Mechanic’s Tale: EGR Equals Grrr!





I was working on a basic ’97 Ford F-150 last week. It was powered by the mediocre 4.2-liter V-6 engine, rather than the mighty 4.9-liter in-line six that used to make a Ford six-cylinder truck better than everyone else’s V-8.


The complaint was a rough-running engine, with the check-engine light on. A quick test drive revealed a dead miss at light-to-mid-throttle that felt ever so much like a classic ignition problem (bad plug, wire, or ignition coil). The computer popped out a code 303 (misfire on the number three cylinder). A few minutes later José was showing me the number-three plug, which had a clear crack down the porcelain insulation.


A set of plugs, a fuel filter, the computer memory cleared, and off I went. Until ten minutes later when the misfire started again and the check-engine light began flashing frantically to warn of a catalyst-damaging misfire. I should have recalibrated my diagnostic procedure at that point but I had so convinced myself of an ignition problem that I returned to the shop and spotted a point where the number-three plug wire was clearly directly against an exhaust tube and burned.


I repaired the wire to no avail. I tried a coil pack. I even went to the wire on the other side of the engine (in this ignition system, one coil fires two cylinders so that high resistance on one wire can give a miss on another). The miss was now so persistent I could duplicate it by simply bringing the engine speed up. Finally with José in the truck holding it at its most violent miss I poked around under the hood looking for an arcing wire. Out of desperation I unplugged the vacuum line to the EGR valve. Immediately the truck smoothed out.


I am no stranger to EGR valves and have seen many drivability symptoms disappear when they are unplugged, but not violent dead misses. Then I began to remember something old and something new.


Bad medicine


Many medicines taken by people and animals are simply carefully calibrated doses of poison — strong enough to kill the invading disease whether it’s a virus in a human or heartworm in a dog, but not strong enough to seriously harm the host’s body. If emissions are the disease, EGR is the carefully calibrated poison that cures them. EGR stands for Exhaust Gas Recirculation. In order to reduce emissions of NOx (oxides of nitrogen), the peak combustion temperatures inside the cylinders had to be reduced. Unfortunately many of the ways of doing this (lowering compression ratios, valve overlap, and overly rich mixtures) made engines run badly, inefficiently, and with higher emissions of HC and CO.


But it was discovered that a small amount of exhaust gas (poison) recirculated back into the cylinders via the intake manifold at light throttle and cruise conditions could solve these problems without hindering performance too much. Now let’s get this straight. Engines don’t want to breathe spent exhaust gas — it’s poison to them. But if the dose is calibrated right it will reduce the emissions without hindering performance noticeably.


The early systems were quite simple. Most V-8 engines already had an exhaust crossover port that forced exhaust to travel from one cylinder head to the other through the intake manifold to facilitate better engine warm-up. So it was no trick to run the exhaust port up to an adjacent intake port with the EGR valve bolted over both ports, sealing them until the EGR valve opened, allowing the exhaust gas to recirculate into the intake just under the carburetor. The EGR valve itself was operated by a vacuum diaphragm with a spring carefully calibrated to balance against the exhaust backpressure and control how quickly and how much the EGR opened. Vacuum was provided from a ported source on the carburetor, as the throttle opened. Usually that fed into a thermo-vacuum valve that wouldn’t allow vacuum to pass until operating temperature was met. (EGR can cause a cold engine to stall). In the lab EGR must have seemed a miracle but on the street in the real world it was causing all kinds of performance problems from hesitations to surges to outright stalling.


It didn’t take long for all the problems to come out, leaving the best mechanics stumped, but quickly they discovered that unplugging the vacuum line on that big funny valve behind the carburetor made all the bad symptoms stop. So dutifully new EGR valves were ordered and installed at great expense but the symptoms persisted. Now the mechanic was in double trouble, having sold the customer what he thought was the fix at great expense but the problem persisted. So quickly a BB or plastic pellet was pushed deep into the vacuum line to prevent the EGR valve from ever opening. Soon every mechanic had a goodly supply of varying sized plastic pellets in his toolbox (they didn’t cause engine damage if sucked in and were so much more tasteful than a crude sheet-metal screw hanging out of a vacuum line).


