Front coolant tank (Page 4/4) fieroguru MAY 16, 11:14 AM While there are always multiple ways to accomplish the same task, I have also helped a lot of people work through overheating issues on various engine swaps, and 80% of the time, they were using an engine mounted expansion tank that was improperly installed. So while some have had great luck with an engine mounted expansion tank, many have had issues, so results vary widely... Unless you want to offer free tech support for everyone wanting to install a rear mounted expansion tank to walk then through the do's and don'ts, the default recommendation should be to keep the coolant system process as close to stock as possible. The 4 main issues... Where does air get pushed out of the system. Where does coolant get pulled back in. Where & how does air get pulled back in. How much air gets trapped in the radiator - remember, coolant enters the top, exits 1/3 from the bottom, air is lighter than water, so 2/3s of the radiator can & will trap air unless it it properly managed. Most front engine cars have the radiator vent tube (located at the top of the radiator) to the expansion tank to keep air out of the radiator. Blocking off the Fiero radiator vent tube creates the risk of air entrapment. The fiero uses a fill cap at the engine... not a radiator cap. It doesn't have a pressure rating, it doesn't allow coolant to purge, and it won't allow air to be sucked back into the system as it cools down (these are things a radiator cap has/does). You can use a radiator cap in the rear as long as you properly manage the coolant release from and air intake into the coolant system... but this is also easy to mess up. If the rear fill/radiator cap and cap/expansion tank are properly setup and never pull air into the system, you can get by with defeating the front radiator purge (but it must also be defeated properly (vent tube plugged). But if it ever pulls air back into the system the air could be pushed to the radiator and trapped forever until the radiator cap up from is released. How likely this is depends on the engine components, how the coolant is routed to the rear expansion tank, and how the coolant is pushed to the radiator. Both Raydar and Rickaddy88's sytems work and they are happy with its performance, both have the potential risk of trapping air in the radiator, but Raydar's setup has more inherent risk than Rickaddy88's. Here is why: If Raydar's engine bay tank was ever allowed to pull all the coolant from the reservoir (his tank is small), it would then pull air into the system and into the large coolant hose. This hose is after the thermostat and is the high point of the system, so the air will stay tapped in the top of the hose until the very next startup. Then any air in that hose would be forced to the radiator with coolant flow and trapped. Without a functioning air purge on the radiator up front, this air would be trapped forever, until he loosened the front radiator cap. Rickaddy88's LS4 setup would be less prone to this as the thermostat setup is on the coolant intake side (not engine exit side), the coolant exit from the engine is very low, the expansion tank is much larger, the high spot on the coolant hose to the radiator is a coolant passage within the LS4 water pump (hose connection is vertical), access for air pockets to rise from this hose is unrestricted by the thermostat, and there is a hose at the top that goes to the expansion tank w/o any restriction. He also modified the LS4 fill cap (engine mounted) to be just a fill cap so no air can be pulled in. So upon shut down the engine and the coolant cooling off, any air still on the engine side of the lower hose going to the radiator is free to float to the top of his much larger expansion tank. So his setup has less risk of pushing air to the radiator, but without a radiator air purge, if any does make its way to the radiator, it too will be trapped until the radiator cap is released. Lambo nut: with a Countach replica your radiator is nearly horizontal which would make the air entrapment issue in the radiator less. Reason for this is that as you stop the car, the coolant will be able to slosh to the front/top of the radiator and displace the air to the rear and pass under the coolant exit port and allow the air pocket to enter the return hose to the engine. So based on all this discussion, installing a rear mounted expansion tank can be made to work, but is easy to mess up, normally requires more parts to buy, more work to install and remove portions of the coolant system, adds more weight to the rear of the Fiero, and most often eliminates the radiator's ability to self purge air from the system. For me... I much prefer to keep the coolant system process as stock as possible stock so the radiator has the ability to self purge any air that makes its way to the radiator every heat cycle. This also keeps # of parts, costs, installation effort, points of failure, and additional weight on the rear of the Fiero to a minimum. theogre MAY 16, 03:43 PM quote Originally posted by fieroguru: For me... I much prefer to keep the coolant system process as stock as possible stock so the radiator has the ability to self purge any air that makes its way to the radiator every heat cycle. This also keeps # of parts, costs, installation effort, points of failure, and additional weight on the rear of the Fiero to a minimum. Not quoting the entire post... but I do agree w/ this part. In summery of several posts including mine... Air gets trapped in radiator after opening to fill and often after heat cycling and will stay there w/o the "auto purging" Fiero and most cars have. In general terms the radiator acts like wide slow river allowing air to rise and stay there but most other parts often are more like white water and literally can push air out of most places. Is why Fiero Tstant house and cap are full of coolant after a few heat cycles on cars that work right. If that isn't true, the coolant system has problems. Adding extra tank in back of Fiero often won't help even when "done right." If that tank doesn't trap 100% of air then the radiator will get any left and fill it w/ air when you disable front overflow. More air in there the less cooling for the whole car. About the only way is maybe use vacuum filling equipment like shown in above video. Even that won't save you if you have the tiniest air leak allowing the system to suck air after engine shutdown. Iffy Tstant or Rad caps, WP seal, or ever a hose joint anywhere can let air in and not leak coolant you can see. While we say Air because most think Air after filling... Air may not be only thing. Very tiny head casket leaks etc can put fumes in the coolant system and those fumes will normally purge thru same way.