89 Honda Prelude failing emissions rich

If you have a fuel sensor check that out. Could need replacing. Also when was the last time you changed the air cleaner filter. A dirty air filter will cause that as well as it can't breath properly. Just some ideas.

There are procedures for adjusting the idle mixture and speed.
I would start with those. Perhaps that resllve both the idle issue and the emissions issue. See procedures at the link below.
Repair Guides Idle Speed And Mixture Idle Speed AutoZone com

If the car still fails the test after there adjustments, please post the test results.

cold engine idle surge. (problem)
RPM range of surge not stated
300-700 or 800 to 1500? (range is critical to diagnosis)

bad fuel
running summer fuel in winter (running old fuel?)
needs a tune up bad, see the list in the operator guide.
idle speed controller,sticking or EGR valve same. (clean them)
location not stated, so cant guess about local air temps.?
no other symptoms, lack of engine power? etc?
was it ok last week, or better, when was the last time it was OK?
1week, 1year?
surge , causes, on a tuned engine might be:
1: too lean. so lean the ISC can not control idle. rpm low, stalling
2: too rich, flooding, stalling, spark plugs foul (look at them)
3: weak compression in 1 or more cylinders.
4: bad spark.
5: air leaks, in the plenum causing , ISC to lose control.
why high RPM?

especially?
well that means it fails hot too.
and there is a simple tests. for hot surge.
is engine fully hot? 180F and holds solid there?
say rpm is about 800
and you turn on head lights, heater fan and (not AC) and defrost.
the RPM must hold or the ISC is failing.
ISC -= idle speed controls. (all EFI has this function)

Emission test by codes reader? or smog? Any codes?

Hi there:
I suggest check this links, according with the fuel system:
- Fig. 62: Vacuum hose routing-1989 carbureted Prelude (except California)
- Fig. 63: Vacuum hose routing-1989 carbureted Prelude (California)
- Fig. 64: Emission component schematic-1989 carbureted Prelude
- Fig. 65: Vacuum hose routing-1989 fuel injected Prelude


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The most likely cause is a combination of two things. I suspect that your catalytic converter has become less effective over the years and should be replaced. Also, the oxygen sensor is responsible for trimming the fuel ratio, and high CO indicates that the engine is running rich. I would start by changing the oil as it may be contaminated now, then replacing the O2 sensor. Then replace the cat. if it still fails. The oxygen sensor could really just be considered maintenance at this age.

Replace cat. The mechanic was correct.

Yes, that can be caused by an O2 sensor.

Valves, fouled plugs, etc., would usually cause "missing", not necessarily rich mixture.

It is running rich. Verify the readings are within specs from the O2 sensors, they should fluctuate up and down once the car is hot, constant high reading is rich run, very low is lean or defective sensors, also clean the throttle body and air-bypass valve, changing the oil can help as well. Do you have any OBD 1 Codes in the system memory?

This prelude model on my experience the cause of in emission either map sensor near the throttle body and the small hose is connected. Next the coolant sensor below the distributor w/ two wires and the fuel pressure is cause by bad fuel pressure regulator at the end of fuel rail or the fuel return line closed. I hope this will give help to you.