Engine rattle at partial or light throttle
 

I've been chasing an issue on my truck for years now. After much googling, reading, troubleshooting and eliminating possibilities, I'm at a loss.

Engine: 1995 4.9l with MAP and about 240,000KM.

Symptoms: Under partial or light throttle, I get an awful rattle which sounds like it's coming from the top end. It's worse when warm but still occurs when cold. If i give it a blip of throttle, or lightly press the throttle it will rattle. If i accelerate hard, it's fine until I let off the gas, and as I lift off, it rattles. This issue didn't occur when I bought the truck and only seems to have gotten worse over the past couple years.

Best way to describe the sound is a loud consistent paced rattle, that is a combination of a card in bicycle spokes, and shaking marbles in a tin can.

I've asked a mechanic about it and he claims it's piston slap, but I don't think that's correct after watching numerous piston slap videos. It doesn't sound or act like knock either as it's far too loud and doesn't occur under load. I've changed the power steering pump, tensioner pulley, belt and just replaced the water pump because it was seeping. So it's not pulley related.

I'm considering pulling the valve cover and checking torque specs on the rockers. Can't think of anything else at this point. Any input would be greatly appreciated.

I have seen cases of a warped bellhousing starter spacer plate contacting the flex plate. Check that.

Also, remove the front accy drive belt completely and try that - just to be 100% sure its not FEAD related.

Hi cBigganz,
Welcome to the forums.

Have you ever had the valve cover off? If not, I'd definitely start there.
It's a little more involved with the EFI since the intake is in the way, but isn't an overly difficult operation.

It'd be good to check over the valve train to see if there are any visible issues. You might have something broken, loose, etc. Dunno. If you're so inclined, you can also run the engine without the valve cover on to see if it's easier to hear/identify the sound. Just know oil will splatter around a bit, which can be messy.

Also, what transmission do you have? I had a bit of clunking/clatter in an engine at low RPMs that sounded a bit like marbles in a can and it turned out to be a combination of loose transmission mounts and engine mounts allowing the whole system to twist. New bushings and mounts and it went away.

Thanks for your reply. I haven't had the valve cover off. The truck has an e4od. Mounts could be it too as the engine does twist some with a throttle blip. Sound seems to be coming from the valve cover area but its hard to pinpoint.

I was thinking about pulling the valve cover and firing it up to see what happens like you said if I couldn't find any other answers. Good to know that won't wreck things.

Here's a video of the noise. It's dark out so you can't see anything, but audio is good.

Hrmmm.... That's definitely an odd sound.

It reminds me of another I had when the bolts that hold the cam thrust plate against the block came loose, which allowed the camshaft to walk back and forth. The sound was the cam gear rubbing against the timing cover. Made an odd growling sound that would come and go.

Took the cover off, tightened down the bolts, and all was well. Some time later I had the oil pan off and inspected the cam and it looked no worse for wear.

I would go with a piston slap or pin.
Try and pull 1 plug wire at a time and see if it goes away.
IIRC with no spark/fire in the bad piston hole it will not make noise but could be wrong too.


I can tell you it does not sound good and not anything on the top end that I have heard of before.
Dave ----

Have you tried retarding the ignition timing or trying a tank of higher octane fuel?

Is it a five speed? Does it make the rattle sound in every gear?

sounds like rotating assembly to me from the posted video...but videos/sound clips can be misleading. I'd try to isolate it further by trying the spark plug wire trick like fuzzface suggested.

Thanks for the additional ideas. I'll try pulling a plug wire at a time tonight.

I have tried higher octane fuel, but haven't tried retarding timing. I'll try to get my hands on a timing light and do that.

Definitely does not sound good. Still runs though, so if something simple doesn't fix it I'll just drive her till she dies.

My 300 sounds EXACTLY the same. I've been trying to identify the source of the noise for a year or so now. Mine is an '86. I completely eliminated the feedback carb system. I'm running an HEI distributor with a good timing curve and a non-feedback carb with a good tune. I thought it was too lean and too much timing, but that didn't fix it. I've tried different oil additives and 20W-50 oil with no difference.

Power is definitely a little lacking on mine. I tried to isolate a cylinder causing the noise by doing a power balance test by unplugging wires one at a time. All I found was that # 1 was weak. A compression test showed 85 psi in #1 and 120-135 in all the others. A wet compression test raised the numbers to about 185 psi on all of them. The engine is obviously weak and worn out. #1 probably has broken rings. I wonder if that could be the noise.

It seems #1 is a problematic cylinder in these engines. I decided I'd pull the engine for inspection and hopefully a simple ring and bearing rebuild. But, for now it's running okay and gets me around albeit slowly. I'll just run it like it is until I can rebuild it, or until it blows up. Let me know if you find out what's causing your noise! I let you know if I ever find out what was causing mine. I wonder if you have a weak #1 like I do...

Would it be the Air Bypass Diverter Valve. Part of the smog pump. My 1993 Air Bypass Diverter Valve rattles when I first press on the gas at low RPMs.

Not on mine, I removed all the smog stuff.

Tried retarding timing, rattle is still there. Pulling each plug wire doesn't seem to isolate it aside from causing the engine to run rough as expected. Must be something internal and I don't care to pull the engine on this old girl.

Looks like I'll just be driving it this way till she dies. Thanks for the input guys.

I arrived at the same conclusion with mine. I installed an oil pressure gauge to make sure I have some oil pressure/ I have about 40 psi at cold start up, right around 10 psi per 1,000 rpms when warmed up, and 15 or 20 psi at hot idle. It has more oil pressure than I expected, so I'm just going to drive it until I pull it to rebuild or replace it.

Yesterday morning I had the truck warming up on the fast idle for 5 or 10 minutes. When I came out to leave I could hear the rattling from the house! I think it's a rod knock. All the way up and down the highway it rattles at part throttle around 2k VERY loudly. On deceleration and acceleration it's perfectly quiet.