Question about 1994 GMC Sierra

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Bad crankshaft sensor on a 1994 GMC k1500 5.7L TBI Auto?

'94 GMC Z71 4x4 automatic
5.7L TBI
57K actual miles on the truck.
New spark plugs 3 mos ago.
Check engine code: 12 -- O2 sensor reading lean condition

I get an intermittent 'bouncing/flutttering' of the Tach at about 2400-2900rpm. It feels like a cylinder dropping out, but not completely. I have had a plug wire come loose and it doesn't feel like that. It's more subtle. Shortly thereafter the check engine light comes on. Keep driving and it goes back off, tach bouncing stops. There is a noticable loss of power as I cannot maintain my normal speed while towing. (65mph) Outside temp does not seem to affect it. (Phoenix, AZ) Alternator voltage, Engine temps all normal.

I am suspecting a failing crankshaft sensor, O2 Sensor, or both. I am changing out the rotor,cap & plug wires because they are due, but I doubt this will fix it.

Point me in the right direction. If you need more detail, email me. Please don't reply with a vague, wide sweeping suggestion.

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  • fixya198 Feb 05, 2010

    I'll do that right now. I have a small propane torch I've used in the past for this. Do vacuum leaks come and go? It is intermittent.

  • fixya198 Feb 06, 2010

    I'll do that right now. I have a small propane torch I've used in the past for this. Do vacuum leaks come and go? It is intermittent.

  • fixya198 Feb 06, 2010

    A quick correction. The code is 44 - O2 sensor read lean for 20 sec. Code 12 is the start/end indicator, (sorry). The 44 code is resident in the ECM. I have cleared it once last week but it came back over the weekend. No joy on the vacuum leak check. I found where the crank sensor should be. I am going to test it and the O2 sensor to see if they are within spec.

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If your o2 sensor is reading too lean then you have a vacume leak spray brake cleaner on all your vacume hoses one at a time if nothing then spray it around your intake manifold gasket they are usually the culprit

Posted on Feb 05, 2010

  • terian_5 Feb 05, 2010

    brake cleaner evaporates fast and has less of a smell than wd 40

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