By Brian Manley
Once in a while we all have a customer that has been to other shops for a particular repair, only to leave with the same concerns that are unresolved. One such customer arrived at my door recently with an '87 Toyota pickup truck and a failed emissions inspection report for high carbon monoxide (CO). She had been to other shops and spent a nice sum of money attempting to get her truck to pass “the test.” What I found while investigating this failure was surprising.
Before I ever opened the hood, I took the truck for a test drive to feel for misfires, watch for visible smoke, check for overheating and get an overall feel of the performance. This truck felt a little sluggish, and I could see a hint of black smoke coming from the exhaust on a hard acceleration.
Back at my shop, I popped the hood and checked the V.E.C.I. decal. Sure enough, it was a California-certified truck for use at low altitude. What does that mean to us here in Colorado at a mile in the sky? Plenty.
Figure 1 shows the first thing that caught my eye during the visual inspection - the battery terminals. Our state emissions test facility once ended up with a ping-pong car - a Honda - that failed multiple times and went back to the repair facility time and time again. Ultimately, voltage drop across the negative battery cable caused the PCM to misinterpret the O2 sensor readings, which forced the mixture rich, resulting in a gross CO polluter. Case in point, Figure 2 shows the voltage drop across the positive battery terminal with the engine idling; 1.45 volts! I've found too many loose, corroded connections to ignore anything as basic as the current state of this battery. Could this be causing the emissions failure? In some cars, it may, but not in this case.
Before cleaning the cables or removing the air cleaner, I baselined the exhaust gas readings with my 4-gas analyzer. CO was 8.1 percent under load with 450 ppm HC, and around 10 percent CO2. I definitely had a rich engine here. Knowing the emissions repair history of some of these Toyotas, I removed the air cleaner to perform further visual checks. Figure 3 shows the “V” cut in the hose that connects the electronic bleed control valve (EBCV) to the carburetor. Why the cut? This application is somewhat unique in that it meters filtered ambient air into the intake manifold for air fuel control. Another tech thought opening this up to outside air would create a lean condition. He was right; it does, but it's not the kind of repair that reflects well on our industry.
These electronic and vacuum systems are subject to tampering and misconnection, so I also checked for correct vacuum routing and electrical connections, rubbed-through plastic vacuum lines, and pellet gun BBs in the HAC hoses. That's right. Some tech working on the vehicle may have left a BB, used as a self-styled “anti-hesitation” valve, in a vacuum hose. Some of these vehicles also came equipped with a high-altitude compensation valve, which should be checked for function. The throttle/idle switch should also close at idle and open off idle.
My next step was to check the feedback control circuit for function while on a road test. The O2 sensor gave me a constant rich reading of 900 mv, but back at the shop the response and calibration tests proved the sensor was able to function properly. On the road test, I also found that the EBCV valve was fixed at 66 percent; the PCM was trying as hard as it could to lean out this carburetor, but to no avail. Figure 4 shows the failed emission test results. The CO failed at 43 grams per mile on a 30 gpm standard. I knew I had to cut my 8.0 percent CO readings at least in half to make this truck pass the test. I began looking for other sources of unwanted fuel, such as the canister purge system, contaminated engine oil, or a leaking power valve. I finally determined that the engine could probably pass the emissions test at sea level, but, as we've experienced with many low altitude certified vehicles, this carburetor would need to be re-jetted.
I have also seen a bad distributor vacuum advance diaphragm on these Toyotas cause low system vacuum and a rough-running engine on this system. I've also seen bad auxiliary accelerator pump (AAP) diaphragms that will cause extremely rich-running engines. This diaphragm is used for cold enrichment, so if it is ruptured you'll see a rich condition until the engine warms, unless the TVS that controls the AAP is stuck open, then you'll have a constantly rich-running carb!
I removed the air horn from the carb and pulled out the main jet. It had a “111” on it, which meant it was about .044". I grabbed my collection of jets and searched for a slightly smaller number. I found one that had a “104” on it, making it about a .041" jet. I decided - also through experience - this would bring my CO numbers down without compromising the performance of the truck. Figure 5 shows the old jet, along with my carburetor gauge set I use for checking jet sizes. I installed my new jet and a new gasket, then performed another loaded 4-gas test. My CO dropped to 3.1 percent, my HC went to 200 ppm, and my CO2 improved to 13 percent. I was satisfied that I'd found the root cause, but I went for another test drive with my lab scope hooked up to the O2 sensor wire. I found that the system was now in fuel control, and when you're chasing after a failure like this, nothing makes you happier.
I drove the truck down to the emissions test facility for my customer's free retest and watched while they drove onto the dynamometer. Figure 6 has the Toyota all set for the loaded dynamometer test. I watched the drive trace on the computer screen while the inspector took the truck through the four-minute test cycle. Figure 7 shows the trace at 50 miles per hour.
Did the truck pass? It did; with a 5 gpm reading on a 30 gpm scale! The performance of the truck actually improved, and the hint of black smoke that was coming from the pipe on hard acceleration had disappeared. One warning: Re-jetting should only be performed after all other systems on the vehicle have been evaluated and are in proper working order. This includes, but is not limited to: basic mechanical condition of the engine, basic engine adjustments, evaporative emission systems, catalyst integrity, computer control systems and vacuum routing.
When “Electrical Jets” are too “Big”
Sometimes, fuel injected vehicles can exhibit the same type of symptoms. We saw a 1991 Chevrolet Caprice, with a 305 CID engine and TBI fail for high CO in much the same way as the Toyota truck. In Colorado, this vehicle has an initial I/M 240 failure rate of 32 percent. Failing multiple retests, the average reading for this car was 40 gpm, ranging from 16 gpm to 62 gpm. To isolate the root cause of this pattern case failure, Chris Chesney of DTEC and state technicians systematically evaluated all PCM inputs and outputs on one of the failed vehicles using a lab scope, a scan tool and a dynamometer. All serial data appeared normal at idle and at cruise, and no check engine light came on. Many sensors and components had been replaced on this car with no measurable improvement in emissions. With the vehicle loaded on the dynamometer, the fuel injector waveform showed an intermittent asynchronous mode when compared to the distributor reference signal (DREF). After condemning and replacing the fuel injectors, the Caprice passed multiple retests and came close to the Federal Test Procedure (FTP) CO emissions requirements.
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