My cold start this morning:
17*F
WTS light on for 19 seconds.
Then controller signaled to turn the glow plugs off after an additional 21 seconds....
The controller requested a grand total burn time of 40 seconds!!!
I typically start cranking at 20-30 seconds and it fires right up (anything less and I begin to have a hard time), then I let the plugs remain on for a few more seconds to clear up any rough idling.
And yes my controller works just fine. When the temperature outside gets warmer this burn time will be dramatically shorter, proving the controller is doing its job. For example, when its 32*F the wts light usually goes off after 12 seconds and will run the plugs for an additional ~13 seconds.
In my mind this is telling me that perhaps the plugs are designed to run longer than people think when the temperatures are cold. Again, my controller didn’t want to turn the wts light off until after nearly 19 seconds! Thats a considerable amount of time compared to what people say the max on these plugs are, and thats just the the pre-glow cycle and how long its takin for the glow plugs just to heat up to starting temperature...
Another thing to note is that the 6.9 idi system is very different and will light up much quicker than the 7.3’s and it is much more prone to burning out plugs than the 7.3 idi system for this reason.
Just to put it out there too, this doesn’t mean I’m recommending people go out and start running their plugs for this long, quite the contrary. I’m just trying to breakdown contradictions in the evidence of some preexisting convictions and statements about the glow plug system and what its limits are, because the math just never seemed to quite add up in my mind. If you’re running the stock system you’re likely running your glow plugs for longer than the 15 second rule when its cold, and thats totally fine.
In fact, my original reason for deciding to bypass my controller was out of concern that my glow plugs were darkening prematurely after using the stock systems judgments all winter (although they test and work fine still). So, I wanted to take that judgment into my own hands, but determined also that the controller communicated useful information that I still wanted, and also that it sets a good benchmark for what the absolute limit of the gp’s is for those “worst-case-scenario” kinda cold starts where its sub-zero, blizzarding and I’m somewhere remote. With this system I get essentially the best of both worlds and actual visible data and comparisons and therefore can understand what works best and most efficiently for my truck in different circumstances.
So, in closing, sure I strongly disagree with the statement that these gp’s can’t run any longer than 15 seconds without burning out. They can and will glow continuously for 40 seconds, repeatedly without burning out. I’ve done it all winter, and most with the controller do this whether they know it or not. Do I think you should? Definitely not if you don’t absolutely need to, but on occasion if you need to, sure.
I believe the 8-15 second rule is a good, safe standard for most trucks and circumstances, but not a limit, if you need more then run them for longer, just know that longer glow times will always eat away at their lifespan significantly, theres just no way around that. But I mean, you’re essentially doing the same exact thing as running them continuously for 30 seconds when you cycle them again and again and again. They don’t cool off as fast as you’d think, a 15 second cycle time x2 with a brief cool off in between is literally exactly the same as 27 seconds of continued burn when its all said and done. 5-8 seconds pulsing seems to be a better, more efficient strategy in my mind if you’re truly worried about the lifespan of your glow plugs, but that only works if your truck will start that way. Mine rarely will because it’s pretty darn worn out in the compression department, therefore I have to compensate with my glow plugs, until I get it rebuilt.
The main thing I’d say is if its cold try to crank the engine over with the plugs still glowing and keep them running briefly as the truck idles. A quick shot of glow, turning then off, and then cranking it over or waiting around to start it will rarely ever work. The engine either starts from fuel being directly ignited from the hot tip of the glow plug + compression, or from enough chamber heat to facilitate a full compression ignition. If the tip isn’t glowing then you’re solely relying on cylinder heat that you’ve collected, and you don’t build up chamber heat in a frozen 85 pound iron cylinder head without a longer gp cycle.
Thats my 2 cents, hopefully some find this thread informative to them. Information on the glow plugs and controllers in these trucks seem to be super confusing and contradicting and sort of all over the place IMO. A whole lot of assumptions and personal preferences and experiences and not a whole lot of cold, hard evidence and proof. This thread is still far from being actual proof or evidence of anything, but is as close as I can get, for now.
In the future I plan to do a more in-depth thread showing exactly what the solid state controller does with the glow plugs out of the truck and show how it reacts to different inputs and scenarios, then compare zd9 glow plug life spans using different starting strategies and mechanisms. So stay tuned for that.
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