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Thread: Sudden power loss at WOT 4k RPM...?

  1. #1
    Spending money on cars Neeek's Avatar
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    Sudden power loss at WOT 4k RPM...?

    Have an odd one that I realise is going to be very hard to diagnose online, but what the hell...

    Turbocharged BMW M30 3.5 six, running 7psi, big injectors, aftermarket ECU (Megasquirt), everything working well.

    So at WOT, when I hit about 4k RPM the engine basically has a fit - revs drop, no power. Lift off and all is well again. Lean on it slowly (like 2/3rds throttle and feed it in) and there's no such problem. AFRs are in the low 12s as this is happening, ignition map is conservative under boost, I've probably taken too much timing out of it but I consider that to be something safe in my tune for now. I'm not running big boost, so I think it *may* be spark related. Gap too big with sudden onset of boost? Ignition system is stock, but in good condition as I replaced everything before I turbo'd it.

    Car runs great otherwise, and I can of course drive around this problem very easily because I don't need to got WOT on the street.

    Any ideas?

  2. #2
    Non Compos Mentos Gammaboy's Avatar
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    regap smaller and try again?
    If that's the problem, put a MSD on it
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    Arrogant wankeler Slides's Avatar
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    Does sound like ignition.

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    Compulsive modifier awdmoke's Avatar
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    Fuel pump not keeping up?
    Do you have a fuel pressure gauge on it?
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    Coils on the plugs, or separate? Maybe breaking down or leaking at WOT... Maybe just losing enough spark when boost is full on that it's too much for it to overcome?

  6. #6
    Spending money on cars Neeek's Avatar
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    Quote Originally Posted by Gammaboy View Post
    regap smaller and try again?
    If that's the problem, put a MSD on it
    Yeah, that's my first port of call. Trouble is they're a royal cunt to get to now..! I confess I didn't re-gap them from when I changed them before the turbo was slapped on.

    Quote Originally Posted by Slides View Post
    Does sound like ignition.
    I thought it probably would be.

    Quote Originally Posted by awdmoke View Post
    Fuel pump not keeping up?
    Do you have a fuel pressure gauge on it?
    Nah, can't be that as if I get on it slower everything is fine. If the fuel pump were at fault, it would do this without smashing it open to WOT, would it not?

    Quote Originally Posted by nutttr View Post
    Coils on the plugs, or separate? Maybe breaking down or leaking at WOT... Maybe just losing enough spark when boost is full on that it's too much for it to overcome?
    Separate, old school set-up, everything is less than 6 months old. Again, seems odd that it only does it when I smash it... gradual increase in throttle and it's not a problem.

    I guess I'm going to try and pull some plugs at the weekend. Fuck, that'll be fun...

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    Tried clicking "ignore" on the "Danger to manifold" warning?

  8. #8
    Non Compos Mentos Gammaboy's Avatar
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    Probably easier to put a CDI on it than change the plugs :P
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    Had this same problem on my 1jz poopra, it had the stock fuel pump from 83 which couldn't keep up with the demands of the 1jz under boost @ wot, 2/3 throttle it would rev out but WOT it would break down, new fuel pump (walbro 255) and it was golden.

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    Spending money on cars Neeek's Avatar
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    Quote Originally Posted by kedderz View Post
    Had this same problem on my 1jz poopra, it had the stock fuel pump from 83 which couldn't keep up with the demands of the 1jz under boost @ wot, 2/3 throttle it would rev out but WOT it would break down, new fuel pump (walbro 255) and it was golden.
    Interesting. So sudden increase in fuel demand did it. Might investigate a fuel pump upgrade as well as re-gapping plugs.

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    Opens Guido's Avatar
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    If the plugs are that much of a sea hunt to get to, maybe put some Platinum Denso plugs in so you can forget about it for a while?
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    Spending money on cars Neeek's Avatar
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    Because I'm lazy and haven't done the plugs or the pump, I managed to get a log of what the car's doing (other than the obvious) when I smash the loud pedal.

    Basically it's a fuel cut. Fuel actually stops, AFR hits peak lean, etc. Very weird. Did some Googling and it seems I'm not alone. In the Megasquirt software, there are some sections on fuel enrichment for accel and decel, and because I don't have a variable TPS the ECU is seeing a big increase in MAP, no change in TPS and it goes "yep, odd, let's cut fuel".

    Disabled fuel cut by entering a different number in the box, as per what others have done when faced with similar issues, we have fixed the problem. Now pulls cleanly to redline and rev limit with the foot to the floor. And I'm enjoying this new fangled WOT business...

    Tuning by Internet, gotta love that.

  13. #13
    Registered User Milkman Don's Avatar
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    Does it actually lean out though or just a 1s & 0s anomaly?

    We toyed with a spare LS t/b I had on mates 535, couldn't think of a reason wouldn't work just never got to it. Worth a look.

