Engine vibrates after high speed pass

**408WS6** · 12-15-2010, 07:30 AM

Just like the title says. I was making a pass at the 1/4 mile, doing probably 6700 rpm - 6800 rpm, and after I let off, engine started vibrating. So I put it on the trailer and took it home. Started it up, still vibrating. At idle, it's ok. But rev it up to out 2000-2500 rpm, it starts vibrating like a cylinder is down.

Anyone have any ideas about what it might be? Oil pressure holds solid at 60 psi, there is no knocking or pinging, the engine revs up like there is no loss of power. My trap speed was what it should be -125 mph Well, I was expecting 126 mph. So if something happened, it happened near the finish line when I was hauling ass. Dad thinks I hurt something in the valve train - bent valve or perhaps a pushrod.

What do you guys think?

**FasstChevys** · 12-15-2010, 07:36 AM

Just a "thought"............are your motor mounts good? I know that'd be too good to be true, but any vibration is bad..........just brainstorming here.......

but your Dad is probably on the right track......did you grind off a cam lobe?

**408WS6** · 12-15-2010, 07:57 AM

Originally Posted by FasstChevys

Just a "thought"............are your motor mounts good? I know that'd be too good to be true, but any vibration is bad..........just brainstorming here.......

but your Dad is probably on the right track......did you grind off a cam lobe?

That's what I think it might be, the motor mount. I got the poly motor mounts, but hoping thats all it is. The motor sounds fine.

**Knight** · 12-15-2010, 09:08 AM

Possible misfire maybe? With no dropoff of power or noises it would be unlikely, just throwing it out there.

**408WS6** · 12-15-2010, 11:09 AM

Originally Posted by Knight

Possible misfire maybe? With no dropoff of power or noises it would be unlikely, just throwing it out there.

yeah, I'm going to check the PCM to see if it is throwing the misfire code. I haven't done that yet. But no, to the best of my knowledge, and the way the engine is responding to the throttle, there is no drop in power. But sometimes when there is a misfire, it's kinda hard to tell. Definitely a possibility.

**UTVols98** · 12-16-2010, 04:04 AM

Sounds like a motor mount. Check that and the transmission crossmember.

**408WS6** · 12-16-2010, 05:38 AM

I checked the compression in the front 6 cylinders (the back 2 are a bitch, but will get around to it). They checked all good, ranged from 190-240 psi (which seems high to me). I am thinking it's the mount also. Maybe when I launched it really torqued the motor enough it ripped a mount.

**FasstChevys** · 12-16-2010, 11:11 AM

Originally Posted by 408WS6

I checked the compression in the front 6 cylinders (the back 2 are a bitch, but will get around to it). They checked all good, ranged from 190-240 psi (which seems high to me). I am thinking it's the mount also. Maybe when I launched it really torqued the motor enough it ripped a mount.

I hope I'm right! Let us know!

**408WS6** · 12-19-2010, 11:51 AM

Without a doubt, I have figured out what is wrong. 6" long crack in the bell housing of the TH400 transmission.

**Willfully Armed** · 12-19-2010, 04:25 PM

Nice!

**blackbirdbrain89** · 12-22-2010, 04:57 AM

Originally Posted by 408WS6

yeah, I'm going to check the PCM to see if it is throwing the misfire code. I haven't done that yet. But no, to the best of my knowledge, and the way the engine is responding to the throttle, there is no drop in power. But sometimes when there is a misfire, it's kinda hard to tell. Definitely a possibility.

o have it pop up a srs light ull need a ton of miss fire hits. u may have to use some thing to look for a miss

**tnthub** · 12-23-2010, 10:52 AM

Originally Posted by 408WS6

Without a doubt, I have figured out what is wrong. 6" long crack in the bell housing of the TH400 transmission.

Make sure you have a blanket or a shield when you fix it. I had a piece of my tranny stuck in my leg a few years back and it wasn't very much fun.

**408WS6** · 12-23-2010, 07:37 PM

Originally Posted by tnthub

Make sure you have a blanket or a shield when you fix it. I had a piece of my tranny stuck in my leg a few years back and it wasn't very much fun.

Damn!!

**408WS6** · 12-23-2010, 07:51 PM

Here are some pics of the carnage;

New bell housing:

**Knight** · 12-24-2010, 09:55 AM

You could have saved some cash by fixing the old one with JB Weld lol. New one looks nice. Agree on adding a shield too.

**408WS6** · 12-24-2010, 10:12 AM

I actually did use some JB weld, LOL. But the added secuirty of the SFI approved bellhousing gives me a warm and fuzzy. I think I might probably go ahead and spend the extra 80 bucks and get a tranny blanket. I'm not really up to those HP levels, but it can't hurt anything.

**Knight** · 12-24-2010, 02:49 PM

True. It couldn't hurt.

**MrMasterCraft** · 12-24-2010, 07:23 PM

Originally Posted by Knight

Agree on adding a shield too.

Ever seen this video?

[ame]http://www.youtube.com/watch?v=hJtJUVyEyYY[/ame]

**Firebirdjones** · 12-26-2010, 01:30 PM

Originally Posted by 408WS6

Here are some pics of the carnage;

New bell housing:

Good man. When I saw the broken bellhousing I was going to suggest the easy fix of a bolt on SFI housing. You'll be better in the long run.

**Rk ws6** · 01-06-2011, 06:44 PM

Hey check the length of your drive shaft sitting at ride height it should have atleast a inch of play to go into the transmission when it bottoms out in the trans that is what cause cracks like that in th350s and th400s.