Re-hashing an old problem

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worzel
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Car model: 1991 525i Auto
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Re-hashing an old problem

Post by worzel » 05 Sep 2016, 12:08

Hi out there

Never pat oneself on the back until you've done a definite fix!

I thought I'd resolved the problem of an intermittent idle fault after discovering a minor split (underneath and out of sight of course) in the intake elbow boot.

However the issue has decided to revisit me. For those not familiar with my earlier posts on this subject in brief what happens is that on coming to a halt (1991 525i auto incidentally) there is a moment of surging for one or two seconds. No dramatic variation in the revs (only about a hundred or so) and without exception every time this happens the idle returns to normal- until I come to the next stop.

Now I know I sometimes have odd flights of fancy but in my mind I'm linking what happens to braking. Daft connection maybe but the surge ONLY occurs after braking and only when the engine is at normal running temp.

As a test I tried using the handbrake only to bring the car to a stop (from a fairly slow speed obviously) and result was no surge.

So- I'm trying to figure out what's happening. The servo operates correctly ie deplete the system of vacuum, foot on pedal, start engine and observe the pedal sink a bit. Servo also holds vacuum after switching off engine.

What I've investigated in addition to above-

Inline one way valve- operates correctly
Vacuum pick up hose- renewed with correct spec hose

Now I'm pretty familiar with braking systems so I can't see how anything else can be at fault- well to cause this problem anyway.
Or- maybe I'm looking in the wrong area since the fault only occurs when the engine is hot.

Lots of head scratching but no useful ideas on my part.

Also- not related to above are there any fellow owners out there located in the Liverpool/Merseyside area who'd recommend a mechanic they've personally used to good effect (ideally one with personal experience of working on the E34 range).

No problem with the local specialists here but I'd prefer to find a smaller outfit than the ones I've used as I sometimes get the impression (maybe wrongly) that they're not too interested in digging too far into a car to trace a fault such as this one.

Regards and thanks



John

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Re: Re-hashing an old problem

Post by M60NJP » 05 Sep 2016, 18:18

This is probably stupid suggestion of the week, but have you checked the pedal box? It's vanishingly unlikely - but possible - that the brake pedal is slightly snagging the throttle?...................

worzel
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Joined: 12 Dec 2015, 10:31
Car model: 1991 525i Auto
Location: Merseyside

Re: Re-hashing an old problem

Post by worzel » 05 Sep 2016, 19:51

Hi

Thanks for your quick thoughts on this.

Any suggestion welcome- in fact your idea is one I've not thought of so I'll def investigate tomorrow.

Lots of past suggestions have revolved around air leaks being the problem. Fair enough- these can produce odd symptoms but I'm pretty sure these can be ruled out because of what I've done/checked so far.

I don't know about fuel injected cars- being more familiar with twin sidedraught setups- but any air leaks on carb fed engines tend (in my experience at least) to produce the worst symptoms when cold (fuel not being burned particularly efficiently on a cold engine) with the problem easing as the engine warms up. The opposite is occurring here- when cold the problem either doesn't exist or is somehow being masked- possibly by the ecu somehow compensating.

I'd emphasise that in a sense the idle quality is not the problem- upon braking and stopping if a surge occurs it last only briefly then the idle is fine even if idling for long periods. The problem only resurfaces when next stopped. There are no misfires etc thru the gears as the box changes up, downchange operation is fine and the engine never stumbles (at least it doesn't now- it used to do so slightly and very occasionally).

All ideas welcome


Regards


John

worzel
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Posts: 36
Joined: 12 Dec 2015, 10:31
Car model: 1991 525i Auto
Location: Merseyside

Re: Re-hashing an old problem

Post by worzel » 10 Sep 2016, 09:53

Hi- again

Apologies if this is becoming boring but----

Standing back from the problem (and thinking!)- since in normal use ie acceleration and steady throttle there are no problems I'm making a bold assumption that the problem of intermittent surge at idle is linked to something that changes when the car is stopped.

The only things I can think of that change are-

The amount of air being fed into the engine as a result of servo operation
The throttle body/TPS position
The amount of fuel fed to the engine when the throttle is closed

With regard to these I'm pretty sure the servo and its connections are not the problem and since I've just swapped the TPS on the car for one that is known to work I can rule out that as a cause.
So one possibility might be the volume of fuel- or is this too much of a wild guess to be feasible- or have I overlooked other changes that occur when the car stops?

All contributions gratefully received.


Regards and thanks- incidentally I've checked the idea of the sticking pedal and unfortunately this doesn't appear to be a problem.


John

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Lewy
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Re: Re-hashing an old problem

Post by Lewy » 10 Sep 2016, 12:56

Could this be more of an electrical issue where there's something loose which shows up mostly under the force of breaking?

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Re: Re-hashing an old problem

Post by Lewy » 10 Sep 2016, 12:57

Does this help? http://www.bimmerforums.com/forum/showt ... p?t=635660" onclick="window.open(this.href);return false;

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worzel
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Posts: 36
Joined: 12 Dec 2015, 10:31
Car model: 1991 525i Auto
Location: Merseyside

Re: Re-hashing an old problem

Post by worzel » 12 Sep 2016, 09:50

Hi

Thanks for the input.

Some interesting suggestions. The brake servo is, I'm pretty sure, operating correctly. I've actually removed the inline air valve and off the car it works fine and holds vacuum. The fluid level is unchanged in the reservoir so no leaks etc.

It's possible that the suggestion of something becoming detached under the force of braking might be the problem but I've still to figure out what that might be as just what might shift enough to disturb a contact isn't obvious.

It's the worst type of fault to trace because it's not consistent and it disappears after a second or two after stopping- it would be easier to resolve if it was constantly present.

The only item I've not looked at is the water temp sender/transmitter. Am I correct in saying this not only registers temp but also has other functions that might impact on running?


Regards and thanks


John

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