Ticking/Rattle on acceleration

[Gator]

When I give the truck gas in the mid range it has this rattle like sound, to me it sounds like a chain is hanging loose and whipping around. When you get into higher RPM (4-5k), it's a fast pace, and very loud tick. My girlfriend has also described the sound similar to water gurgling. Earlier today, I had to run it to high RPM in 3rd, and after wards it just advanced to the fast pace ticking sound.

I'm not sure what the issue is because I've replaced all lifters, pushrods and 1/3 of the rockers in August 2015, and they have less than 200 miles on them. The trans went out in January 2016 and I recently had it rebuilt and it still makes the sound. The carb has been recently tuned too, along with timing, so I don't believe it is pre detonation. The motor is a 400 SBC, with the trans being a TH350.

I wanted to see if anyone had any ideas before I brought it to a shop.

 

[callmewaylon]

Sounds like you've got some pre ignition "pinging" going on. Probably need to back the timing off some. What's your initial timing? Is your vacuum advance working? Does it happen at lower rpm when the engine is under load or just at mid to higher rpm when under load?

 

[Gator]

Sorry for replying late, been busy with school and work.

I took it to a shop and it got diagnosed as detonation issues. The shop report says "Reset base timing to 8* BTDC and reworked vacuum advance to ported vac. instead of manifold vacuum. Has blow by and low compression which may cause detonation issues. Recommend higher octane fuel."

I took 91 octane in it and it still makes the knocking noise. Only difference is the truck can hit 30 mph without it being obnoxious like it was prior. It starts around 1500 RPM. The shop recommends driving through a tank or two to resolve it. Would this be correct or is there more work that needs to be done? I'm concerned about damaging the motor from doing this.

 

[1987 GMC Jimmy]

How many miles does this motor have? I hate to say this, but blow by AND low compression sound like failing piston rings on whatever cylinders they saw issues. They should have done a leak down test. I don't see why it'd take a couple tanks of 91 for anything unless there was water in the fuel. Plus, if there's pinging at eight degrees then it should be retarded between 4-0 to see if that helps. Did they say what it was before? I'd say eight is a good spot most of the time. Some people can get a little more and some can't. Regardless of what the ignition situation is, it sounds like you'll be needing a rebuild soon.

 

[Georgeb]

How many miles does this motor have? I hate to say this, but blow by AND low compression sound like failing piston rings on whatever cylinders they saw issues. They should have done a leak down test. I don't see why it'd take a couple tanks of 91 for anything unless there was water in the fuel. Plus, if there's pinging at eight degrees then it should be retarded between 4-0 to see if that helps. Did they say what it was before? I'd say eight is a good spot most of the time. Some people can get a little more and some can't. Regardless of what the ignition situation is, it sounds like you'll be needing a rebuild soon.

Agreed. I believe they set it to eight. If I'm not mistaken having the vacuum advance on ported will add advance when accelerating thus contributing to the problem. If you connect to manifold vacuum it will drop the advance out when the throttle is opened and should reduce the pre-ignition issue. Of course a readjustment of the idle mixture will be in order as well. this of course doesn't deal with the worn out-ness of the motor but it should help. Additionally if that doesn't help there may be severe carbon deposits on the pistons that are adding to the issue. With that I would do a seafoam treatment down the throat of the carb to try and clean it off.

 

[Gator]

I don't believe the timing was readjusted from before today, so it should have been about 9 degrees.
They mentioned the piston rings or valves possibly being tired and causing the compression issues. I was lied to when I purchased the truck so I'm unsure of the mileage (was told it was freshly rebuilt).
I'll try moving back to the manifold vacuum. I did notice it to be a little quieter when cruising on the highway (55 mph 2-2.5k rpm) than when it first became as loud as it is.
The seafoam is a good idea too, I've been considering it.

 

[marks86]

before you do anything. Take a good flashlight and some free time and go through the entire engine bay, and under the truck and just look around. you will be amazed at what you see. If you don't find anything do a leakdown test.

 

[Georgeb]

before you do anything. Take a good flashlight and some free time and go through the entire engine bay, and under the truck and just look around. you will be amazed at what you see. If you don't find anything do a leakdown test.

