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Discussion Starter · #1 ·
(Car has full and proper service history and just turned 70,000 miles)

Description of problem:

1. I start the 840, drive normally, at low revs, I could drive for 200 miles at 60 or 70 mph with no problems;

2. If I use kickdown and really start to use that M62 engine, within a few minutes, I feel a slight loss of power and the engine is running not so smoothly. It is now almost impossible to get the car over 100mph;

3. I bring the car back to base and it is now idling unevenly as if one or more of the cylinders has dropped out;

4. I go out to car the next morning and all is fine, all 8 cylinders in harmony;

5. Symptoms will return if I demand high performance as in step 2;

6. Plugs, plug coils, mass air-flow sensor, oxygen sensor, crank sensor - all checked or replaced - engine pressure tested - all in order;

7. Computer reads "misfire on cylinders 5 and 7, possible catalyst damaging;"

8. Has anyone got any suggestions about what I should try next? Some friends have suggested a restricted fuel filter, restricted injectors, or a catalyst on its way out.

I will be very grateful for your thoughts on what I should try next.

Many many thanks,

Lloyd
 

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Hmm, these types of faults can be difficult to diagnose.

After driving at high speed, say, for 20 minutes, do you have catalyst overheat (smoke, crackling noises from the exhaust, etc)?

If so, fuel from the misfiring cylinders is sent into the exhaust system indicating that a fuel supply fault is unlikely.

One thing that can cause the symptoms you describe is an internal breach in an ignition cable. If the cable has been taken off a plug by pulling the cable instead of the connector this can sometimes happen. If the ensuing gap is small there will be an internal spark but the cable will work. If you stress the ignition system at high speed the ignition pulse can no longer bridge the gap and you will get a misfire. The plug will foul up with fuel, which is the reason it will not resume firing even when the speed is reduced. Only after it has had time to dry will it start to work again.

A marginal rotor/cap can probably cause something similar but based on your description my first thought is on some ignition cables.
 

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Discussion Starter · #4 ·
Were the codes read using a BMW specific scanner?

And the sensors that were changed, were they genuine BMW items or aftermarket, if so were they a good make?
Dear Ronnie,

Thanks for your time - the scanner is BMW specific and the replacemnt parts were genuine.

The fault is always on the same bank of cylinders, i.e. 5, 6, 7, 8 all have shown up at different times on the fault codes.

Today, I had a 200 mile round-trip to do. I cleared all the faults from the computer before I started out. I kept the revs low, and had a very pleasant drive, including motorway driving at 70/80mph. No problems.

On my return journey, I was free of of problems until I had to wait for about ten minutes at idle in roadworks. It was a warm day (well warm for Northern Ireland) and the sun was belting down on the black bonnet. Air-con was switched on. When I went to move off, I felt the misfire. I checked the ECU when I arrived back home and it showed up misfire in cylinder 8. The car was firing on all 8 after being parked for about hald an hour at home.

I am working on the theory that if it is sensor related, then these sensors must be in pairs, because Cylinders 1 to 4 never misfire. There are two pre-catalyst oxygen sensors and two post catalyst oxygen sensors i.e. a pair for each side of the V8. I am considering swapping these around, or disconnecting one at a time to try to emulate the symptoms.

I'll let you know how I get on - any advice in the meantime about my idea for checking these is appreciated,

Best regards and many thanks again.

Sincerely,

Lloyd
 

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Discussion Starter · #5 ·
Hmm, these types of faults can be difficult to diagnose.

After driving at high speed, say, for 20 minutes, do you have catalyst overheat (smoke, crackling noises from the exhaust, etc)?

If so, fuel from the misfiring cylinders is sent into the exhaust system indicating that a fuel supply fault is unlikely.

One thing that can cause the symptoms you describe is an internal breach in an ignition cable. If the cable has been taken off a plug by pulling the cable instead of the connector this can sometimes happen. If the ensuing gap is small there will be an internal spark but the cable will work. If you stress the ignition system at high speed the ignition pulse can no longer bridge the gap and you will get a misfire. The plug will foul up with fuel, which is the reason it will not resume firing even when the speed is reduced. Only after it has had time to dry will it start to work again.

A marginal rotor/cap can probably cause something similar but based on your description my first thought is on some ignition cables.
Dear Talyinka,

I cannot answer your question about prolonged high-speed, because I am too afraid of losing my licence!

Please see my reply to Ronnie below. Cylinders 5, 6, 7, 8 will misfire but on a random basis, for example, today it might be 8; tomorrow 5. This would seem to rule out an individual plug cable, but I will investigate the rotor cap.

