Friday, January 6, 2012

Fuel Injector Heat soak

This one has taken me quite a while to narrow down.  The symptom is that with a heat-soaked start, such as when I stop at the grocery store for 20 minutes, the car will idle significantly leaner.

The usual suspect is heat-soaked MAT but I found that even with MAT correction completely turned off I was still having the issue.  The curious thing was that even at identical injector pulsewidths it was still idling much leaner.  For instance, at normal operating temperature my idle PW is 2.2ms and will be at around 14.4 AFR.  During a heat-soaked idle it will still be at 2.2ms but up around 16.0 AFR.


I started to hypothesize that the fuel injectors themselves were getting heat soaked and that the extra resistance in the copper coil windings was increasing the latency, therefore less of the 2.2ms PW was going towards delivering fuel.

I found some literature that showed injector latency can increase as much as 100ns which is .1ms.

If my idle PW is 2.2ms, that means the actual squirt is around 1.1ms with my normal 1.1ms latency.  (squirt + latency = 2.2ms).  If the latency is increased by .1 ms that means the squirt is reduced from 1.1ms to 1.0ms, a 10% reduction and certainly enough to explain the lean idle!
A plot of injector resistance with time after a hot shutdown.

I plotted injector resistance with time after a hot engine shutdown and found that the resistance does increase significantly.  I'm guessing it's the fuel that cools the injector when the engine is running.  As soon as the fuel flow stops it heats up, increases the resistance which increases latency.

I was hoping that I could correlate heat soak of the MAT with heat soak of the injector but I don't think it's good enough to use for software compensation.

For now, there's nothing that can be done to compensate for this directly.   Know that it exists and is not the same as MAT heat soak.   I don't use closed-loop EGO at idle because it creates more instability than it solves.

I would also recommend insulating the fuel rail since the fuel itself can boil causing similar issues.

1 comment:

  1. Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector.

    Common Symptoms- When a fuel injector fails, you may experience hard starting, misfires, or an engine that lacks power. The fuel injectors may also leak fuel.
    Common Misdiagnoses- Fuel injectors may be replaced when the problem is actually dirty fuel filter(s), low engine compression, worn out glow plugs, failed fuel supply pump, defective glow plug relay, improper injector pump timing, and/or poor fuel quality.

    Injectors Set of 8 LS1 / LS6 / LT1 / EV1 Style High Impedance Injectors