ok I got a 1986 359 pete with a 400 low flow cummins engine. It has a new water pump, thermostats, hoses, and all system cleaned, also radiator rebuilt. It will idle and never heat up, if you drive it down road it will hear up in a couple miles. I took the plug out of top of water manifold with engine running, seemed like it was not much if any flow, then I took radiator cap off and started flowing and pushing water out of hole, put cap back on and cut flow off. Just trying to figure out what would cause radiator cap to make system pull vacuum and almost immediately with taking cap on and off. Thanks
Sounds like its air locked. The tiny line from the highest point in the cooling system to the radiator tank might be plugged. That high flow was junk from day one. Any high flow from a 855 will bolt on unless you have stc.