The Experiment - 525i Questions

[Ferret]

First post!

Firstly I'll explain what 'the experiment' is...

In january I was given a rather beautiful looking '91 525i (non-vanos) in alpine green or whatever it's called. Given... lol... more like had the keys thrown at me in disgust because the owner was fed up with it. Local dimwits at the garage had diagnosed it with a blown head gasket and replaced it at a cost of around £800... only to have it suddenly start blue smoking and ejecting water out of the bores when the spark plugs were removed...

Over the next few months I stripped it down leisurely and took the head off, to find the head came away from the crank case nearly in three pieces the cracks were that big.

I spent the next two months being frustrated trying to find a replacement head that I'd not have to chop my arm off to pay for it... all to no avail. I was trying to do this on a shoe string and replacement heads in scrapyards were always there because they'd failed...
Eventually I discovered that the heads from the 2.0 are an exact plumbing match for oil and water etc, though have a smaller combustion area. Practically handed a 2.0 head I decided to experiment, after all I had a dead 525 on the drive and it wasnt going anywhere. Result is that it ran quite happily, noticably more thrashy and powerful in the low revs... but lots of black smoke/unburned fuel in the high revs.

However, almost immediately it developed an idle misfire that vanishes the instant you open up the throttle. I dont think it's a piston ring/valve issue because occasionally it runs almost spot on with only a smooth, but slightly wobbly idle expected from a high compression engine, not a misfire.
Cylender 3 repeatedly carbon fouls up sparkplugs, that when you take apart are covered in fuel, not oil. However, as there's a nut missing off the manifold behind cyl 6 I have a suspicion air is leaking into that cylender. They're a strange 9mm thread that you just cant really get with encapsulated washer, so havent had chance to replace it yet.

Hrm... does anyone think a leak into 6 could cause a problem with 3?

One of the problems I'm having is because of the compression and misfire it's really piddling oil up the breathers - when you top out the rpm, the car occasionally blue/white smokes. Again I know it's the breathers because I've pulled one off when revving the car up, that and the manifold is full of oil. I've got this in hand though because I'm making up a t-junctioned breather that's going to drain back into the sump.

Does anyone know if the brake light circuits are housed inside the LKU? Just I dont want to go pulling it apart to find there's no brake circuit in it, looking for the cause of the irritating gong 'BRAKE LAMP'

One last technical issue I'm having, and someone might be able to help here - after retro fitting the OBC, both buttons on the dash have packed in... (and stay packed in when you remove the OBC) I've got the stalk, but not fitted it yet. Has anyone else come across a similar problem? Should the OBC display on the dash under its own control without the stalk?

Any thoughts ladies and gentlemen, would be greatly appreciated.

[Morgenster]

Is the head really that different? Bore increase is 4mm whereas stroke increases by 9mm when going from 2.0l to 2.5l. Just curious.

[Ferret]

Yeah, despite the bores being almost no different, the combustion areas on the 2.0 head are quite a bit smaller. I dont know if the compression ratios on the 2.0 and the 2.5 are different, but to fit a larger volume of air/fuel mix at the same compression as the 2.0, the compression areas of the 2.5 head have to be larger.

The valves on the 2.0 and input ports are also noticeably smaller.

[SnakeyesTx]

Could you just stick a MAF from a 2.0 in place of your 2.5 to alleviate the overly rich mixture? Also, verify if rates of the injectors between the 2.0 and 2.5?

[Ferret]

As far as I'm aware, the airflow and charge will be the same... the engine's still taking in 2.5 litres of charge, just compressing it into a smaller area. Therefore I shouldnt really need to adjust the mixture in any way, the difference in running should easily be taken up by the current MAF, Injectors and Lambda...

[Paul in NZ]

is cylinder three injector leaking or faulty?

[yaofeng]

The brake light circuit, similar to my SAAB's, is always hot. It does go to lamp control, which provides only the monitor function I beleive. Between the battery and the brake lights the only devices in between are the fuse, the brak light switch at the pedal and the lamp control.

[Ferret]

Thanks guys,

Paul - I've gotta rip the manifold off this weekend anyway - will have a butchers and see if it's leaking with the manifold off... though I'd be rather irritated if that was causing it.

yaofeng - Thanks, I'll inspect all the bits and get connectors cleaned and reseated this weekend as well.

[Ferret]

Hmmm, okay, I'm now suspecting the rings on that cylender...

Though why I dont know...

After lifting the oil cap while the cars running, the systems blasting wispy white smoke out of the filler in exact time with the misfire. The misfire is now working it's way up to mid range rpms and the smoke is increasing in volume with the rpm. There's two scenarios here...

A) Cylender 3 is totally knackered and needs a ring rebuild. - Need to check with a compression guage but havent got a twin cam one at the moment.

B) Cylender 3 Fires straight after Cylender 6... and cylender 6 is known to probably have an air leak... if cylender 6 is misfiring lean perhaps the air/fuel mix being forced through the block is shunting up through the breathers making the next cylenders cycle run rich?

It's generating a /lot/ of crank case pressure at the moment, and a scary amount of smoke out of the oil filler...

Thoughts anyone... I'm really not in a mood to rebuild this bugger again?

[SnakeyesTx]

When you put the new head on... did you re-deck the block? Its possible that its so warped that the leaking compression is blowing into an oil passage? When my head cracked, the block deck warped. I went ahead with a whole other motor rather than putting on another head, and stripping the block down to parts since that would basically end up being a total rebuild.