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Old 06-26-2011, 06:39 PM   #1
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Default Misfire when on boost

Started having some issues on the dyne during the week, pretty much what happening is, when the car comes on full boost it splutters and misses, around 5k rpm. Almost as if it's hitting a limiter. Although if you part throttle it, it will drive through, but once you hit wot it dies.

So, I've changed the plugs, gaped them, change the fuel pump, changed the coil.

So any other suggestions? Really stumped on this, and want it sorted so I can get back on the rollers to finish the mapping!
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Old 06-26-2011, 06:45 PM   #2
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Do you have a wideband? How are the AFRs?
Whats your timing map like?
What spark plugs are you running?
What fuel are you running?
Have you tried running down the tank and getting fuel somewhere new?
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Old 06-26-2011, 06:49 PM   #3
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What coil setup are you running and how much boost? What are your spark plugs gapped at?
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Old 06-26-2011, 07:23 PM   #4
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I do have a wideband, afr's are around 13/14 at that rpm, boost would be coming up to around .8 bar when it starts to miss. Normal 95 pump fuel, has had a full tank through it. Not sure On timing/ spark tables, the dyno'r would know that stuff.

Could it be a dodgy injector or fuel pressure reg?

It drives perfect up to the point when it starts to miss
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Old 06-26-2011, 07:35 PM   #5
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Overboost protect? (eg: the ECU cuts off fuel and spark if your MAP exceeds a certain limit.)


A log would be quite helpful.


Probably unrelated (unless you've got one bad injector) but those AFRs would scare me at .8 bar. You're definitely on the lean side of what most folks consider safe.
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Old 06-26-2011, 09:11 PM   #6
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Do you have the prediction enabled in the ignition settings? I was having this same problem and after searching the forums I disabled the prediction and this solved it for me.

Although for me it was doing it when revving the engine out of gear and it sounds like you're only having this problem under load?
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Old 06-26-2011, 09:21 PM   #7
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14.x:1 AFR in boost?
Are you intercooled?

I want to make sure its not detonation before getting into ignition problems as often times people dont know what detonation sounds like and mistake bad detonation for "missing" or "breaking up"
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Old 06-27-2011, 11:18 AM   #8
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Okay guys, went for a spin there, I am not seeing 13 14 on full tilt, its actually more like 11, my mistake sorry!

I think it could be my tps, dont think Im getting a signal from it! So Im going to look into it and see what I can find, hopefully thats it!

If anyone else has had a similar issue, please help as wiring and the likes really hurts my face!
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Old 06-27-2011, 03:15 PM   #9
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I can't imagine how TPS would be causing this- lots of folks running without one.

I still want to see an MSQ and a log proving to me that you're not hitting overboost cutoff.

That said, I had a problem similar to this on the red car back when I was still running a stock CAS. Misfire only when under heavy boost, observable in the logs as a momentary loss of trigger integrity (sudden, huge RPM spike). I never captured the data to actually prove it, but I always suspected that the culprit was noise on one of the CAS wires, being induced into it from the injector wires, which would be running at their maximum average current at the high duty cycles seen only at WOT operation. No amount of filtering ever solved the problem, so I just made the whole CAS go away.

Hopefully the problem is simpler in your case, but there's about a thousand different things that could be wrong, and a map and a log are going to be a helpful first step in diagnosing this.
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Old 06-27-2011, 05:53 PM   #10
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Okay I played around with boost controller and dialled the boost down to .5bar, no it revs cleanly though, but once I go higher it hits the problem.

Checked over boost protection, and it's set to 16Psi, or 1.2bar? So don't think it's that. Will try a See if I can do a log tomorrow, not reAlly sure how, but I'll figure it out.
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Old 06-27-2011, 05:57 PM   #11
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To take a log you do the following.
1. Click the "Data Log" tab in TunerStudio.
2. Click "Start Log".
3. Drive around, when you get the symptoms hit space bar. This will cause a mark in the log so we know where to look.
4. Clock "Stop Log"
5. Upload log and MSQ.
6. PROFIT
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Old 02-18-2012, 11:44 AM   #12
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Hello ladies and gentlemen, just bumping this thread back up again! Havent had the car out since the summer. But now that the weather is getting abit better i pulled it out of hiding and started working on it again.

So need to get the above problem sorted so i can go and get the mapping completed.

Today i change the tps and put a different coil into it, with no joy.

I remember my mapper saying that he wasnt receiving a signal in the megatune from the tps and that it may have been the issue.

THe main problem at the moment is that i dont have my megatune laptop at the moment, its broke, so i cant uplaod any logs for you guys to look at!

But if anyone has any ideas or areas to start looking into then let me know. As i really want to get this probelm sorted and start driving the car again.
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Old 02-18-2012, 01:13 PM   #13
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Can you at least share your MSQ file with us?
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Old 02-18-2012, 01:36 PM   #14
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try updating your megasquirt firmware. I had some random misfires that went away after I updated
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Old 02-22-2012, 06:20 PM   #15
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You didn't mention what you have for ignition or spark plug gap (or I missed reading it). That RPM is usually the peak torque which can screw up with your spark if it is not strong/big plug gap.
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