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.
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
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.
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.