R107 Hesitation on Acceleration

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A hesitation, flat spot or stumble when you press the accelerator is one of the most common R107 driveability complaints. On K and KE-Jetronic cars, the cause is usually a lean condition when the throttle opens, or an ignition problem.

Causes

• Vacuum leaks: unmetered air leans the mixture.
• Warm-up regulator (K-Jetronic): wrong control pressure gives a lean or rich mixture during acceleration.
• Airflow sensor plate: a sticking or misadjusted plate does not meter fuel correctly when the throttle opens. See airflow sensor plate.
• Ignition: a weak spark, worn cap and rotor, or bad plug wires misfire under load. See ignition coil.
• KE-Jetronic EHA: a faulty EHA or control signal affects mixture. See EHA valve.
• Fuel pressure: a weak pump or clogged filter. See fuel filter.
• Injectors: poor spray patterns. See fuel injectors.

Vacuum leaks

The intake air boot, hoses and injector seals are common leak points. A leak after the airflow meter means air enters without being measured. See vacuum leak symptoms.

Warm-up regulator

On K-Jetronic cars, the warm-up regulator controls the control pressure. If cold or warm control pressure is out of spec, the mixture is wrong. Test with a fuel pressure gauge; see fuel pressure test.

Airflow sensor plate

Check that the plate moves smoothly with no sticking and returns to its rest position. Check that it is centered in the venturi and set at the correct height. Misadjustment causes hesitation off idle.

Ignition

Check the cap, rotor, wires and plugs. Check timing. A weak coil shows up most under load.

KE-Jetronic EHA

On KE cars, check the EHA current and the duty cycle. A faulty EHA or control signal causes lean or rich mixtures.

Diagnosis order

1. Check for vacuum leaks.
2. Check ignition.
3. Check fuel pressures.
4. Check the airflow sensor plate.
5. Check the warm-up regulator or EHA.
6. Check injectors.

Quick answers

• Why does it hesitate only when cold? Often the warm-up regulator or cold start system.
• Why only when warm? Often vacuum leaks or temperature sensor issues.
• Can bad gas cause hesitation? Yes, old or contaminated fuel can.
• Is the transmission involved? Usually not, but check the Bowden cable; see shifting.
• Can a dirty air filter cause hesitation? It can contribute; check it first because it is easy.
• Will a fuel injection cleaner help? It may help slightly; it will not fix a mechanical fault.