Simple, But Strange

Ever since OBD arrived in the early 90s, many techs have gotten a little rusty in their diagnostic skills. Trouble codes don’t always tell all, but they’re close enough most of the time that a sleepy tech can get it right without thinking too hard. Here’s a little weird one that shows how the on-board system can sometimes tell you the wrong thing, or nothing at all.

The vehicle was a mid-90s GM N-car with Oldsmobile’s Quad-4 engine and a careful 185,000 kilometres with regular maintenance. Drivability was good, the customer was happy, but at the car’s required Ontario Drive Clean inspection, it failed with high NOx levels. Evap, CO and HC levels were not just below threshold, they were excellent. Sounds simple, right?

The owner reported no issues and the MIL wasn’t set, but the tech scanned the car immediately. The only historical code was an O₂ sensor issue, although the cross-counts and normal closed-loop operation showed normal real-time data. This vehicle uses an older one-wire sensor and in this case replacement was a logical, low-cost place to start. Retesting showed that NOx levels went up, not down. A thorough road test with another tech reading the data in real time showed totally normal operation. Spark plugs had been replaced along with a misfiring coil pack the previous season and had less than 5,000 kilometres on them. Filters were changed on schedule, as was engine oil and compression was excellent. What gives?

Tracking this phantom problem required going back to basic principles. Drivability was excellent and the low levels of HCs and CO showed good combustion and proper injector pulse width feedback, even with the possibility of a lazy or intermittent O₂ sensor. The replacement sensor, however, seemed to make things worse, not better. The answer required some thought about how nitrogen oxides are formed in the combustion chamber.

Fuel burns with air at or around the stoichiometric 14.7 to 1 ratio producing CO, CO₂ and especially in worn or malfunctioning engines, unburned hydrocarbons. Air is only 21 per cent oxygen, but it’s 78 per cent nitrogen, which his inert and won’t combine with oxygen. Nitrogen isn’t inert, however, at the high combustion pressures and temperatures inside an engine cylinder at TDC. NOx forms at high cylinder compression and temperatures. This

well maintained engine wasn’t carbon-fouled and ran well on low-octane fuel without codes, and the scanner didn’t show abnormal knock sensor activity, so it’s unlikely that compression was up. What about temperature? The cooling system appeared normal, with stable coolant temperatures well within limits, so that seemed unlikely. Pulling the plugs told the story. Cylinders three and four showed the telltale signs of serious overheating: a white glazed electrode tip and serious erosion of the ground strap, even though the plugs were double platinum. Since the coil pack fires 1-3 and 2-4 in a waste-fire setup, that wasn’t an issue. A close look at the back of the cylinder head, however, told a story. Small white deposits on the head and on the exhaust manifold suggested a tiny coolant leak from a cracked water jacket. Why did the cooling system perform normally? A can of “stop leak” did the job during a road trip a few months before and was quickly forgotten. In this case, the plugs suggest that the goo had restricted coolant around the back cylinders, which, combined with an aging converter, explains the high NOx. Why did NOX go up with a fresh O₂ sensor? Contrary to popular belief, excessively lean mixtures create cooler combustion chamber temperatures, which is why EGR is so effective, diluting the incoming charge. If the old sensor was failing lean, it’s possible that there wasn’t enough fuel to generate high NOx or CO readings. And Quad-4 engines don’t use EGR, so it could go unnoticed, especially if the careful driver didn’t notice the power loss. Replace the O₂ sensor and air-fuel ratios return to normal, giving the hot running cylinders more fuel to burn and create the additional heat needed for excessive NOx. It’s not an everyday kind of failure, but the kind of thing that can eat up valuable time in your bay. NOx means heat, period. To control it, combustion temperatures have to stay down, either by careful feedback control, EGR or both … and the converter has to mop up what’s left. Sometimes on-board diagnostics just aren’t enough.