cylinder compression

[Mainebronco]

I've got a 1968 bronco with an original stock 289.  I'm not an auto mechanic or even much of a past tinkerer.  anyway. I had the cylinders tested and one of the 8 has low compression, about 60 pounds while the other 7 are all about 125 pounds.

Question: what might be causing this?  And what's needed to repair each of the possible problems that might be causing this?  what kind of costs should each repair run me?

Also, am I ok to drive as is for a while?  If yes, what other damage can come from driving as is? 

thanks very much.

 

[Rons beast]

Hey Maine,

There could be a few things causing the low compression. Bad news is most are all internal, ( broken ring, burnt, broken, bent valve, burnt piston, blown head gasket cracked head or block..etc..) 

There is a possibility that a valve could be stuck open.   Then the valve may be able to be freed with an application of WD40 in the spark plug hole, and on the valve stem, and a tapping with a hammer on the top of the valve. This only requires removing the valve cover. Caution the valve spring is under tension, and WD40 is flammable!    If this were the problem you would be lucky. But.... there is a good likelihood that it would reoccur.

These problems almost always require a removal of the cylinder head.

However!!!  You are talking about an engine that is 46 years old!  You should consider a complete rebuild. Parts are still available and relatively cheep. I would inquire with a local rebuilder as to cost.

Driving it like it is, is not advised.

Good Luck

 

[Bully Bob]

What Ron said plus..,

Curious, if you had this tested by a neighbor or an auto shop..?  Point being a shop would/should give you a report showing before & after shooting oil in the bad cylinder. If the compression went up with the oil, then there's an issue with the piston/rings, etc.

If it didn't go up, there's a problem in the head. This would be the best scenario.

Rebuilding the heads on a hi-time eng. works for a while however, in some cases, the additional pressure on the rings will eventually lead to them being weakened.

The best cure with the least expense would be to fix the bad valve(s) in the weak cylinder. Assuming there's no other issues, this eng. would serve you for quite some time but eventually a rebuild would be in order.

The shop owes you a re-test with the oil if that wasn't done. Also, good shops will hook the engine to a scope & read ea. cylinders performance as well.  It's possible this eng. was rebuilt once already some time back.

Driving around town is likely OK. I wouldn't get up on the super-slab 'til it's fixed.

Once you know if it's a valve or deeper in, a piston/rings issue, you can ask a few shops for a quote. Prices vary all over the country.

 

[Mainebronco]

thanks very much for the feedback.  I had a reputable shop check it out for me.  It's still there for a couple more days.  Had them do a few other checks too.  I'll ask if they shot oil in the subject cylinder and got the same results.  Thanks again... 

 

[Bully Bob]

Sorry, don't mean to be picky but, I think I'd scold them a bit for not giving a full report or test, if in fact, they didn't test the rings.

(not telling you is a "red flag")

I might even suggest; "Should I be going elsewhere for these services.??"

Compression & vacuum tests are just simple/basic tests.!  And ALL results must be reported to the customer.!

Come back & tell us what they quote you for any repairs B4 you commit.

Let them know you're on line with a Bronco owners site that is helping you with decisions.

M5 has a list of prices paid for eng. work by geographic area.  He will likely spot your post. 

Good luck,

B