Help with compression test #’s and spark plug reading

[1984 t-type]

So here’s the scoop, my vacuum numbers have been low ( around 9” ) since I got this car a few years back and I have just kinda dealt with it. But I’m thinking it is the reason for my not so good brakes and now I want to track this issue down. In that time I have blown the drivers side head gasket and replaced both gaskets, have chased a High BLM issue which went away when I switched over to the Cruz vacuum manifold and intake spacer kit and now my BLM’s are in check. So today I decided to do a compression test and here are my numbers, took some pics of the plugs also if anyone can tell me what they are saying. I recently installed a new coil pack because 1 of the banks went out and have new plugs ready to go back in. Car runs on E85 at about 20lbs of boost, I will turn the boost up in the spring to around 25lbs or so when I take it in to have it tuned.

 

[1984 t-type]

I have been doing some reading since i did the test and my throttle was not open, does this make a difference ?

 

[earlbrown]

It makes all the difference in the world. You can't get accurate compression readings if you're starving the cylinders for air.

And those plugs have too many miles on them to be read accuratly. (#1 does look like some fuel glazing gong on though)

 

[1984 t-type]

It makes all the difference in the world. You can't get accurate compression readings if you're starving the cylinders for air.

And those plugs have too many miles on them to be read accuratly. (#1 does look like some fuel glazing gong on though)

Thanks for your input. Hey whats fuel glazing ? I will get back out there in the next couple days and retest with the throttle open, also do you think the 120 reading is too low if 150 was my high ?

 

[earlbrown]

It's when you've fould a plug and end might end up with incomplete combustion. Yes, a 30# swing is more than you want to see.

 

[1984 t-type]

It's when you've fould a plug and end might end up with incomplete combustion. Yes, a 30# swing is more than you want to see.

Ok so i know i need to retest first but lets say there is still a 30# difference on cylinder 6, where do i go from there ?

 

[earlbrown]

find out why the bad hole is leaking

 

[dank GN]

Do a leak down test it will tell you a lot more info then just a compression test . It will tell you where you are losing that compression. Rings,through the coolant system ect . Google leakdown and get one from harbor freight for cheap

 

[No disintegrations]

Ok so i know i need to retest first but lets say there is still a 30# difference on cylinder 6, where do i go from there ?

If you squirt a few drops of oil in the cylinder the reading may increase. If it does it points toward piston or ring problem. If it stays the same it indicates other stuff lol, like leaky valves or gasket problems.

 

[1KWIKSIX]

In addition to an open throttle blade . A warm engine will give better results too

 

[1984 t-type]

If I warm the engine and open the throttle blade let’s say all the numbers go up 5-10 #s, I would still be out of spec correct ? Going to get a leak down tester first thing in the morning when the stores open back up. I have read up on this some today but have not seen anyone mention what percentage of leakdown is acceptable, what range should I be in for a healthy motor ?

 

[ScottHeidinger]

https://manuals.harborfreight.com/manuals/62000-62999/62595.pdf


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[ScottHeidinger]

That’s the manual for the harbor freight one. HTH

I know sometimes it’s hard to get to #6. Hope that maybe the tool wasn’t all the way in to make a good reading otherwise reading the manual should give you a good idea on causes.


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[1984 t-type]

Yea #6 was the worst one of them all but I’m good at it now. Just hoping it’s nothing too serious of a problem with the engine, it runs and drives great to me just has a low vacuum reading and not so great brakes.

 

[ScottHeidinger]

You have a check valve in the vacuum line going to the booster?


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[1984 t-type]

No not in the line only at the booster, should there be one in-line also ?

 

[earlbrown]

You're fine. It's the plastic 90 that hooks the hose to the booster.

Do you have a large face vacuum gauge? If so I bet the reading will have a sharp downwrd tick. (old old old school tuning)


Also, you need to cap that dipstick tube. If you don't have a dipstick a vacuum cap will do it.

 

[1984 t-type]

Yea just pulled the dipstick to see if I heard air in there, I couldn’t get a leakdown tester today so I pulled the valve out of the compression tester and set my regulator to like 15psi just to see if I noticed anything.

 

[No disintegrations]

I'd like to see a larger supply hose to the vacuum block. Or a large dedicated hose to the brake booster.

 

[1984 t-type]

The supply hose to the vacuum block is what Cruz provides with the kit, i cant go larger without changing out the fittings. The line to the booster is 3/8'' cant go any larger there either, 3/8 should be what all the vacuum brake conversion cars use.