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I drive a Toyota Corolla with a 1AD-FTV engine. This car has become progressively harder to start over the years. Even at 10°C it might take two tries. It still starts without too much trouble, if you ignore the glowplug light (which goes off almost immediately), count to 10 slowly, then floor the accelerator and start. At -2°C it takes a good 20 minutes of repeated trying before it fires. Once the car is warm there are no issues at all.

I checked the glow plugs and they are all fine.

The other issue that sometimes crop up, which also hints towards a sticky SCV, is a slight hesitation (a flat spot) when accelerating sharply. This is intermittent.

What I've done so far is looked at the fuel rail pressure. Apparently the Denso system uses PWM to control the SCV and this should not fluctuate too much. So my question is, is the picture below reason for concern? It shows FRP fluctuating between 30Mpa and 42Mpa at idle. I don't have enough experience to know if this is normal.

FRP vs RPM

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This looks like a vacuum leak or valves sticking, as the air pressure is bouncing all over the place. However, it could also be the mass air or map sensor. To check this you or a friend could perform a leak and leakdown test. If this is found to be normal then you could try and find the sensor I spoke of at a local salvage yard, on a car that went into said yard, in running condition. If the swap solves the issue by the new one. I say to do it this way because those sensors are not cheap. If the swap does not work and the leak tests, are normal then most yards have a 30 day or so return policy.

  • It's a Diesel. No vacuum as with a gasoline car. In fact it is a turbo diesel so quite the opposite. The chart shows fuel rail pressure. The car does however have both a mass airflow and a manifold air pressure sensor, so that is something I can check. I still suspect low fuel pressure is the main culprit for the hard starting. – izak Dec 4 '18 at 6:57
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    There should still be vacuum at idle. I am used to kpa referring to airflow, but yea if that graph is fuel pressure. Then the fuel system needs to be checked. It should be a lot more stable. – Michael Prokopec Dec 4 '18 at 7:03
  • Right! I remember now that the manifold pressure does drop to around 0.7 bar at idle, which is normal when the turbo isn't making boost. Also, it seems the fuel pressure at idle should be between 30Mpa and 40Mpa, but I note that it is a lot more stable once the engine is warmer. Just trying to confirm if it is the SCV... as that is a common problem with Denso systems, and also a common hard-start reason for AD engines, and ALSO a lot cheaper than what comes next! Thanks for the answer! – izak Dec 4 '18 at 7:08

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