Q: I have a mystery symptom with my 1994 GMC Suburban 350. On a cold start it does not show the mystery symptom. Once it gets warm, the idle drops to near stalling when I am in reverse and take my foot off the gas. Also, if I feather the gas to creep through a parking lot, it chugs and runs rough. If I hit the gas to accelerate, it runs fine. It had a different ailment two years ago that turned out to be the coolant sensor. The TPS, IAC MAP sensor and PCV have been replaced. Any ideas?
A: Your last sentence looks like the tiles from a Scrabble board! If you hadn't mentioned replacing the coolant sensor, I would have suggested it — and still do. The engine is obviously running too rich once warmed up. The increased need for a richer air/fuel mixture when cold masks the symptoms. It might be worth having a scan tool monitor the coolant sensor signal as the engine warms up from cold. Our 1996 Tahoe had a similar issue some years ago. Turns out the coolant sensor provided accurate info up to a certain operating temperature, then sort of "stalled," causing the engine to continue to operate in open loop without oxygen sensor feedback. This may or may not trigger a "Check engine" light on your pre-OBDII engine management system.
Other potential suspects include the fuel injector unit and fuel pressure regulator, both located in the throttle body. The fuel pump in the tank delivers roughly 18 psi to the TBI unit, where the pressure regulator keeps fuel pressure between 9 and 13 psi. Too much fuel pressure reaching the injector unit could cause these symptoms as well. As could low electrical resistance in the injector coils. As could a very sluggish oxygen sensor — slow cross-count rate — that may not be able to keep the air/fuel ratio properly trimmed. And, check for a significant vacuum leak to or at the power brake booster.
Q: On my 2007 Hyundai Azera, the "Check engine" light came on for a fuel level sensor P2067 fault. The dealer wants $750 to $800 to fix it. My fuel gauge is still working and I set my trip odometer to 0 every time I fill up. The dealership turned the light off. Should I have it fixed?
A: Has the check-engine light come on again? The P2067 code is set when the signal from the sender is inaccurate for two consecutive driving cycles. Since the fault has apparently not recurred twice in a row and the fuel gauge is working, there is no continuing fault. The issue may have been caused by an intermittent electrical connection in the circuit. Access to the fuel pump assembly, including the fuel level sensor, is through an access panel in the trunk.
As long as you're keeping track of mileage and range, I'd suggest waiting and hoping the fault doesn't recur. By the way, my ALLDATA database indicates that the fuel level sender can be replaced independently from the pump itself, which would reduce the repair cost dramatically.
Q: Is there something magic about 2,200 rpm? I have noticed for years that nearly every car I have ever driven runs at 2,200 rpm at 60 mph. Four-, six- or eight-cylinder, automatic or manual, gas or diesel; turbocharged or not, they all cruised at 2,200 rpm. The only exception I ever saw was an ''88 Acura Integra five-speed that cruised at 3,000 rpm. What's up with this?
A: My 1995 BMW 325i also cruised at 3,000 rpp. Not so much anymore. Our 2007 Impala cruises at well under 1,600 rpm, my 2007 Corvette barely turned 1,500 rpm at 60 mph. Even our VW Passat stays below 2,000 rpm at that speed.
But your point is well taken. Engineers try to coordinate gear ratio, tire size, etc., to allow engines to operate in their most efficient rpm range at cruise speeds. With the combustion efficiency of today's engines, the efficient rpm range is lower than in the past.
Paul Brand is the author of "How to Repair Your Car" and "How to Repair Your Truck and SUV," published by Motorbooks.