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Old 09-11-2011   #1
90_gtr
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Default Would a misfire cause a knock?

I have a slight misfire at idle and when it misses I can feel the engine stumble and hear a knock noise from the head area. I bought the engine blown up and rebuilt it.

It now has larger forged pistons that I though may have been causing the knock noise but it's not a steady knock and it's only at idle and after the car has warmed up for a bit.

I think the misfire is causing the knock, does this make sense?
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Old 09-14-2011   #2
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Default Re: Would a misfire cause a knock?

Can you give some more info?
Does it sound like only one cylinder?
Forged pistons usually do make a knocking noise but it tends to get better as they warm.
Who built your engine?
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Old 09-14-2011   #3
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Default Re: Would a misfire cause a knock?

It almost sound like spark knock but that's usually is only higher in the rpm range. I think I found my problem though.

I start the engine let it run for a bit until the temp gauge starts to move then it will start doing the rough idle/miss/knock. It's not real bad but you can see the engine jerk and the hood prop rod shaking. So I've tested all my coil boiling hot(0.9), then cold and there all at 0.7, within spec.

So when it started doing it again I grabbed each injector clip and wiggled it. The knock would stop for a second then come back. I grabbed the harness loom for all the injectors and started moving it around and the noise would go away then come back. So I think I have a loose or dirty connection at the injectors. gonna change the fuel filter tonight and get some contact cleaner for the injectors.

I know it's not an internal engine knock because a rod/main/piston knock would do it at all rpms and would be a steady knock. This is more related to the miss/rough idle and only knocks when it will miss or stumble(only at idle).
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Old 09-14-2011   #4
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Default Re: Would a misfire cause a knock?

Good to hear. Good luck! Hopefully it's a free fix.
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Old 09-14-2011   #5
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Default Re: Would a misfire cause a knock?

Rplaced the fuel filter and cleaned all the injector pins and connector pins. Still same knock sound at idle. It's runs alot smoother now but still goes knock..knock..........knock...............knock..k nock?????

I don't get it. If it was a bearing, piston or valves making nose wouldn't the noise be steady, not erratic like my problem? I can set the idle at 1800 and there is no knock and all through the rpm range.

I took the manifold off when I first got the car running because the idle was steady at 1500rpm, AAC screw was already all the way in and tps was adjusted way down to try and bring the idle down. I bought the engine this way but blown, so now I'm dealing with someone elses high idle problem before they blew the engine.

Anyway I found that all the throttle plate screws way out of adjustment. I back them off with the linkage disconnected and got it to where the plate would stick in the bore and then turn the screw in 1/3 turn to keep it sealed. Put everything back together and the idle was at 650, adjusted tps and AAC valve, idle is a perfect steady 1000rpm. Since then I've had this knock noise, i don't have anything loose under the intake manifold, the engine also only had like 30kms since the rebuild. It might have taken some time for the break in period to develop this knock.
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Old 09-15-2011   #6
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Default Re: Would a misfire cause a knock?

that sounds strange but hopefully a bearing didn't get put in backwards or something. The only reason why I would say that is when your increase revs your increase oil pressure/flow which will help take up the slack in the wiped bearing. That happens. I really hope that isn't the case.
It's so hard to tell without hearing it.
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Old 09-15-2011   #7
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Default Re: Would a misfire cause a knock?

All the bearing where installed in the right position. The noise started happening or I started to notice it once I took the manifold off to set the throttel plates. But also before the manifold came off the idle was high so I might not have noticed it until the idle was brought down to normal.

I've been searching all over google for engine noises and nothing is like what I have. Every engine noise I've heard on google is all through the rpm range and get faster and louder as rpms go up. Mine goes away and is not steady. I tried to take a video and post it up but it just posts as a picture, not a video.

Anyway tonight after work it's going up on the hoist to check for a loose bracket or maybe and exhaust leak??? If I can't fix the problem the engine is coming out over the winter and I'll just get an r33 engine or maybe even an r34. I really think it's something to do with the bore that I got done and the wiseco pistons. The machine shop didn't really understand why the pistons were "warped" as he said??? Again it wasen't a small block chev so really in this farm town where do you take it to get rebuilt?
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Old 09-15-2011   #8
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Default Re: Would a misfire cause a knock?