I would appeciate your comments on my reply to Ronnie as pasted in below.

Many thanks for your time,

Sincerely,

Lloyd
***************************************************
Quote:
Originally Posted by Ronnie
Were the codes read using a BMW specific scanner?
And the sensors that were changed, were they genuine BMW items or aftermarket, if so were they a good make?

Dear Ronnie,

Thanks for your time - the scanner is BMW specific and the replacemnt parts were genuine.

The fault is always on the same bank of cylinders, i.e. 5, 6, 7, 8 all have shown up at different times on the fault codes.

Today, I had a 200 mile round-trip to do. I cleared all the faults from the computer before I started out. I kept the revs low, and had a very pleasant drive, including motorway driving at 70/80mph. No problems.

On my return journey, I was free of of problems until I had to wait for about ten minutes at idle in roadworks. It was a warm day (well warm for Northern Ireland) and the sun was belting down on the black bonnet. Air-con was switched on. When I went to move off, I felt the misfire. I checked the ECU when I arrived back home and it showed up misfire in cylinder 8. The car was firing on all 8 after being parked for about hald an hour at home.

I am working on the theory that if it is sensor related, then these sensors must be in pairs, because Cylinders 1 to 4 never misfire. There are two pre-catalyst oxygen sensors and two post catalyst oxygen sensors i.e. a pair for each side of the V8. I am considering swapping these around, or disconnecting one at a time to try to emulate the symptoms.

I'll let you know how I get on - any advice in the meantime about my idea for checking these is appreciated,

Best regards and many thanks again.

Sincerely,

Lloyd
 

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I can't recall how advanced the 840ci DME(ECU) is but on some, if using a AUTOLOGIC or BMW dealer scanner it will tell you exactly at what revs each fault code flagged up, what the mileage was, when the last time the fault occurred e.t.c, this may be beneficial to you in tracing your fault :)

And swapping the O2 sensors is a viable option to try :)
 

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Dear Lloyd,

I hadn't realised from your first post that the misfire changed position albeit remained in the same bank. It definitely seems to be temperature-related.

An intermittent signal from the cam position sensor could lead to misfire but that would most likely affect more than one cylinder at a time I should think.

A missing signal from the coolant temperature sensor, leading the motor control computer to think the engine is cold and hence operate the 'choke' could cause misfire but on both banks.

I assume of course that you are using the correct type of spark plugs (I have encountered incorrectly labelled spark plugs more than once - recognised brands too - actually cost me a racing engine once - which the spark plug manufacturer subsequently paid for).

Water ingress into cabling or connectors at high engine temperature could be a possible cause. Does the thing lose coolant at all?

Like Ronnie I would suggest you find somebody with an Autologic to try to pinpoint possible areas at fault, either by reading fault memory or by driving the car with the computer connected. The problem is that the diagnostic computers rarely find the difficult faults on their own. You always end up looking, feeling and ohm'ing...:D

Just some suggestions. I'm sorry I can't be more specific but my own E31s have M70 engines so I don't have much experience with the eight cylinder ones.:lol2

EDIT: It just struck me - though this is not the first thing you would consider - that an E-box fan failure could cause this type of fault at high engine room temperatures. If the E-box fan fails to start the Motronics could overheat and cause this type of random fault.
 

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Discussion Starter · #8 ·
840Ci Misfire at High Speed (Same engine as E39 540i)

Does the e-box fan run all the time or is it switched by temperature?

What I have done since your last post is as below:

Opened e-box
Could not be sure whether fan was running or not - I need to remove the e-box housing to verify this
Disconnected the two relays and checked for corrosion - all clear to the eye
Disconnected the two DME units and checked for terminal corrosion - to the eye all looked very fresh and clean.
Reconnected everything and sealed e-box with the four screws
Ran disgnostics - it picked up everything I had disconnected.
Cleared these faults.

Took car out for a 'hard drive' - using kick-down, and generally trying to raise engine-bay temp and cause misfire.

No misfire!

Too early to say - but is there a chance that disconnecting the DME has reset it? (It appears to have a permanent supply from the battery as one of the fault codes was 'battery disconnected')

If the fault returns, I will check the e-box fan and also meter the resistance of the engine coolant sensor in the e-box. (Ohms reading taken from Bentley manual)

Will keep you posted - many thanks again - keep me posted on any brain-waves you may have....

With best regards and thanks,

Lloyd
 

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The fan is controlled by switch B210. It is supposed to switch on at around 45 degC and off at around 35 degC.

