Re-starting after fuel runs out... learned soemthing today.

[genphreak]

Hi all,

Just had the second episode of having trouble trying to start an M50 powered e34 after the fuel had been run out.

In both cases
- I'd parked on a slight slope, left the car and then been unable to start after being away for a while.
- the fuel was critically low.

After adding fuel, as much as 9 litres, the engine would not start after prolonged cranks/cranking. I even tried jacking the car back to level.

The solution?

1. Remove the fuel relay (the middle, 4 prong one on M50 (vanos and non-vanos) engined 520i and 525i e34, possibly M20, M30 and e36, e46 and e39/M52 as well), jumper the high current wires (#87 and 30) and listen to the glorious sound of fuel gushing through the engine's fuel pipes and purging the air.
2. Start the car (even more glory; immediate vrrrroom!)
3. Remove the jumper and re-sinsert the relay with the engine running.

When cranking one may not be able to hear the fuel in the pipes but it was very loud. I doubt the pump was running at all when cranking previously.

But the question is why?

I'd cycled the ignition several times before cranking
Did a few cranks, not just one long burst
Left up to 10s between cranks, some with ignition on

The ECU has to initiate fuel. As I understand it, it will do so if

- the engine is rotatating (cranking or running), (ie. input from the cranks position sensor required)
- the crash relay is not triggered
- the trans lever is in P or N (if auto), or clutch depressed if manual (though maybe not all, mine does not have this feature)

So Dear Bimmernut Boffins; Why?
Is there some fuel system pressure check that is falsely positive?

Cheers! Nick

[shogun]

there is a check valve originally on top of the fuel pump inside the tank, if that fails, all fuel goes back into the tank, Bill R. shows here how to install a check valve on top of the tank
http://www.bimmernut.com/forum/5-ser...d-535-her.html
You can use any fuel check valve with that diameter which can stand at least 3-4 bar. Usually the shops which handle bio fuel parts have them.

Have you checked if the fuel pump is loose inside the tank and maybe floating?

[genphreak]

Thanks Erich!

On the car that just did this, I replaced the pump (with a Treperformance one) a year ago. I think this one has a valve in the pump too, (have not verified it yet). Will need to t the line and check the fuel pressure on shut down. Since replacing it, the pump has worked perfectly- no starting issues, no strange behaviour really- until I ran the tank dry!

However if it was the check valve- or even a lack thereof; why would jumping the relay allow the car to fire?

NB. I will pull the pump again soon (and check the location/see if it's moved or is floating as you suggest). I could add a return valve too. The gauge is not working accurately- which I expected after installing it. One of the downfalls of installing an aftermarket pump is that it is smaller and the vertical positioning of it and the gauge can end up different.

BTW; This car is an ex-Japan 11/1990 RHD e34 M50b25 525i (one of the first M50 e34s; BJ38700). I've had it for years and converted it to a 5 speed. The automatic expired at 220,000km and I've had to re-plumb the intake side of the engine to get it to run property. It has 2 lambda sensors, one before, and one post-katalyst. I disconnected the post kat sensor and that solved the "check oxygen sensor" error on the dash (I have read that the post-cat sensor is not really used (???).

Now it is throwing the error again- so I'm going to replace the pre-cat sensor.

Also I've been told the Air Conditioning system is charged with R12, but there are no stickers. Does that sound right for an e34 made for sale in the Japanese market in 1991?

[shogun]

the socalled post cat sensor is a temperature sensor, which was required in Japan then for all cars. In case the cat was clogged and got really hot, there came a warning.
On my E32 there is under driver side (LHD) rear seat a special relay for that. Here a pic I made, 870 degree C, and then the check control warning comes
ImageShack® - Online Photo and Video Hosting
No need to remove the sensors, just remove the relay. Usually the fault for the warning is a defective relay.

1991 should be still R12, but that you can easily check, the connectors are different R12 <>R134a.
I did the change in 2006 like shown here
http://www.meeknet.co.uk/E38/AC/Index.htm
The only item I replaced was the dryer and I made a long vacuum on the system with a self built vacuum pump. Still going strong in 2013, although I never changed the O-Rings, in these many years I only recharged 200 gramms = 1 can.

[genphreak]

Thanks for the tips on the post cat temp sensor. Great to know.

Sweet. I did just buy a 4cfm pump to install my home air conditioner. If I get a gauge set I can check and re-charge the refrigerant in the car systems. My local indie is always happy to remove it and recharge after I've ripped into the dash or other parts of the AC. Just did the tx valve (aka expansion valve) and cabin filter which on a RHD car is not easy. Had to remove the steering column and use very good 3mm allen keys to remove the TX valve bolts which took a lot of torque to crack (which are very hard to get to and impossible to see). If you can get to them, you don't have to remove the pipes that link through the firewall but its worth doing as the green seal rings are all at the age where they need replacement. So far I've done all of them, and am replacing the compressor (and of course the dryer) as well as I had a chinese replacement sitting here. The TX valve had a little restriction in one of the refrigerant pipes which the new valve wouldn't accept. If you compare them, they look different inside, so I'm hoping the new one is a proper replacement. The old one was pretty dirty, so it'll be interesting to see if how it goes.

Will do the other car's system soon too (replacing all the o-ring seals- and do the TX valve and dryer). It'll be interesting to see if it works as well as the one with the new compressor! Both are M50s, one a 1990, one a 1995, so it should be a good comparison.

