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At idle, I am getting a consistent ~32 psi on my fuel pressure gauge;

When I step on the gas, pressure drops to 28 psi.

Is this normal?

Ok....... my car revs fine, but idles strange I'm trying to fix.
I've gone through almost everything..

I'm this _ close to having this car run right.

Sometimes at idle its nice and quiet, other times its got a bad case of the grumbles. Only thing I can think of is a faulty fuel pressure regulator, but my readings are consistent, not erratic, so I'm *stumped*.

{EDIT: I would change the name of this post topic if I could.
My car's idle is smooth in so far as my RPMS are pin straight on the tach. There is no jumping around. But sometimes the engine shakes and the car sounds DIESEL-ISH.
Other times its quieter. I really don't get it at all. Timing is also spot on to factory spec. Removed all EGR stuff so not sure if it makes a difference.}

I checked, and there is no fuel in the FPR's vacuum tube.

When I pull the vacuum line off, my fuel pressure goes to about 38 psi.

When I rev the engine, the pressure is constant at 38 psi.

When I connect the tube, and press the throttle, my pressure drops to 28 psi while I'm holding the throttle down.

Any input much appreciated!!

Also, does anybody know how to bench test the vacuum limiter aka deceleration valve? I think its stuck close, but not sure. In any event, this item can't be causing a rough idle, can it?

Please help!

I think I read somewhere that my pressure should be going up.

see http://forums.bimmerforums.com/forum/showthread.php?1818923-M30-Fuel-pressure-test-results

or

http://www.hiperformancestore.com/Motronic.htm

They're for motronic engines but the basics are the same.

...tom



-----Original Message-----
From: CBradio <gabynyc@hotmail.com>
To: tech_forum <tech_forum@firstfives.org>
Sent: Thu, Jun 5, 2014 5:30 pm
Subject: Re: Fuel pressure question

#AOLMsgPart_1_e267cf0f-d2db-4898-85ad-8cfed2b64b21 td{color: black;} Ok....... my car revs fine, but idles strange I'm trying to fix.
I've gone through almost everything..

I'm this _ close to having this car run right.

Sometimes at idle its nice and quiet, other times its got a bad case of the grumbles. Only thing I can think of is a faulty fuel pressure regulator, but my readings are consistent, not erratic, so I'm *stumped*.

I checked, and there is no fuel in the FPR's vacuum tube.

When I pull the vacuum line off, my fuel pressure goes to about 38 psi.

When I rev the engine, the pressure is constant at 38 psi.

When I connect the tube, and press the throttle, my pressure drops to 28 psi while I'm holding the throttle down.

Any input much appreciated!!

Also, does anybody know how to bench test the vacuum limiter aka deceleration valve? I think its stuck close, but not sure. In any event, this item can't be causing a rough idle, can it?

Please help!

I think I read somewhere that my pressure should be going up.



1977 530ia









Post generated using Mail2Forum (http://www.mail2forum.com)

Should go to 43.5 psi I believe with the vacuum unplugged. Basically 3 bar.

based on what I'm reading, I think my regulator may be faulty.

I bit the bullet and ordered a new one-

I guess we'll see if I have a working spare soon.

Keep you guys posted, thx so much for the input!!!

So I bought a new fuel pressure regulator;
It really seemed to make very little difference when watching the gage.

I do suspect my fuel pressure is low.

Does anybody know if the injectors get a little quieter if the pressure is too low? Maybe I'm just used to it, but I don't hear that sewing machine type sound so much anymore.

I also noticed my fuel pump sometimes makes a loud whining noise at idle.
I will always hear it, but after some driving on the highway, it sounds louder.
Its easy for me to hear because my back seat is completely out.

May bite the bullet and buy a pump, see what happens.
Or maybe I'll just buy a bullet..

Where is the vacuum to the FPR located?

On the intake manifold, to right of throttle body.
(Not on the throttle body)

That's the right place. Before you buy a new pump, check for pressure while crimping down on the return hose. Don't just take a vise grips to it though and clamp down hard, it should be capable of over 100PSI and it will blow the hose off somewhere, probably back by the fuel pump or filter. But clamp it some and watch the gauge.

when I did this, my fuel pressure guage went cuckoo and spun all the way around.

So I'm guessing my fuel pressure is good?

According to BMW manual, spec is 35 psi (2.5 bar) for my car.
It says operating pressure is about 28-43.

It seems I'm hanging around 28-30. If I should be at 36psi, is 5 psi a big deal?

Fuel pressure does drop off after a while with engine off, so I'm assuming check valve in my fuel pump is done since I just changed the FPR.

The fuel pump does whine a little harder at idle at some times than others.

If its not the fuel pump, maybe I have some weird vacuum leak somewhere (oil pan, crankcase?).

I did notice driving around on the highway today that I'm getting about 8 mpg.

I will check compression again tomorrow too.

Other than consuming tons of gas, car runs good at highway speeds, a real cruiser. At idle it shakes and the exhaust sounds like sh**. I timed to spec but at idle in park, ~700-800 rpm, it doesn't really run right unless I advance the timing a bit.
I also notice more power when driving.

Also, my oil smelled like gas in the past, I just changed it. I'm sure if I keep this up, i will ruin this batch too.

Any input based on the above.........always appreciated.
I've learned so much here its crazy.

just a thought: maybe its not my fuel pressure at all,
maybe my vacuum retard item is too strong or something?

I wonder

My fuel pressure was low on my e12 and my e12 based e24 because the thick cloth braided hose from the tank to the pump needed to be replaced(my e12 didn't come with an in tank pump). It was cracked and partially collapsed.

I just ordered a fuel pump . I guess we'll find out guys!
My car was also sitting a few years, and the first few months the fuel pump fuse blew inexplicably a couple of times. Fingers crossed.

But after reading a bunch of stuff last night, I'm starting to suspect my vacuum retard may be off. Maybe its retarding the ignition too much at idle.

I have a spare distributor I will swap out today. Will post results.

no change with that......

I think my car did run a touch smoother if I adjusted to 1000rpm with vacuum tube disconnected.

Timing the car though, I notice spark jumping from the #1-wire-pick-up-clamp of the timing gun to the wire. I'm guessing my wires are toast, even though I changed them just last year. Maybe the overeheating toasted them.

I guess we'll see. Thanks gents for all your help; you especially Mike, you are a legend.

{EDIT} AH HA!!
I think after all this, my issues are not related to fuel pressure at all:
My wires are copper core wires with resistive ends.
I am using a petronix ignitor ii. Apparently, I need suppressor wires not these copper core ones, or I will get interference. Maybe, just maybe this is the issue I've been dealing with all along. If so, I fully expect my car will run like a champ once I replace the wires. WE WILL SEE!!!! :

SOME NEWS:

I replaced the fuel pump with a new one-
Results: no perceptible difference in fuel pressure; although, this pump certainly does not whine like the other one. Still at 32 psi.

I will note for any NOOBs changing their pump in the future, the 12mm hose may have TONS of gas in it, even after you depressurize the system.

One surprise, maybe. I fully expected with the NEW pump and NEW Fuel pressure regulator that my fuel pressure would not drop down to zero overnight; but it does just the same.

Does this add up to me having a leak in my fuel line somewhere ... or a leaky injector? Or is it normal for the pressure to fall down to zero with a new pump and pressure regulator? Somebody please give me some input here, if its not normal, I will pull the fuel rail and check the injectors...... they were just replaced with some remanned ones in september I think, and I maybe put 60 miles on the car since then........

I hope all this will be useful to someone, someday.

{Still waiting on some fresh wires.}