3.1 TBI Running rich

[Meridivs]

After a strong rev. up of the engine (during an emission test) our 3.1 Vin D Pontiac TS started to run rich.

Now there's an awful smell coming from the tail pipe, lots of gray smoke, the snow in the back is getting black and the engine is hesitating when you press the gas pedal.

I've just changed the EGR valve but this didn't improve things at all.

The plugs, Bosch, changed 2 years ago, are fouled (just cleaned them up). Oil, changed 2 months ago, is already diesel-black.

The air filter, spark wires (ACDELCO), distributor cap, rotor and ICM are changed recently. Cooling system is ok (new thermostat).

Any ideas where we should go from here?

[siKKa]

o2 sensor. replace plugs with ac delco. thoes expensive plugs are sh*t.

[geoffinbc]

Injector leaks happen on old engines. Take the air cleaner off and inspect the injector spray pattern. Look for drops and breaks in the cone. Have someone shut the engine off and look for drips after it is shut off. If you see any of these signs it is time to replace the injectors.

[Meridivs]

Thank you for the ideas.

Forgot to mention that injectors were changed 2 years ago. I will check them anyway.

O2 is probably as old as the car itself. I have a replacement ready but it won't budge for the moment. It probably needs something more than a good wrench...

[siKKa]

the 02 wouldnt budge in my old 3.1 either. i got it effin HOT. held at 2500 rpm in park for about 5-10 min, then shut it down, soaked it with penetrating oil and it cracked loose. another thing you can do is break off the top of the sensor and put a socket on it. thats always my last resort, and its never failed.

[Ed_Mc.]

It could also be your ECT (Engine Coolant Temperature) sensor going bad, this'll cause extreme rich-running as well. Disconnecting the wiring harness from the sensor causes the ECM to go into "limp" mode which may well be better than operation with the failing/failed sensor's input.

This is the sensor screwed into the intake manifold, not the temperature gage sensor which is mounted to a cylinder head.

Check the sensor's resistance with a meter to tell if it's working properly. Here's some info that may help:

http://www.rockauto.com/catalog/x,carcode,1261730,parttype,2188

http://www.wellsve.com/sft503/Counterpoint2_2.pdf

http://www.sonic.net/~mikebr/efi/plots/mat_vs_res_adc.html

http://www.diy-efi.org/gmecm/component_info/sensors.html

http://webcache.googleusercontent.com/search?q=cache:eKkcm7npzVYJ:www.mightyautoparts.com/pdf/articles/tt67.pdf+GM+coolant+sensor+resistance+chart&hl=en&ct=clnk&cd=1&gl=us

HTH.............ed

[Meridivs]

Temperature sensors

Thank you Ed.

This car has two temperature sensors - one on the upper part of the engine, just below the TBI unit and another at the left-back of the engine.

I have a spare back one but I will try to use an ALDL OBD I to USB cable to read the actual sensor/engine parameters on my laptop before changing more parts.

[CNCguy]

I will try to use an ALDL OBD I to USB cable to read the actual sensor/engine parameters on my laptop before changing more parts.

Excellent decision. That's a great place to start. Check ECT, IAT, MAP and O2 readings.

[pocket-rocket]

Yeah, and a normal O2 reading will swing high and low at a fairly fast speed. The slower it does it, the older and more worn out the O2 sensor is.

[Meridivs]

ALDL Diagnosis

We finally got an ALDL OBD I to USB cable and diagnosis software to go with it.

Fortunately we have two Pontiacs TS with the same 3.1 TBI engine setup to compare so here are the readings:
1. the good Pontiac sensor data

http://www.pieseamerica.com/GOOD_LOG.htm

2. the bad Pontiac data

http://www.pieseamerica.com/BAD_LOG.htm

I have marked in orange the times when the engine died completely (usually when coming to a stop at a traffic light).

Can anyone come up with some hints from this data?

[Meridivs]

Changed O2 sensor

Just changed the O2 sensor & spark plugs.

You can find the new ALDL Log at http://www.pieseamerica.com/NEW_O2.htm

I have marked the times when the car died at idle, usually when coming to a stop after braking.

So far EGR, MAP, O2, ICM, filters and all the ignition components were changed without bringing any change in the engine's behavior at idle. Over 650rpm/s everything is great - pulls fine, mileage is good, no hesitation or stalling.

Any idea on what to look-up next is greatly appreciated!