More help wanted with my Beemer !

Banjo013

Hey guys - I'm back !

I've got a 1995 BMW 520 auto. Haven't had it long - already solved one problem I had by fitting a new throttle position sensor.

Now here's some more symptoms ...

It's very slow on accelleration and absolutely drinks petrol. When first started from cold, it goes like the clappers - accelleration is brilliant. Then about two minutes into every journey, accelleration just dies. Won't pick up speed going up hill. Gears seem to shift up far too early.

Now I'm thinking oxygen sensor. Bad compression also crossed my mind, but I've kinda ruled that out because like I said, when it's cold it runs brilliantly. I am aware that the oxygen sensor doesn't kick in until the engine warms up a bit. Therefore I think that might be the reason why it always takes a few minutes for the symptoms to arrive.

So am I right you think ? Any comments or possible other explanations ?

lomb

definately start by buying a compression tester for the engine. these are 50 quid approx. check each cylinder in turn. if this checks out move onto the next step. unfortunately it is impossible to do a proper diagnostic without correct tools. theres a mobile diagnostics checker in the sticky, i would ask him or another auto electrician to run a fault code scanner on it, and then check all main sensors such as oxygen, tps, mass air flow, crank position sensor. then eliminating that, the cats could be screwed, and blocked. or it could be vacuum leaks, or a slipped chain on the cam.
there isnt any cheap way to do this unfortunately, if its the cats and o2 sensor like it may well be, it could cost 1500 euro to put right professionally. perhaps merlin car auctions may not be a bad idea as u should get most of your money back with very little effort and let someone else worry about it.

lomb

Banjo013 wrote:

Hey guys - I'm back !



Now I'm thinking oxygen sensor. ?

how many miles has the car done, has it ever had its cats replaced? they could be restricting the exaust gases on warm up ie something expands etc hence car loses power hence more accelerator applied. i would have it fixed pronto b4 you blow the cats if they havent already been blown.
u could always get under it and remove the o2 sensors, heat the tip in a flame while measuring resistence.
could even be a coolant temperature probe, or an ecu problem, could be anything really.

Wheelnut

If it's good cold and then bad, then it's obviously temperature related. So why not check out the temperature sender? There is likely to be a second unit for the ECU which is seperate from the one for the temperature guage.

lomb

Wheelnut wrote:

If it's good cold and then bad, then it's obviously temperature related.

yeah but this only means that something changes with heat. so it could be any sensor/ ecu/ mechanical problem that changes with temperature ie absolutely anything. for example lets say the crank position sensor doesnt like heat and fails when warm and starts sending dodgy signals to the ecu what does this mean?
so basically could be anything. it needs to be done in a professional way to fix it which usually means it will cost money. i would def pay an auto electrician to diagnose it after i had done a compression check. its 100 euro well spent. otherwise its a wild goose chase.

AMurphy

As we do not know when it is "drinking petrol", I'll assume all the time. Cold, hot and in between.

Anyway, on anything that changes performance between cold and hot, I'd check the temp dependant items.

Thermostats, being the simplest.
Next Engine (EFI) temp sensors. You car probably has a collection of them about eh thermostat housing. But without a manual, I cannot give you temp resistance specs for any of the sensors.
disconnect each one in turn and see what goes whacko. You could have one each for the AC/Fans, EFI and gauge.

O2 sensors and EGR are time (EFI) temp sensor dependant, typically they are not used for the first 10 min of operation.
Easy to check the O2 sensor on those ones, if you can find the connectors/wires and have a DVM. If not, how many miles on it? Then can die anywhere between 45 and 120 K, typically about 60K.
Any idea if you have an EGR on this thing?.

However, "drinking petrol" could also be associated with another item, which I suspect you have on your EFI. Fuel pressure regulator. If it either sticks shut or springs a leak, it will result in excess fuel pressure or simply excess fuel.
Again, assuming your EFI has one, pull the rubber hose off the regulator and plug the hose. see if the reg. leaks fuel, if so change.
If not, get a hand vac pump and check it changes the fuel pressure.... engine note will change.

No, I'm assuming that all the ignition parts are in good shape and correct spec, ie not the wrong spark plugs, etc.

It appears this is an AT machine. Any reason the suspect the problem is engine and not AT?. if nothing else, you can force it into the lower gears manually and re-check.

Faulty cam/crank sensors/ignition sensors would cause it to run badly, misfire, cut out.... not simply drink petrol on a continuous basis.

While you are at it, check you have no intake vac leaks of any sort.
Check and insure your oil cap and dipstick seals and OK.

528i

could be a MAF (mass-air-flow) sensor ? very expensive if it is.. you wont know for sure until diagnosed with an ecu fault code reader. anything else is just guesswork, but dam fine guesswork so far

AMurphy

You reminded me of something there 528.
Check your Air filter is not plugged up with... anything, gunk, water, oil. Should only take 1 minute.

Banjo013

Just by way of a bit of background ....

I've already installed a new mass airflow sensor, a new throttle position sensor and the air filter is new.

