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Old 18-03-2021, 22:23   #1
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Injector pump vacuum gauge reading

I have a vacuum gauge installed on my engine’s fuel supply line between the injector pump and the secondary fuel filter. It reads about 4.5 inches of mercury ( which I think converts to about 2.2PSI) and doesn’t seem to change much with engine rpm. Is this level of vacuum normal? I cannot find any info anywhere on what the low pressure fuel vacuum should be.
The engine is a marinized 1980’s VW Jetta 4 cyl diesel with a Bosch injector pump and no lift pump other than the injector pump.
I am curious to know what the reading should be as I’m experiencing some difficulty in cold starting and one of the things my repair manual says to check is the fuel supply vacuum. But it doesn’t say what the actual vacuum pressure reading should be.
Also not clear to me is whether or not the low pressure side of the pump should remain pressurized when the engine is not running?
Thanks.
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Old 19-03-2021, 01:31   #2
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Re: Injector pump vacuum gauge reading

4.5”of mercury equates to slightly over 5 feet of water which in turn equates to 4 feet of diesel fuel ( approx) so if your tank is 4’ below the injector pump there’s no unexplained restriction that is causing that gauge reading and no way to change it other than installing an electric lift pump close to the tank top. If the tank is higher up then there may be a restriction to flow caused by the two suction filters.
I’d be happier with less vacuum .......but if the engine runs ok at 4.5”Hg and stays at that, why worry about changing it. Just be ready to do a quick filter change if it climbs above that reading though. Is there a manual priming pump on the Jetta?, how do you bleed the air when you change filters?
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Old 19-03-2021, 11:09   #3
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Re: Injector pump vacuum gauge reading

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Originally Posted by skipperpete View Post
4.5”of mercury equates to slightly over 5 feet of water which in turn equates to 4 feet of diesel fuel ( approx) so if your tank is 4’ below the injector pump there’s no unexplained restriction that is causing that gauge reading and no way to change it other than installing an electric lift pump close to the tank top. If the tank is higher up then there may be a restriction to flow caused by the two suction filters.
I’d be happier with less vacuum .......but if the engine runs ok at 4.5”Hg and stays at that, why worry about changing it. Just be ready to do a quick filter change if it climbs above that reading though. Is there a manual priming pump on the Jetta?, how do you bleed the air when you change filters?
Thanks Skipperpete. I’m not trying to change the vacuum, just trying to see if the current reading of 4.5 inches is “normal” for this type of engine and pump. My thinking is that if 4.5 was higher than it should be, then presumably there could be a restriction somewhere between the tank and the pump. If it were lower than it should be, this might indicate an air leak somewhere. My problem is I don’t know what it should be, so I don’t know if my reading of 4.5 indicates a problem or not.
FYI, the pump is about 24” above the bottom of the tank, so if what you’re saying above is correct, then I should have sufficient vacuum to feed the pump? Also, what prevents the fuel from draining back into the tank once the engine shuts off? Is there a check valve in the Racor filters?
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Old 19-03-2021, 11:15   #4
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Re: Injector pump vacuum gauge reading

"Is there a check valve in the Racor filters?"
My question: Is there an airleak in the system? If there is no air IN there is no fuel OUT.
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Old 19-03-2021, 21:01   #5
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Re: Injector pump vacuum gauge reading

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Originally Posted by sailormed View Post
"Is there a check valve in the Racor filters?"
My question: Is there an airleak in the system? If there is no air IN there is no fuel OUT.
My answer to your question is “I don’t know”. That is one of the possibilities I am trying to determine with the aid of my vacuum gauge.

Do you have an answer to the question I asked concerning a check valve in Racor filters?
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Old 19-03-2021, 22:12   #6
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Re: Injector pump vacuum gauge reading

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Originally Posted by skipperpete View Post
. Is there a manual priming pump on the Jetta?, how do you bleed the air when you change filters?
The only manual priming pumps are those on the top of the external Racor primary and secondary filters. The injector pump is self bleeding. Cranking the engine for 30 seconds or so after a fuel filter change purges any air in the system.

