Rpm related vibration

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pjc360
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Rpm related vibration

Post by pjc360 » Sat Feb 10, 2018 4:03 pm

Ok guys I’m back, I’ve got an rpm related vibration that I can not figure out.
Engine is a 300hp crate 360 magnum in a 91 w150 truck.
This vibration appears at about 2400-2500 rpm in every gear.
Engine was purchased brand new by my father, I acquired it from him and it only has around 25k miles on it when I got it, it’s now got about 30k miles.
The vibration feels like it’s got a rythem to it, it cycles around and goes vrrrm vrrrm vrrrm vrrrm, I can feel it in the steering wheel and in the floor boards and in the seat.
At first I thought it was a drivetrain vibration but I quickly discovered its rpm delendeant ans not speed dependent.
I can be going the same speed and change gears and when the rpm drops the vibration ether goes away or subsides a lot.
I’m running a 2 pin Non lock up A-518 automatic trans with an older LA style nuetral flex plate with a Magnum balanced torque converter.
I have verified the converter is correct, the transmission was re-built a few years back and I haven’t put too many miles on it cause I’ve been slowly putting this truck together in the last few years.
I’m running the La style timing cover and V belt set up which is how the engine came assembled from Mopar as a crate.
I have brand new ps pump and brackets, brand new alternator and alternator brackets, brand new water pump, brand new fan and brand new pulleys.
Belts are straight and tight.
The harmonic balancer is what came on the engine from Mopar as a crate.
The transmission shop that did my transmission made a couple stupid mistakes that cause them to have to pull and install the transmission a couple of times before everything was straightened out and it was good to go.
I’m wondering if they warped or bent the flex plate during the removal and the installation of the trans?
Idk what else to point the finger at here.
Ignition is all brand new, I was fighting a missfire a few backs that I determined to be coming from the crane cams hi-6 cd ignition box I was running.
I took the crane box out and bought an MSD Digital 6 box and wired that in with brand new wiring.
I delegated the trigger and coil wires, the trigger wires run on the passenger side of the truck and the coil wires run on the drivers side of the truck, no where near each other at all.
Distributor is a firecore 50 electronic vacuum advance distributor, it’s brand new and I have the mechanical advance set at 16 degrees.
I’m running 18 degrees of initial timing, 34 degrees total timing.
Plugs are brand new Ngk v-power’s gapped at .040, plug wires are brand new Taylor Thundervolt 8.2mm all 8 wires check out at 40 ohms of resistance per foot.
Coil is a brand new crane cams lx91 which is compatible with the digital 6 box.
I’ve verified timing with 2 different lights, I’ve verified accuracy of timing mark on the balancer using a piston stop, it’s accurate.
I’ve done a warm compression test on all 8 cylinders, all 8 cylinders put out 155psi of cranking compression with carb wide open at 3500 feet above sea level.
What else am I missing here? It has to be in the converter or flex plate right?
I can’t think of anything else and I’m pulling my hair out trying to figure this out.

SchmidtMotorWorks
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Re: Rpm related vibration

Post by SchmidtMotorWorks » Sat Feb 10, 2018 4:37 pm

Does load matter?
For example does it vibrate if revved up in neutral?
http://www.schmidtmotorworks.com Aerospace Machine Work: Prototypes, Tooling, Molds.

pjc360
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Re: Rpm related vibration

Post by pjc360 » Sat Feb 10, 2018 6:37 pm

SchmidtMotorWorks wrote:
Sat Feb 10, 2018 4:37 pm
Does load matter?
For example does it vibrate if revved up in neutral?
I will find out here in a minute and report back.
I gotta go start it and I’ll bring the rpm up to 2500 where the vibration is most pronounced while driving and see what it feels like.

Circlotron
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Re: Rpm related vibration

Post by Circlotron » Sat Feb 10, 2018 9:58 pm

Sometimes hard aftermarket engine mounts can cause that problem. The mass of the engine and the springiness of the rubber mounts will have a natural resonance that will strongly accentuate vibration at a particular rpm. OEM mounts are often relatively soft and so the vibe will occur at below normal running rpm. With my car I found reducing the front tyre pressure from 35psi down to 28psi reduced this annoying situation. No kidding! Had nolathane suspension bushes so that made it worse.

