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ollieh

M50 Idle Problems & Poor Fuel Consumption

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Hi, looking for some help with my current running problems...

I have a '93 525iX, M50TU B25 lump has done about 150k.

The engine hunts for idle speed, it can bounce between 500rpm and 900rpm to as much as 1800rpm and cutting out!

The car has had this problem since I got it, it has been a project and this was one of the things to sort out. The car is now on the road after some work. After some reading it looked like the idle control valve may be to blame, or an air leak?

I took off the ICV last weekend, it moved freely and didnt look too sooted up. I gave it a clean with some carb cleaner and put it back on the car. I checked the surrounding hoses for leaks and could not see anything. The hose from the rocker cover to the intake boot was a bit perished and had collapsed in on itself so I replaced that.

After this the car was much better, not perfect. The engine would now drop to about 600rpm, wind up a couple hundred rpm and then idle steadily at about 650-700rpm.

I drove to work and back for a few days and the car carried on running this way. However this week it has been back to its old tricks, but actually worse than before!

I did some diagnosing last night. I unplugged the MAF and car idled differently, infact more steadily. This would lead me to believe the MAF is sending a signal and probably working correctly. Did the same with the Lambda sensor (car at operating temperature), no improvement. Then unplugged the ICV, engine hunted for idle in a different way..

Other problems seem to be very poor fuel consumption, and lumpy idle from cold - seems to be running on 5 cylinders intermittently

Any ideas?

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Oh yeah,

I did a thorough service a month or so ago. New plugs, filters, oil, engine flush, cleaned the MAF.

After this it still did not idle properly, but did run very sweetly once moving

Looks to still have the original coil packs

With a bit of calculation based on my daily commute for 3 days, the car seems to be doing less than 18mpg...

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Does it still cut out, if so I would look at the cam or crankshaft position sensor as that sounds like a symptom of either one failing

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this afternoon I have changed the ICV for a spare, didn't sort the problem.

Replaced vacuum hoses

Checked for air leaks on the intake boot - couldn't find any

Replaced the O-ring seals on the connectors attached to manifold and rocker cover

checked all the ICV pipes, the long one from the intake boot had the beginnings of a split so I trimmed the end off.

Cleaned out the throttle body

Ran the car and it was a bit better but not solved... then...

found that the non-return valve the sits between the manifold and the brake servo seems to have some sort of vacuum hose attachment snapped off the bottom and looks like the source of an air leak.

maf.jpg

Part number 11 on the below diagram

268.png

The funny thing is that if I put my finger on the hole and block it up, the idle searching problem gets even worse!

What should this connect to, any ideas?

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On some cars there is a small bore pipe from part 11 to a nipple in the bulkhead next to where the heater pipes go through it. Its part of the heating/ventilation system. If you dont find it just block the leak. It wont be doing your idle or mpg any good.

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OK,

Blocked off the air leak on the non-return valve, changed the vac hose between manifold and fuel pressure regulator as that was a bit perished.

This didnt make a blind bit of difference!

I unplugged the MAF again, engine ran much better. Idle was steady at 750rpm, pulls like a train though does look like it is running a bit rich. I guess this is due to the ECU using default settings as the is no MAF signal.

However, with out the MAF that car is a different animal, so looks like it is the MAF that is causing the problems.

Does this sound about right?

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if in doubt with vacuum leaks i think you can use various aerosols around suspect areas, if you have a leak idle will change

i think brake cleaner maybe one such spray (not sure)

mafs can cause issues like this

i had similar issues when i bought my car the PO was convinced the o2 circuit was duff in the dme, it turned out that the o2 heater relay was shot so hed disconnected the o2 due to nasty idle issues

fuel economy with the o2 disconnected also did not better 20 mpg

but definately fix order for me is

icv clean and test, check for vacuum leaks while refitting, make sure hte o2 is good (hard to do with diy tools)

after that then look at the maf, crank and cam sensors (unlikely but just in case) and coils (although coils usually break down in high load situations)

Im always very sceptical about malfunctioning mafs, as far as i can tell unplugging it should yield a change, if it does change then the maf can be assumed good.

