I posted this in another forum and I followed all the suggestions as it was figured there was air trapped within the block. Thought I’d try here and see if any other ideas. The noise comes from the front of the engine under the stat, or water pump area. I have jacked the front of the truck up, installed a bleeder valve on the stat housing and verified the water pump is pumping. Flow through the radiator when the engine is revved up seems normal There was a small amount of air came out of the system but not much. Presently have no stat in it just to make sure any trapped air if any comes out. The noise is identical to water hammer in a steam radiator in a home.
Hoping another set of eyes might have an idea what is going on.
"I just finished rebuilding my 351M into a 408. Everything was replaced new except for the heads and block. Block and heads were boiled clean, then the block was decked and the heads shaved. Everything except for the castings was replaced with new parts (not remans) including the water pump.
The 351 did not have a stat in it when I purchased it and it did not overheat although it ran pretty cold most of the time. There was no water hammer in the engine even if idling for a long time and in the 200*'s
The new rebuilt engine has a water hammer, it sounds like air in the system. It was burped and has been driven about 50 miles and although less hammer (about 50%) it still has the hammer after shutting it down. It does it whether running at normal operating temps 192* or less than. Also noticed the noise is there with the engine running but not as severe.
I have tested the cooling system for leaks; there is none, held 15 #'s. I tested the antifreeze for exhaust gases and it was free of. I did a compression test after running the first 20 minutes and found all to be very close but around 80 (the rings weren’t really seated at this point.) I changed the thermostat for the 2nd time with another bypass stat. I am blowing good heat from the heater all the time."
Thanks for your comments in advance.
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I vaguely recall having a problem with head gaskets when rebuilding an Ford M engine back in the Good Ole Days, @Winger4647. There was a problem with coolant flow passages being blocked on one bank. With an infra-red temperature sensor locating any hot spots would be somewhat simple. Of course if you push the engine hard enough the problem will come out of hiding.
Are you saying all of the cylinders were around 80 PSI when you tested them? If so, it’s time to call a time out because there are issues and while ring seating may be the cause it’s not because it needs to have more miles put on it.
I have a feeling you’re going to be ripping this one back apart. Condolences…
I know radical cams can lower compression due to late intake valve closing, but 80 seems awfully low.
A leakdown test would be more conclusive.
Is the radiator cap letting the system pressurize properly when it heats up?
The radiator hoses should get really firm even before the 'stat opens and the hoses get hot.
If parked on a level surface the engine will purge air when the thermostat opens… UNLESS there is an obstruction in the water jacket. The 4bbl and 2bbl engines used different heads, intake manifolds and gaskets. Also, the correct head gaskets could be installed inverted and obstruct proper flow from the block to the head.
Both block and heads were shaved. This brings up issues about valve stem height, stock length pushrods, and so on.
My opinion is the cause of that 80 PSI of compression needs to be sorted out first.
In my opinion, the water hammer “theory” is a red herring. Water hammer happens when water that has momentum is abruptly stopped and has nowhere to go. The solution to water hammer is to ADD an air reservior so it can absorb the water’s momentum. Bleeding the air from the system would make it worse, not better. Secondly, with the thermostat removed, this should be one less impediment to the water continuing to circulate and gradually slow down after the pump shuts off. Any obstructions in the flow that would cause water hammer would also significantly impede the flow and you observed the flow is good through the rad. Lastly, because you said it happens also while the engine is running, during this time the water is continuously being pumped around and so no chance of abrupt stoppage and resulting water hammer.
The problem IMO lies elsewhere. Some excessive clearance issue with lifter pushrods for example tapping away, perhaps even stuck lifters/valves on top of that causing the hard tap at the final rotation. But not likely water hammer regardless. Good luck, maybe as OK4450 has alluded to, the root cause here is also is the reason for the low(er) compression readings…
Just had a final thought- shaved heads? Piston clearance issue??
Common sense asked what this was in: 79 Ford Bronco
Valves were ordered last at the machine shop after all work was done and measured. Originals purchased were wrong I understand. Head gaskets are marked which end goes up and all cooling passages were open. There are no hot spots externally and the engine is running cool. Compression test was taken before the engine was broken in, haven’t taken one lately but my bet it is fine it runs really strong. The engine only ran 10 minutes the first time and it started to make the noise that is why we stopped and checked exhaust gasses, compression and water pressure leak down.
Used a stethoscope on the engine yesterday. Checked the passages below on both sides and the crossovers and there was no noise. The noise is coming from the water pump. The lower hose has a pulsation in it as well as the 2 heater hoses. This noise in the water pump is identical to water hammer you hear in a steam radiator.
Going to pull the water pump to see if there is something wrong like the vanes installed backwards. I know the pump is working as there is good flow through the radiator and when the engine is revved up the level goes down as it should but it seems like there is cavitation inside the water pump. Going to change one of the heater hoses to a clear line to see what is coming out of the radiator core. Wouldn’t be the first time something was made wrong but it is probably a stretch.
I have been told by various people to check the heater core as this has been an issue in the past with these engines. It is about the highest spot in this vehicle, if not the highest. Yesterday I used a vacuum pump on the engine though (blocked off the radiator) and pulled a couple of gallons of fluid out without any air. Would have thought that would have cleared the heater core guess won’t know until I view whats coming thru it or bypass all together.
Again thanks for taking the time to respond the saga continues!
Ten minutes is plenty enough time to break in an engine and if you have 80 PSI on each cylinder after that 10 minutes then there is a problem.
Ring issue, valve stem height, cam out of time, or pushrods too long after shaving the heads and block; I have no idea but there is an issue.
Eighty PSI sucks and sucks badly if the compression gauge is correct. The rule of thumb on compression is 20 X the compression ratio.
A '79 would be a lower compression smog motor and even in stock form should be in the 180 range. Shaving the heads and block should bump that number up even higher.
I’m curious as to what a vacuum gauge reading would be.
I believe that the heads were ported to the intake crossover at the front and the gaskets could be reversed to block that port. And despite the gaskets being marked TOP, BOTTOM, REAR, etc., they can be incorrectly installed and result in the problem you are having… Just saying.
If I remember correctly there is a plate that is supposed to be installed between the water pump and timing cover that covers the impellers. Make sure it’s there and installed properly.
I believe the culprit was found, won’t know for sure until I take it out today but I let it idle to get hot twice and it did not “hammer.”
I found the clamp on the suction side of the heater core at the pump was not tightened, was just hanging loose. What I don’t understand is how it held pressure when we pumped up the radiator to 15#'s. Needless to say I tried it twice and on the second time I relieved the pressure on the radiator after a couple of minutes and the hammering started, then stopped if I flipped the lever down on the stat.
Going to check the compression today again as we are going to reset everything and lean it out. Will be using the vacuum gauge to do this.
