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Holding fuel pressure overnight, a couple Q's

Posted by Earendil 
While I sit here bored, waiting for parts to arive, and poor Bev up on blocks, I'm looking for things to do. So something I haven't done in a while because it (lifestyle wise) was a pain in the neck, is to put a fuel pressure gauge on the fuel line and watch the pressure over a period of time to see what it does.

So I hook this all up, the system pressurizes to exactly 43psi (spec = 43.5 +/- 0.9psi). Good start to the test. Now I put a timer on it and walk off to grab the bentley to see what the time frame and pressure is I'm looking for. The bentley doesn't have this test. Okay... I go looking on the internet for the specs... nothing. I search this forum....nothing.

Have I gone crazy? I could have swore that how well your system holds pressure was a real test, and is indicative of certain problems.
The problem I suspect right now is leaky injectors, but they've never been a big enough problem to mess with while so many other things are on the plate.

Thoughts?

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1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

10 views and no responses? How unlike you guys. Go ahead and say it, I'm crazy ;-)

For the record, I went ahead and got some results.

with the system completely without modification but for a gauge stuck in right before the gas enters the fuel rail, I got the following:
Starting: 43psi
After 45: 38psi
After 2hr: 30psi

This got my thinking, with the system just as it is, I wouldn't know where I was loosing pressure. It could be back through the pump into the tank, out through the pressure regulator, through the injectors, or (God forbid) a leak in the line its self. So I ran the test again, this time clamping off the fuel line just behind the gauge (blocking the pump) and right after the pressure regulator. This leaves the fuel to leak out through the injectors, or a location that I can visibly check.

Due to life, I let this one sit a lot longer.
Started with 43psi, but after clamping it increased to 46psi.
After 2 hours: 33psi
After 8 hours: 16psi

Remembering of course that I started 3 psi higher than the previous test, this would imply that all leaking is through the injectors.

Since I was concerned about damaging my fuel line by clamping them, I didn't clamp real hard. However after 10 hours of clamping and no visible marks, I think I'll run the test again, clamp them harder to ensure nothing is slipping past, and see if I get the same results.

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1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles




Edited 1 time(s). Last edit at 08/06/2009 02:11PM by Earendil.
rkj
I started doing this leak down test many years ago with my daughters 84 325, at 230k the pressures held within a few pounds overnight and my new car, the 88 325is lost quite a bit more, say half or close to but I know at 220k my injectors are due to be replaced.

Don't forget to plug the vacuum line going to the regulator; you should see a rise in pressure of 5 pounds or so (I forgot the specs, they're in the book). I even left the fuel gauge on and took the cars for rides to see what the road pressures were smiling bouncing smiley

I think the leak down test is a good idea just don't let it drive you crazy B)-

Rick
Quote
rkj
I started doing this leak down test many years ago with my daughters 84 325, at 230k the pressures held within a few pounds overnight and my new car, the 88 325is lost quite a bit more, say half or close to but I know at 220k my injectors are due to be replaced.

Well I'm far from just losing a few pounds smiling smiley
Checked it again with solid clamps. Started at 44 after clamping.
1 hour: 34
2 hours: 28
3 hours 26

so it certainly looks like my injectors are leaking, which would explain the longer starts in the morning, and excessive gas vapor for the first few seconds after starting in the morning.

Quote
rkj
Don't forget to plug the vacuum line going to the regulator; you should see a rise in pressure of 5 pounds or so (I forgot the specs, they're in the book). I even left the fuel gauge on and took the cars for rides to see what the road pressures were smiling bouncing smiley

Now you are talking about a with-engine-running test, yes? I don't think plugging the vaccum hose for a leak down test does anything :-)
I bought some extra fuel line hose which I figure can't hurt to have in the trunk, but also to extend my gauge to a place I can see it in the cockpit. The idea kind of scares me, but shouldn't be any more dangerous (if done right) then the hose already in the car.

Quote
rkj
I think the leak down test is a good idea just don't let it drive you crazy B)-

Rick

Dang, I wish you had told me not to go crazy sooner winking smiley

-----------------------------------------------------------------------------------
1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

rkj
Quote
Earendil
Quote
rkj
I started doing this leak down test many years ago with my daughters 84 325, at 230k the pressures held within a few pounds overnight and my new car, the 88 325is lost quite a bit more, say half or close to but I know at 220k my injectors are due to be replaced.

Well I'm far from just losing a few pounds smiling smiley
Checked it again with solid clamps. Started at 44 after clamping.
1 hour: 34
2 hours: 28
3 hours 26

so it certainly looks like my injectors are leaking, which would explain the longer starts in the morning, and excessive gas vapor for the first few seconds after starting in the morning.

smiling smiley Don't worry about the gas smell on cold start, I think all thirties do that. Your drop in pressures might even be alright.

The symptoms of leaky injectors is on hot starts, not cold ones (its been my experience); when you've been running your car and then stop it for lunch (just say) and then you come out to start up and it takes a lot of cranking to get fire'd and you see some black smoke out of the pipe, that's a signal the injectors are leaking.



Quote
rkj
Don't forget to plug the vacuum line going to the regulator; you should see a rise in pressure of 5 pounds or so (I forgot the specs, they're in the book). I even left the fuel gauge on and took the cars for rides to see what the road pressures were smiling bouncing smiley

Now you are talking about a with-engine-running test, yes? I don't think plugging the vaccum hose for a leak down test does anything :-)
I bought some extra fuel line hose which I figure can't hurt to have in the trunk, but also to extend my gauge to a place I can see it in the cockpit. The idea kind of scares me, but shouldn't be any more dangerous (if done right) then the hose already in the car.


smiling smiley Yes, blocking off the vacuum line to the FPR is a running test, that acts like WOT situation and you should see a small increase in pressure when the vacuum drops.

I just ran my gauge/line up out of the hood and put a wiper on it to hold it in place while I drove the car, with the hood a little open, around the block for some on the road readings. That shouldn't really be necessary but I was fishing for a running condition that turned out to be something else.
Quote
rkj
I think the leak down test is a good idea just don't let it drive you crazy B)-

Rick

Dang, I wish you had told me not to go crazy sooner winking smiley

Not letting these things rule you is easy to say, much harder to live....however B)

Rick
I dont mean to hijack your thread but what do you use to test the pressure and how do you splice it into the system with out causing a leak point. I always use a vacuum pump to measure such things, but would be afraid of damaging the pump with fuel. I would assume you are coming in on the fuel in side and have clamped the return line and as you said behind gauge on the tank side. I am interested bc I had the same problem and it turned out to be losing the pressure at the pump.
Thanks
There are actually "fuel pressure gauges" for sale out there. I don't know what else could be substituted, but the gauges and associated parts are less than I personally would have thought.
You can get everything from $20 ones where the consumer compaints are that fuel melts the plastic on the gauge making it unreadable, all the way up to more professional fancy $100-$200 ones. I'm sure if you wanted to throw $1000 at it you could find one ;-)

The gauge is placed between the fuel pump and the fuel rail. NOT between the pressure regulator and the tank.
Personally I just remove the hose from the fuel rail, add my own piece of hose, and insert the gauge in between.
If this isn't clear enough let me know, and we can start working with pictures/diagrams smiling smiley

-----------------------------------------------------------------------------------
1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

Thank you I now understand.
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