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Old 26-11-2014, 12:40   #1
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What pressure to test coolant system?

I'm trying to track down a possible leak in my Perkins 4-108 freshwater cooled diesel engine. The radiator cap is marked "7 lbs". I've rented a coolant pressure tester and I'm testing it at 7.5 psi. So far no change (and it's been an hour). Is 7.5 psi enough? How long should I continue the test for?

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Old 26-11-2014, 13:55   #2
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Re: What pressure to test coolant system?

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Originally Posted by prof_mariner View Post
I'm trying to track down a possible leak in my Perkins 4-108 freshwater cooled diesel engine. The radiator cap is marked "7 lbs". I've rented a coolant pressure tester and I'm testing it at 7.5 psi. So far no change (and it's been an hour). Is 7.5 psi enough? How long should I continue the test for?

Thanks
I think it's to much. I think the caps are supposed to be about 4psi. Try running the engine at the same time. Get some heat in the system.
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Old 26-11-2014, 14:01   #3
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Re: What pressure to test coolant system?

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I think it's to much. I think the caps are supposed to about 4psi. Try running the engine at the same time. Get some heat in the system.
I'm pretty sure 7 psi is the right cap. I don't know if running the engine and getting it hot without a safety release is a good idea. Plus it increases the risk of getting coolant in my tester.

EDIT: Yup, item #2 in the safety section of the owner's manual says not to use on a hot or running engine.

Two and a half hours now, still holding 7.5 psi. It's looking like I don't have a coolant leak so it may be time to check to see if coolant is leaking into the cylinders.
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Old 26-11-2014, 14:05   #4
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Re: What pressure to test coolant system?

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Originally Posted by prof_mariner View Post
I'm pretty sure 7 psi is the right cap. I don't know if running the engine and getting it hot without a safety release is a good idea. Plus it increases the risk of getting coolant in my tester.

EDIT: Yup, item #2 in the safety section of the owner's manual says not to use on a hot or running engine.

Two and a half hours now, still holding 7.5 psi. It's looking like I don't have a coolant leak so it may be time to check to see if coolant is leaking into the cylinders.
Ok , get it warm then test it. A head gasket might not leak cold.
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Old 26-11-2014, 14:07   #5
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Re: What pressure to test coolant system?

Sounds like you don't have a leak. Remember to test the radiator cap itself as well.
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Old 26-11-2014, 17:00   #6
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Re: What pressure to test coolant system?

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Sounds like you don't have a leak. Remember to test the radiator cap itself as well.
I will, but I'm not seeing coolant coming out of the cap so I assumed it's OK. I also did a quick compression test (by loosening the hp fuel line to each injector) of all 4 cylinders and they seem to be working fine. I'll run the engine hard for an hour or so tomorrow and see what happens.

I'm starting to think that maybe the steam in the exhaust may be from water in the fuel. That would be a big relief...
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Old 26-11-2014, 17:24   #7
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Re: What pressure to test coolant system?

7.5 too high? Crickey, I just tested my land rover at 25 psi. Found about 3 leaks. But I would say 5 psi above the cap opening pressure would be fine.
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Old 26-11-2014, 17:36   #8
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Re: What pressure to test coolant system?

Steam won't come from water in the fuel. Water in fuel will usually damage injectors with catastrophic results. Most likely what you've got is a slightly reduced water flow in the exhaust allowing slightly higher than normal exhaust temps. Check your mixing elbow.


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Old 26-11-2014, 17:42   #9
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Re: What pressure to test coolant system?

So you have steam (white smoke?) coming from the exhaust? If so, most likely there is an issue with the raw water system, blocked intake, clogged strainer, bad raw water pump (impeller), clogged heat exchanger in that order. Is the engine overheating?
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Old 26-11-2014, 17:51   #10
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Re: What pressure to test coolant system?

And to reinforce what Sailmonkey said, check your mixing elbow.

Edit: and white "smoke" out the exhaust can be caused by unburnt fuel. You may have a problem with your injectors, even though the engine seemed to be effected equally. Your injectors could need servicing. When was the last time they were serviced?
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Old 26-11-2014, 17:53   #11
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Re: What pressure to test coolant system?

+1. If the white smoke is steam then likely a cooling system issue. Is the engine actually running hot? Is the exhaust water hot? Normally wet exhaust is cool to warm, if its not hot then its not steam.

As posted, its not water in the fuel...water getting to the injectors is usually catastrophically bad.
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Old 26-11-2014, 19:28   #12
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Re: What pressure to test coolant system?

Thanks for all the feedback, I think I've covered most of the areas I need to to diagnose the engine.

I first saw what looked like steam when I ran it early one windless morning. It started to appear at about 1500 rpm and got worse at 2000+ rpm. I couldn't really see what the coolant level because I have no access from above the engine (it's under the galley sink and counter). I only ran the engine for maybe 30 minutes, but during that time it did not run hot or overheat.

Here's what I did a few weeks ago:

1. I pulled the heat exchanger and had the core cleaned and pressure tested.
2. I re-assembled the heat exchanger with all new rubber boots, hose clamps, hoses, etc.
3. While I had the heat exchanger apart I pulled the mixing elbow and cleaned it. Then I remounted it with a new gasket.

I then ran the engine in the slip up to 1500 rpms and didn't see any steam. The exhaust water is warm, not hot and the engine ran at normal temps.

Yesterday I dismantled the galley in order to gain better access to the engine. So today:

1. I rented a cooling system pressure tester and pressurized the cooling system to 7 psi. After 2.5 hours no change.

2. I cracked each of the hp fuel lines to the fuel injectors and noted a sudden drop in RPMs with each.

My plan for tomorrow:

1. Check the coolant level to see if it has changed.
2. Run the engine hard and under load for at least an hour to see if the coolant level changes and/or if the engine temp rises above normal.

I beginning to think (and hope) that it's either: a) steam caused by hot exhaust hitting moist cool air, or b) unburned fuel (probably caused by dirty fuel injectors or a thermostat that's stuck open). Either of those are a lot easier/cheaper to fix than a blown head gasket.
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Old 26-11-2014, 19:59   #13
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Re: What pressure to test coolant system?

What is the air temperature? If the air is cool, it could just be condensing vapor. Like a car exhaust on a cool morning.
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Old 27-11-2014, 05:34   #14
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Re: What pressure to test coolant system?

I would bet on injectors or just air temp differential. If its not running hot then can't see how it could be steam.
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Old 27-11-2014, 06:10   #15
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Re: What pressure to test coolant system?

You wrote possible water leak. That by itself raises more questions. What is actually happening?

As far as testing.......7 PSI won't hurt anything as long as the cap is rated for holding it. If the sealing surfaces/gaskets can't handle 7 PSI.......you have problems. A couple hours of steady pressure should be enough for investigating.

Perkins cap pressures vary depending on application, and what type of closed cooling system they are using on that particular engine. I have seen a few different closed systems used on the 108's.

I have worked on some Perkins with caps as low as 4 PSI, and as high as 15 PSI. Most people have no idea what cap pressure they are using........or why it is important to begin with.
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