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Occasioanlly Runs On 5 Cylinders After Nearing Redline

ShayneJ

By ShayneJ,
in Engines & Forced Induction

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ShayneJ Explorer

ShayneJ

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Heres the story:

My first motor went pop so I bought a replacement from a friend. Put the motor in, and this motor had a weird issue of occasioanlly running on 5 cyliders when you near redline. Then after 5-15~ seconds it will come good again and run like a dream... 2 weeks after this motor was put it, it also went pop and put a hole in piston 1 but i believe it to be because the boost controller failed and ran a fair bit of boost.

Now, instead of buying another new motor because it was starting to get expensive. I bought a new complete bottom end from somebody on this forum. We mated the head from the motor that just went bang up together with new gaskets. The thing is this new motor is doing the same thing. It will occassionally run on 5cyliders when you rev it to redline. and again, after 5-15~ seconds it will come good again.

We had done a compression test on the motor and got these results: 120 - 125 - 130 - 135 - 140 - 140

I am running splitfire coil packs, which inspected a few days ago and they don't have any cracks.

I have been told it could be something to do with the injectors, but then why would it still run rough once the revs had dropped and come good again not depending on the revs?

I haven't driven to car much at all over the last 4 months, scared that it will break again....

Before you tell me to put it on a dyno, the closest one is 500kms away so its not exactly practical...

Thanks

ShayneJ Explorer

ShayneJ

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I can only seem to get the problem to happen when its under load. And 5-15 seconds isn't enough time for me to pull over, pop the bonnet and start pulling coil pack plugs off, LOL....

I was thinking its a problem with a valve aswell, but didn't want to bias the answers of people on here. Is there an easy way to test without pulling the motor apart?

Anyone else have some input?

Dearlove Collaborator

Dearlove

Posted
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I can only seem to get the problem to happen when its under load. And 5-15 seconds isn't enough time for me to pull over, pop the bonnet and start pulling coil pack plugs off, LOL....

dyno it, might cost money, but cheaper than a new engine :D

ShayneJ Explorer

ShayneJ

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GTR fuel pump

splitfire coil packs

RB20DET computer remapped to run the 25

KKR480 turbo running at 13 psi, making about 320hp at the wheels.

Everything else is standard i think. Was purchased with all mods done. Thing is it never had a problem until the first motor went due to bearings and we put the 2nd hand motor in. Thats when the problems started to arise. Run sweet as before it spun the bearings... Now its just been a nightmare

ShayneJ Explorer

ShayneJ

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My guess is it is valve floating causing the lifters to pump up.

Can you elabortae a bit more? Im not a mechanic and don't exaclt understand what that is... And what is needed to be replaced to fix it and is it exepensive?

ShayneJ Explorer

ShayneJ

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my guess is your injectors are maxing out

I am actually running less boost than with the other motor, and had no issues there. And if it were injectors, wouldn't they spring back to life as soon as you drop below the max cycle, not 15~ seconds after the car is at idle/minimal load?

Edited by ShayneJ
joeyjoejoejuniorshabadoo Grand Master

joeyjoejoejuniorshabadoo

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sorry... i wasn't referring to the problem at hand... was just mentioning it because stock injectors will be dancing around 100% duty cycle to be flowing 240rwkw

Guilt-Toy Grand Master

Guilt-Toy

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are you hitting rev limiter before it starts to miss?

does it only do it when your revving high then go back to idle and low rpm staight away ? does it do it when your revving to a max of 3000rpm?

MBS206 Grand Master

MBS206

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Do a compression test, but put some oil down the bore of each piston just before you go to test it.

If compression comes up higher then it was when dry, you have a dud piston/ring.

If it stays low, valves/head is dud.

Test each dry, then test "wet" (Oiled)

This will pin point obttom end or head in comp issues.

Did you have the head serviced or crack and pressure tested when you had it off?

ShayneJ Explorer

ShayneJ

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are you hitting rev limiter before it starts to miss?

does it only do it when your revving high then go back to idle and low rpm staight away ? does it do it when your revving to a max of 3000rpm?

Yes, it only starts to miss when up high. It wont do it at 3k... And when it starts to miss, even dropping the revs back to around idle it will still miss and complain for a bit and then come good...

Do a compression test, but put some oil down the bore of each piston just before you go to test it.

If compression comes up higher then it was when dry, you have a dud piston/ring.

If it stays low, valves/head is dud.

Test each dry, then test "wet" (Oiled)

This will pin point obttom end or head in comp issues.

Did you have the head serviced or crack and pressure tested when you had it off?

Did the dry compression test, and got results i have in the first post. Will haver to give a wet test in the next few days...

And no, did not get the head tested or serviced.

Guilt-Toy Grand Master

Guilt-Toy

Posted
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this used to happen to me after a race or a big rev limit mission or even after comming off boost on higher RPM's... what is happening is that when you back off the throttle the engine runs really really rich for a second or so because air flow was stopped instantly but the AFM still thinks the air has gone through the engine...

The plugs get flowled up and one or 2 can stop firing untill it heats back up again and drys out then the cyl's start to fire again.

This only happened to me with my very very rich tune, .78 on boost (10.8 11.0 afr) once i retuned it after more km's on the engine to normal AFR's the problem went away.

So i am guessing your tune up top is very rich. chuck it on a dyno and post up the AFR graph to power graph's and i bet your running 11.0 AFR.

Fix the tune, the problem will go away

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