Holding fuel pressure overnight, a couple Q's
Posted by Earendil
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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? ----------------------------------------------------------------------------------- 1989 - E30 - M20 - Manual. Approximately 270,000 miles 2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles 
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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. ----------------------------------------------------------------------------------- 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 12:11PM by Earendil. |
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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 
I think the leak down test is a good idea just don't let it drive you crazy B)- Rick |
Well I'm far from just losing a few pounds 
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.
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.
Dang, I wish you had told me not to go crazy sooner  ----------------------------------------------------------------------------------- 1989 - E30 - M20 - Manual. Approximately 270,000 miles 2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles
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Not letting these things rule you is easy to say, much harder to live....however B) Rick |
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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 ----------------------------------------------------------------------------------- 1989 - E30 - M20 - Manual. Approximately 270,000 miles 2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles
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