What would he be looking for?

 

[marks86]

What would he be looking for?

anything really, loose wires(grounds), broken vacuum lines, leaking gaskets, loose bolts, or cracked metal. I had a nasty noise under my truck and it was the bellhousing bolts had all come loose.

Hes looking for clues to what his problem could be. There is nothing worse than spending all this time and money and turns out one of the plug wires is loose. We all know we have been there.

 

[Georgeb]

anything really, loose wires(grounds), broken vacuum lines, leaking gaskets, loose bolts, or cracked metal. I had a nasty noise under my truck and it was the bellhousing bolts had all come loose.

Hes looking for clues to what his problem could be. There is nothing worse than spending all this time and money and turns out one of the plug wires is loose. We all know we have been there.

Right! Even just checking that bolts are snug and clamps are tight.

 

[Gator]

anything really, loose wires(grounds), broken vacuum lines, leaking gaskets, loose bolts, or cracked metal. I had a nasty noise under my truck and it was the bellhousing bolts had all come loose.

Hes looking for clues to what his problem could be. There is nothing worse than spending all this time and money and turns out one of the plug wires is loose. We all know we have been there.

I found that the oil sending unit next to the distributor is broken, cause the bell housing is covered in oil. I'm going to install a replacement tomorrow.

 

[Gator]

I've installed the new oil sending unit, and filled the oil up a little today. Last night I switched back to the manifold vacuum and I've found that the truck will actually idle at a higher RPM than it was when the shop switched it.
After getting it back from the shop, I've noticed some other issues now. The truck is stalling out, once when I was parking and once when making a turn. During a test drive today, it almost did but I was able to get the pedal down fast enough and it took off. I think this is related, but when the truck is just idling down the road, when you give it some throttle to rev, it hesitates a lot, feeling like its about to stall.

I can take some video of all this if it would help too.

 

[Gator]

So to update the situation:

I installed new exhaust manifold gaskets earlier this week, a mechanic friend suggested an exhaust leak as the cause. That wasn't the issue

I installed a new mechanical fuel pump yesterday morning because I saw a video with a similar sound and that was their issue. That however, was not my issue either.

So I decided to try installing a new ignition coil last night, and that was also not my issue.

Not really sure where to go with it next besides look at replacing the engine. A friend suggested the PCV valve causing the issue but I'm not very sure on that. I've had 3 shops tell me it's pre-ignition/detonation, but no luck with the solution.

 

[MikeB]

If I'm not mistaken having the vacuum advance on ported will add advance when accelerating thus contributing to the problem. If you connect to manifold vacuum it will drop the advance out when the throttle is opened and should reduce the pre-ignition issue.

George, as soon as the throttle blades start to open, ported vacuum kicks in and becomes the same as manifold vacuum. Going to ported is not a good idea, and will not solve a pre-ignition problem if that's what it is. What it WILL DO is make for a rougher idle, and possibly off-idle stumbling, just like on mid-70s cars with a kazillion emission controls.

To determine if the problem is or is not pre-ignition, I suggest disconnecting the vacuum advance hose, plugging that carb port, and going for a drive. Does it still make noise? Is it a function of throttle position and load? If the noise is gone, then the problem was too much vacuum advance at part throttle acceleration. If there is still noise, then either it's pre-ignition due to too much centrifugal advance, or it's mechanical. I'm starting to lean towards mechanical, because 8" initial plus anything even near stock centrifugal advance shouldn't cause pre-ignition.

 

[74 Shortbed]

Ported vacuum pulls from idle on up along with the mechanical advance, so depending on the total you could have up to 50* on acceleration and that will rattle the **** out of it if not melt stuff, on the other hand with manifold vacuum you have vacuum at idle(which gives a smoother idle) and it drops as you accelerate and you're only on the mechanical which should be about 36*-38* depending what the engine can handle, then when you back off to cruise mode vacuum comes up and gives you more advance which gives you better mileage and less heat, ported vacuum is a smog thing and works in conjunction with the rest of the junk, so if you don't have all the smog stuff and you want it to run right, you hook it up to manifold vacuum.