Trying to bleed cooling system, coolant pouring out crossover filler post

I have attempted to bleed the cooling system as I suspect there is an air pocket.

I have elevated the front of the car, then the drivers side 5 more or so inch’s. I have removed both filler caps and the bleed bolt. When I start the engine to begin the bleed I have a massive amount of coolant pouring out of the crossover fill port (center fill port). No coolant comes out of the overflow fill port or bleed bolt hole.

Could it be my radiator is blocked?


BTW Its a 1990 XJS convertible collectors classic.

Seem like you added coolant then lifted the car. I think the best way to do it is to raise the front then slowly add the coolant in order to get all of the air out.

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Hello Geoff - sounds like maybe you have the left side up TOO much - the reason for the elevation, is to allow trapped air to find an upper most exit - as long as the left front is one to two inches higher than the right front you would be okay - make sure that you have the heater valve open (turn heater to full heat position) so that any air trapped in the heater system also runs towards the high point - once you have put in coolant, at the fill point, then close that cap - open the bleed banjo bolt and let air out while running at idle - close the bleed banjo bolt when you see coolant escaping - when you place the car on a level position again, you should notice that the coolant on the side pipe is about two to three inches from the top - this is the pipe with the chained cap (done so that the caps don’t get installed in the incorrect position since the caps are different pressure ratings).

I will follow this process next time.

My caps are both 16PSI, I dont see a “chain”

I always do this, but NOT with the engine running. Seems to get most of the air out every time. I also squeeze the upper radiator hoses a bit. I also have a bleed port in the heater hose, which helps big time.

The reason I was thinking I have air in the cooling system was I was getting a lot of coolant pumping out of the center pressure cap after highway driving. But after the bleed issue here I am starting to think I have another issue. Thoughts?

Thanks all.,

How old is your cap? How hot are your coolant temps getting?

Either your cap is not holding pressure or not rated at 16psi, or you’re overheating causing pressure to build above 16psi?

I swapped caps, replaced caps every time it overflows. I think im overheating. Odd thing is I overheat more at high speed, low speed I can run almost indefinitely.

Hi Geoff,
Welcome to J.L. Here are a couple of observations- some of us (me) are not too smart, so you need to be very specific when you describe what you are doing. For instance-
“I was getting a lot of coolant pumping out of the center pressure cap after highway driving.”
Was the engine still running, or not? Was the cap on the header tank in place? (Header tank is what you are calling the “overflow” tank, I think.)
First, the cap on the crossover pipe fill tube will not relieve pressure. You could have a 7lb cap there, or a 20psi cap. It doesn’t matter. Pressure is released by the cap on the header tank.
If you went for a good drive, and then took the center fill cap off, expanded hot coolant will go everywhere. Bad idea.

Try this. Elevate the front of the car a couple of inches. Loosen, but do not remove the bleed plug. With both radiator caps off the car, without starting the engine, fill the cooling system starting with the header tank. When the header tank is FULL, cap it. Then ignore it. Continue filling the center fill pipe until it is also full to the top. Again, do not start the engine!
Now unscrew the bleed port plug. You should see the water level drop in the center fill tube. Slowly add more coolant to the center fill tube, and you should see coolant coming out of the bleed port. Once you have decent flow there, put plug back in. Top off the center fill port to the top. Cap it.
Now you can worry about whether there is an air pocket in the heater by running the engine with the heater “ON”.
If your air purge system is working as it should, and you know about the “Real” overflow container behind the LHS front wheel, you should make progress. If you have read Kirby Palms “Book”, that would help you figure things out, too!

Trying to remember, a few months ago someone had a similar problem, would only run hot at high speeds, not low speeds. Search the archives, maybe you’ll find it.

Thanks everyone. I will go through things again and post my results.

Hello Geoff - from memory, the caps are different (I will have to check my service manual) - what year is your car?

When my 89 had coolant squirting out the crossover pipe cap, it was because my atmospheric tank hose had collapsed. The atmospheric is the one behimd the fender. You can check by trying to blow air through the hose that’s connected at the top of the expansion tank.

The cap at the header tank should be 16 psi. The cap at the crossover pipe is irrelevant.

Hi Jeff, did you have overheating issues as well? I may have multiple issues.

I have also found the top left (N.A. driver side) radiator hose put on by the prev. owner is a larger diameter than the stock. Will this cause issues?



I seem to remember someone thinking their centrifugal advance in the distributor was not working, thus causing too retarded timing at higher RPM, making the engine run hotter only at higher RPM? So it wasn’t a cooling issue, but an engine putting out too much heat issue?

I could be way off though…

1990 is Marelli, so how could the timing be off at higher RPM?

I don’t see how a larger diameter hose would cause the issue. If your cooling system was not working right, you would be overheating at idle.

Went and check service manual and saw it stated filler cap is 20psi and header is 15psi therefore the chain on header cap so as not to interchange - this pic is of the page from my service manual for the 5.3L HE -

reading the refill procedure, the steps are to fill at the filler (middle neck) until the header neck is full (approx. 1 to 2 inches from top is what I found best), close header cap, fill filler neck until approx. 1 inch from top, then close filler neck.

From your description at the beginning, it sounds like maybe the water pump is pumping coolant out the crossover pipe fill point

So I pulled the right hand (N.A. passenger) side thermostat housing and found the thermostat jiggle ping was at 6:00 position. Would this be enough to cause overheating issues?