Last weekend, I decided to replace my glow plugs. Although incredibly reliable, I find the 7.3L IDI to have an incredibly finicky glow plug system; even with the solid state controller. I decided to bypass the controller to go with a momentary push button for increased reliability and this is what I found:

The orange ones are bad

The orange ones are bad.

Old glow plugs 8k miles.

Old glow plugs 8k miles.
















Is it good, bad, neither? I don’t know. Generally, the black seemed to just be soot that would easily wipe away. However, there was evidence of some coking and the tips of a couple glow plugs had swelled. It seems that the bad plugs had the most buildup.

Something all waste oil burners need to be aware of is coking. Unwanted harmful buildup of unburnt fuel and dried hot oil. It can kill your turbo, performance, coat your pistons, and limit your injectors. If you notice more smoke, white at idle or black at throttle, less power, or hard starting, you may have coking issues.

One of the inherent problems with WMO and other waste oils is that they usually have a high flash point and a high auto ignition temperature. As a reference point, diesel has a flash point of 144*F and an auto ignition temp of 410*F. Likewise, RUG has a flash point and an autoignition temperature of -6*F and 480*F respectively. Most of the waste oils that I currently use have a flash point of around 400*F, however, since they are not intended for use as fuels, they do not list autoignition temperatures, but from some safety bulletins it seems like they are in the 500-750*F range.

Autoignition- Minimum temperature at which the fuel vapor will spontaneously combust without other external ignition sources.

Flash Point- Minimum temperature at which an external ignition source will cause the fuel vapor to combust. It is important for a diesel to have a high flash point and a low autoignition temperature. Our numbers are not too far off and it seems that adding RUG may actually help bring the autoignition temperatures of our blends down a little, although it hurts by bringing down the flash point as well.


I’m still working on a solution to coking, and although I may never find it, I do believe that it can be mitigated. Sometime in the near future, I plan to pull my injectors to inspect and clean them. My hope is that the tips are easily cleaned and that the pintles are not ruined.