Azurefine69

I have a 98 Silverado, 5.7 Vortec, basically stock with 58,000 miles.

The performance dropped and the engine sounds like it's pinging from bad gas or has valvetrain noise when cruising about 2200 or so rpm. It is not throwing any codes and I have cleaned the injectors, new plugs, fuel filter, cap rotor, wires and put the stock tuning in the computer and it still runs bad.

The next step I can think of would be to pull the valve covers and see if I have a loose rocker arm or a bent pushrod.

If anyone has had a similar problem or has any suggestions I would appreciate it.

Thanks
Dave

96SCTA

Does the sound come from the lower part of the motor (if you can tell) It may be a lifter. And it is drying out at those constant low rpms. Does it get better when you rev it up higer(does the sound go away some)

Micah

Pro Stock John

iTrader: (34)

Big exhaust leak, like an exhaust manifold?

I have the same engine in my 99Tahoe and I have 121K on it and it runs strong.

-Joseph-

iTrader: (12)

Are you low on coolant? The intake gaskets are notorious for leaking, and sometimes they leak coolant to the inside of the engine. When Dex-cool and oil mix they make a sticky glue type substance which makes the lifters stick and cause really bad valvtrain noise. Especially when the engine is cold.

Azurefine69

It's not an exhaust leak, no low coolant and I just changed the oil. It doesn't sound like it's a lifter and like I said it only has 58,000 miles. I thought maybe I over revved it and maybe bent a pushrod.

One thing else I noticed was when I did the tune up on it the throttle body had alot of carbon from it running rich with the Hypertech tuning. I cleaned the throttle body and put the stock tuning back in, and cleaned the injectors.

Maybe the EGR is sticking from the carbon and causing the timing to be off?

I did not check the timing after I did the tune up.

Any thoughts?
Dave

-Joseph-

iTrader: (12)

Those engines get allot of sludge in the intake, thats not too abnormal. I guess the EGR could be sticking, Ive had to replace a few before, you would prob get a few codes from that though. That wouldn't be all that hard to check, they are easy to pull off, just a little harder to reinstall since the gasket is hard to hold in place at the same time.

The Ign timing really isn't adj. but you can adjust the camshaft sensor in the distributor (just like you would set dist timing), it should be set as close to 0 as possible using a scan tool. This also would set a code if off far enough.

Azurefine69

Well I looked for the EGR Valve today and I couldn't find it. Do the look like the old type and do you still check them by putting a load on them to see if the diaphram moves and disconnect the vacuum to see if it still pings?

Thanks
Dave

-Joseph-

iTrader: (12)

Its at the front of the intake close to the thermostat, its black and electronic now. You can control the opening of it with a TechII from a dealership, but it won't help if there is a piece of carbon holding it open.

Gangly

a stuck EGR wouldnt cause that much of a power loss. if you have a loud knocking/tapping, it wouldnt be an egr either. something internal is causing the noise. you could also be getting a misfire for some reason but you checked just about everything to make sure that it isnt an ignition issue. was there any residue in the oil when you changed it? how high were you revving the motor? if you did over rev it and bend a pushrod, that would explain the loss of power and the tapping you hear. best way to check is to pull the valve covers off and listen to each of the rockers with the motor on. you will hear a loud tapping if a pushrod is bent. best thing to do however is pull the rocker arms off and check the pushrods(roll them on a piece of glass) to make sure they arent bent. reinstallation and adjustment of the rocker arms are easy so it shouldnt take more than 2 hours to check and reinstall everything.

Azurefine69

Ok. I ran the Top Engine Cleaner in case it had a carbon knock with no results.

I went by a friends shop who has 25-30 years of mechanic and racing experience.

He said it did not sound like valve train problems but probably a cracked piston and when the motor is revved past 2,000 rpm the knock sensor is probably kicking in and retarding the timing.

Well I'm going to still check the pushrods before I pull the engine.

I'll let you know the results.

Azurefine69

Well guys the verdict is in. It's a cracked piston!
I guess it got bounced off the rev limiter one too many times.
Put a scanner on it today. No knock sensor codes and timing was 0 and only moved 1 when revved hard. We listened to it with a stethescope and everyone agreed it sounded like a cracked piston.
Next I'll pull each plug wire and put a load on it to see if I can pinpoint which piston.
Unless some extra money appears I'll just replace the one piston but it would be nice to put a 6.0, LS1 or bore and stroke the 5.7.

Thanks for your help.

Dave

Gangly

do a leak down test and find out which piston. its easy to find out that way. also, if the piston is cracked or damaged, it could possibly scratch a cylinder wall so check the cross hatching on the cylinder with the damaged piston. if the crosshatching is fine and there is no apparent damage other than the piston, I would just replace the piston and not worry about anyting else. however, i have NEVER heard of a piston cracking because a motor was revved to high. i would definatley make sure its the piston and not a bent valve that is tapping and causing a loss of power which is a frequent occurance from over-revving and floating the valves. if the cylinder isnt holding pressure, it could also be from a bent valve. just make sure you arent skipping any steps when checking to see what the cause is.

Simpleman59

Bars, works wonders, same problem, put some in yesterday, and it stopped it, can't even hear it now, (136K) miles on it! Read instructions.

ACE1252