Questions and Answers

Buick Park Ave Stumbles And Hesitates Q. I am working on a 1985 Buick Park Ave with a 3.8 liter engine, VIN code 3. It has an automatic transmission and just over 104,000 miles on it. It is fuel injected, non-ABS, power steering, cruise control, and A/C. It has rack and pinion steering. The problem I am having is a rough/erratic idle, searching for idle, and an occasional stumble at highway speeds and with the car in overdrive.

The car will also hesitate and sometimes pop back through the intake when I go to accelerate from a dead stop. Another symptom I have noticed is that if the car is cold it will start almost immediately but when hot it has to crank a bit longer to hit and then will put out a small puff of grayish blue smoke.

I have put a new engine in the car (#1 rod bearing was scored badly and knocking), new computer but not new EPROM's, new Mass Air Flow sensor, new fuel pump, fuel filter, fuel pressure regulator (slightly higher pressure then stock one but still in spec), EGR valve, EGR control solenoid, all new vacuum lines, 6 new injectors, Coolant Temp Sensor, did a sweep test on the TPS with an oscilloscope and found no dead spots, tested the IAC motor, new O2 sensor, ignition module, coil, rotor, cap, wires, plugs, tested the pick up coil, a thorough cleaning of the throttle body, new PCV valve and grommet.

Checked ground leads on engine and battery cables for corrosion and resistance and found none. I tested the vacuum leads for leaks and found no significant ones. I even went as far as to block off the vacuum port and EGR port on the intake to eliminate the chance that this was an external vacuum leak but this made no difference in the vehicles performance.

I also redid the intake gasket and checked for intake leaks but was unable to find any. The computer doesn't read any DTC's but with the snap on scan tool it appears that the computer is trying to lean out the engine to the point of rough idle. I would appreciate any advice you could give on this problem.

A. Sounds like you may be on the right track with the 02 reading. I would try to pull a large vacuum line off while watching the O2 reading to see how it reacts. Then spray some carburetor cleaner into that same vacuum line to see if the O2 goes lean to compensate for the rich condition. Does it have an A.I.R. pump? Sometimes they can be pumping when they aren't supposed to and really mess you up. You just need to see if the 02 sensor is able to realize if it's seeing a true rich or lean problem first. Then you'll need to find the vacuum leak or excess fuel problem.