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I will be keeping an eye on the other Grampian Forums and adding here any interesting information regarding engine questions and problems (inboard and outboard) from these other sites plus any information you want to provide on engines that others might find of interest. Please contact the Webmaster with any suggestions. 


  1. Atomic 4 Water Pumps
  2. Water Pump Impeller Replacement
  3. Sticking Valves






  1. Atomic 4 Water Pumps

Here are some questions and suggestions regarding the water pump on an Atomic engine.


The Atomic4 on my Grampian 30 overheated on its first day in the water this year.I started with first replacing the rubber impeller which had two vanes broken off that we stuck in the water pumps outlet.

This did not solve the problem.

Then I cleaned out the thermostat Housing (it was full of black sludge) and replaced the thermostat.

This made the engine run at 160 at idle and very low load. But once load was slightly increased the engine overheated.

Next step I acid flushed the engine as per Moyer Marine's manual's recommendation and afterwards hooked up a pressurized hose in the marina to my T fitting which attaches right next to the through hull fitting at the beginning of the system and flushed the system this way.

After this flushing it took the engine a while to start up but now the waterflow at idle is greatly improved and the engine temperature remains stable at medium and slightly higher loads with a fairly strong flow of water coming out of the exhaust.

However, if I really crank up the engine and increase the load on the engine suddenly to high rpms the flow of water is reduced, the engine temperature rises rapidly and steam starts coming out the exhaust. If I reduce the load to low or idle the temperature drops quickly to 160 and if I then increase the load to low medium it drops further to 150 area.

I ordered as well Moyer Marine's by-pass kit and a new thermostat housing but I am hesitant to put it in as I feel it might not solve the problem. Since the overall flow of water gets reduced at higher rpms it seems not to be a problem of water not finding its way through the head.

Does this problem sound familiar to anyone? Is it possible that the waterpump capacity gets somehow reduced at higher loads?


A few comments (assuming you have a late model A4):

1. Connecting tap water pressure to the inlet of the pump might have caused some damage to the pump. If you re-read Moyer's manual, you'll note he describes connecting the hose to the 1/8" pipe just forward of the alternator pulley. Also, most experienced A4 people recommend 'pressure flushing' the block/head combo and the manifold separately - and in both directions. And pinch off the bypass (with a padded vice-grip) when 'pressure flushing' the block/head.

2. Was the thermostat removed for this 'pressure flushing'? If not, I'd suggest removing it to check for debris under the stat.

3. The water pump flow normally would increase with increasing rpm but ...

4. The flow you are observing at the exhaust is the combination of the bypass flow and the cooling flow thru the block and head. The reduction at higher loads suggests to me that the thermostat is starting to block the bypass (as intended) but there is probably some blockage in the block and/or head resulting in lower cooling flow than intended.

5. Since you have already acid flushed, I'd recommend that you 'pressure flush' again, in both directions and do the block/head separately from the manifold. Bypass pinched, stat removed. ALSO, DO NOT connect the manifold water outlet to the usual exhaust connection when you flush. Run it overboard through a separate hose. If you run the water through the exhaust, you run the serious risk of having it back up into the engine and enter one of the cylinders thru an open exhaust valve. [This can also happen if you crank the engine for extended periods with the normal water inlet open.]

6. A word of caution (tho' not likely your current problem): it is fairly common for flakes of rust to accumulate at the outlet of the manifold after flushing (acid or not). But this restricts the total flow, not just the cooling flow.

7. Lastly, I'd strongly recommend that you join the Atomic 4 list which has 400 + members and many with decades of A4 and other engine experience.(Bob Wieber)

I have a Volvo in my G30, and some years ago I had same problem you are going through now.

I tried every solution possible, with no results.

Then  when I changed the impeller again, I lost the thin seal of the cap, so I applied a very thin coat of silicon for hot junctions. Believe it or not, the problem was solved and it  is still working perfectly  today with the same pomp and impeller.

Here is the reason:

The old pumpís cap was shredded (microns) because of the contact with the impeller and lost the "hermetisism", (to be hermetic) so, when you increased the load you lost the capability of suction and pumping.

