156 v6 slightly misfires on idle - Alfa Romeo Forum
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(Post Link) post #1 of 17 Old 17-07-16 Thread Starter
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SOLVED ! 156 v6 slightly misfires on idle

Hi guys. Recenlty i've encountered a problem with my v6. Fuel consumption went up, idle is kinda erratic and you can hear the engine slightly misfires.. This happens mostly when the car is hot, otherwise it pulls well and runs good. I guess it's overfueling since exhaust gases are pretty strong.

Car is Alfa 156 3.0 v6 swapped from 1.6 TS 6 months ago. It uses 2.5 setup from 156 with ME2.1 CF2 ECU, 1 lambda and 3.0 CF2 injectors.

Things i've tried so far: MAP swap, changed all 6 coil packs (used), cleaned spark plugs (all 6 sooty), changed fuel pump (used), changed ECU with other ME2.1. Checked for air leaks..
Things left to try: new lambda, injectors change.

I've recently acquired another 2.5 v6 CF2 for spares and it runs and idles like a dream. So i've hooked them both up and checked the parameters. Here are the results..

http://i.imgur.com/cXfxbGm.png

So 2.5 shows higher air kg/h rate than 3.0 on idle. Lambda on 3.0 doesn't make sense while on 2.5 it behaves as it should..


Any thoughts ?

Last edited by moth3; 24-07-16 at 15:35.
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compression test it so you can rule out pitted valve seats.
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(Post Link) post #3 of 17 Old 17-07-16 Thread Starter
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Unfortunately i'm not able to do a compression test at the moment.. At least not until proper tools arrive
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Sounds like wrong camshafts timing.
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A slight missfire just at idle can be a valve not sealing properly, usually an exhaust valve and as you seemed to of ruled out most of the possible other electrical causes and its happening when its hot this is looking likely.
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Your lambda plot from the 3.0 is all wrong - should look like the 2.5, flipping every second or so.

See here:

Alfa GTA Sportwagon - Page 1 - Readers' Cars - PistonHeads

Assuming power is still good at speed, I would be suspecting camshaft timing.
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(Post Link) post #7 of 17 Old 19-07-16 Thread Starter
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Yes, i can see the lambda reading is messed up. Car is jacked up at the moment as i was trying to undo the lambda for a half an hour, but it wont budge. Had to take a break and cool off.

Timing is the last thing on the list to check since i've done the timing myself couple of months ago when i was assembling the engine. Proper tools were used and everything was done by the book. I'm not saying it's impossible that timing is messed up but i'd like to eliminate all of the other easier possibilities.

Thanks for your input.
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Does the trace of front and back lambdas look the same? Lambda 1, pre-cat are the interesting ones.
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(Post Link) post #9 of 17 Old 19-07-16 Thread Starter
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It's CF2, only one lambda. It's pre-cat.
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Easiest way I have found to get the Lambda's out is to remove the pipe from the car, stand on it. Then use a 6 sided crows foot spanner head (22mm I think) and a 2ft breaker. It won't even put up a fight.

Of course, you could just swap the manifolds over complete with lambda and see if the misfire moves with it.
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(Post Link) post #11 of 17 Old 20-07-16 Thread Starter
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Changed the lambda, no improvement.

New readouts:

http://i.imgur.com/70NJFWV.png

I went ahead and checked 2.5 for ECU faults and found there is a lambda heater fault so perhaps 2.5 readings are not accurate. But still, it idles great, while 3.0 shakes a bit and "skips" every now and then.

EDIT:

Tomorrow i'll change the injectors with spare ones i have. I guess it might be leaking injector.

Last edited by moth3; 20-07-16 at 20:05.
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(Post Link) post #12 of 17 Old 21-07-16 Thread Starter
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Well this was a waste of time.. Changed the injectors for the spare ones and they made it worse than before. Idles like it's lost a cylinder and lacking power. Lacking a lot of power.

On the other hand, injection times went from ~3ms with old injectors to ~5ms with the new ones. MAF is reading a bit higher values, similar to 2.5 one. It could be the new injectors or i have done something to it when i was replacing the plenum and intake pipes.

