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Discussion Starter #1
Hi all,


I'm having a frustrating issue that I'm hoping you guys can help me resolve. My engine vacuum is just terribly low at cruise.

At idle, my engine vacuum is around 19 - 20. Very nice and tight.
Around town in 2nd (2200rpms), I can hold 17hg at 35mph. 3rd gear (1200rpms) is around 14hg.

On high speed city streets, I can hold 45mph (1500rpms) at around 14.
On the normal highways, I can hold 55mph (1800rpms) at around 12 - 14hg.

But once I get to 65, it starts to drop sharply.

65mph in 3rd (2200rpms) in the flats is around 8 - 10hg. Downhill, and it'll go up to around 14.
If I shift into overdrive (which I hardly do), vacuum will stay around 8 - 10 until I have to do any hills.

If I want to go the speed limit and go 75mph. In 3rd (2500rpms) engine vacuum is around 6 - 7hg.
If i shift into overdrive (1800rpms), engine vacuum is around 2.

Remember, this is on flat, even road. No hills, no accelerating. Just maintaining speed.




For starters, this is my 1981 Bronco with a 300 I6. It's has a 4bbl intake, performance manifolds, 2.5" exhaust out the back, and a port and polished head. The carb is a Holley 600cfm.

Most of the ignition, including the coil, spark plugs, wires, and distributor are new.

I've had it checked at a local brake shop for dragging disk brakes and they said no. I replaced the drum brakes, as well as the e-brake cables last year.

The cat is only a few years old, and I don't have any other symptoms of a clogged cat.

I've had the same symptoms with both a 390cfm and a 600cfm carburetor (almost identical vacuum readings). Both carbs are tuned nicely and give ample power. I've also had the same symptoms with two different engines as I've swapped the original 300 out of this Bronco in place of another one. Both had great compression with around 150 - 155 down the board.

There are no vacuum leaks, I've done extensive testing for them.

It has PLENTY of power. At cruise, I'm at around 1/8 - 1/4 throttle. Just very very low vacuum. I can accelerate from 60 - 80 going uphill if I feel like it. There's no shortage of power.


I have two Broncos, the other is nearly identical. It has the exact same exhaust setup from the engine back, the same tires, almost the same intake (an open plenum vs. a dual plane), and the same carburetor. At 65mph it is at 16hg. The only difference is it has a 3.55 rear end vs. a 3.00.



Any ideas on what's causing this? Something that could be dogging the engine and making it work harder? Something I'm not thinking of? For a nice healthy engine, my cruise vacuum (especially on flat roads) should easily be above 15, and I am having to feather the gas to keep it out of the single digits. Something's not right.

Thoughts?
 

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Discussion Starter #3
That sure seems like it as far as vacuum is concerned. I have no other symptoms of a clogged cat that I know of though.

As stated above, my cat is only about 3 years old. If it was my cat, what would cause it to clog so quickly?
 

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What type of cat? I would guess, just poor construction materials, those missing tools you can't find and blamed the neighbor. You could disconnect the cats and check vacuum. Maybe someone put a potato in the tailpipe.

My other guess would be distributer not advancing correctly, though I would guess different/more symptoms. Maybe an internal vacuum leak not sure if a 300 can do that.
 

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Discussion Starter #5
Yeah, disconnecting the cat would require a cutting torch or a pipe cutter, sadly.
Although, I could probably pull out the oxygen sensor. Would that be enough? Or would that cause other issues? The O2 sensor is for my own readings and has NOTHING to with engine performance (aka. ECU).

I know it's not the distributor. I've checked the whole range of timing with a timing light and a vacuum pump, and have gone through two distributors.


Oh, the cat is: http://www.summitracing.com/parts/WLK-15038/
 

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The more it lugs the less vacuum. 1800rpm at 75mph is crazy low for a full size vehicle that weighs 6000lb. Idea would be around 2400 rpm at that speed and would prob get better mileage. Likely a combination of high geared and a tired engine. Older engines make less vacuum as they age and then start to get blowby soon there after.

PS. check your pcv valve could be leaking.
 

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Discussion Starter #8
The more it lugs the less vacuum. 1800rpm at 75mph is crazy low for a full size vehicle that weighs 6000lb. Idea would be around 2400 rpm at that speed and would prob get better mileage. Likely a combination of high geared and a tired engine. Older engines make less vacuum as they age and then start to get blowby soon there after.

PS. check your pcv valve could be leaking.

That's usually why I stay in 3rd gear instead of overdrive (2200 @ 65), but on that note, this is also the I6 and not a V8. They have quite a bit lower RPM band than the V8s and 1500 - 1800 RPMs at cruise is not at all unheard of.
When I stated what my RPMs were in overdrive, it was only for comparison. I usually don't use it since my vacuum is so low. But a vacuum of 8 - 10 at 2200RPMs @ 65mph? Something doesn't add up right there. My other Bronco is at 16hg @ 65mph, and the RPMs aren't that much more.

Back when I had the 1bbl on there, I'd be at 1600 RPMs all day long on the interstate and get 18 - 20mpg. No idea what the vacuum was though since I never checked that sort of thing until I put the 4bbl on.


The engine is very strong and tight. It has 155 compression in all six cylinders and the head was professionally rebuilt only a few months ago.
PCV valve is brand new.
And last I had it weight, my Bronco sits right at around 4000lbs. I actually got it to 3800 when I took the top, rear seat, and spare tire rack off. :thumbup Very light for a Bronco.
 

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Discussion Starter #9
I would think pulling the 02 sensor would work.

Read THIS
I'll give that a shot and see what happens. That's an easy way to check at least.
 
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