When I  replaced the seal and changed it for a very thin silicon coat (for hot junctions) the impeller  recover ed the hermetism (to be hermetic). (Ruben Castillo )

Check and see if there is a heat exchanger in the starboard storage locker bolted in next to the wall of the ice box.
If there is not one you are most likely 100% raw water cooled.
If there is one then you most likely have a raw water/ fresh water system, and the heat exchanger works like a radiator.  The fresh water circulates through the engine for cooling and then back to the heat exchanger. The raw water circulates through the heat exchanger as well, drawing the heat of of the fresh water... then the raw water exits thru the tailpipe port.
If this is the case, check the water level in that and replace that radiator cap. If the water level is low, fill it, run the engine with the cap off and look to see if there is water circulation in the exchanger. You should be able to see it moving very obviously once the engine warms up. If it is not circulating, you many need to look at the fresh water pump on the back side of the engine.
I say "look" with tongue firmly imbedded in cheek. It is on the stern end of the engine on the starboard side of the transmission housing...and you need eyes on the ends of your fingers to remove the cover gasket, replace the impeller and clean the water inlet and outlets of rubber debris.
I had the same problem when i bought my G30 15 years ago and it took me forever to find the problem. once that impeller was replaced it worked like a champ

Are you using the original propeller ? Changing it can have serious consequences... like overheating & is it possible the either there is debris in the strainer or that the raw water intake is collapsing at high draw? (John Waddell)


  1. Water Pump Impeller Replacement

Any boat engine needs to have the water pump impeller replaced every 3-4 years. If you do it before any of the rubber fins break off its pretty simple. If any parts came off and you don't find them in the water pump housing, there is a chance they have become lodged in the engine and can create a blockage and over-heating after your impeller has been replaced.

Lay a large folded up towel or blanket on top of the engine and go into the engine compartment from the inside of the boat, head first, laying your chest on the towel. It's tight work, but possible. You can temporarily remove the distributor cap to give you more visibility to work. The other alternative is to go in from the back using the opening to the engine compartment from the starboard cockpit seat compartment.

I recommend replacing the 4 standard Philips head screws that hold the water pump cover on with stainless torx head screws. They are easier to get tight, so the cover won't leak, and back out the next time, especially at the bad working angle. (Jeremy Thompson)

It is rather unlikely that the cam will need replacing. However, when you remove the cover, just use a vise grip to grab the impeller and pull it out along with its drive shaft. Then you replace the impeller and reverse the process. It is a rather tight fit so be persistent. And the drive slot must be aligned with its drive pin. If you have trouble with that, the entire pump can be removed using a long extender on a socket wrench. But it is a bit tricky to reinstall the lower bolt.

Don Moyer at Moyer Marine offers a longer bolt to avoid that problem. See www.moyermarine.com. Also, if you don't have his manual, get it. It's the best investment you can make. Also, if you are not on the Atomic 4 list, I recommend you join it. It is a very active list with very knowledgeable members. (Bob Wieber) (Back)


  1. Sticking Valves

Got a call this week from a friend with  Atomic IV  trouble.

After a motor run, he could not restart. The engine would turn over rapidly as if the flywheel was not connected to the crankshaft.  Had spark, therefore the rotor was turning, i.e. not broken crankshaft. Found 3 valves stuck open. Removed the spark plugs to view the valves. Sprayed carburetor cleaner and pushed the valves down manually (carefully) with a screwdriver via the plug holes and rolled her over by hand several times on the advice of Moyer Marine. Valves freed up immediately and running fine now.

Moyer mentioned that with elevated exhaust pressures they have been seeing increased glazing on intake valves. Also a result of unleaded fuel I believe, leaded fuel was important for valve stem lubrication. I think he will be obtaining some Marvel Mystery oil additive from Moyer Marine as well. Added some valve cleaner to the fuel in the meantime. Thanks to the people at Moyer Marine for their phone help. Thought I'd pass this on, I understand this is not uncommon on Atomic IVs. (Greg Seaman)  (Back)