As for old injectors, i've checked them for leaks and there isn't any. Tomorrow i'll check the spray pattern and flow rate on all 6 and fit them back if everything is fine. Changing the rings could be a good idea as well.
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Stick with the 3.0 injectors on the CF2 ecu - the 2.5's run lean at high revs as standard, with the 3.0 injectors it should be about right.

I had a similar issue with my cf2 156 when it was running a GTA engine. I went through everything before we found the ecu had failed. The wiring connector had worked free from a coil pack, which burnt out the low tension output inside the ecu.

But you say you've tried a spare ecu?

And now that you've changed the lambda the only thing that springs to mind is an issue with the engine wiring harness - they have been known to fail.
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(Post Link) post #14 of 17 Old 21-07-16 Thread Starter
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Quote:
Originally Posted by Chris155 View Post
Stick with the 3.0 injectors on the CF2 ecu - the 2.5's run lean at high revs as standard, with the 3.0 injectors it should be about right.

I had a similar issue with my cf2 156 when it was running a GTA engine. I went through everything before we found the ecu had failed. The wiring connector had worked free from a coil pack, which burnt out the low tension output inside the ecu.

But you say you've tried a spare ecu?

And now that you've changed the lambda the only thing that springs to mind is an issue with the engine wiring harness - they have been known to fail.
Thanks for your input.

I forgot to mention that the spare injectors are also 3.0 CF2 white ones but it seems they are clogged since they were off car for too long. Also one interesting thing.. Before exhaust was watery, with water dripping and now it's dry. Also fumes are not that strong so it could be running lean.

It's currently running on a spare ECU, also 2.5 CF2 one. Same symptoms so i guess we can rule out the ECU. Tomorrow, old injectors are going back and i'll have the tools for the compression test.
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(Post Link) post #15 of 17 Old 23-07-16 Thread Starter
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Well, seems like i killed the car. Today i've replaced the injectors with the old ones and did a compression test. I've removed the main pump/injector relay in order to disable the ignition and fueling. After the test, placed everything back and had a little surprise. There is no power to the ECU, no power to the pump, no code key test light and no injector light (check engine) test light. Also throttle body is not buzzing as it should when the ignition is on.

I've searched around a bit and it all comes down to that one relay that controls the pump/ignition/injection. So i've replaced it once, twice, 5 times with 5 different ones.. Nothing. Then i've cut off and replaced the box (brown one) that holds it, still nothing. Checked all the fuses on main box and the caged box above it, all good. I should mention that the main relay is heating up a bit after having the ignition on for a while.

I'm running out of ideas, and i'm not too happy about replacing the whole wiring harness all over again.

Any idea if it's possible to bypass that relay and do a direct 12v ?
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(Post Link) post #16 of 17 Old 24-07-16 Thread Starter
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Just tested the other car's relay with the probe lamp. There is constant power wire from the battery, ignition driven +, negative and the lead to the pump/ecu. On my car, ignition driven + works, but the constant power wire acts like a negative lead.

How can pulling and replacing a relay mess up the wiring so bad ?

EDIT:

Got the power back to the constant power from the battery, the fuse on + was blown. But the wire to the pump/ecu is showing short curcuit to negative. It's blowing the 30A fuse on the battery lead assembly.

EDIT:

SOLVED !!

Looks like i wasn't very careful when putting back the coil packs and i've accidentally clamped one of the wires which made a short circuit and kept blowing the fuse..

As for the erratic idle/misfire problem it seems that it's solved as well.. The idle is good and stable, engine doesn't skip. The solution for that could be one of the following:

There was a broken cap on one of the injectors which may have caused the injector spray pattern deformation, replaced it. There was also a leftover evaporation box from when it was a TS engine, replaced it with the v6 one which has the evaporation valve on it. It was throwing an ECU fault before since on TS one the was no place for the connector.

Connected the old ECU back, cranked and it fired right away.
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Last edited by moth3; 24-07-16 at 15:43.
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Well done, glad you've fixed it.
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