When I start my truck the engine idles up and then drops down(when its warmed up) I know this is normal and it's the IAC that does that. The problem is when it does that I hear a noise that almost sounds like a knock or ratle only does it as the engine kicks up and drops in idle then it's gone. If I shut the truck off and start it right away it doesn't do it.. if it sits for 1/2 hour it will do it. My buddy(a mechanic) says it sounds like one of the main bearings is going in the bottom end. He told me to try starting it with the belt off to elminate any of the belt driven components.
could it really be a main bearing?? the engine has 138,000 KM's on it. It runs great and has lots of power. My buddy had put in my cam,lifters, intake & new timing gears and chain about 5 months ago and he couldn't believe how clean the engine was internally.

If it is the main bearings how big and how expensive of a job is this?? he says if I'm going to change the main bearings I might as well change all the bearings in the bottom end and install a high flow oil pump..

any ideas??
btw it's been doing this for a good 3 or 4 months that I can remember.. just never thought much of it.

thks

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'88 C1500 350 TBI
Belltech 2/4 Drop,
Nitro Drop Shocks,
3.73 Rear gear ratio,
Edelbrock performer TBI intake, performer cam, timing chain,
Hiflow cat with Rhino dual 2 1/2" out the rear.

 

does it get faster with engine rpm? if so then maybe it is a exhaust leak at the manifold?

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it's hard to tell because it happens so fast. it's like the idle goes up down and the noise is gone. I am going to try it shortly to see if it does it on the initial cold start. I normally use the remote starter to start it first thing so I never noticed.

 

Alot of sounds can happen right at startup, because of lack of lube and oil pressure and all that. One of the most common ones is a cam/lifter noise, if you slowed it down it would probably sound like a squeak rather than a knock; while common and "normal", it indicates that wear is occurring every time the engine starts up. I've seen estimates that as much as 90% of the wear in flat-tappet systems occurs within 10 seconds of startups.

Try changing your oil filter to a AC/Delco PF-1218, which has an anti-drainback valve to keep the oil in the passages inside the motor from beng able to run back down into the pan while the engine sits. That helps sometimes.

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"So many Mustangs, so little time..."
ICON Motorsports

 

I have a 87 SC camaro with a truck 350 in it and i have almost the same noise but mine comes from the fly wheel to torque converter

what could this be caused from ( the torque converter bolts are tight)

 

hmm.. I know mine has been doing this for about 5 months now and hasen't got any worse. I never thought anything of it t'ill my buddy said it sounded like an an engine knock. You 'd think if it was something in the bottom end it would get worse. I spoke to my cousin( he is also a mechanic) tonight and he said he'd have to hear it. He also told me that if it was rod bearings that there 'd be other wear and to replace them would just be a band aid fix for now. He said to do it right pull the engine out and tear it down..

I don't think I want to get into that.. I'd be better to just buy a crate engine and drop that in..

if anyone has any advice please pass it on..

thks

------------------
'88 C1500 350 TBI
Belltech 2/4 Drop,
Nitro Drop Shocks,
3.73 Rear gear ratio,
Edelbrock performer TBI intake, performer cam, timing chain,
Hiflow cat with Rhino dual 2 1/2" out the rear.

 

ya mine has been doing the same for about 5 months to, its just weaird it will knock when idealing but as soon as i hit the gas and keep it at a higher RPM it dont knock.

 

yeah mine only does it for 2-3 seconds on startup.. if it sits for 20 mins or more.. I've elminated belt driven components.. I'm really starting to believe it's in the bottom end somewhere.. I always get the oil changed at the dealer.. it has an acdelco filter on it right now.. not sure if it's the type with the valve in it or not though.. maybe I'll get the oil changed and request that type of filter tomorrow and see if it goes away.

 

thechnically could I unplug the injectors and crank the engine off and on of course t'ill the oil pressure builds up and then hook the injectors up and try starting it.. If it doesn't do it then I could say it's in the bottom end??