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When I start my car, a belt seems to slip and then the tachometer reads zero RPM but the engine keeps running, but has weak power and only runs at low RPM.

Any idea what's going on?

The Symptoms:

  • When first started, or after sitting idle and trying to accelerate, the engine idles unevenly and there's a loud squeal like a slipping belt.
  • After a minute or so, suddenly the idle stabilizes and the squealing stops, the tachometer and temperature gauge drop to zero. These all happen at once, but the engine continues running.
  • Power is very weak and (or because) the engine never runs faster than 2k or 3k RPM based on sound (tach says zero). Starting from a stop feels like I'm in a manual trying to start in third gear letting the clutch slip. The effective top speed is around 40 mph (but I haven't tried to push past this). Putting the transmission into D₃ or 2 helps with the startup acceleration.
  • Engine light is on,
    but I haven't had the code read yet. Will be heading to autozone soon to have it read
    but the OBD-II reader at Autozone gave an error (reader error, not engine light code).
  • Squealing belt appears to be the alternator or power steering belt, based on this diagram. Power steering works fine and the belt felt firm. I didn't check the alternator belt because I didn't know it was right there until I came inside, but the battery is holding a charge fine.

The Car:
2002 Honda Civic EX, Automatic, 110k miles.
Timing belt, water pump, timing belt, and spark plugs were all replaced 500 miles ago.

The Story:
This first happened while I was driving on the interstate, and after a stretch of congested traffic, I hit the gas to return to cruising speed. I noticed the battery light come on and the tachometer read zero, so I thought the engine had stalled. Tried to start it but it didn't (because it was running, but at the time I didn't realize), then I turned it off and it started again, and that's when I heard the squeal for the first time, and when the tach went to zero again I realized I still had some power, so I exited the highway quickly and took surface roads.

The Theories:
Edited post to add some ideas from other research and talking to friends.

  • The power steering and the battery seem fine, so it seems unlikely that the belt is the source of the issue. But why would it slip?
  • No tachometer, uneven idle, and strange engine speed control suggests perhaps a sensor is busted. I found two, but am not really sure which to try:

    Seems like the one on the left is probably for the speedometer and the one on the right probably sources to the tachometer? This could explain the uneven idling and refusal to shift. This wouldn't explain the temperature gauge showing zero though, and I don't get why it would make a belt slip.
  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Jan 29 '18 at 22:34
  • As soon as you get the codes, put them on here. That will help start the troubleshooting. – CharlieRB Jan 30 '18 at 16:00
  • Thanks, @CharlieRB. We had trouble reading it at Autozone. Not sure if my OBD or their reader was the problem, but there was an error. Will take it to Advance today. Added some more thoughts to the post in the meantime. – askewchan Jan 30 '18 at 16:55
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    Those are both vehicle speed sensor for the speedometer, not engine speed (RPM). RPM is likely being read from the crankshaft position sensor or camshaft position sensor. From all that you are describing, it sounds like the computer may be going bad. That could explain why they could not read the codes. If it were me, I would take it to a shop that has the full diagnostic suite because you will probably spend a lot of money chasing a ghost at this point. – CharlieRB Jan 30 '18 at 17:08
  • Turns out you're probably right, @CharlieRB. Took it to a shop and they think it's the computer. Thanks for saving me the trouble of trying to solve it myself! – askewchan Jan 31 '18 at 23:51
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Took it to a reputable mechanic who spent a while troubleshooting and thinks that the computer is going bad. To fix it I have to take it to a Honda dealership due to proprietary software :(

A disappointing answer, but so it goes.

  • You don't have to take it to a Honda Dealer, take it to a good auto-electrical specialist, engine tuner or independent specialist as they will be able to program a used ECU or even repair it. Often independents or electrical specialists have far more knowledge and can do far more than a main dealer – Terry Gould Feb 1 '18 at 13:27

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