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It is very hard to start when it is cold. Below 50 degrees. What could it be? This is a 2003 Toyota Avalon XL. Like it is laboring making all sorts of rattling noise. Yesterday when it was about 49 degrees, It cranked after numerous attempts, but the rpm kept oscillating between about 300 and 1000 for a few minutes until it stabilized at about 600.

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    Is this a diesel? – Pᴀᴜʟsᴛᴇʀ2 Jan 3 '16 at 13:55
  • @Paulster2 do Avalons come in diesel trim? – Zaid Jan 3 '16 at 14:56
  • @Zaid - I doubt it, but it sure sounds like a diesel cold start issue. Just making sure. – Pᴀᴜʟsᴛᴇʀ2 Jan 3 '16 at 15:10
  • If you pump the gas a bit while cranking, does it start easier? It sounds to me like the Idle Air Control Valve (IACV). When the engine is cold, it should idle at about 1500-2500 RPM and slowly drop down as the engine heats up. – rpmerf Jan 4 '16 at 13:52
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I believe the key clue here is the fact the engine runs rough for a few minutes. This tells me that the cold-start enrichment, where the air-fuel mixture entering the cylinders is slightly rich, isn't taking place.

There are a few possible reasons for this:

  • insufficient fuel pressure

    To rule this out, have the fuel rail pressure tested to make sure that the fuel delivery is as per spec.

    If fuel rail pressure is lower than expected, the following would be my culprits:

    • clogged fuel filter
    • weak fuel pump
    • malfunctioning fuel pressure regulator
    • a leak in the fuel line
  • partially-clogged injectors

    These would prevent cold-start enrichment from taking place.

    If this is to blame, you might be able to get away with just running a bottle of fuel injector cleaner through the fuel tank. If that doesn't work the injectors may need to be taken out for more thorough conditioning.

  • unmetered intake air

    This would result in a lean mixture on most engines since they operate in open-loop mode while warming up (for the first few minutes).

    Culprits to consider here:

    • underreading mass air flow sensor (MAF)
    • a post-MAF intake air leak

I'd recommend you to start with the test for fuel pressure. Analyzing the fuel trims using a scan tool/OBD-II will quickly help you identify whether an air intake leak is present.

Running fuel injector cleaner through the fuel system can be done regardless.

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This could be due to the engine ECU not knowing the air or coolant temperature due to a faulty sensor. A temperature reading is important to the engine ECU, to allow it to adjust fuel mixture at startup.

There is usually an air temperature sensors built into the MAF sensor (if it has one) and there will be a coolant sensor screwed into pipe work next to cylinder head somewhere.

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I support what has been said make a scan on the vehicle first bcos I believe there shld be a check engine light on ur DB before proceeding to any repair.

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    The engine ECU is not always able to identify a faulty sensor. It is possible that a sensor could be giving an incorrect reading and yet the reading still be within a valid range. In this case no error light will be illuminated, but the engine ECU will not function correctly. – HandyHowie Jan 3 '16 at 22:00

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