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E36 (1991 - 1999)
The E36 chassis 3-Series BMW was a huge hit among driving enthusiasts from the first moment the car hit the pavement. The E36 won numerous awards over the years it was produced and is still a favorite of many BMW enthusiasts to this day! -- View the E36 Wiki

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  #1  
Old 04-16-2009, 06:18 AM
Khaito Khaito is offline
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Location: Perth, Scotland
 
Join Date: Mar 2009
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Mein Auto: BMW 1993 316i E36 M43
Unhappy Excessive amount of vacuum in fuel tank?

I'm not sure if this is the problem but it seems like it, tried searching the forums too but I'm not finding anything similar posted so far.

Recently replaced my fuel pump with a new one - as a process of elimination for another problem - and in the process knocked a wire off for the fuel gauge. Soldered it back on, problem solved. A week later, fuel gauge dies again and upon taking the fuel pump assembly out discovered that the sender unit was broken and the leg that sits on the bottom of the tank was bent (!). Checked the siphon assembly on the passenger side tank and same problem, the leg was bent and the slider on the fuel sender was shoved right to the top and the plastic was broken, bent etc.

This has only happened recently as two weeks previous we had both out and they were fine. Do not believe it was when we were putting them back in as we fixed them up as best we could on a temporary basis and put them back in and immediately took them out again to check and they were fine. The legs that stand in the bottom of the tank had to be put in a vice to straighten them so it's not so easily done.

I'm back down at my parents house today, to use their garage, and I've taken both units out again 5 days after putting them back in and both legs are bent again and the siphon side sender unit is all shoved to the top and bust again. The only thing I can think of is excessive vacuum in the tanks, which would imply the EVAP system isn't working properly at some point (if it has one? if not, what does it do with fuel vapor?). On the other hand though, I have tried shoving the bottom of the tank myself and it really doesn't move easily, so I don't really believe it would be vacuum compacting the tank which is bending the legs on the sender units.

The vacuum can confirmed when you try to open the fuel cap on the tank. If the car hasn't been running then the cap comes off easily and it doesn't have any vacuum behind it. If it's been running for a while, even just idling, then the vacuum is so strong it's some effort to get the fuel cap off.

So I'm sort of at a loss as to what's causing this. I think it's related to issues I'm getting intermittently whilst driving though, where upon coming to a stop or almost stop the car lags when accelerating, you put your foot down and it struggles to get going, almost feels like fuel starvation for a few seconds.

I found this site: http://www.bmw3-5.org.ua/page-161.html
And I'm wondering has anyone A; had this problem? B; do E36s have systems like in the site linked above? Or is that only in newer models? If not, does anyone know what it has instead?

As far as I understand there's not meant to be ANY vacuum in the tank as it has a breather pipe that comes out at the fuel cap. And there is meant to be a purge system that relieves the vacuum, no? I'm wondering if maybe the tank is collapsing at some point as there isn't really any other way the legs on the sender units could be getting bent.

My car has a 1.6 litre M43 engine, 1993 model so ODB I.
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  #2  
Old 04-16-2009, 07:35 AM
E36 Phantom's Avatar
E36 Phantom E36 Phantom is online now
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Had a reply and deleted it all by mistake. Dammit.

Short version:
-First off, BIG THANKS for actually describing things in detail with backstory and everything. Getting info out of people is like pulling teeth sometimes.
-I suppose it is possible, though unlikely, that you're getting such a strong vacuum that it's collapsing the tank. I have heard of it happening on other cars, but it's VERY rare. The E36 tank has so many ridges in it I'd be surprised at it, but it sure sounds like that's what's happening.
-Yes, you've got an activated carbon container and EVAP system just like that diagram illustrated. I don't believe this would be causing your excess vacuum, but I could be wrong.
-Check your vent hose and fuel pressure regulator. You can check the pressure in the tank, as well, with some simple fittings. I have specs for US cars, but I don't know what proper spec should be for a 316i. I do know that other OBDI cars should be at 3.0 bar +/- 0.6.
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  #3  
Old 04-16-2009, 07:53 AM
Khaito Khaito is offline
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Location: Perth, Scotland
 
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Mein Auto: BMW 1993 316i E36 M43
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Originally Posted by E36 Phantom View Post
I suppose it is possible, though unlikely, that you're getting such a strong vacuum that it's collapsing the tank. I have heard of it happening on other cars, but it's VERY rare. The E36 tank has so many ridges in it I'd be surprised at it, but it sure sounds like that's what's happening.
Yeah, it would explain the problem with it choking when trying to pull away from a stop after it's been running for even 5 minutes as if it had excessive vacuum in the tank and then the fuel pump was trying to pump more fuel out, it might cause fuel to be (basically) sucked out the engine via the return pipe? It seems like a bit of a long shot though.

