Light throttle missfire

My Beat has a missfire problem.

At no load the engine will missfire as the revs pass through the 2-4000rpm range. A gas analyser indicates a slightly rich mixture which tends to point at ignition and incomplete combustion.

Out on the road it will missfire at very light throttle at almost any rpm. It feels like a fuel cut as you come off the throttle, then an ignition problem as you go back on or hold. Everywhere else it runs crisply. The caps in the ECU look fine. When I get a chance I'm going to go round with a scope looking at the various sensor signals.

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Originally Posted by Moriniman

The caps in the ECU look fine. .

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Id still put my money on it being an ECU problem.

Your problem sounds very similar to one I diagnosed to a faulty ECU, thou that was running noticably rich at idle as well.

Is your check engine light working correctly? e.g. comes on with ignition on and off after 5 / 10 seconds.

Any codes stored when diagnostic plug bridged?

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Originally Posted by Steve_M

Is your check engine light working correctly? e.g. comes on with ignition on and off after 5 / 10 seconds.

Any codes stored when diagnostic plug bridged?

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Check light is OK, the only codes I've seen were 9s when I was adjusting the ignition timing and mistook a very crisp straight red line on the flywheel as the timing mark. It turned out to be rust! Pushing the vehicle in gear found the right marks (which don't match the pictures, but what's new?).

I've changed a fair few failed caps in my time and I've left the ECU accessible for now. It's my daily driver at the moment, so I'm a little wary of playing with the ECU without a backup to hand. I do have a megasquirt on the bench, but it's set up for another project at the moment.

Fixed I hope!

I pulled the ECU again this lunchtime and planned to pull the caps one by one, I have a component tester here at work that will show up failing or out of spec. electrolytics.

Glancing at the 220uF cap I noticed that it was already a 105 centigrade rated part, as was the nearby 33uF. Somebody has been in here before! Then I noticed the big stain and corrosion on the inside of the lid from where a cap had blown its top.

I didn't like the look of the soldering done previously, so I took out the 220uF cap. Underneath the cap it was a bit of a mess, so I gave it a little squirt of PCB cleaner (Safewash 2000) and left it to sit. With the flux and electrolyte stains out the way, it looked like one of the pads for the capacitor was no longer connected to it's track. Some work with a scraper and fibreglass brush confirmed that this was definitely the case. The repair was no better than having a failed cap in there! I bridged the damage and put the cap back, then I re-did the other poor soldering before using the board cleaner again to get rid of the flux (highly recommended). With such an obvious fault found I've ignored the other caps for now. I made a note of the values and will order up a set to be on the safe side.

I didn't have time to actually drive the car, but the missfire that was very noticeable even at no load seems to have gone. I'll know for sure when I drive home.

Nice work!

Hope it's not one I did! Did it have an eeprom in a holder too?

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Originally Posted by witsend

Hope it's not one I did! Did it have an eeprom in a holder too?

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No, no Eprom socket fitted. Is there still an upgraded map available?

I'm delighted to say, missfire completely cured! So much more pleasant in traffic and the fuel economy should be better too.

I may take it up to Stafford this Saturday to look at another Morini race bike, then to Darley Moor on Sunday (15th) where my existing 1954 Morini Settebello is being given an outing.

I still have 1 eeprom I think, can anyone copy them?

i would very much like an eeprom at some point if there's a way of getting a new batch (does it remove the rev limiter tho? - id be a bit scared if it did!)

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I still have 1 eeprom I think, can anyone copy them?

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I half noticed a part number on the board, but didn't pay it much attention. If it's a plain Eprom or EEprom, then I should be able to read and copy it. If it's a uP with integrated Eprom/EEprom then I should still be able to read it unless it's been protected.

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i would very much like an eeprom at some point if there's a way of getting a new batch (does it remove the rev limiter tho? - id be a bit scared if it did!)

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It's unlikely that the 10,000rpm hard limit would have been lifted.

A rough rule of thumb is that you can get a total variation in power output (without missfire) of around 15% by changing the air/fuel ratio. Maximum power is generated running richer than the 14:1 stoichiometric mixture needed for the catalytic converter to be most effective. You still sometimes see people saying 'lean is mean'. That's a holdover from the carburettors days and lack of dyno facilities where getting the correct mixture could be quite tricky. They actualy mean going from a very rich mixture to a less rich mixture. The excess fuel helped prevent engine damage by supressing detonation and providing a cooling effect to valves and pistons. You're only going to gain a tiny bit of power by remapping the fuelling because 14:1 is reasonably close to optimum anyway.

There's a lot more power in the ignition map, up to around 40% variation. Maximum torque is generated when peak cylinder pressure occours about 20 degrees after TDC and the ignition timing can be adjusted to achieve this depending on load and rpm. The problem is that you might run into detonation (knock) first and you can destroy an engine very fast that way. A dynamometer and/or a sophisticated knock detection system is pretty much essential if you're going to press the limits. Cars that have knock detection and adaptive timing can take advantage of high octane fuels and give a performance boost. Likewise they can run on a low octane fuel without damage. Manufacturers mostly leave a generous margine for engine safety and emissions complinace, so an upgraded ignition map can give you a significant boost in performance.

Some re-maps mostly give the impression of more performance rather than an actual increase. Making the throttle response a bit abrupt and introducing a surge can trick you into thinking you've got more power, when all you've done it make it less tractable. Even on track tractable often wins, on street I'll take it every time.

On the board it says 27C256 Eprom

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Originally Posted by Steve_M

On the board it says 27C256 Eprom

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Got loads of those! We have more than 100 glasshouse controllers at work. Each one uses three 27C256's to hold the firmware. Big stock of (used) spares on the shelf!

Onecall (Farnell) only list the one time programmables (PROMs) these days. These are actually a better choice as EPROMs don't have an infinite life. We have had equipment on site that has suffered from 'Bit Rot' in as little as 15 years. I don't think we've ever had to scrap anything because of it, there's always been another unit to copy from, but we do try and take an image from an EPROM and back it up somewhere.

I'd be happy to put a chip in the reader and burn some copies. I'd also be happy to replace caps for anyone not confident of doing it themselves.

Hey Paul, drop me a line to wits.end #ntlworld.com (do the obvious with the #!) with an address and I'll pop it down to you.

From memory it removes the limit and improves torque a bit.

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Originally Posted by witsend

Hey Paul, drop me a line to wits.end #ntlworld.com (do the obvious with the #!) with an address and I'll pop it down to you.

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Email sent.

Hi there
Can I ask if you ever managed to copy the EPROM
Thanks

Doh! Now where did I put them and who was going to do them?

I don't know where you put them, but I was offering to make the copy. I did drop you an email, don't know if you ever got it.

I've never managed to get notification emails from this forum to work BTW.

Hi Paul, I've found the eeprom now, drop me a line to Paul.Whittaker2.pw# Gmail.com (replace the # with @ obviously )

If you're still OK to copy these. I'd like 10 doing if possible! (let me know the costs)

Paul w

If at all possible,I would like to buy one as well!