Rattling On Acceleration
 

Here is the issue I'm having and all the observations I have made. It's long, but I'm trying to paint a clear picture.

I have a 1990 GMC Sierra with a 350 (didn't want to join another forum for something so similar to a Camaro, which I also have). Just about every time I run the truck at about 3/4 throttle and up I will hear a rattling sound, then the service engine soon light will come on. There seems to be a loss of power after this happens. The next time I run the engine the service engine soon light will be off until I give it gas again, it rattles, then a second later the light will come back on. The rattling only happens once, almost every time I run the truck. If I'm light on the throttle sometimes it doesn't happen.

I went to O'Reiley's, they scanned the truck, and he said it was throwing a code for low timing voltage or some **** like that. He also told me from his own personal experience that when he had a rattle like this his timing was off. He turned the distributor and the rattle went away.

I just put a new air cleaner on and it seemed to help a bit, but the rattle still happened and there was still a loss of power. Next I'm going to replace the plugs, wires, cap, and rotor because it probably needs them anyway. Any recommendations on what to get for these? I may replace the whole distributor. If I'm not mistaken the module that controls the electronic advance is inside the distributor. Replacing it would also rule that component out.

Some more info. The cat seems original and there is a newer muffler clamped on. If I find out that the cat is clogged would hollowing it out throw anything off. I have heard both yes and no.

Originally I thought this was a fuel system problem. I can see dried fuel around the outside of the throttle body.

And one more thing. When I'm sitting at idle in neutral and I blip the throttle really fast it will sometime pop. Which I believe is either a rich or lean condition. Don't know which. It still happened after I replace the air cleaner.

Any advice on this would be great.

Re: Rattling On Acceleration
 

The ESC system isn't working. The rattling you hear is the forced knock test in the ECM adding spark advance to create detonation (which it is). Since the ECM doesn't get any knock reported it sets code 43, pulls timing (gets sluggish), and sets the SES light.

The ESC system consists of the knock sensor and knock filter (ESC module), plus the wiring between those and the filter and ECM.

RBob.

Re: Rattling On Acceleration
 

Thanks!

So I can rule out the TPS, fuel pump, and injection? Just making sure. This is a pretty specific set on symptoms, so it wouldn't surprise me.

The knock sensor feeds a voltage to the ESC, if I'm not mistaken. How could I test which is the culprit?

Also, I would like to check initial timing. How do I prevent the computer from advancing it while I check?

Re: Rattling On Acceleration
 

I am not sure on a truck but on a Camaro there is a single grey wire coming out of the ECM harness on the passenger's side that ties right back into the loom with a plug in the middle of it. This is the wire you unplug to set timing. It will throw a code so you have to pull the battery cable to reset the code when finished. If you can data log you can tap a screwdriver handle on the block next to the sensor and see if you get a knock count to show up.

Re: Rattling On Acceleration
 

The knock sensor is located in the engine block, passenger side, down by the pan rail in front of the starter. It may not even be plugged in, or the connector is old and crumbled.

You can measure the knock sensor resistance between the center pin and the sensor body (or engine block). Should be about 100K ohms (100,000).

RBob.

Re: Rattling On Acceleration
 

Thought I'd give some updates on the progress of this. I was poking around under the hood today and found that the line from the charcoal canister to the throttle body was cut. I replaced it, unplugged the battery to reset the computer, and the SES light still came on under hard acceleration. I know it seems unrelated, but vacuum leaks can cause all sorts weird **** to happen.

Unrelated to the rattling issue, the truck also has been having a problem with a squealing serpentine belt, especially when it's cold. Took a gander around the belt and notice a lot of oil and also some pretty worn pulleys. Since the only place that oil could have come from would be the power steering pump I pulled the cap, and, sure enough, the fluid was low and the seal on the cap was all messed up. Going to replace the cap and the belt, meticulously clean every pulley, replace any bad ones, and check for any bad bearings. Might as well replace the tensioner while I'm at it, the truck has 150,000 miles and it's probably original.

Re: Rattling On Acceleration
 

Interesting... I've also observed rattling pre-EBL and aft. Easily observable when engine cover is in place but not so much when not installed.

I've been thinking it's my odd custom headers rattling against frame members or something... but same rattling was there with old headers and stock cam & lifter set.

I'm not getting any SES lights or reported fault codes. Maybe EBL and its ESC test procedure is working then. Is there a way to change how often this is done ?

Seems mine, against all odds, is behaving properly. Those are pretty much last factory installed engine control components in addition to TPS and MAP in place...

Re: Rattling On Acceleration
 

If you have a 7747 there is a patch called ESC done that is part of the highway mode patch. I think the simpler thing for the OP to do is just fix the problem. There are only 3 main parts to the ESC system and only 4 wires to test in the whole system. Knock sensor, ESC module and ECM. I would first disconnect the ESC connector and verify 100k ohms between the blue wire and engine block to check the integrity of the knock sensor and wire. Then with the key on check for 12vdc between the black and pink wires. Finally ohm out the wire between the ESC and ECM. If all the connections test good try a new knock sensor and make sure its correctly torqued. The rattle you are hearing is the ECM advancing the timing 22* to make the engine knock to test the ESC system.

Re: Rattling On Acceleration
 

The EBL doesn't have the forced knock test. That piece of code was eliminated a long time ago. Should hear an high performance engine knock with that test, it was scary...

RBob.

Re: Rattling On Acceleration
 

Should hear what that forced knock test sounds like in a 10:1 engine in a van with the engine cover off!:eek: Sounded like 12lb cannon shot being bounced in a metal trash can. That was the first pass (+11* IIRC) on a sucessful test, not the 22* portion.

Re: Rattling On Acceleration
 

Still haven't tested that sensor yet. I've been working on locating a fluid leak that's getting on my serpentine belt.

Re: Rattling On Acceleration
 

Ok, to update, I have begun a myriad of seal replacements on the truck to get rid of a perpetual leak.

Whilst under the truck I have located what I presume to be the knock sensor, just forward of the starter. There is no plug attached to it. Just above the starter there is a loose plug dangling in space. I cannot tell where the plug is coming from, but it's oval and black. The hole in the sensor is white and round. What's going on here? If pictures will help, I will post them by request.

Re: Rattling On Acceleration
 

Plug the connector into the knock sensor and see if the forced knock test now passes...

RBob.

Re: Rattling On Acceleration
 

I did some searching and the plug I found is the one that should be connected to the knock sensor. What should I use to clean the plug and the sensor that won't affect the continuity or damage them?

Re: Rattling On Acceleration
 

CRC Electrical contact cleaner

Re: Rattling On Acceleration
 

Cleaned the plug and sensor with the CRC Electrical Contact Cleaner and so far no check engine light!

I also fixed a few oil leaks that were saturating my serpentine belt. Feels good to be squeal, leak, and check engine light free!