92 "new" 4 bolt tbi knocking

[vwrabbitgtd]

I'm baffled here and just not quite convinced to pull the pan yet. It's a 92 Chevy 4x4, Auto 4 spd tranny and a nice "new" 350 4 bolt TBI that just went off warranty about a week before it started making noise

Anyway, here's the deal: when the internal timer on the motor's warranty hit 0, the truck started running like crap. Under cold acceleration it was popping out the exhaust (have a very loud dual exhaust system, so you hear any imperfection). Then started noticing at idle that it has a little knock that sounds like it's coming from the pan.... the knock isn't very consistant... sometimes it's on every revolution and sometimes vanishes for minutes at a time. Now today, I noticed it's knocking under acceleration-- or at least with my foot more than half-way to the floor and sounds like it's on every revolution... but again, pull off on the side of the road and crawl under it and the knock is intermittent. Oil pressure is fine and steady and the truck idles pretty decently, but lacking a little power now... some hills I used to accelerate on needs to downshift to hold speed.

Do I tear it apart, just pop for another crate motor or is there a chance I've got something going on in the ignition with detonation?

[airdale94]

Originally Posted by vwrabbitgtd

I'm baffled here and just not quite convinced to pull the pan yet. It's a 92 Chevy 4x4, Auto 4 spd tranny and a nice "new" 350 4 bolt TBI that just went off warranty about a week before it started making noise

Anyway, here's the deal: when the internal timer on the motor's warranty hit 0, the truck started running like crap. Under cold acceleration it was popping out the exhaust (have a very loud dual exhaust system, so you hear any imperfection). Then started noticing at idle that it has a little knock that sounds like it's coming from the pan.... the knock isn't very consistant... sometimes it's on every revolution and sometimes vanishes for minutes at a time. Now today, I noticed it's knocking under acceleration-- or at least with my foot more than half-way to the floor and sounds like it's on every revolution... but again, pull off on the side of the road and crawl under it and the knock is intermittent. Oil pressure is fine and steady and the truck idles pretty decently, but lacking a little power now... some hills I used to accelerate on needs to downshift to hold speed.

Do I tear it apart, just pop for another crate motor or is there a chance I've got something going on in the ignition with detonation?

Popping out the exhaust sounds like cam timing (ignition). I would pull off the front cover and verify the chain isn't stretched. You said the engine was new but diddn't say how many miles were on it. I would think internal components on a good crate motor would have a considerable warranty.

[vwrabbitgtd]

Originally Posted by airdale94

Popping out the exhaust sounds like cam timing (ignition). I would pull off the front cover and verify the chain isn't stretched. You said the engine was new but diddn't say how many miles were on it. I would think internal components on a good crate motor would have a considerable warranty.

I guess this is where the "junk" comes in: I bought the truck specifically because they had just set a new motor in it. The manufacturing company didn't typically transfer waranties, but considering it had less than 100 miles on it, they were willing to transfer to a 12K deal. the motor has just a little over 14K on it now.

I did give the truck a full tune up the other night, in hopes to find a bad plug or something, but everything looks perfect. With the air cleaner off and under load and playing with the throttle seems like it's actually popping out the intake and the loud exhaust is just deceptive in making me think it's an exhaust backfire.

[formulaross]

You might have a bad intake valve. Try doing a compression test and see if you have a bad cylinder. Might just need an early valve job.

[vwrabbitgtd]

Originally Posted by formulaross

You might have a bad intake valve. Try doing a compression test and see if you have a bad cylinder. Might just need an early valve job.

Did the compression check and just about everything you can imagine... even hooked it up to a computer. The noise is definately coming from the back of the motor... either #7 or #8 rod bearing or the thrust bearing wasn't set right. I'm driving (can you believe it) the truck that runs like a champ, but knocks, up to a place about 100 miles from here for a nice "mild" performance motor. Looking at about 300 HP and around 375 lb/ft of torque for about $3,000 and a 75K mile nation-wide warranty-- sure in hell beats my previous 12K warranty with the motor blown less than a week after the warranty expired!