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1991 Accord fails smog with high HC and CO

My wife’s 1991 Accord, with 275,000 miles, recently failed California smog @ 15mph with elevated HC and CO values. NOx was well within the limits. It had been a while since I gave the car a ‘real’ tuneup, so I did the works: plugs, wires, cap, rotor, O2 sensor, PCV valve, air filter. We re-tested the the car, and the numbers were lower but it still failed HC and CO at the 15mph test. The car passed with flying colors at 25mph.

Both myself and the smog tech noticed what seemed to be a misfire during the 15mph roller test. It was quite clear from the exhaust note, as well as the occasional shake of the engine during the test that something wasn’t quite right. I took the car home, and performed a couple simple diagnostics to see where we were at.

I checked engine vacuum at idle and it was very steady at about 21. I also performed a basic vacuum test on the EGR by connecting a vacuum pump directly to it and applying 10 inches of vacuum. The car did not stall, but did stumble as expected. For the heck of it, I left the EGR disconnected (plugged the vac line) and went for a drive. The symptoms were completely gone. It drove very smooth at all speeds. I reconnected the EGR, and went on the same test drive. Sure enough, at light throttle loads I could feel the misfire around 1700-2200rpm. I could not reproduce the issue at WOT.

After scouring the internet, I found a link to a Honda TSB regarding clogged EGR ports in the intake manifold. The diagnostic process outlined in the TSB matched up with my observations, thus it is likely I’ll be performing the port cleaning as outlined.

My question, though, is this: Since the car passed NOx emissions just fine, I assume that the ports might be partially clogged and not fully clogged. If one or two ports are completely clogged up, could that cause a misfire and thus the elevated HC and CO values without elevated NOx?

If this is a big red herring, what else should I check to figure this misfire out?

The engine is running rich, especially at low speed. There are two sides to a rich running engine: 1. There is more fuel than air; 2. There is less air than fuel (ratio-wise).
I think you could get a little more air flow through the air intake and on into the combustion chambers. Use Throttle Body spray cleaner to clean the throttle plate, throttle bore, and idle air control valve air passages.
It’s possible that the engine coolant temperature sensor (ects) is indicating a cooler than actual temperature. Then, the engine computer adds more fuel, causing a richer running engine. You could measure its resistance, and compare those ohms figures with the ohms/temperature chart in the repair manual.

If it passed at idle, the injectors and valves must be OK, so have it tested with the EGR line plugged and see what happens.

I think your EGR valve is initially opening abruptly because it is sticky.
Too much EGR causes a misfire, thus high HC and CO, while NOx stays low.
The EGR valve is able to close, and it does at idle and full throttle.
Remove the valve and spray carb cleaner while working the plunger back and forth.

Great suggestions. This car uses a MAP rather than a MAF, but I will check the throttle plate for buildup and give it a good cleaning.

Checking the coolant temp sensor is also a great idea! Thanks.

There was no idle test, only 15mph and 25mph on rollers. It passed at 25, but failed at 15. I’ll see if the place that tested me is willing to spend time to do it with the EGR plugged. Time is money to those guys …

It will likely fail NOx with the EGR hose plugged.