The Erratic Suburban

Dear Roy, The vehicle is a 1990 Chevrolet Suburban 1500, VIN 1GNER16K8LF118915, with a 5.7L engine and a four-speed automatic transmission. Mileage is 160,000; a new long block and transmission were installed at 103,000 miles. The Suburban has been running great until three weeks ago.

Test And Repair Data: No codes. Fuel Pressure: 14 psi Timing Set: zero degrees TDC Compression: 130 psi on all Oil Pressure: 6 to 9 psi @ idle Engine Vacuum: 15 to 20 with erratic idle

This truck started acting like it jumped time. It had an erratic idle and low power; it stumbled and missed during acceleration.

I have replaced the fuel filter, knock sensor, fuel pump relay, IAC, TPS, MAP sensor, O2 sensor, Coolant Temp Sensor, Throttle Body and injectors, ignition coil, sparkplugs, wires, cap, rotor, the ignition module and the electronic spark control module. Some of these replacements were known-good components from another engine.

The pickup coil passes ohm testing and generates 0.5 volt steady during cranking. The distributor drive gear is good. Base timing is on the money, and exhaust restriction has been ruled out. The EGR valve is seated, and the vacuum line is plugged. PCM grounds back-probed from PCM harness connector are good, as are all circuits to the distributor module, ignition coil and ignition feed. Power is steady to the fuel pump and coil.

Disconnecting the EST lead to set timing manually has no effect either way; timing goes to manual and sets Code 42 when disconnected. Manually advancing or retarding timing at the distributor results in stall/backfire. When I unplug the vacuum line from the MAP sensor, the truck straightens up and idles high, but nicely, with a vacuum leak, so it has no acceleration under load, of course. Plugging the MAP vacuum line richens the mixture, and the truck still idles high but smooth, though it still has no acceleration under load.

The O2 readings went rich and the PCM promptly set Code 34 (MAP LOW). I reconnected the vacuum line to the MAP, and the engine died. I restarted the engine, unplugged the MAP electrical connector and the engine settled down a bit. Then the PCM set Code 33 (MAP HIGH), and the engine died again.

I recall that timing on these engines is about 12 to 14 degrees BTDC with EST enabled, but a timing light shows timing four to five degrees BTDC with EST enabled, while the idle is unstable. I have tried unplugging the knock sensor and the E.S.C. module to no avail. I have checked the circuits for the knock sensor and the E.S.C. module and all is well. Clearly I am getting timing advance at idle, but I don't understand why.

I'm ready to try another PCM, but any advice you can offer will be much appreciated.

Harold Burson, owner/technician, ASE Certified Auto Master Burson Automotive, Spring, TX

Dear Mr. Burson, Looks like you have been pretty thorough in your diagnostics. From the symptoms that you describe, I suspect one of two problems that were common on these vehicles.

There were a rash of distributors with worn shaft bushings causing erratic ignition timing. Carefully check that distributor.

GM also experienced ECMs with bad solder joints during that time period, which also will produce the symptoms you describe. Typically, tapping on the ECM with your fingers while the engine is running will produce erratic engine operation on these defective units.

Losing Our Minds

Dear Roy, The cooling fans and A/C condenser fan on a 1995 Buick Century Special (VIN 1G4AG55M556464695, with 3.1L engine and automatic transmission) are not turning on.

We replaced the three-wire temperature sensor, but still no luck. I have three ASE-certified techs trying to figure this one out, but they can't even pull a code because no one has an OBD I reader.

The car runs and shifts smoothly, and all options seem to work. The "Service Engine Soon" light is on, but we just can't seem to pinpoint the problem. Do you have access to the schematics for this vehicle? It would be helpful to follow wiring to locate the problem.

Thanks, Stan Kozlowski, Tucson, AZ

Dear Stan, In as much that both fans are inoperative, I would first check the 10-Amp eng/acc inline fuse and the rust colored fusible link (F) located at the starter motor. These provide ignition and battery power to both fan relays.

If power is now present at the pink and the red wires at both relays, ground the dark blue wire of the pusher relay and the dark green wire of the primary relays. The relays should click and the fans should run.

If the relays click and power is provided with no fan operation, check the ground circuit to both fans located on the engine near the firewall.

If all this checks out, it appears that you have a couple of bad fan motors.