Mechanical Fuel Injection Adjustment
⚙Tools Needed
- CO exhaust gas analyzer
- Tachometer
- 3mm Allen key (for mixture screw)
★Supplies Needed
- None typically required
Step-by-Step Instructions
- 01
Warm Engine to Full Operating Temperature
Start the engine and allow it to idle until the coolant temperature gauge reaches the center mark (approximately 80-85 degrees Celsius). This is critical because the K-Jetronic system behaves very differently when cold versus warm. The warm-up regulator enriches the mixture when cold, so any adjustments made before the engine is fully warmed will be inaccurate. Drive the car for at least 10 minutes if possible to ensure the catalytic converter and oxygen sensor (if equipped) are also at operating temperature.
- 02
Connect the CO Exhaust Gas Analyzer
Insert the CO analyzer probe into the tailpipe at least 12 inches deep to get an accurate reading. If your car has dual exhaust, test both sides separately. Turn on the analyzer and allow it to calibrate. Note the current CO reading at idle. Factory specification is typically 1.0-3.0% CO for non-catalytic converter models and 0.5-1.5% for catalyst-equipped models. Record the baseline reading before making any adjustments.
- 03
Locate the Idle Mixture Adjustment Screw
The mixture screw is located on the airflow sensor plate housing (the air filter assembly area). On most K-Jetronic Mercedes models, it is a 3mm Allen head screw recessed into the air sensor housing. It may be covered by a tamper-proof plug that needs to be carefully pried out. This screw controls the position of the air sensor plate at idle, which directly affects the air-fuel mixture ratio.
- 04
Adjust Idle Speed
Before adjusting mixture, verify that idle speed is correct. Connect the tachometer and check idle RPM. Factory idle speed for most V8 models is 650-750 RPM (check your specific model). The idle speed is adjusted via a separate screw on the throttle body. Turn it clockwise to increase idle speed and counterclockwise to decrease. Set idle speed to the middle of the specified range.
- 05
Adjust CO Percentage
With the engine at operating temperature and idle speed set correctly, use the 3mm Allen key to turn the mixture screw. Turning the screw clockwise (richer) will raise CO%, and turning counterclockwise (leaner) will lower CO%. Make very small adjustments—an eighth of a turn at a time—and wait 30 seconds after each adjustment for the reading to stabilize. Aim for the middle of the factory specification range.
- 06
Check Warm-Up Enrichment Operation
After setting the warm idle mixture, verify that the warm-up enrichment system is functioning. This cannot be fully tested with a warm engine, but you can verify the warm-up regulator is receiving current by checking for voltage at its electrical connector. On the next cold start, observe whether the engine starts easily and idles smoothly while cold, then settles to the correct warm idle. Difficult cold starts may indicate a warm-up regulator problem.
- 07
Road Test and Final Verification
Take the car for a thorough road test of at least 15-20 minutes. Check for smooth acceleration from idle through all RPM ranges, absence of hesitation or stumbling, and proper response to throttle inputs. Pay attention to behavior during deceleration and when returning to idle. After the road test, recheck CO% and idle speed, as the readings may have shifted slightly. Make any final fine-tuning adjustments as needed.
⚠Warnings & Safety Notes
- 01CO exhaust gas is odorless and lethal. Never run the engine in an enclosed space. Always perform this work outdoors or with professional exhaust extraction equipment.
- 02The mixture screw is extremely sensitive. Over-adjustment can cause the engine to run dangerously lean (risking engine damage) or excessively rich (fouling spark plugs and catalytic converter).
- 03Do not adjust the mixture screw unless you have a proper CO analyzer. Adjusting by ear or by spark plug color alone is unreliable and can damage emission control components.
- 04On catalyst-equipped models, running excessively rich can overheat and destroy the catalytic converter, which is an expensive replacement.
Frequently Asked Questions
Can I adjust the mixture without a CO analyzer?+
It is strongly recommended to use a CO analyzer for accurate adjustment. While some experienced mechanics can set the mixture approximately by ear (listening for the smoothest idle), this method is imprecise and risks running too lean or too rich. CO analyzers can be rented from auto parts stores or purchased affordably. Some emissions testing shops will also let you use their equipment for a small fee.
My CO reading is way out of spec and the mixture screw has no effect. What else could be wrong?+
If the mixture screw has little or no effect on CO readings, the problem is likely elsewhere in the CIS system. Common culprits include a leaking cold start valve (causes persistent rich condition), incorrect fuel system pressure, a failed warm-up regulator, vacuum leaks (causes lean condition), or a damaged air sensor plate. Perform a complete fuel pressure test and vacuum leak check before attempting further mixture adjustment.
What is the correct idle speed for my specific model?+
Idle speed varies by model and engine. Common specs: R107 450SL is 700 RPM, R107 380SL is 650 RPM, R107 560SL is 650 RPM, W126 500SEL is 650 RPM. Always refer to the emission control sticker under the hood or the factory workshop manual for your specific year and engine. Altitude can also affect idle speed—cars at high altitude may idle slightly faster.
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