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Re: Rpm related vibration

Post by Geoff2 » Sun Feb 11, 2018 4:32 am

Flex plates are hard to check, but harmonic dampeners are easy. Check that you have the correct dampener for the 360 [ as opposed to a 318 ].

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Re: Rpm related vibration

Post by pjc360 » Sun Feb 11, 2018 6:17 pm

Geoff2 wrote:
Sun Feb 11, 2018 4:32 am
Flex plates are hard to check, but harmonic dampeners are easy. Check that you have the correct dampener for the 360 [ as opposed to a 318 ].
Harmonic balancer is correct, it’s the same harmonic balancer that came on the engine as a crate from Mopar.
This engine was at one point in a different truck, and it never had vibration issues in that truck.
I put the engine in this 91 and it’s had this annoying rpm related I ration at 2500 rpm ever since.
It hasn’t ran for a few days cause I haven’t been driving it.
I just fired it up and am letting it warm up and I’m gonna check to see if I can feel the same vibration while it’s in park.
I have pictures of my converter and flex plate I can post up and I have compared my converter to pictures of other magnum balanced converters and they look identical to me right down to how the weight is shaped.
At this point I just don’t know what else it could be.
Engine mounts are brand new, I bought them from My local Napa, they are just standard basic rubber replacement engine mounts.
The last time I drove it I climbed under the truck and watched the converter and flex plate and I couldn’t see anything that jumped out at me as far as wobbling or out of balance looking, but I’m 80-90 percent sure I could hear a faint metallic sound coming from the converter/ flex plate area.
I put my ear to the oil pan and I couldn’t heat any noise coming from inside the oil pan, but when I pulled my ear from the oil pan and made my way towards the converter area I could faintly hear it again.
Exhaust is kind of noises under the truck with hedders and the noisy 6 blade flex fan.
So it was hard to pin point exactly where it was coming from but I know I heard some kind of faint metallic sound from that area.
The truck is warming up now and I’m gonna go out and bring the rpm up to 2500 and see if I can feel the same vibration I feel when driving it at 2500.
The vibration feels like it spins with the engine cause it has a rythem to it and it rolls around in a perfect pattern if that makes sense.

pjc360
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Re: Rpm related vibration

Post by pjc360 » Sun Feb 11, 2018 6:41 pm

So I just got back from holding the throttle at 2500 rpm in park and the vibration isn’t as intense as I was anticipating it to be.
But there is a rough spot around 2500 rpm in the engine and I opened my door to listen to the exhaust and I could hear popping out the exhaust???
Wtf is going on with this thing?
Brand new MSD Digital 6 ignition box, brand new crane cams lx91 ignition coil, brand new Taylor Thundervolt 8.2mm wires that all ohm out at 40 ohms of resistance per foot.
Brand new Firecore 50 electronic vacuum advance distributor, mechanical advance is set at 16 degrees in the distributor with two medium advance springs that being timing all in at 2500 rpm with 18 degrees of initial timing and 34 degrees total timing.
I’ve verified accuracy of timing mark on the balancer with a piston stop, it’s accurate, verified timing with 2 different lights that are within a 1/2 of a degree of each other.
I’ve done a warm compression test on all 8 cylinders, all 8 measure at 155 psi cranking compression with carb wide open.
I am so lost here and have no idea what’s going on.
I did notice this vibration seemed to change a little when I changed back to the medium advance springs in the distributor, it seemed to get slightly better.
Also when I remove the vacuum advance it seems to get a little better as well.
But is still there, just not quite as intense.
Could different timing settings change how an imbalance in the engine feels?
I am very close to throwing in the towel with this engine and pulling it and putting in a different 5.9 magnum.
Cause this just isn’t making any sense to me at all.

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Re: Rpm related vibration

Post by BigBlocksOnTop2 » Sun Feb 11, 2018 6:48 pm

Start the engine in darkness with the hood up and look for spark jumping. Look around the spark plugs.

