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Engine Discussion of preventative/corrective maintenance and other technical issues regarding your coach's engine.

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Old 09-25-2011
DW SD DW SD is offline
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Default Racor Gauge - holds vacuum when engine shutoff?

After reading about 20 threads without the answer, I thought I'd post.

Why does the racor vacuum gauge not drop to zero when the engine is off?

It stays at 3 in HG when the engine is off. I pulled the gauge, separated the vacuum line from the back of the gauge and the gauge dropped to zero. I think there is a check valve in there. True?

After changing all three filters (Racor + 2 spin - on) within the last 200 miles, I still get 5 in HG of vacuum at idle and up to about 7 in HG under load. this is not quite into the yellow, but still seems high to me, given brand new filters.

Are my readings normal?

Doug
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Last edited by DW SD; 09-26-2011 at 09:31 AM. Reason: OOOOPS - had the wrong units. Correct to in. HG
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Old 09-25-2011
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Quote:
Originally Posted by DW SD View Post
After reading about 20 threads without the answer, I thought I'd post.

Why does the racor vacuum gauge not drop to zero when the engine is off?

It stays at 3 psi when the engine is off. I pulled the gauge, separated the vacuum line from the back of the gauge and the gauge dropped to zero. I think there is a check valve in there. True?

After changing all three filters (Racor + 2 spin - on) within the last 200 miles, I still get 5psi of vacuum at idle and up to about 7 psi under load. this is not quite into the yellow, but still seems high to me, given brand new filters.

Are my readings normal?

Doug
Doug,
Those are exactly the readings I am getting on my 80FC31SB, including the
3, (I believe it is inches of mercury vacuum, at least that is what it says on my gauge). since I fixed an old cracked fuel line that was leaking air a couple weeks ago.
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Mesa, AZ
1980 FC 31 SB

THE SHORT BUS
(She's Just a Little Slow)
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Old 09-25-2011
rrueckwald rrueckwald is offline
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Hey, you guys are on to something. My gauge used to read around 3, but about a year ago it slowly decreased to about 1. I changed the filter with no change on the gauge. From your post it looks like I'm sucking air. Dave, where did you find the leak?

I also found the Racor was 1/3 air, another clue. If the air leak is before the Raycor, the gauge should not be affected. It must be on the outlet side of the Racor. Maybe the lid gaskets, except they are all new. Puzzling. Seems like I ought to find that one before the trip to RATS. Any help would be appreciated.
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Old 09-25-2011
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Turbokitty Turbokitty is offline
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On my FC the 3 psi is the residual system pressure caught between the lift pump and the fuel cut off solenoid upon shutdown. The lift pump will not be energized until oil pressure is sensed. 2 stroke Detroits don't have a remote electric lift pump though? Is the 3 PSI the static head pressure left in the system between the injection rack and the fuel tank via a check valve?
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Timberlake Products Group, Inc.
1985 PT36
1983 FC35RB Left me for Atlanta!
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Last edited by Turbokitty; 09-25-2011 at 10:06 PM.
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Old 09-25-2011
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Originally Posted by rrueckwald View Post
Hey, you guys are on to something. My gauge used to read around 3, but about a year ago it slowly decreased to about 1. I changed the filter with no change on the gauge. From your post it looks like I'm sucking air. Dave, where did you find the leak?

I also found the Racor was 1/3 air, another clue. If the air leak is before the Raycor, the gauge should not be affected. It must be on the outlet side of the Racor. Maybe the lid gaskets, except they are all new. Puzzling. Seems like I ought to find that one before the trip to RATS. Any help would be appreciated.
Ron,
My leak was a bunch of cracks around the end of a hose just before the hand pump on the Cat 3208. Since you have a later unit I would look for cracks in any rubber fuel line.
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Mesa, AZ
1980 FC 31 SB

THE SHORT BUS
(She's Just a Little Slow)
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Old 09-26-2011
Clueless Clueless is offline
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Quote:
Originally Posted by rrueckwald View Post
Hey, you guys are on to something. My gauge used to read around 3, but about a year ago it slowly decreased to about 1. I changed the filter with no change on the gauge. From your post it looks like I'm sucking air. Dave, where did you find the leak?

I also found the Racor was 1/3 air, another clue. If the air leak is before the Raycor, the gauge should not be affected. It must be on the outlet side of the Racor. Maybe the lid gaskets, except they are all new. Puzzling. Seems like I ought to find that one before the trip to RATS. Any help would be appreciated.
The racor having air in it sounds similar to what John Hall was experiencing when we were all in Frankenmuth. I think we saw the air coming in by holding a light on or behind the filter while revving the engine. Bill Pape furnished some magic thread dope.

Hopefully John or Bill will chime in.

Call me Ron if you need help lookin'
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82 FC 33 SB SOLD
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Old 09-26-2011
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With the help of a bright flashlight and younger eyes than David's or mine, we were able to see bubbles through the dark glass. I had previously changed the fuel line and sealed the terminals that penetrated the glass, but the pipe dope solved the problem.
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1988 Narrow PT40
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Old 09-26-2011
rjtoles rjtoles is offline
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The gauge in our coach was disconnected and capped off. I recently put a new one in, but not sure what the readings should be for "normal". When I run the engine it will increase in reading depending in the engine acceleration up to a point that will take it into the yellow at times is this normal? The filters on the coach were replaced about 3500 miles ago could they need replacing already?
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Old 09-26-2011
DW SD DW SD is offline
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Thanks to all for weighing in. Are others with 80s PTs seeing the same #s as me? Suspect it is a good sign that that Dave Ward sees similar on his FC. However, the engines are quite different and even fuel systems different, too.

Note, I made the mistake of not reading the units on the gauge properly and adjusted my first post to in HG.

It does sound like Ron has a leak in his fuel system. Certainly, I would guess at a fitting. Have you tried tightening fittings adjacent to the racor separator? Or using pipe dope there? From what I can see, some are compression fittings and some are pipe. I'd only put dope on the pipe fittings. The compression just check with a firm hand and your combination wrench.

Thank you,

Doug
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2004 M380
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Old 09-26-2011
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Bill Pape Bill Pape is offline
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The last two rally repairs were on a 6v & an 8V, replaced the fuel line to the Racor, then cleaned all fittings and used the pipe dope available at any plumbing store, Good for high temp, gas,water, fuel. Check inside the brass fittings for hair line cracks. careful wrenching on that last full turn , it can always be re-tightened later if still leaking.
The racor filter is always a high suspect for leaks, replace gaskets. Use a light behind the racor bowl, sometimes better in the dark, check for tiny bubbled especially around the wire connectors & drain valve. Some owners replace the Racor with a disposable spin-on filter.
Our 88 FC racor gage holds a vacuum when parked, a slight positive pressure when idling, and yellow under heavy load.
We use a 10 micron Blue racor filter and a 2 micron Cat filter on the 3208.
I do not know what the DD racor gage reads, and some gages only read a vacuum.
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