Don't forget to check the muffler bearing or the spark plug sequencing valve!
Who built yours?
Sussex engine builders are supposed to be VERY good. There is another really good place in the Annapolis Valley not far from me (Armstrong's) who are extremely good as well.
I hope you find something simple!
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Old 09-15-2011   #9
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Default Re: Would a misfire cause a knock?

I checked it out tonight at work with a stethoscope, running. It's cylinder 5's exhaust valve guides. That's why it's only at idle, or slightly above. It runs perfect, other than the weird knocking at idle. All the gauges work, lights, windows, even the power locks work.

I'll drive it this way until winter anyway, store it and build or get a new head for the spring. At least get a few months of driving it, the heat works.

Last edited by 90_gtr; 09-15-2011 at 05:55 PM.
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Old 09-17-2011   #10
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Default Re: Would a misfire cause a knock?

do a compression test just to make sure ! should be equal and above 100psi , worth doing just in case , if you were using more than the standard boost pressure the miss fire could be a buggered piston ring , happened to me and MANY others

Last edited by rasored; 12-23-2011 at 09:00 AM.
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Old 09-17-2011   #11
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Default Re: Would a misfire cause a knock?

I did a compression test first thing when the engine was in the car. 160psi on all 6. The pistons and rings are wiseco and brand new, not even 1000km yet. I'm still running stock boost and haven't really even stepped into yet, until a figure the noise out first.

I'm going to pull the valve covers tomorrow and check to see if anything dropped inside or is rattling around. Then run it without the valve covers to see exactly whats going on. I've done some more searching and if I had a bad valve guide, it would be burning out and smoking. It's not blowing smoke so I'll have to check it tomorrow or pull the head off.
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Old 12-23-2011   #12
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Default Re: Would a misfire cause a knock?

Bump ... any more news on your knocking
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Old 05-02-2012   #13
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Default Re: Would a misfire cause a knock?

It's been awhile but I still have this noise slightly. I've changed the oil a few times now and only hear this knock t ype noise once in awhile. It does have a rough idle when cold and I haven't figured it out yet.

I replaced the coolant temp sensor, o2 sensors, cleaned and re-soldered all maf sensor pins, set tps again, checked for vacuum leaks with a pressure tester set to 10psi in 3" intercooler piping, it held pressure and i did not hear any leaks. I even hooked up our evap tester with the smoke and didn't see any smoke coming out anywhere.

Anyway I have a CAS and oem ecu coming in the mail soon and will update this rough cold idle when I figure it out. When you google search "rb26dett rough idle" there seems to be all kinds of people with this same sort of issue and alot of people just leave it alone, I can't stand it!! I have even tried 3 different spark plug sets ups and gaps from 0.6mm to 1.1mm and there was no change in performance or idle roughness.
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Old 05-02-2012   #14
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Default Re: Would a misfire cause a knock?

I hope this link works. here is the knock noise, if you listen close enough you can here the tick.........tick,tick,,,,,tick,, in that type of rthym. You can also hear the the valve noise that is normal for these engines. I'm more concerned about the other not so common random tick that is louder than the normal valve noise. If I unplug the coil to that cylinder #5 or #6 in the vid is goes away but after more testing it doesn't matter what coil is unplugged it's still there.


http://www.youtube.com/watch?feature...Bfs5-fSKI&NR=1

http://www.youtube.com/watch?feature...Lq1z3fZ3U&NR=1

Last edited by 90_gtr; 05-02-2012 at 08:10 PM.
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Old 05-04-2012   #15
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Default

At first I was wondering if it might he the rattle that forged pistons tend to make. I can't watch the vids from work.
Then I read the rest of the thread and it doesn't seem to fit.

Last edited by bobbo; 05-04-2012 at 05:36 AM.
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