91 Caprice - l am having problems on acceleration, loss of power, backfiring, check engine light, stalling. Had to repair shop three times and they have not fixed it. Is there a scanner that can diagnose the problem? It is 8 cylinder, 5.0 engine 305.

  • What are the codes? May 8, 2016 at 2:47
  • I'm not sure what vintage the engine control computer is for this car. Best recommendation for you is to purchase a paper copy of the service manual (Haynes or Chilton) These should be available at your local autoparts store, for around $30. Another place to find the manual is Ebay. That manual will tell you how to read the computer codes. Question: How many miles are on this car?
    – zipzit
    May 8, 2016 at 5:31
  • I put a new motor in it at 100,000 . It now has 110,000 miles on it. May 8, 2016 at 11:18
  • Last time my old 91 GMC did that, one of the 2 TBI fuel injector harnesses came loose.
    – Moab
    May 8, 2016 at 14:48

1 Answer 1


There are scanners that will talk to OBD1 computers, but most shops (I would guess) don't have the equipment anymore. I got rid of my OBD1 equipment last year. I've seen ≤ 2 cars that required me to use it in the past 4 years.

Maybe a GM dealer would still have the equipment. You would have to call around and ask.

By jumping pin A to pin B on the ALDL the check engine light will flash codes for you.

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A list of codes for an F Vin 5.0 from http://www.troublecodes.net/gm/90-91_50f/

13 Oxygen sensor voltage stays between 0.35 and 0.55 volts for 60 seconds. Engine was at normal operating temperature and had been running for at least 2 minutes after start with a throttle angle above 5%.
14 Coolant temperature sensor signal indicates a temperature of over 266° F for 3 seconds.
15 Coolant temperature sensor signal indicates a temperature colder than -38° F for 3 seconds.
21 TPS voltage was above 2.5 volts for 3 seconds when the MAP sensor signal showed manifold vacuum to be 15 pounds or more, or TPS was above 4.8 volts at anytime.
22 TPS voltage was under 0.2 volts for 2 seconds when the engine was running.
23 Manifold air temperature sensor signal showed an air temperature below 22° F after the engine has been running for 5 minutes.
24 When engine speed was between 1400 and 3600 RPM and throttle position angle was less than 2% for at least 4 seconds.
25 Manifold air temperature sensor signal showed an air temperature over 302° F for 2 seconds after the engine has been running for 2 minutes and vehicle speed was greater than 5 MPH.
32 EGR open command did not change the fuel mixture when the command was given during off idle operation. EGR opening should cause the mixture to be slightly richer because of the low oxygen content of exhaust gas.
33 MAP sensor signal voltage was too high (low vacuum) for 5 seconds when throttle opening was under 4%.
34 MAP sensor voltage was too low (high vacuum) when engine speed was over 1200 RPM, or when TPS was over 21% when engine speed was over 1200 RPM.
41 Faulty or incorrect MEM-CAL.
42 The EST signal did not change when the ECM applied bypass voltage to the ignition module.
43 Knock sensor signal was above 3.5 volts or below 1.5 volts for 0.5 seconds.
44 Oxygen sensor voltage was under 0.2 volts for 50 seconds of closed loop operation.
45 Oxygen sensor voltage was over 0.7 volts for 50 seconds with a throttle angle above 2%.
51 MEM-CAL error.
52 CALPAK error.
53 ECM sees a battery voltage of more than 17.1 volts for 2 seconds.
54 Fuel pump voltage was less than 2 volts for 1.5 seconds since the last reference pulse was received.

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