheap, and the most common single cause.
2. The intake boot
The large accordion section between the airflow sensor and the throttle. Cracks develop in the folds where you can't see them with the engine off. Squeeze it and work the folds open while you look.
3. Injector seals
The o-rings where the injectors seat into the head. These harden and shrink. Symptoms lean toward a rough idle that improves as the engine warms and the rubber expands.
4. The crankcase breather hose
Runs from the valve cover to the intake. Same failure mode as everything else made of rubber in 1987 — it goes hard and splits, usually on the underside where you won't spot it.
With the engine idling, spray a small amount of carburettor cleaner around each suspect area, one at a time. If the idle speed jumps when you hit a spot, you've found it. Work systematically and give the idle a few seconds to settle between sprays, or you'll chase ghosts.
Do this outside, keep it away from anything hot, and don't empty the can into the engine bay.
Idle still hunting and no leaks anywhere?
Now look at the idle control valve itself.
These get gummed up with carbon and oil vapour over decades. The valve is trying to regulate airflow through a passage that's slowly closing up, so it overcorrects, and the idle surges up and down rhythmically instead of sitting still.
The quick test: spray a cleaner into the intake with the engine running and see whether the idle smooths out. If it does, you've found your problem. Pull the valve, clean it properly with throttle body cleaner, and reinstall it before you spend money on a replacement. They're often fine underneath the muck.
One more pattern worth knowing
Rough only when cold, perfectly smooth once warm?
That's a different fault entirely. Look at the thermo-time switch and the cold start valve. The system enriches the mixture for a cold engine, and when that circuit misbehaves you get an engine that runs badly for the first few minutes and then sorts itself out.
Don't chase vacuum leaks for this one. A leak doesn't care what temperature the engine is.
What not to do
Don't start with the fuel distributor. It's the most expensive part of the KE-Jetronic system and the least likely thing to be wrong. It's a precision mechanical unit and it generally outlives everything around it. People replace them because a forum post said to, and then still have a rough idle plus a lighter wallet.
Don't adjust the mixture screw to make the symptom go away. You'll mask a leak and lose your baseline. Fix the actual fault first, then set the mixture.
Don't replace parts in price order hoping something sticks. Every fault above has a specific test. Run the test.
The short version
| Symptom | Look at | Cost |
|---|---|---|
| Rough idle + ABS light | OVP relay — check its fuse first | $1–80 |
| Rough idle, no ABS light | Vacuum leaks — idle hoses, boot, injector seals, breather | $10–60 |
| Rhythmic surging, no leaks | Idle control valve — clean before replacing | $0–200 |
| Rough cold only, fine warm | Thermo-time switch / cold start valve | Varies |
Work top to bottom. The order is deliberate — cheapest and most likely to most expensive and least likely, which is the opposite of how most people troubleshoot these cars.
The rest of the car
Mick DeLane writes the Heritage Wrench field manuals — diagnostic-first repair guides for classic cars worth saving. The W201 190E Field Manual covers this and the rest of the car in 93 pages, with every torque figure traced to a factory or professional source.
Get the W201 Field Manual — $27