#7 cylinder not registering compression

[Miker]

Guys:

I went into my first time diagnostic of a cylinder compression check on the great '69 my cuzz sold sold me and I went to checkin' on what that fffff noise I was hearing could have been and like some of you guessed, somethings amiss with my #7 cylinder. The most compression it would register is about 20 (barely enough to move the needle)

All other cylinders: two of them fired up to 140, two of them fired up 145 and three of them fired up to 150.

Soooo, any ideas or tricks of the trade to figure out what's up with #7 the most minimally invasive way to the most invasive way (ie removing the head).
Thanks

 

[bax]

You have a burnt valve. You will need to pull the head to fix it. You could put a scope inside the bore but there is really no need. You will see it after the head is removed.

 

[cs_88]

When I bought my Bronco, I had no compression in my #1 cylinder. It seems that I had a valve out of adjustment. I installed a shorter pushrod, I think .060", and it brought the compression up with the others.

Just a thought.

 

[nrramse]

One way to confirm a bad valve is to squirt a little oil down the spark plug hole. If the piston rings are bad, the oil will temporarily seal the ring and the compression will go up. If its the valve, the compression will remain low.

Another way to confirm a bad valve is to attach a vacuum gauge. If the neede flutters, then a valve is likely no good.

Newell

 

[Broncobowsher]

Minimal invasive...
Pull valve cover and check things out.
With that cylinder at TDC on firing stroke the valves should be completely closed. You should be able to move the rocker arms around a little. If one is super tight, it isn't closing.

Another good one, even less invasive. Put #7 at TDC firing stroke. Many compression testers (the 2 part ones anyway) have a common compressed air quick connect. You need to go in and remove the schrader/check valve fom the hose. Now you can blow compressed air into the dead cylinder. You will hear it hissing out very easily in one of 3 places. Carb, exhaust or oil fill cap. First 2 are valves (see above) last one is a very dead piston. This is the crude leakdown test. The fancy ones use a pressure regulater and an orfice. the pressure leaking past will cause a pressure drop across the orfice and the bigger the pressure drop the more leakage there is. At this point we don't care about how much leakage, it is nearly all of it. We just want the leaking compressed air to tell us where it is going.

Unless you got super lucky and someone didn't put the engine together right and the valve has always been too tight. You have internal damage that will require the head to be removed. Don't matter if it is in the head or the piston, the head is coming off to find out and assess the damage.

 

[bax]

Remember Miker got this bronco and drove it home like 500 miles. When he left it was running good. He developed a miss close to home. This is why I think it's a valve, not a tight valve train or adjustment issue. valve spring? Worth a look for sure.

 

[Miker]

well more like a 1,050 miles. AND, I trailered a bare frame and a 5.0 half way home with it with a u-haul trailer - all total about 1,800 lbs. (I think I'm a dumb-arse for thinking she could handle it). I'll pull the valve cover off tomorrow and let you know what, if anything, I could find.
Thanks

 

[gumbydood]

I had a broken intake valve spring that registered as no compression.

Keep your fingers crossed...

G'dood

 

[DirtDonk]

If nothing is obvious at first glance, you can pull a plug and put some pressurized air into it (assuming you have a compressor of course) and listen to where the air goes. If it spits out the exhaust manifold, look to the exhaust valve to see what's up.
If you hear it come out of the intake/carb, look to the intake valve.

Of course, for this to be a valid test, you'll need to put that cylinder on it's compression stroke first. Either by counting degrees of rotation from #1TDC (1.5 turns of the crank I think?), or watching the valves to see if they''re both closed.

It won't cure the problem, but will at least point you in a direction of attack.

Good luck. As always, keep us informed. So we can share in your frustration anyway!

Paul

 

[VT_Don]

Broncobowsher has offered some excellent advise in accurately diagnosing the problem.

I had a similar fault on a fresh rebuild that caused some serious hair pulling. Discovered an oil baffle, rivet head on my cast aluminum, Cobra valve covers was contacting a roller rocker arm very slightly. Enough to cause a miss on the effected cylinder.

Lesson learned, when combining aftermarket parts, don't assume anything is correct.

 

[Miker]

Staying with the concept of needing to remove the head-- These heads have very cool outside the frame Hooker headers that were installed around '72, so my additional concern is breaking off a head bolt into the head for removal. What's the best advice to coax these things out? Sprayed Blaster on it last night inanticipation of what's ahead. Any other tricks?
Thanks

 

[Miker]

Okay, couldn't stand it, went out in my damn robe this morning with a cup a Joe, pulled the valve cover, stared it a while as if that were going to miraculously fix my problem, and give 50 me extra horsepower etc. - surprise, surprise, that didn't work. So,,,

I did notice that the exhaust spring was in fact compressed so I thought on the off chance that that's where the compression stroke was, I'd simply take off the rocker relieving any pressure from the push rod to see how much the spring rebounded thinking that if it was burnt and stuck, it would stay put. Well, as I unscrewed the rocker, the spring came back up pretty much in alignment with the other springs. I know some have said the rings could have gone, but I really think that I would have been able to register some pressure readings. I guess I'll try the squirt of oil into the cylinder trick this evening to see if that builds any pressure readings, then that should maybe help narrow it down.

Wish I had the magical "Bronco fixin Fairy Dust" I suspect I could sell it for more than Tony Montana could get for his "product" Arghhhh,,,,,

 

[Broncobowsher]

Was the exhaust valve in the exhaust cycle when you backed the rocker off?
I guess you could actually completely remove the exhaust rocker and check compression.
Intake won't work the same way, you still need to feed the cylinder to give it something to compress, not letting it out the exhaust will cause running issues but you can still check compression that way.

 

[76fomoco]

I am looking a similar problem on my GTO did you find out what caused the problem?

 

[Miker]

Burnt exhaust valve

 

[tasker]

easy fix! have it done by saturday will ya! ;D

 

[Miker]

I'm still gonna drive it down, others have said it shouldn't hurt it any further than it already is,,

 

[tasker]

that's my boy

 

[Steve83]

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