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2.4 Broken crank symptoms?

1941Galant

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Aug 21, 2004
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Charles Town, WV
What are the symptoms of a broken 2.4 crank? I've been getting crazy amounts of knock lately (42 counts while in 2nd gear at 2k rpm w/ a 13.5:1 A/F ratio and maybe 13-14% throttle), and tonight I started getting this jackhammer noise while being in low RPMs w/ giving it decent throttle. An example would be to think of accelerating from 8-10mph in 2nd gear. This noise also causes a lot of knock, and appears to be coming from low in the engine. I have already checked the mounts, my alternator (relocated to back of block), and the axle carrier bearing, and they all appear to be fine. I am also getting more knock in the upper RPMs than I was before, at half the boost. The clutch seems to work the way it should, and the knock seems to be better if I easy into the throttle rather than just mashing it, the knock sensor is also new.

I was on DS-Map for a little while trying to tune the car, and it wasn't going so well, but I wasn't going nuts with the boost, and my A/F ratio was pretty rich (13.5:1 out of boost, and 10:1 in boost, and I only saw 4-5 psi max, but the norm was 1-2). I switched back to my keydiver chip and 2g mas tonight so I could take the car on a trip tomorrow.

Any comments or suggestions are much appreciated, I will be tearing into the car tomorrow sometime.
 

curtis

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May 4, 2003
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Clarksville TN
Damn it Hill you broke my car..... /ubbthreads/images/graemlins/banghead.gif

Man sounds like a spun rod to me more than a broken crank.
What was the oil pressure? What does the oil look like? hopefully no chocolate milk. Don't go tearing into it crazy until you see whats up... you may be able to drop the transfer case and exhaust then the pan and start inspecting there if you find its down there you can pull the caps without pulling the head off but thats risky ....sorry brother welcome back to my world. /ubbthreads/images/graemlins/banghead.gif
 

1941Galant

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Aug 21, 2004
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Charles Town, WV
I didn't even think about a spun bearing, then again it is late and I am pretty annoyed. I'll drain the oil in a day or two once I get the garage free again, but from what I saw in the head, the oil is still nice and clear. I was going to take the car apart for a new clutch, and a nicer oil pan, so I can just investigate then. I was actually looking for a reason to ditch the 2.4, I just wish it would've held out until after tomorrow.

Edit: I forgot to say that my oil pressure gauge never worked, I was planning on sending it out for testing/replacement when the car went down for the clutch and oil pan.
 
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slugsgomoo

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Oct 16, 2003
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Tacoma, WA
does engaging/disengaging the clutch affect it? I've seen a lot of horrible noise issues on the big stroke motors from loose flywheel bolts /ubbthreads/images/graemlins/dunno.gif

Other than that, I'd guess rod bearing problems rather than broken crank, since if the crank breaks the motor isn't going to be running most likely- as one of the ends of the crankshaft won't be turning with the rest of it. /ubbthreads/images/graemlins/rofl.gif
 

Rausch

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Dec 21, 2004
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Cleveland, OH
Rod bearings usually have a hollow knocking sound. They tend to go away under load, and are loudest under part throttle. If it's bad enough, it won't matter what load/RPM you are at, it'll make a hell of a racket. /ubbthreads/images/graemlins/frown.gif
 

1941Galant

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Aug 21, 2004
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Charles Town, WV
Slugs, thats a good point on the whole crank not turning thing, and the clutch being depressed has no effect on it. I just fired the car up, and it is pretty quiet while idling and light reving. I pulled the e-brake, and put it in reverse and tried to go, and of course there is a large knock coming from the engine. I guess all I can say is sh*t happens.
 

1941Galant

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Charles Town, WV
Quoting Rausch:
Rod bearings usually have a hollow knocking sound. They tend to go away under load, and are loudest under part throttle. If it's bad enough, it won't matter what load/RPM you are at, it'll make a hell of a racket. /ubbthreads/images/graemlins/frown.gif



This knocking sound does go away after I get above 2500 rpm or so, and you don't hear it in the upper rpms.
 

1941Galant

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Aug 21, 2004
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Charles Town, WV
Well, I guess that is a little relief, I thought it was something worse. The car will be fixed over the next few weeks, I'm not going to be a ***** and part the car out or sell it.
 

BoostedAWD91

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Mar 1, 2007
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Danville,Pa
agree with u Nate, i would check pullies too just incase Hill. when i first started my car after the rebuild i had this bad knocking noise and it sounded like something internal was broke and smashing and it just turned out to be the crank pulley bolts came loose
 

I would be willing to bet its loose flywheel bolts also. The best way to fix them from coming loose again(if that is your problem) is to drill the head of the bolts and saftey wire them to each other.
 

I broke my 2.4 crank once. Sounded exactly like a loose flywheel but that's because it was. I broke it on the flywheel side of the #4 journal. The crank did not 'come apart' but it was essentially in 2 pieces. I've also had loose flywheel bolts on a colt. Both sounded the same, but clutch engagement was erratic in each case.

If your clutch is okay, I'd lean more toward a rod bearing.
 

1941Galant

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Aug 21, 2004
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Charles Town, WV
A little update on this thread, the flywheel was good and tight, and none of the bearings, rod or main, appear to be spun. There is some slight wear on the rod bearings, but nothing abnormal for a higher hp 4 cyl. It looks like there is very fine aluminum in the oil pan, so I am going to move onto looking at the cam caps. So much for picking up Crankwalk's shortblock for this thing, I guess it is definitely going in the RX-7 now.
 

Kenny_Kline

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Dec 27, 2007
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789
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Seekonk, MA
OK now after reading the thread, LOL, it sounds like its not a "broken crank". I would drop the pan and inspect the rod bearings. I pet you they are shot. I have also seen wrist pins fail but not as often as a bearing issue. Regardless, your going to need to rebuid it or get another short block assuming everything else is fine like oil passages, oil pump, etc.
 
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