TheSamba.com :: View topic - Vanagon California Smog Running Rich in Idle

Tue May 20, 2025 4:39 pm

On my second round of CA smog testing my 1991 Vanagon failed due to running rich during idle. The technical said that it showed high Hydrocarbons and CO at 950 rpm. It passed the smog tests that aren't done during idling.

Has anyone had a similar issue? Other than taking a guess at manually adjusting the screw on the AMF unit, I'm a little at a loss.

Tue May 20, 2025 5:22 pm

is the AFM tamper plug still in place? if so...
change yer oil.
change yer spark plugs.
change yer plug wires, cap, rotor.

come in after a hot drive and i'll wager you pass.

Tue May 20, 2025 9:04 pm

Can you post the readings? The non-judged gases like O2 and CO2 are useful in figuring out why your engine isn't passing. We also need to see the margin of fail.

A few HC over the limit is probably an old O2 sensor or small vacuum leak causing an occasional misfire. A few hundred HC over the limit is probably a bad spark plug/wire/ignition issue.

Lots of ways to spend money without solving the problem… Let's see the printout. :P

Robbie

Thu May 22, 2025 4:54 pm

Not sure if the OP passed yet or is still working on it. I 2nd the recommendation of changing the motor oil, new oil filter, air filter, spark plugs if that hasn't been done. If the distb cap is old, swap that out and maybe plug wires too. Fresh gas with maybe a splash of octane boost wouldn't hurt. Fresh is best when it comes to smog tests. Motor timing good? Then run the motor nice and warm, 20 minutes at speed at least. Keep it running as much as possible till/through the test. Keeps the cat hot. Good luck.

Thu May 22, 2025 5:03 pm

Post up that report so help here guides others.

Chump don’t want no help, chump don’t get da help

- Barbara Billingsley

Fri May 23, 2025 8:21 am

Thanks for all the great feedback! Here is the failed smog report as requested,

Fri May 23, 2025 4:35 pm

My Range Rover was failing smog and I was lucky enough to have a smog tech that was good. He said: that unburned fuel is your injectors. I replaced them and passed! Got Lucky. It never hurts to ask, sometimes those guys know what they are doing.

I use chatgpt for a lot of diagnosis help. I hope it's not annoying but I think the checklist provided here makes sense:

Smog Test Diagnosis: Vanagon Fails at Idle but Passes at 2500 RPM

High RPM (2649) Results Summary:

* CO2: 15.30 percent — Indicates very efficient combustion
* O2: 0.30 percent — Very little leftover oxygen, shows complete burn
* HC: 81 ppm — Well below max of 180, shows no significant misfire
* CO: 0.34 percent — Slightly elevated, but overall a clean burn

This means the engine is burning fuel efficiently under load and moderate throttle.

Idle Test Results (Failing):

* HC: 203 ppm (max is 120) — Too much unburned fuel
* CO: 0.25 percent (max is 1.0) — Still within limit, but elevated compared to average
* O2: 2.4 percent — Indicates possible lean misfire or exhaust leak
* CO2: 13.5 percent — Slightly lower, suggests less efficient combustion

This pattern, passing at 2500 RPM but failing at idle, usually points to issues that only show up under light load or low engine speed.

Most Likely Problems:

1. Dirty or Leaky Fuel Injectors
At idle, precise fuel delivery is critical. Dirty injectors may cause uneven or excessive fuel, resulting in rich or lean misfires.

2. Weak Ignition System (Spark Plugs, Wires, Coil, Cap)
Misfires are more likely at idle when cylinder pressure is low. Weak spark leads to unburned fuel and poor combustion.

3. Vacuum Leaks
More noticeable at idle. Can cause a lean mixture that leads to misfire and higher emissions.

4. Idle Mixture or Air Control Issues
The mixture might be too rich or too lean at idle, even if idle speed is within normal range.

5. Weak or Failing Catalytic Converter
May not be effective at lower temperatures or low exhaust flow. Works better under load.

6. Sluggish Oxygen Sensor
A slow-reacting sensor gives poor feedback to the ECU, causing incorrect fuel adjustments, especially at idle.

Why It Passes at 2500 RPM:

* Fuel mixture is more stable
* Combustion chambers are hotter, improving fuel burn
* The catalytic converter is fully active and effective
* Higher RPM reduces the impact of injector or ignition imbalances

DIY Diagnostic Checklist:

1. Inspect and Clean Fuel Injectors
Use injector cleaner or remove and bench test them. Replace any that leak or spray unevenly.

2. Check Ignition Components
Inspect and replace spark plugs if worn or fouled. Replace spark plug wires, distributor cap, and rotor if old. Test the ignition coil for weak output.

3. Check for Vacuum Leaks
Inspect all vacuum hoses and connections. Spray carb cleaner around the intake manifold while idling; RPM change indicates a leak.

4. Inspect Idle Air Control and Mixture Settings
Ensure idle air control valve is clean and functioning. Check for proper idle mixture adjustment if applicable.

5. Test Catalytic Converter
Check for a temperature increase across the converter using an infrared thermometer. Tap lightly to check for loose internals.

6. Measure Fuel Pressure
Ensure pressure is within spec at idle and under load. A faulty pressure regulator can affect idle mixture.

7. Test or Replace the Oxygen Sensor
Check response time using a scan tool. Replace if the sensor is old (over 80000 miles) or reacts slowly.

8. Verify Ignition Timing
Use a timing light to check and adjust base timing if needed.