With regard to disconnecting computers it is always good practice to disconnect the battery first. When taking these connectors apart I routinely clean them with contact cleaner, which can be acquired from suppliers of computer and audio service consumables.
 

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Discussion Starter · #10 ·
Dear Talyinka,

Unfortunately, the problem has returned. As you know, it is intermittent, and therefore quite difficult to pin down, unless I get hold of AUTOLOGIC or Main Dealer diagnostics.

To answer one of your previous questions, the cooling system does not lose any coolant.

One other piece of information: the problem seems to be worse when the fuel tank is low. The fuel filter has been changed but to no avail. The problem is still on the same bank of cylinders.

A few people have suggested that it may be a fuel restriction problem, hence worse on low fuel, however I wonder does the fuel pressure actually drop when the level lowers? Surely the fuel pump propels at a constant pressure? What do you think about a restricted injector or a restricted injector feed rail on the affected bank? Can these be checked and cleaned?

My brother is a qualified mechanical engineer (not on vehicles) but he does do all my servicing and has always done first class work, free of charge! My point is, that if you give me a technical description of a procedure for checking the injector system, he will be capable of understaning detailed mechanical instructions.

Sincere thanks once again for your time and help.

Kind regards,

Lloyd
 

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The fuel level should have no bearing on the fuel pressure unless it's running out.

And cylinders 5, 6, 7 and 8 are the passenger side bank so this is the area to look into.

I doubt it will be low fuel pressure as it should bring up a fault on all cylinders, or a dodgy injector or fuel rail.
 

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Like Ronnie I would consider an injector or fuel supply (mechanical) problem unlikely to be the cause. What we are looking for is something which is specific to the right bank and to individual cylinders but not tied to only one specific cylinder. The fault is also temperature related.

Good candidates: missing ignition or missing injection cycles caused by missing or erratic electrical signals of one sort or another. How many ignition coils does this engine have - one or two? If two, swap them and see if that causes any changes. If not I still tend towards suspecting one of the DMEs (oh, does it use one DME for each bank? Must remember to buy one of these 8-cyl e31s to learn what I'm talking about :D).

Whether or not it uses two DMEs I would still make sure that the E-box fan works. Where in the world are you? Perhaps we could take it for a ride with my MoDIC hooked up.

Finding one of these elusive intermittent faults is a bit like gaining knickers removal rights to the hot redhead down the road - you persevere, persevere, persevere :rofl.
 

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Discussion Starter · #13 ·
Good news gentlemen - Problem solved!

Hello everyone,

At last the mis-firing problem has been identified and solved.

I had left the car in my garage until I got my annual leave. Then I decided to investigate the one option left, i.e. the exhaust catalyst on the near-side.

I removed the exhaust section containing the catalyst and when I held it vertically in my hands, little pieces of the remainder of what was a catalyst began to fall down around my feet in a pile of grey/brown chunky pieces.

I completely evacuated the exhaust box which contains the catalyst until it was see-through from one end to the other. I re-fitted the empty pipe to the exhaust system and sealed the joints.

The 840i is now running like a kitten, although the exhaust note has changed slightly. I'm not sure whether I like this new note and in any case, to maintain the integrity of the car, I intend to fit a new catalyst in the near future. Does anyone know whether the nearside Vee exhaust wil meet the MOT emissions test?

So, it appears the crumbled catalyst was restricting the exhaust gases, particularly under high performance demand, causing the nearside Vee to misfire on a random basis.

Obvioulsy this didn't appear on any of my own prinmitive diagnostic kit, but I guess for anyone having the same problem, the catalyst could be tested by appropriate equipment. The only clue I had that the catalyst was causing the problem was that the fumes from the offside exhaust were sweet but the nearside were noxious, but this didn't immediately indicate to me that this was the cause of my problems. Also, occasionally, I thought I could hear a rattle in th eexhaust, but it was intermittent. I even used a sound meter to sample both exhausts and the results were similar.

Thanks guys for your help, particularly Ronnie and Talyinka.

Best regards,

James

PS. Does anyone have any suggestions for a competitive supplier for an E31 nearside catalyst?

Good luck and safe driving and thanks once again for keeping the faith!
 

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HI . My 840 CI 4.4 did this. wrecked everyones heads but I had a hunch it was a CAT problem. Had the exhaust canister opened only to find the CAT core had dislodged and partially blocked the exhaust gases on this side. Causing gas fouling on plugs and sending oxygen sensors wild also.
I cut both CATS out and she now performs perfect :) This Car had been to BMW with the problem and even they missed this?
Hope that helps.
 
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