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If you have a vacuum leak after the MAF, it can seem like a duff MAF. Its has already metered what air is supposed to be flowing into the engine and then more is introduced. If you are using brake cleaner to test, it needs to be the flammable stuff. Have you done the test to see if the wire glows after you switch off? I'd also want to check the fuel pressure is right and that the FPR is working correctly. I'd also disconnect the battery and start afresh as the ECU might be getting confused. TBH I'd get the codes read too. You are going to have some stored as you have disconnected the MAF and the O2 sensor so they need to be cleared and re-read. Might save you chucking more parts at it and hoping.

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Im always very sceptical about malfunctioning mafs, as far as i can tell unplugging it should yield a change, if it does change then the maf can be assumed good.

Indeed, I am sure many MAF sensors are needlessly replaced.

However, in my case unplugging the maf did yield a significant change - the problem went away. When the MAF is connected the car wont idle steadily, runs a bit lumpy, stumbles off idle, occasionally feels like it is briefly hitting a rev limiter at various RPMs, has poor fuel consumption...

With the MAF unplugged the engine is as sweet as a nut, pulls well, idles solidly at 750-800 rpm ish. As I say, its seems to over-fuel a little but I imagine that is due to the ECU assuming a default value for intake air mass flow.

I have replaced all the vac hoses, cleaned and checked the ICV although it did not show any sign of being clogged up, blocked up the vacuum leak on the NRV between manifold and brake servo

I have an O2 sensor removal tool and a DMM, so may be able to do some kind of rudimentary test. Though on disconnection of the O2 sensor, the symptoms did not go away. As I said before, these problems occur when the car is started from cold which would lead me to believe the O2 sensor isnt the problem

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Oh yeah, been disconnecting the battery systematically between tests to allow the ECU to "refresh" as such.

I don't think the MAF is the "hot-wire" type, but instead uses a little flap and potentiometer arrangement (so I'm told), so when I cleaned it with carb cleaner it probably did nothing at all. Cleaning the MAF was the first thing I did, made no noticeable difference

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dont rule out the o2 sensor

the car learns via the o2 sensor (and i assume the maf) over time

so your car gets upto temp, o2 switches in, sees too rich and starts to lean off, eventually it will find the sweet spot and your idle and cruise mixtures will be correct (wide open is ignored as it uses set dme values, possibly tweaked by maf readings)

now, lets say your o2 sensor is duff, either not readin or providing hte wrong reading (check o2 heater relay btw at same time) your dme could be learning to be too rich/lean and hte icv is struggling to maintain the correct mixture AND idle speed (one will effect the other) also it is getting readings from the maf, which do not coincide with what the O2 (is incorrectly) reading. Therefore makes more of a mess or may store a code. the fact that unplugging hte maf does make a difference either tells us hte maf is reading correctly ,OR incorrectly. Either way, as far as im concerned, the O2 sensor will have a vastly greater impact on idle.

Likewise, the crank pos sensor could be feeding spurious readings, but that is easier to test than an o2 sensor, i tend to hook a meter up and move the toothed wheel past it and check for change. (not sure how official a test that it but it suits me)

Btw i had a nasty idle issue on mine and it WAS the o2 sensor, but only the heater relay, the PO had not checked the operation of that. THe heater relay will cut in when the o2 sensor cools, generally at periods of idle or on start up.

no, your maf (if you have a tu) is not a hot wire, its something different like a thin diapghragm which gets distorted as the airflo passes it (i believe) non vanos had hotwire. thats the difference between motronic 3.3 and 3.3.1. However you marf isnt (should not be) a flapper, that is only on m20 and m30 e34's

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Fitted a new Bosch MAF on Friday, problem solved - car is running really well. Absolute pleasure to drive

Thanks to all the posters, cheers for your help

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After last week I can report the car is much more reasonable on fuel and is running like it should - it is easy to see why BMW straight sixes have such a good reputation. I'd be using even less fuel if it wasn't so satisfying to let the engine wind up to 5 or 6k!

Having inspected the O2 sensor, I am thinking about replacing it anyway. It looks like it has been on for a very very long time and I would imagine it is not working as well as it should. Bosch ones are only 68 quid so may as well swap it as it is a serviceable part.

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