SL/R107: Running rough when warm with K-jetronic
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Newbie
Joined: Dec 2023
Posts: 1
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380 slc
Running rough when warm with K-jetronic
Hello!
I have got a MB 380 slc from 1980 that has the following symptoms: The car starts fine when cold, and runs pretty well. It seems to be firing on all cylinders. Idle is around 1500 rpm in a couple of minutes after cold start.
The car runs fine when warm, while driving, but after it gets up to working temperature the idle rpm, goes under 600 rpm and It starts stumbling at idle. Even stalls.
When the car is hot it doesn’t start until it has cooled down – around 20 minutes later…
This is what I have done:
None of the above has changed anything…
I have measured the pressures:
Whats the next thing to check? What could be causing the absence of residue pressure and low pressures at the WUR?
Would be really grateful for any help or ideas…
Regards Jens
I have got a MB 380 slc from 1980 that has the following symptoms: The car starts fine when cold, and runs pretty well. It seems to be firing on all cylinders. Idle is around 1500 rpm in a couple of minutes after cold start.
The car runs fine when warm, while driving, but after it gets up to working temperature the idle rpm, goes under 600 rpm and It starts stumbling at idle. Even stalls.
When the car is hot it doesn’t start until it has cooled down – around 20 minutes later…
This is what I have done:
- Replaced fuel accumulator
- Replaced fuel pump
- Replaced fuel filter
- Replaced o-rings at fuel pressure regulator valve
- Checked the Cold start injector for leakage – which it doesn’t..
None of the above has changed anything…
I have measured the pressures:
- Cold controlpressure is 0,0 bar at 5 degrees C
- Warm control pressure is 2,5 bar after ca 5 minutes and it doesn’t go any higher
- System pressure is 5,1 bar
- Residue pressure 0,0 bar 5 seconds after shuting of the fuel pump.
Whats the next thing to check? What could be causing the absence of residue pressure and low pressures at the WUR?
Would be really grateful for any help or ideas…
Regards Jens
Junior Member
Joined: Dec 2023
Posts: 29
Likes: 9
C-Class
Hi JensMB, I hope this will help. Given the symptoms you've described and the actions you've already taken, it sounds like you've covered several potential culprits. However, there are a few additional components and checks to consider:
Considering these checks might help in identifying the root cause of the idle and stalling issues, especially when the engine is warm. If these checks don't resolve the problem, seeking the expertise of a professional mechanic with experience in vintage Mercedes-Benz models might be the best next step. They could perform more comprehensive diagnostics and pinpoint the issue accurately.
- Warm-Up Regulator (WUR): The warm control pressure not increasing beyond 2.5 bar could indicate a problem with the WUR. It might not be adjusting the fuel mixture properly as the car warms up. A malfunctioning WUR could cause the issues you're experiencing with the idle and stalling when hot.
- Idle Air Control Valve (IACV): The IACV controls the idle speed of the engine. If it's malfunctioning or sticking when the engine reaches operating temperature, it could cause low idle RPM or stalling. Cleaning or replacing the IACV might be necessary.
- Vacuum Leaks: Check thoroughly for vacuum leaks. Leaking vacuum hoses or gaskets can lead to issues with engine performance, especially at idle and when the engine is warm.
- Engine Coolant Temperature Sensor (ECT sensor): A faulty ECT sensor might not provide accurate temperature readings to the engine control unit, resulting in incorrect fuel mixture adjustments. This could affect idle performance when the engine is warm.
- Oxygen Sensor (O2 Sensor): A failing O2 sensor might not provide accurate readings to the engine control unit, leading to improper fuel-air mixture adjustments, especially during warm-up.
- Throttle Position Sensor (TPS): A malfunctioning TPS might not communicate the correct throttle position to the engine control unit, affecting idle speed and performance.
Considering these checks might help in identifying the root cause of the idle and stalling issues, especially when the engine is warm. If these checks don't resolve the problem, seeking the expertise of a professional mechanic with experience in vintage Mercedes-Benz models might be the best next step. They could perform more comprehensive diagnostics and pinpoint the issue accurately.
Newbie
Joined: Dec 2025
Posts: 1
Likes: 0
mercedes benz 380slc
Hello!
I have got a MB 380 slc from 1980 that has the following symptoms: The car starts fine when cold, and runs pretty well. It seems to be firing on all cylinders. Idle is around 1500 rpm in a couple of minutes after cold start.
The car runs fine when warm, while driving, but after it gets up to working temperature the idle rpm, goes under 600 rpm and It starts stumbling at idle. Even stalls.
When the car is hot it doesn’t start until it has cooled down – around 20 minutes later…
This is what I have done:
None of the above has changed anything…
I have measured the pressures:
Whats the next thing to check? What could be causing the absence of residue pressure and low pressures at the WUR?
Would be really grateful for any help or ideas…
Regards Jens
I have got a MB 380 slc from 1980 that has the following symptoms: The car starts fine when cold, and runs pretty well. It seems to be firing on all cylinders. Idle is around 1500 rpm in a couple of minutes after cold start.
The car runs fine when warm, while driving, but after it gets up to working temperature the idle rpm, goes under 600 rpm and It starts stumbling at idle. Even stalls.
When the car is hot it doesn’t start until it has cooled down – around 20 minutes later…
This is what I have done:
- Replaced fuel accumulator
- Replaced fuel pump
- Replaced fuel filter
- Replaced o-rings at fuel pressure regulator valve
- Checked the Cold start injector for leakage – which it doesn’t..
None of the above has changed anything…
I have measured the pressures:
- Cold controlpressure is 0,0 bar at 5 degrees C
- Warm control pressure is 2,5 bar after ca 5 minutes and it doesn’t go any higher
- System pressure is 5,1 bar
- Residue pressure 0,0 bar 5 seconds after shuting of the fuel pump.
Whats the next thing to check? What could be causing the absence of residue pressure and low pressures at the WUR?
Would be really grateful for any help or ideas…
Regards Jens
MBWorld Fanatic!
Joined: Jan 2010
Posts: 3,117
Likes: 782
NGL450 w/EORP, S550
Like set above, check for crud in the secret filter on the fuel divider, as well as in the WUR screen. Last place to check for crud is the injectors themselves. They have a micro filter, which could very well be the issue here. If you can still get a hold of new ones at like 15 bucks apiece it’s much better just to buy new injectors than to try to back flush the old ones. They were 15 bucks last time/last year when I was messing with my last M117. Injector buckets, and O-rings on the other hand, though are NLA last time I checked so be very careful, removing said injectors.
Depending on your year, the pressure sound OK Ish but it’ll be individual configuration driven. No residual pressure is a result of a bad accumulator, but as long as you have a healthy pump, the pressure should read normal when running. If below spec, you can look at shimming the regulator or buying a new pump. The accumulator also evens out the pressure spikes and troughs so that may also have some minor effect but once running it doesn’t really. It’s just there to aid in hot starts.
Depending on your year, the pressure sound OK Ish but it’ll be individual configuration driven. No residual pressure is a result of a bad accumulator, but as long as you have a healthy pump, the pressure should read normal when running. If below spec, you can look at shimming the regulator or buying a new pump. The accumulator also evens out the pressure spikes and troughs so that may also have some minor effect but once running it doesn’t really. It’s just there to aid in hot starts.
Last edited by Max Blast; Dec 13, 2025 at 07:27 PM.
