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Unknown Vibration

Ss76bronco

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Dec 5, 2019
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56
Getting a weird harmonic vibration in neutral that becomes most apparent at 1200-1600 ram's. Kind of has a frequency to it. Not a steady sync with rpm. Block was from a 84 cougar. 302. Put a 50 oz imbalance flex plate and harmonic balancer. Tranny was rebuilt. C4. Don't seem to notice it when driving. Totally at a loss. Anyone have any ideas?

1976 bronco
 
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DirtDonk

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47,737
Has it done this ever since you put it together? Are you sure an 84 from a cougar is 50 ounce?
It probably is, but there were a lot of discrepancies in those first years of transitioning between both the imbalance factor and the different firing orders.

Have you removed the accessory drive belt in front and started it up? Just to eliminate anything external of the engine at least in front.
 
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Ss76bronco

Ss76bronco

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Dec 5, 2019
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Has it done this ever since you put it together? Are you sure an 84 from a cougar is 50 ounce?
It probably is, but there were a lot of discrepancies in those first years of transitioning between both the imbalance factor and the different firing orders.

Have you removed the accessory drive belt in front and started it up? Just to eliminate anything external of the engine at least in front.
I think it has done this since I built it. I got a lot of confusing info as to the 50 or 28 oz, but most sources said it should be 50. Plus, why would it seem to get better after 1600 rpm. I haven't disconnected the belts, but I guess it's worth a try. I did add add a Delphi pump, but I doubt it's that. Thanks!
 

DirtDonk

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You’re right that it should not stop vibrating after a certain RPM. Should just get worse as it goes higher. Just checking on some of the specifics because we don’t know anything yet.
 
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Ss76bronco

Ss76bronco

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You’re right that it should not stop vibrating after a certain RPM. Should just get worse as it goes higher. Just checking on some of the specifics because we don’t know anything yet.
Well your advice nailed it! It disappeared after belts were off. It is the fan/and or pulley. I added a high volume flex o lite fan. Something must be wrong with it or the pulley is damaged. Thanks! Think it's time to switch to electric. Runs very hot with this engine. 350 hp 302. Any recommendations on an electric set up?
 

DirtDonk

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Well, even though I like electric fans, I would be remiss if I didn’t recommend the explorer front set up first. It’s still a mechanical fan but it’s got an amazing amount of pulling power. Factor that along with a pretty efficient water pump and super nice alternator and power steering set up, not to mention extra fan clearance because everything is shorter than the standard bronco stuff, and it’s definitely the winner.

Otherwise, our WH Spal fan is very nice. But I wouldn’t install it if your running hot issue is more than just airflow. A lot of times Broncos have overheating issues for the reasons.
Some of which include, incorrectly reading temperature gauge, old worn out water pumps, not enough ignition timing, or any one of several other common issues.

Cooling Broncos as a whole book in itself!
 

DirtDonk

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I forgot to say congratulations! Wonderful news that it’s not something inside.
 
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Ss76bronco

Ss76bronco

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Yes, I was very happy it wasn't the engine! I kind of like the old school look so I just may go Electric. The 5044 looks like the best set up, is that the one your referring to?
 

DirtDonk

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Yes, one of them at least. I also like using a fan only, such as the #5049 or #5051 and creating my own deeper shroud.
Unfortunately deeper mounting and shrouding is not always possible in a Bronco (much of the time!) so the #5044 was a solution to that problem. It's still deeper than other aftermarket shrouds (to promote a little more pulling through the entire radiator core) but still fits within the narrow Bronco space. (measure yours to make sure though, as things may have been changed without your knowledge).

This goes back to the Explorer setup though. I hear you on the old school look, but sometimes gaining that extra 3/4" of space in front of the water pump snout trumps all other factors!
I think V-belts are cool, but serpentine with wide belt in a compact package is the winner winner chicken dinner of almost every Bronco modification.
So at least keep it (and it's expense and trouble) in mind as you're living with your Bronco.
And definitely make sure everything else is optimized so you're not spending money AND still fighting an overheating issue.

Remember that a good mechanical fan setup is almost always stronger at pulling air than a good electric setup (except at low rpm of course). Which means that while electric has some distinct advantages over mechanical, it won't overcome a problem engine cooling scenario. Seen it many times in person and here on the forums.
Not really trying to talk you out of electric (like it might sound) because like I said I'm a big fan;) I'm just making sure you've seen all the pros and cons and have already been down your own road of discovery for why your Bronco is running hot.

Have fun!

Paul
 

jmhend

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Dec 25, 2007
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2,738
I think it has done this since I built it. I got a lot of confusing info as to the 50 or 28 oz, but most sources said it should be 50. Plus, why would it seem to get better after 1600 rpm. I haven't disconnected the belts, but I guess it's worth a try. I did add add a Delphi pump, but I doubt it's that. Thanks!
This is exactly why we all ask and reply with the silliness. The vibration should have gotten worse but Paul nailed it with a simple "remove the belt". In some cases it doesn't make sense but the community here has seen some really weird stuff.

Glad the issue was identified now good luck on a decision of how to resolve it.

My .02 is an explorer front dress too. I had an electric fan for years but hard to beat a good mechanical setup that works well.

Sent from my SM-N960U using Tapatalk
 

Yeller

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If you are having cooling issues with the flex fan, I'll say as others, no not go electric, it will not be magically better. No electric fan is going to out perform the explorer fan or your existing flex fan, simple physics. Some of the latest OEM fans challenge that thought process but fitting it, controlling it, having a charging and electrical system up to feeding it is a whole other can o worms.
 
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