First engine start of the day...?

[ChuckBlack]

Hey guys, I usually work on larger fleet engine systems so small engine comes with a whole lot of others set of skills.. reguarfless, I've had my van for soon 2 years now, now @ 160000klm and had no engine issues. I've been wondering why when I start the van in the morning, first start, the engines RPMs drops to 600, then 30 seconds later will rise at 650 give or take... it happens usually after long periods without being started. Just by curiosity? Perhaps it's normal but just wondering if anyone else's van does the same??
Thanks!

[ChuckBlack]

Alright never mind wondering why it does. The ECM reads a cold engine and slows the rpm down for so that oil pump will not cavitate itself out. Slower rpm till The oil pressure is up and running then rises to 650+-50...

[Growlerbearnz]

Not sure where you read that, maybe it referred to one of the newer Direct Injection 4D56s? On our primitive 4D56's there's no automatic idle speed compensation. Idle is low when cold because the engine's not making much power, that's all. If your cold idle suddenly jumps by 50rpm it's probably when the glow plugs turn off- the plugs draw a lot of load from the alternator, which slows the engine down.

The glow ECU doesn't control anything but the glow plugs. The throttle position sensor on the injection pump is only there to tell the glow ECU what you're doing with the throttle- the ECU will turn the glow plugs on a bit more if you're trying to accelerate while the engine's cold.

I'm not a huge fan of letting an engine chug when it's cold- it's hard on the bearings. I prefer to set the idle speed a bit higher (like a modern vehicle would do automatically) using the idle speed control on the left of the steering column. Looks a bit like a choke knob. When you first start the engine use the accelerator to bring it up to 750rpm, pull that knob out until it stops, then twist it to the right. That'll keep your idle speed up a bit until the engine warms up. (twist the knob to the left to release once the engine's warm).

[ChuckBlack]

Thanks Growl! I figure it does that... just that some other light duty trucks have a similar set up. When I first turn the key on, I get 10V at the glow plugs, then I start the engine, for about 5 more seconds te glow plugs are still at about 5V, then they drop to 0V. I was thinking that the drop in the engine rpm was due to a temp sensor, indicating that the engine is cold, lowering the rpm then continues to run the glow plugs to have not as much smoke and keeping on heating the fuel, cylinders etc etc... but yes you're right on the answers... as usual thanks again!