    Also don't forget the cams in these things are hollow, they do have a tendency to snap on stock engines.
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    Spending money on cars Neeek's Avatar
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    It actually leans out, yes - goes from a fat 12 to a very thin 22 in less than half a second - because fuel cut will do that.

    Well, it did, but doesn't any more.

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    Registered User JZK25's Avatar
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    There is a TPS that fits onto the throttle to give proper variable signal. Can't remember what it was off but I think it was a kia.
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    Spending money on cars Neeek's Avatar
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    Yeah, Al - it's on the list of things to get. Apparently an auto 535 one works too. Just that manual 535s didn't get a variable TPS, obviously not needed, for German reasons no doubt...

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    Spending money on cars Neeek's Avatar
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    Fucker's doing it again. My Internet tuning trick didn't work after all. Sadface.

    In letsseeifthisworks news, I have procured a variable TPS from an auto E34 this afternoon. Will splice it in sometime this weekend and then MS should get a signal, and perhaps this will solve the problem of the mismatch between MAP and TPS when I give it the beans.

    Then, fuel pump.

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    Sudden power loss at WOT 4k RPM

    What would cause a Loss of power and misfire at wot. I put in new plugs and wires acouple of months ago and it ran fine this problem just started to occur. I inspected the plugs,wires, and coils all seem to be fine. Has anybody had this problem or know what it could. Oh yea another the car will shift and accelerate fine at 1/4 throttle but when flooring it, it will sputter crazy.

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    fuel pump

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    Hungry Hungry Hippo Tripper's Avatar
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    Would throw a fuel filter at it first, as its the cheapest thing. Obviously not when you are doing your days work with the finger in the hole.
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    Mate had a similar sounding issue, turned out part of the induction system prior to the turbo was collapsing and cutting of air supply. Actually I've heard of it twice one was a silicon hose collapsing the other a cheap plastic air scoop. GoPro is your friend, set it up to watch while you replicate the issue.

  22. #22
    Purist, whats that? Jason Broadhurst's Avatar
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    Post a datalog please. It's likely one of many inputs to the ecu tripping out, or a random setting in the MS. They have lots of sneaky settings to check for.
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  23. #23
    "The bitches, are wet" Sketchy's Avatar
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    Fuel pumps in these things are typically overkill (036 Bosch I think, same as VL turbo, maybe even an 044 on the big bmw's?) and having driven it, the cut is savage so my money would definitely be on internal MS programming. When it's not doing that shit its a proper rapid Q car, you'd be hard pressed to even tell it's boosted when driving it. Very quiet, no whooshy turbo spool or wastegate noises whatsoever.

    Got that variable TPS wired in yet guvna?
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  24. #24
    Spending money on cars Neeek's Avatar
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    Nah, will try and get the new TPS in this weekend. If I have time, I'll chuck the Walbro in too.

    JB, can send you the log if you like, but you might need some software to view it. Perhaps that long overdue tinker is in order?

  25. #25
    Purist, whats that? Jason Broadhurst's Avatar
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    Yeah for sure. Email me the log for now and we will do some work if I don't find anything.

    The log is likely a CSV so I can open it in MATLAB.
    Jason Broadhurst

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  26. #26
    Purist, whats that? Jason Broadhurst's Avatar
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    Jason Broadhurst

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  27. #27
    Spending money on cars Neeek's Avatar
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    I may have found the problem here...

    I've not touched anything to do with Boost Control in Megasquirt, largely because I don't run any form of it on the car, only running to wastegate spring pressure, no solenoids or anything, so it's all mechanical. For a laugh, I went in and had a look to see what was in there and found out that despite there being no boost control settings activated (good), there's a little bitty section on overboost protection by means of a fuel cut. Interesting.

    So when looking at the log I took when the car misbehaved, I have a gentle pull in first with no problems, getting to a MAP reading of about 145. Then when I nailed it in second, boost peaked at a MAP of 153 (7.7psi or so) which is good considering my wastegate spring is 7.5 or 7.75 (I forget which). Problem is, the overboost protection kicks in at a MAP of 150, as set by the initial tune I got from DIY Autotune.

    In a proper lightbulb moment, I plugged in the laptop to the car and set it to a MAP of 168, which is a smidge under 10psi. I'm going to be running the kids around this afternoon to rugby and dancing, so whilst I haven't tested it yet I'm quietly confident that this has been the issue all along.

    I shall report back.

  28. #28
    Spending money on cars Neeek's Avatar
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    Haha... that was it.

    Many whoosh-fucka noises up the top end now, Sketchy. Case closed, I hope...

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    Registered User PXL265's Avatar
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    So where are the pics of skids whilst dropping kids off to rugby and dancing to impress all the soccer mums???

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    Quote Originally Posted by PXL265 View Post
    So where are the pics of skids whilst dropping kids off to rugby and dancing to impress all the soccer mums???
    Too busy staring at their tits and arse, sorry...

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