Vacuum at Primary Filter

[lpdsn]

Over the weekend, I fitted a vacuum gauge between the primary filter and lift pump. It was just a cheap one aimed at the boy racer (land-based) market, but all seems well and it looks pretty good.

Anyway, the question is, what sort of vacuum should I expect at this point in the system?

At full revs, tied up, I'm getting about 5lbs/sq in. However, there was one blip where it slowly rose to 10lbs/sq in over about 90 seconds before falling back quickly to 5. No change in engine revs. 4 Cyl Yanmar.

I have been experiencing problems with blocked filters. It certainly isn't diesel bug as the filters look very clean. I would use a lot of white diesel from south of the border (often works out cheaper than 60:40) as well as red, so I'm obviously suspicious that there is some truth in the stories of bio diesel blocking copper pipes, and this eventually clogging up the filter. I've a run of about 6-7 meters from tank to primary filter.

There's no issue with the lift pump & secondary filter - if I connect the pipe to the lift pump to a can (as has occasionally been necessary) it can empty a 20l can in about 30 mins at normal revs, which means well over 90% is going down the return pipe.

 

[pvb]

Generally, 5lbs/sq in would indicate a filter starting to block, or perhaps a filter wrongly sized for the overall fuel flow (your can experiment suggests 40 litres/hr, and a regular CAV 296 filter is rated at max 45 litres/hr).

 

[lpdsn]

Generally, 5lbs/sq in would indicate a filter starting to block, or perhaps a filter wrongly sized for the overall fuel flow (your can experiment suggests 40 litres/hr, and a regular CAV 296 filter is rated at max 45 litres/hr).

Thanks. That's very helpful. I only need 6litres/hr at full throttle according to the engine manual - actual consumption at cruising revs seems to be about 2-3 litres/hour.

So far it is consistent with the theory that something is forming in the copper pipes - tank seems pretty clean. Very plausible that the filter has started to block, or the pipes aren't clear.

I've overdosed a bit with Marine 16 Diesel Fuel Complete, so I'm hoping that cleans out the pipes at the cost of only a filter or two.

Do you have any suggestions at what vacuum I should change the primary filter as a preventative measure? I'm sure I'll eventually find my own figure through trial an error, but easier to learn from others.

 

[pvb]

Thanks. That's very helpful. I only need 6litres/hr at full throttle according to the engine manual - actual consumption at cruising revs seems to be about 2-3 litres/hour.

That's just what the engine burns. But you also need to add on the amount that is sent back to the tank - it all comes through the filter.

Do you have any suggestions at what vacuum I should change the primary filter as a preventative measure? I'm sure I'll eventually find my own figure through trial an error, but easier to learn from others.

I'd think 8 lbs/sq in.

 

[lpdsn]

Thanks.