I gather from your previous response that you think 4.5 “ of mercury is high, suggesting there could be a restriction somewhere in the system. What should the pressure be and should it vary much with engine rpm? Thanks
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Old 19-03-2021, 22:29   #7
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Re: Injector pump vacuum gauge reading

Coming from a car, the Jetta injector pump is probably designed to function as it's own lift pump. But that's not what the usual diesel does. They have low pressure fuel delivered to the injector pump, not vacuum. If you're having starting problems, it's cheap to add a 12v fuel pump. And that adds the fuel pump valves that keep fuel from going back to the tank when the engine is shut down.
Having a vacuum in the fuel line after shut down is the same as trying to suck the fuel out of the Bosch pump. Any slight air leak will allow the fuel an easy time of escaping the injector pump.
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Old 19-03-2021, 22:30   #8
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Re: Injector pump vacuum gauge reading

This thread has got me curious - where is the fuel lift pimp situated or is your injection pump pulling a vacuum and you don't have a lift pump?

EDIT: cross posted with Lepke!
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Old 20-03-2021, 09:24   #9
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Re: Injector pump vacuum gauge reading

Quote:
Originally Posted by osprey877 View Post
I have a vacuum gauge installed on my engine’s fuel supply line between the injector pump and the secondary fuel filter. It reads about 4.5 inches of mercury ( which I think converts to about 2.2PSI) and doesn’t seem to change much with engine rpm. Is this level of vacuum normal? I cannot find any info anywhere on what the low pressure fuel vacuum should be.
The engine is a marinized 1980’s VW Jetta 4 cyl diesel with a Bosch injector pump and no lift pump other than the injector pump.
I am curious to know what the reading should be as I’m experiencing some difficulty in cold starting and one of the things my repair manual says to check is the fuel supply vacuum. But it doesn’t say what the actual vacuum pressure reading should be.
Also not clear to me is whether or not the low pressure side of the pump should remain pressurized when the engine is not running?
Thanks.
I'm old enough to speak 80's VW Diesel. The Bosch pump has an internal lift pump. What vacuum reading you have depends on how far below the pump your tank is. I'd change the filters, blow back the line and see what kind of vacuum you have as a base line. BUT you'll need to either block off the vent and gently pressurize the tank to get fuel back up to the filter OR pour some fuel down the line from the filter. It's also helpful to fill the filter with either clean diesel fuel or better Lubro Moly Diesel Purge. It will take cranking forever to get the line and filter filled back up with fuel unless you prime it somehow. With clean filters and line use the vacuum reading as your base. Anything about 2" above your base would make me want to look at changing the filter. The rest pressure is zero unless there is a check valve in the filter which they usually don't have.

Another way to do it is spray something like spray oil (WD 40) into the intake and run the engine on that. I do not advise doing this unless you have some experience.

If you have starting issues look at the glow plugs or clean your injectors (Diesel Purge). The lift or low pressure is usually not an issue as the pump will run on what's in it when it shut off and will run the engine for a few seconds.

As others pointed put you can install an electric lift pump if you tank is more than about 3' below your engine. A typical auto installation lifts the fuel less than 2'.
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Old 20-03-2021, 10:00   #10
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Re: Injector pump vacuum gauge reading

You also might have a bad glow plug. That will cause the hard starting. Easiest way to check the glow plugs is with an infrared thermometer. With a cold engine activate the glow plugs. Then use the infrared dot and aim it at the base of each glow plug. Glow plugs are good or bad there's no in between. Any temp rise at all means the glow plug is OK.
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Old 20-03-2021, 20:03   #11
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Re: Injector pump vacuum gauge reading

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Originally Posted by osprey877 View Post
The only manual priming pumps are those on the top of the external Racor primary and secondary filters. The injector pump is self bleeding. Cranking the engine for 30 seconds or so after a fuel filter change purges any air in the system.



I gather from your previous response that you think 4.5 “ of mercury is high, suggesting there could be a restriction somewhere in the system. What should the pressure be and should it vary much with engine rpm? Thanks


Scubaseas and Lepke both clearly defined the problem re the VE pump suction/ feed and the logical solutions to the “hard starting from cold”
There was a post on this forum where a Racor filter with the inbuilt manual priming pump had a blocked internal non return valve but I had no luck relocating that thread.
Yes I think 4.5” hg is a bit high and would prefer zero to maybe half what you have on the gauge ( you won’t ever get less than the lift height without an electric supply pump or a day tank) but Bosch do point out that the VE injector pump needs no additional feed pump in automotive applications.
Pete.
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Old 20-03-2021, 20:07   #12
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Re: Injector pump vacuum gauge reading

Oops, here’s a screen shot of the Non return valveClick image for larger version

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