Quote:
Originally Posted by E36 Phantom View Post
Check your vent hose and fuel pressure regulator. You can check the pressure in the tank, as well, with some simple fittings. I have specs for US cars, but I don't know what proper spec should be for a 316i. I do know that other OBDI cars should be at 3.0 bar +/- 0.6.
Fairly certain the fuel pressure regulator is fine, the petrol pressure is fine at the fuel rail at the fuel injectors.

Does anyone know if there SHOULD be a vacuum in the fuel tank when the car is running? I'm away to put my car up on ramps and run it for a while so I can see if it is the tank collapsing at any rate.

Is there an easy/easier method of checking the vent pipe? So far got the driver side rear wheel off and the mud flaps etc off and I can managed to get the vapour tank thingy out but other than the two hoses going into it from either tank and the one going out, there ain't a whole lot to be looking at. Also, why bother with a vent pipe that has an air tight fuel cap that goes on top of it? Seems a bit redundant? Unless it's only for venting when the car is being topped up?

I've bought a new siphon + fuel sender assembly and a new fuel pump assembly to replace them both but I'm avoiding putting them in incase they both just end up mangled.

I just discovered the charcoal cylinder is apparently US only, for your emissions control, so I'm not sure what the EU/UK equivalent is in that case. Frustrating! Haha.
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  #4  
Old 04-16-2009, 09:07 AM
Khaito Khaito is offline
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Location: Perth, Scotland
 
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Mein Auto: BMW 1993 316i E36 M43
Quote:
Originally Posted by Khaito View Post
I just discovered the charcoal cylinder is apparently US only, for your emissions control, so I'm not sure what the EU/UK equivalent is in that case. Frustrating! Haha.
Hrmm, I take that back, seems there is meant to be a carbon canister in the engine bay and it's.. missing from my car? Not sure about that though! But it's meant to be in the right hand side of the engine bay - on the right hand side when you're looking into it from the front.

But if it is missing, it seems they've plugged the pipe from the expansion tank straight into the purge valve so the carbon canister isn't there to fill up and isn't there for the expansion tank to be plugged into and isn't there to be plugged into the purge valve.

Any ideas what the expansion tank is for? For when the fuel heats up and expands, I'd presume?

I can imagine the fuel tank is NOT meant to have a vacuum as that would work against the fuel pump, you would want air pressure in the tank to help push the fuel round. I'm struggling to get an answer anywhere on that front though.
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  #5  
Old 04-16-2009, 10:32 AM
Khaito Khaito is offline
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Mein Auto: BMW 1993 316i E36 M43
Okay, so the fuel tank breather valve is stuck open, so it was creating a mass vacuum in my tank which, it turns out, IS the culprit for the mangled assemblies. My poor car is missing the activated carbon filter, so I'm going to go hunt the scrap yards tomorrow to see if I can get one on the cheap, and I'm going to speak to a couple of places about getting the DME (or ECU if you prefer :P ) reset to A; fix that valve hopefully (if not, replacing time), and B; reset the airbag light I turned on by removing the steering wheel last week. :P

HOPEFULLY, I've hit the nail on the head, or it's back to the drawing board. But this would explain the lack of power after coming to a stop once the car's been running for a while as it needs to create the vacuum to cause the problem. I've just test run it around town, crawling along to try and get the rev/power/acceleration loss with the valve disabled and the hoses plugged and it ran fine and there were no popping/cracking noises from the back when the car was shut off. So fingers crossed that this solves it once i get the bits.
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  #6  
Old 04-18-2009, 02:51 PM
Khaito Khaito is offline
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Location: Perth, Scotland
 
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Mein Auto: BMW 1993 316i E36 M43
:d

Just an update. Carbon filter IS for US cars only it turns out. So disabling the valve temporarily until I can get the ECU reset has fixed the problem, I may possibly have to replace the valve though, will find out next week if it's an ECU problem or not.

I also changed the oil, oil filter, air filter and I tried changing from Bosche +4 sparks to NGK BKR6EK copper ones and my car is running smooth as silk - I think any difference from changing the type of spark plugs was negligible though, think just fixing the vacuum problem and a change of oil + filter were the biggest differences.

Chuffed to bits though! Just got to get the ECU reset, bearings replaced on one of the rear wheels and the problem with the dash shorting fixed and then it's just bodywork stuff needs done which I'll be doing myself anyway.

But I don't know how I would live without Halfords. :P

Last edited by Khaito; 04-18-2009 at 02:58 PM.
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