I've bought a new O2 sensor now - just need to fit it. Only thing is I'm not sure if it's the correct one - there are two types, a Bosch and a Siemens. Unfortunately they are not interchangeable and it's impossible to tell which one is the right one without taking the old one off.

I find the car to be very difficult to work on to be honest. For example the O2 sensor sits on top of the cat about a third of the way down the exhaust pipe - a totally inaccessable area !

I did a compresson test also - looks like cylinder no.5 might be weak - all the others are up around the 190psi mark - the no.5 came in at just 150psi. So I may have to consider a broken piston ring, maybe a leaking valve or a shot head gasket. I guess this could potentially be the cause of my problems, however it doesn't hold water for me really cause like I said, it goes like a rocket when the engine is first started.

Anyway, keep the ideas coming, I'll continue with my quest !

MercMad

Okay even though one of your cylinders is lower on compression than the others that indicates wear but wont give you the symptoms you have.

If it drives normally when cold then heats up and drives poorly AND drinks fuel then to me this indicates the cold start system is switching ON but not off. I reckon it IS a temperature sender that is faulty. When the car heats up the mixture is too rich and the performance will be poor.

You have spent quote a lot of money at this stage, these "swop and try" diagnostics eat money.

Where are you based ? Is there a friendly dealer around (not likely) or a independent with diagnostic equipment ?

Interceptor

I'd get it to a diagnostics centre before you spend all your money on bits you don't need - it sounds like it might be fuel starvation to me and you don't mention changing the fuel filter or checking fuel pressure.

'c

AMurphy

Banjo013 wrote:

....

I've bought a new O2 sensor now - just need to fit it. Only thing is I'm not sure if it's the correct one - there are two types, a Bosch and a Siemens. Unfortunately they are not interchangeable and it's impossible to tell which one is the right one without taking the old one off.
......!

What's not interchangable about them?. length, Thread (I'm only aware of one thread type for all of them) or the end connectors.... Easily, switched with a soldering iron and pliars.
How many wires on it, given the year, probably 3 or 4.

Insure there is anti-sieze grease on the threads of the new one, before installation. If not, get some on there.

Anyway, it is probably a 22mm hex. So get yourself a 22mm ring spanner and is the connectors at the end fo the wire do not pass through the eye of the ring cut a slot in it to allow the wires to pass through.
You might have to heat and bend the handle of the spanner to make it avoid the bits and pieces in there. (now you got a special tool)
Unless it has been in there since 95 and well rusted in place, they are not normally difficult to loosen, just difficult to get at.
(clean the dirt off the area before you remove the old one).

Put some anti-oxide compound (tune-up grease) on the connector contacts and work it onto the cavities.


Usually they ahve a ring seal on st the base, so there is no need to overtighten the new one to get it sealed.

Save the connector off the old one, they make useful test connectors for the wiring lom.

lomb

AMurphy wrote:

What's not interchangable about them?. length, Thread (I'm only aware of one thread type for all of them) or the end connectors.... Easily, switched with a soldering iron and pliars.
How many wires on it, given the year, probably 3 or 4.

Insure there is anti-sieze grease on the threads of the new one, before installation. If not, get some on there.

Anyway, it is probably a 22mm hex. So get yourself a 22mm ring spanner and is the connectors at the end fo the wire do not pass through the eye of the ring cut a slot in it to allow the wires to pass through.
You might have to heat and bend the handle of the spanner to make it avoid the bits and pieces in there. (now you got a special tool)
Unless it has been in there since 95 and well rusted in place, they are not normally difficult to loosen, just difficult to get at.
(clean the dirt off the area before you remove the old one).

Put some anti-oxide compound (tune-up grease) on the connector contacts and work it onto the cavities.


Usually they ahve a ring seal on st the base, so there is no need to overtighten the new one to get it sealed.

Save the connector off the old one, they make useful test connectors for the wiring lom.

hi AMurphy, do u know if the cats have to be removed b4 removing the o2 sensors, as they are on top of the cats, and persumably there isnt any access above them.

also if 1 cylinders pressure is down, u need to find out whats normal, and i would only proceed if it is. otherwise consider auctioning it off. having said this a motor can be bought cheaply but whats to say the new used motor isnt fooked (remember the nikasil problem and bmws particularly the 520 are usually driven very bad and cars arent serviced regularly here)

AMurphy

Some sensors are in dam awkward locations, but usually with a modified/custom bent spanner you can get around most obstacles.
Remove a couple of hangers and bend it down a bit for access, if necessary.

I do not know the exact locations of the sensors on this exhaust.

What is this a straight 6cyl.? then it would have only 1 upstream sensor. not sure about a downstream one on this year, probably not.
If it ihas 2, change the upstream one. (one closest the engine).

Wheelnut

Go back to what I said and stop wasting money! It has to be temperature related and Mercmad has put his finger on the second stage of the solution. If the temp sensor for the ECU is not working, it will not switch off the cold start device. The second alternative is that the cold start device is seized for some reason. This would account for good performancecold, bad performance hot and drinking petrol!

If the cold start device is an air valve it is probably seized, if it is an enrichment device it is probably the sensor.