95 Burb 5.7 TBI rattle when warm under load at specific rpm

[moondog1]

Hi everyone!
I'm back asking for help from you gurus. Any help or direction would be great.

My truck, 95 Burb k2500 5.7 tbi, has a rattle under the hood under light load only. Not in netrual, can't power brake high enough to make it to do it, but between 1300-1600 rpm it sounds like a valve is rattling. It has developed in the past 100 miles, not directly after but I did put it through a weekend of towing about 500 miles. Truck has 166k miles.

At first I thought it was a belt tensioner, then saw the water pump was leaking so replaced both of those, I went ahead and replaced plugs and wires, it's got good looking oil and normal levels. I also changed the icm and coil for the hell of it. No change.

I pulled the serpatine belt off to see if it made the noise go away. Noise is still there, same rpms. I pulled the driver's side valve cover off and tried to wiggle each rocker, there was nothing obvious. I had a stuck rod and a bent pushrod in a CJ7 once but I don't know squat about tearing apart motors and how things should be but I'm trying to learn. I pulled the valve cover off the passenger side thought I found a loose rocker arm, or slightly more wiggle. So I watched a few videos and tried to adjust the valves on passenger side. Opened them up until clacking and slowly closed it until clacking stopped then a hair past that. Maybe half a turn.

This seemed to make zero difference.

I don't know what to do next. This is my main way to haul my family so I'm trying to fix it right. Most mechanics here are 6 weeks behind and that's a long wait. Any ideas what to check next? This just recently developed not after any changes. Just a weekend of towing then a week later I notice this weird rattle. Thanks in advance.

 

[moondog1]

Hi everyone!
I'm back asking for help from you gurus. Any help or direction would be great.

My truck, 95 Burb k2500 5.7 tbi, has a rattle under the hood under light load only. Not in netrual, can't power brake high enough to make it to do it, but between 1300-1600 rpm it sounds like a valve is rattling. It has developed in the past 100 miles, not directly after but I did put it through a weekend of towing about 500 miles. Truck has 166k miles.

At first I thought it was a belt tensioner, then saw the water pump was leaking so replaced both of those, I went ahead and replaced plugs and wires, it's got good looking oil and normal levels. I also changed the icm and coil for the hell of it. No change.

I pulled the serpatine belt off to see if it made the noise go away. Noise is still there, same rpms. I pulled the driver's side valve cover off and tried to wiggle each rocker, there was nothing obvious. I had a stuck rod and a bent pushrod in a CJ7 once but I don't know squat about tearing apart motors and how things should be but I'm trying to learn. I pulled the valve cover off the passenger side thought I found a loose rocker arm, or slightly more wiggle. So I watched a few videos and tried to adjust the valves on passenger side. Opened them up until clacking and slowly closed it until clacking stopped then a hair past that. Maybe half a turn.

This seemed to make zero difference.

I don't know what to do next. This is my main way to haul my family so I'm trying to fix it right. Most mechanics here are 6 weeks behind and that's a long wait. Any ideas what to check next? This just recently developed not after any changes. Just a weekend of towing then a week later I notice this weird rattle. Thanks in advance.

If it were something with the cam shaft or valves out of spec wouldn't it do it in neutral? Could this be a spark ping? I don't know if I know what that sounds like . I've refueled lately and don't suspect low octane or bad fuel? Could this be a tbi thing? Sorry if my questions are dumb.

 

[moondog1]

Update . I just went and unplugged the MAP sensor and it didn't run good but definitely got rid of the sound. Maybe I'm mistaking valve rattle for pinging? I'm getting a new MAP tomorrow hopefully!

 

[Griffin Meeks]

Ping does happen under load, so it is possible. An easy way to check would be add an octane booster or some premium to the tank. If the sound goes away, it's probably pinging.

I'd take a look at the exhaust system before spending money though (if you haven't already). I've found a lot of odd rattles and squeaks like that to be the result of loose things on the exhaust moving around. My 4.3l has some heat shielding on the manifolds that starting to come loose, maybe something similar is happening to yours? It could also be one of the exhaust hangers, and the sound is just carrying to the engine bay via the frame. I had that happen to me with a Camry I was working on one time. Oddly enough, that only made noise when I had the engine under load too.

 

[PlayingWithTBI]

If you're getting detonation (pinging) you should be able to pull the spark plugs and look for "peppering" on the porcelain.
http://dynamicefi.com/SparkPlug.php

Or hook up a scanner and see if you're getting KCs (Knock Counts) and/or spark retard due to KCs.

 

[moondog1]

I will check the plugs I just replaced.
I swapped the map sensor and it seemed to completely get rid of the knock or what I was calling valve rattle.

How ever now it doesn't want to idle very well in gear or after driving and pulling off it will sometimes sputter and die. Also after first cranking and shifting to reverse it will die slowly without throttle. All this sense replacing the MAP sensor. It does run and drive much better. It also has a check engine light so I need to get that checked and cleared.

 

[Erin]

Keep us posted. I have a similar issue.

 

[moondog1]

Keep us posted. I have a similar issue.

So turns out I unplugged the MAP vacuum hose when I replaced. Didn't secure it well enough I guess.

Guess what?! my entire initial problem is back. It's spark knocking at low rpm. 1400-1600 rpm.

So it doesn't spark knock with the MAP sensor unplugged. But does with it plugged in.

 

[PlayingWithTBI]

So it doesn't spark knock with the MAP sensor unplugged. But does with it plugged in.

It sounds like you may be getting too much SA (Spark Advance) when the MAP sensor is connected due to your SA tables based on MAP vs RPM. Did you set your initial timing to 0 BTDC with the ESC bypass wire disconnected? On a 95 it s/b under the dash near the blower motor on the right side.

 

[Schurkey]

Flexplate cracking is epidemic on these engines.

Consider the possibility that with the MAP sensor unplugged, the engine didn't run well enough to excite the flexplate.

I bought a core short-block engine to rebuild for my '88. This engine was removed from the vehicle due to a "rod knock"; once the thing was out and hanging from the engine hoist, they saw that the flexplate had broken in a ragged circle around the six bolts holding it to the crankshaft.

As you've already looked into, cam/lifter/rocker wear is a potential source of rattle.

"Ping" would be easily traced using a scan tool, by looking at knock sensor data. Connecting a scan tool is rarely a bad idea; and the same for verifying fuel pressure.