[Mutley]

This may answer your question on why the car will not start after running out of fuel. If you put the key in and turn it immediately the pump will be unable to deliver enough fuel to purge the system so will not start at all, what you need to do is just turn on the ignition and leave it for at least a minute in order to purge the lines,( removing the relay and jumping the terminals has the same effect, as you found out) a more detailed explanation may help you below.

The Motronic electrical system is controlled by a single system relay with dual contacts. A permanent voltage supply is made to relay terminals 30 and 86 from the battery positive terminal. When the ignition is switched on, the ECU earths terminal 85 through ECU terminal number 36 which energises the first relay winding. This causes the first relay contacts to close and terminal 30 is connected to the output circuit at terminal 87. A voltage supply is thus output at terminal 87. Terminal 87 supplies voltage to the injectors, ECU terminal 37, ISCV and the FTVV when fitted. In addition voltage is supplied to the second relay contact. When the ignition is switched on. the ECU briefly earths relay contact 85b at ECU terminal 3. This energises the second relay winding, which closes the second relay contact and connects voltage from terminal 30 to terminal 87b, thereby providing voltage to the fuel pump circuit. After approximately one second, the ECU opens the circuit and the pump stops. This brief running of the fuel pump allows pressure to build within the fuel pressure lines, and provides for an easier start The second circuit will then remain open until the engine is cranked or run. Once the ECU receives a speed signal from the CAS, the second winding will again be energised by the ECU, and the fuel pump will run until the engine is stopped.

[Rustam]

This reminds of my earlier days of having a similar issue, requiring the similar solution of jumper wire. (Perhaps this was same problem don't remember.) I remember seeing a note somewhere that pointed to a "defect" in the wiring to the fuel relay, wherein the TWO wires sending (1) the engage voltage to the relay and the (2) the engage voltage to the pump were switched in some cars for some reasons. (Hence the relay is actuated by the current meant for the pump, yet carrying to the pump the current meant for the relay).

The note said that this was virtually not a problem. It is relevant to note that some relays (as I understand) do in fact have the prongs switched as well, in the same manner, that is in some relays the prong that performs function (1) matches the placement of other relays' prong that performs function (2).

Please consult the outer wiring and the inner designations of the relay prong functions to ensure proper connections. I used Bentley Repair manual for the wiring schematics.

Also, on a relevant note - the fuel relay that I use is a cheap relay bought at an electronics parts store for some 5 bucks or so, it has worked in the manner of a fuel relay forever without the need for the expensive OEM replacement. It does same thing indeed, no troubles, no extraneous expenses.

[genphreak]

This may answer your question on why the car will not start after running out of fuel. If you put the key in and turn it immediately the pump will be unable to deliver enough fuel to purge the system so will not start at all, what you need to do is just turn on the ignition and leave it for at least a minute in order to purge the lines,( removing the relay and jumping the terminals has the same effect, as you found out) a more detailed explanation may help you below.

The Motronic electrical system is controlled by a single system relay with dual contacts. A permanent voltage supply is made to relay terminals 30 and 86 from the battery positive terminal. When the ignition is switched on, the ECU earths terminal 85 through ECU terminal number 36 which energises the first relay winding. This causes the first relay contacts to close and terminal 30 is connected to the output circuit at terminal 87. A voltage supply is thus output at terminal 87. Terminal 87 supplies voltage to the injectors, ECU terminal 37, ISCV and the FTVV when fitted. In addition voltage is supplied to the second relay contact. When the ignition is switched on. the ECU briefly earths relay contact 85b at ECU terminal 3. This energises the second relay winding, which closes the second relay contact and connects voltage from terminal 30 to terminal 87b, thereby providing voltage to the fuel pump circuit. After approximately one second, the ECU opens the circuit and the pump stops. This brief running of the fuel pump allows pressure to build within the fuel pressure lines, and provides for an easier start The second circuit will then remain open until the engine is cranked or run. Once the ECU receives a speed signal from the CAS, the second winding will again be energised by the ECU, and the fuel pump will run until the engine is stopped.

Thanks Mutley, but I certainly did give it time to prime (but maybe not exactly enough (30s) though I cranked for 40s or so after that so it should have been fine)

Appreciate the info though. Where is it from btw?

[genphreak]

This reminds of my earlier days of having a similar issue, requiring the similar solution of jumper wire. (Perhaps this was same problem don't remember.) I remember seeing a note somewhere that pointed to a "defect" in the wiring to the fuel relay, wherein the TWO wires sending (1) the engage voltage to the relay and the (2) the engage voltage to the pump were switched in some cars for some reasons. (Hence the relay is actuated by the current meant for the pump, yet carrying to the pump the current meant for the relay).

The note said that this was virtually not a problem. It is relevant to note that some relays (as I understand) do in fact have the prongs switched as well, in the same manner, that is in some relays the prong that performs function (1) matches the placement of other relays' prong that performs function (2).

Please consult the outer wiring and the inner designations of the relay prong functions to ensure proper connections. I used Bentley Repair manual for the wiring schematics.

Also, on a relevant note - the fuel relay that I use is a cheap relay bought at an electronics parts store for some 5 bucks or so, it has worked in the manner of a fuel relay forever without the need for the expensive OEM replacement. It does same thing indeed, no troubles, no extraneous expenses.

Interesting stuff mate, thanks.

Nice footnote too; I've always wondered why there are so many specialised relays in a BMW, good to know the Fuel Relay is not one of them!

[Rustam]

Yes. Just have to be careful that the prong functions are correct on the cheapo replacement.