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The Big Burp


Larry Brown
Well... It appears that the total sum of my overheating problems was a large air bubble in the chamber on the front of the motor! The first overheat was probably due to the filter being clogged and after it was cleaned every subsequent failed test was probably due to the lack of a burp. With all these Perkins 4-108 owners I'd think that would be everyone's first guess. Don't you guys have to do this on yours? I guess it must be one of those things that everyone thinks is too simple and that I would certainly have known this. ENGINES SHOULDN'T HAVE TO BE BURPED! ;-)

I'm looking at the bright side... At least I don't have to put it over my shoulder and tap it on the back...

For those unfortunate souls out there who aren't in the know as I myself was not... After an overheat on the 4-108 you should loosen the plate along the rectangular cylinder in the front of the engine along the top left. Fill the exchanger with its fresh water and coolant mix, close the cap on the exchanger and start the engine. Air will slowly escape from the plate until fresh coolant starts exiting. As the coolant starts to escape tighten that plate back down and shut down the engine. The exchanger should be cool enough to open the fresh water cap again. Obviously, if the exchanger is fairly hot you should let it cool down etc. You should find you can now fill another 1/4 to 1/3 gallon of water/coolant mix.

The plate I'm describing is on the apparatus labeled Perkins above the raw and fresh water pumps. The plate has three bolts. Mine has a valve, but I didn't feel it was working and just loosened the bolts. It also took some time for the water to exit and I was wondering if it was supposed to. It did finally come out as a friend had described.

I certainly don't want to jeopardise anyone by describing how to do this on line. If any of you old salts out there know how people can screw this up based on my poor attempt at describing it, please jump in.



Larry Dyer
FWIW: I've had to burp the plumbing from the engine to the hot water heater, but never the engine itself. Just run it a while with the cap off the heat exchanger, and keep adding coolant. In this same thread, it took me a couple of years (with another boat) to track down an overheat problem. The P.O had made the transition between two different hose diameters with a PVC fitting. Repeated tightening of the hose clamps had the ID of this to about 1/4"
Glad you solved your problem.


Terence Thatcher
Thanks Larry. and don't feel stupid. I have run Perkins engines since I was a kid, and I didn't know about burping. I have been lucky all these years, I guess.


Hey Larry,
Will running the Perkins with the header tank cap off achieve the burp? In your post it said "heat exchanger".
Thanks in advance,


Dave Ahlers
I have had to unscrew the rear temp sender (or warning light) at the aft most part of the head *when refilling* coolant - cold. Otherwise, it would airlock and quickly over heat. I had the header tank in the front of the engine.