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Re: Rpm related vibration

Post by blown265 » Sun Feb 11, 2018 8:28 pm

Gday mate.
If changing the timing curve changes the vibration, it wouldn't be unreasonable to suggest that it's tune related, and not necessarily mechanical.
It may not be caused by the timing either- the fuel map (injected?) could need attention, particularly with popping in the exhaust.
Regards
Paul

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Re: Rpm related vibration

Post by tresi » Sun Feb 11, 2018 8:50 pm

Double check the torque convertor to flex plat bolts. Check engine to transmission bolts and all engine mount bolts. Don't rely on another shops work. There's a truck in my yard with a new transmission and 1/2 the cross member bolts are gone. All bolts that you didn't put a wrench on are suspect.

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Re: Rpm related vibration

Post by Geoff2 » Mon Feb 12, 2018 4:36 am

Bringing rpms up to 2500 rpm no load would bring in the vac adv. If it is the Mopar dist, the VA is on a pivot/overcentre mechanism. Check that the reluctor teeth are not contacting the p/up coil during vac adv operation as this causes a no-spark condition.

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Re: Rpm related vibration

Post by pdq67 » Mon Feb 12, 2018 6:04 am

I have posted this about everywhere over the years so here goes again.

My old junk301 that was made out of, well junk parts, had three harmonics that it would go through until it was up top. I could shut my eyes and tell what my rpm's were while driving it back then.

It was NOT BALANCED and made out of --

A worn out, stock bore, SJ 327 block;
Pistons were new 1/8" over, 1/8" tall, 1/2 round dome cast with steel struts 283, "2x4 and Fuelie", jobbers from W/JCW that cost all of $39.95 back then. Early '70's here!! They must of had .012" on the cylinders!!
A '55 265 truck engine crank;
who know's what thicker 283 rods, not the skinny, "265/later 305", jobbers;
Ground hell outta '55 265 little valve heads with no sparkplug cooling trough and of course "Z-28" springs;
A salvaged junkyard -097 cam and a coffee can for its solid lifters;
My 350SS's Q-Jet carb., intake, and exhaust manifolds;
My 350's 11" c/pp and flywheel and M-20..

The high-strung little jewel would hit 7500 with new points and a week later like only 7,000 after the point spring wore in.

It was a real hoot rpmming!! Probably never made 325 hp, but it was more fun than John Law would allow!

Sure it knocked and hammered and used/leaked oil, but it was sure fun!!

He, He!! It had a bad habit of tossing it's damper running down the Hwy. So the last time it happened I lost it and just went to a junkyard and got another one and went on.. It would go out the front under my radiator in front of my pass side tire and usually cut a path through the weeds along the edge of the road.

I bet that you have an internal balance problem of some sort, but finding it?? It might be as simple as a tweaked fuel pump pushrod??

Good luck,

pdq67

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Re: Rpm related vibration

Post by mag2555 » Mon Feb 12, 2018 7:04 am

It would not be the first time that one or more bad injectors made for ruffness in a small rpm range!

pjc360
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Re: Rpm related vibration

Post by pjc360 » Mon Feb 12, 2018 10:17 am

Engine is carbureated, not efi.
I have checked reluctor to pick up coil post gap with a brass feeler gauge with and without vacuum advance using a hand held mity vac pump.
The gap is .008-.010 which is within spec per the instructions with the distributor.
Engine mounts were replaced by me and everything is tight, transmission work was done by a transmission shop but the converter to flex plate bolts are tight and none of them are missing.
I thought it was an imbalance issue at first, but now I’m not so sure... since I only feel it at a specific rpm and I can hear what appears to be a miss while it’s running rough in that 2500 rpm area.
Maybe I should start with pulling all the plugs again and doing an up to date compression test and make sure all 8 plugs are good and none of them are fouled.
I was having a problem with cylinder 8’s plug fuel fouling all the time a few months back but I thought I had fixed the issue by replacing the ignition box.
Maybe there is an issue on cylinder 8, but I did remove the valve cover on that side a couple months back and didn’t see anything un-usual with the valve train and I did a compression test on cylinder 8 and got 155psi.
This is just so frustrating.

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Re: Rpm related vibration

Post by grandsport51 » Mon Feb 12, 2018 1:32 pm

PJ, Do you have access to a drive on lift
If so raise the car ,leave a helper behind the wheel
Jack up rear wheels and rev to 2500 checking both in gear and out of gear at your
critical speed now carefully isolate your vibration
With a screwdriver to your ear ,check mounts
To chassis, radiator support etc . If the vibe is strong you should easily be able to locate it.
Dave B
LIGHT 'EM UP

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