Those of us who knew the importance of emission control felt bad and tried hard. Sometimes a vacuum delay valve could be put in the EGR vacuum line to slow down the action of the EGR valve without eliminating it. Also, just rendering the EGR valve inoperative often resulted in the engine pinging, because the EGR did serve to lower high combustion temperatures that caused ping. Out came the distributor wrench and the timing light retarding the timing two degrees at a time, in the hope of eliminating most of the ping without hampering performance too much.


It was an awful time to work on cars. And always we in the aftermarket felt a sort of guilt that we simply weren’t up to the task of diagnosing and repairing these cars. Eight months at a Ford dealership taught me otherwise, where the mechanics knew even less then we did and had no qualms in taking hundreds of dollars of the customers’ money and sending them out running the same as before. So what was really happening?


Too much too soon


EGR was not the only emission system that came on-line in the mid-Seventies. Catalytic converters became standard in 1975 along with unleaded gas. Carburetors were running much leaner than before and timing advance was much greater under cruise and light load. When the car was perfectly new everything seemed okay or at least tolerable. But the minute it got a few miles on it, say 12,000 to 45,000, the catalytic converter would be 25- to 50-percent clogged. Now we had no way to know or test for that, or even suspect it. My guess is most of the manufacturers knew it but with a five-year 50,000-mile emission control warranty mandated by federal law, they weren’t talking.


The result of the restricted exhaust flow out the tail pipe was that the EGR would become hyperactive due to the increase in exhaust backpressure, quickly overcoming the spring in the EGR, causing a much greater exhaust recirculation than calibrated for. The slightest touch to the throttle, and the EGR was flung wide open and the increase in exhaust pressure had it flowing double or triple what was calibrated. Of course unhooking it took care of the hesitation or stumble it would cause but with the lean fuel mixture, high amount of timing advance at cruise, and a half-clogged catalytic converter trapping exhaust heat in the engine, outrageous pinging occurred, helped along by the new 87-octane unleaded fuel.


And there were other problems. EGR would get fouled with carbon and stick open, causing the engines to stall on deceleration. Or the passages in the valve or aluminum mounting plate would burn through, causing a perpetual rough idle. Sometimes the passages would clog up completely, rendering the valve inoperative, saving the mechanic the trouble of disconnecting it but leaving the car with a death rattle ping going uphill. Fortunately, EGR, like everything else, has gotten better.


EGR has improved with electronic engine controls. The typical system now uses a vacuum solenoid controlled by the computer to precisely regulate the timing and amount of EGR. The solenoid receives a full-time vacuum supply from the engine. The computer energizes the solenoid by grounding it, but it pulses the solenoid, grounding and ungrounding it many times per second. And the system is monitored by the computer, which may utilize a backpressure sensor or an EGR position sensor. The computer periodically tests the EGR system by opening the valve at a time when it will go unnoticed by the driver and looking for the expected results at the oxygen sensor and the map (vacuum sensor). Any time the computer doesn’t see the EGR behaving as expected, the service-engine light soon comes on. General Motors uses an entirely electronic EGR valve using electromagnetic solenoids to precisely position the EGR. Even when the EGR fails, ping doesn’t seem to be an issue anymore. I think they are using much less EGR and not counting on it for ping control.


Back to the future


So what was happening on my Ford F-150 with the EGR system that caused the number-three cylinder to miss but gave me no EGR code? The system on this vehicle has individual cylinder ports for EGR. Every one of them except the number three port was partially or fully clogged, forcing the full measure of exhaust gas meant for six cylinders to flow to number three only, causing a fatal overdose of the poison, killing combustion in the cylinder and causing a miss. Since the computer still saw the full measure of desired EGR, it did not register an EGR code but did record the cylinder misfire. No system is perfect. We still see a fair amount of EGR problems but they’re all quite fixable. The backpressure sensors, which are circuit boards, are exposed to corrosive exhaust gas and water. The electronic GM valves fail quite often, and with smaller passages, clogging is always a danger. But at least we can make them run right without having to resort to BBs, block-off plates, and other forms of trickery.


Doug Flint owns and operates Tune-Up Technology, a garage in Alexandria,Va.


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