Compression Check Results; Please advise?

[Bilgamesh]

I have a '74 Bronco with a stock 302. It has 135K mi. on it and idles roungh. I bought the compression tool and found I have the following compression in sequence from cylinder 1-8: 90, 30, 115, 160, 10, 130, 160, 130.

My stepdad the shadetree mechanic said it could be blown head gaskets, valves or rings. My question is how would one know if a gasket, valve job or rings would do the trick, or a complete rebuild would be required? The engine does not smoke, and all plugs burn clean.

Thanks

 

[Broncobowsher]

#1 looks a little seak
#2 is bad
#3 is OK
#4 is real good
#5 is completely gone
#6,7,8 are good to real good.

From those results, it's more then just one problem. Both sides of the engine are affected.

Now for the other question, you did follow a small block FORD cylinder numbering, right? If you followed a CHEVY cylinder numbering then the 10 and 30 could be next to each other, that would be a bad head gasket. Maybe you followed the firing order?

I will ask another question, you did crank the engine for several revolutions per each sylinder, right? Those numbers show a wasted engine with major problems, if it was measured right.

 

[Bilgamesh]

Yes, several revolutions; Ford V8 cylinder #s, not firing order.

When you say wasted engine, are you suggesting not to bother with the items I mentioned and just replace it?

Thanks

 

[Doyle]

With those readings and that mileage, I believe a rebuild is in order. Has this block been rebuilt before? Most don't advise going over 4.030 bore on a SBF rebuild.

 

[bax]

Well with a zero reading or even a 30 I will have to say burnt exhaust valves. It is very hard to get a zero unless there is a hole somewhere. Im talking burnt valve like 1/4 of the valve is not there at all. Pull the heads and let the fun begin.

 

[Broncobowsher]

OK, sounds like you did the test right. I was hoping you did the test wrong and a retest would come up with better results.
I would pull the engine (or at least the heads to start with) and start looking around. I would expect broken compression rings, burned valves and/or gaskets.

If you want to play around a little more before you take it apart, do a quick leak down test. Don't really care how much it is leaking, you know it will leak a lot with those numbers. My compression tester has a schrader valve I can remove. This lest me hook it to shop air. With a piston at TDC ready to fire, put shop compresed air on top of the piston. Listen for where it is leaking to. Few options;
Intake, burned intake valve
Exhaust, burned exhaust valve
oil filler/cranckcase, rings and pistons
neighboring cylinder or outside of the engine, head gasket

Keep the radiator cap off while you do this test as well. If the water in the radiator starts to rise with air pressure in one cylinder, you are leaking into the cooling system. Head gasket, cracked head or cracked block

 

[SaddleUp]

Do a wet test and see if it changes. Squirt some oil in each cylinder and test it that way.

 

[Bilgamesh]

Too late for leak down and wet test. I pulled the heads. The pistons and cylinder walls look good from the top, no scoring, play or slap evident. The cylinder walls have only a paper thin ridge of wear at the top, almost none. How do I tell if it has been bore before. Honestly, they look like they have never been off. The valves look a little rough, but none just eaten away. 4 of the smaller valves are receded intop the head quite alot more than the others, not sure what this means. I am sending the heads off for machine work, and contemplating my next move. I hate to dig into the block...

 

[cs_88]

It is possible that for whatever reason your valves may have been out of adjustment, I ran into a similar problem with my 74, the only way to adjust the valves on my 302 is with different length pushrods, I installed pushrods that were .060" shorter and it made a big difference with mine.

 

[73stallion]

Too late for leak down and wet test. I pulled the heads. The pistons and cylinder walls look good from the top, no scoring, play or slap evident. The cylinder walls have only a paper thin ridge of wear at the top, almost none. How do I tell if it has been bore before. Honestly, they look like they have never been off. The valves look a little rough, but none just eaten away. 4 of the smaller valves are receded intop the head quite alot more than the others, not sure what this means. I am sending the heads off for machine work, and contemplating my next move. I hate to dig into the block...

sounds like either valve seats or it sucked a couple of valves. you probably just found your problem.

 

[Bilgamesh]

What exactly does sucking a couple valves consist of? I guess I am wondering if I should just buy some rebuilt heads, or another set of used ones as a core. Does this also mean there is a good likelihood I can have the valves done and forego the lower end of the engine? - Thanks

 

[mr incredible]

sounds to me that you have a engine that was never hard seated for unleaded gas ,,, the valves literally beat them selves into submission ,, and the seats go south so to speak ,, and start leaking ,,, my bet,,it is the heads and valves ,, but you can test this also , get the heads done , and reassemble heads to block , and install valve train , leave all spark plugs out , do not install the intake or the distributor yet , and only spin it over a few times , per cylinder , as you will have no oil pressure ,, or you can pump the pressure up with a drill between every test , then take another set of compression readings , at least you will not have waisted a lot of time on assy ,, and coolant and all ,,and gaskets , just to find out that the bottom end needed help aslo ,, head gaskets ,,many are reuseable even after torqueing , if they do not see heat ,, but if they seem done in ,, they are fairly cheap to buy ,and can be sacraficed if need be ,, to find out if the bottom end is decent ,,,

 

[regshawn]

Had a similar thing happen to me when I swapped a 69 302 into my 64 Falcon. The unleaded gas plus the soft seats made quick work of the heads. Pulled the heads had them worked over with hardened seats and the works and had no issues again. Well until I dropped it a gear going 110mph......lots of smoke!
Good luck,

Shawn

 

[Bilgamesh]

Thanks for the information. A little research reveals the condition is known as "valve seat recession". Over time the "soft" seats experience micro damage to their surface and eventually recede into the head and no longer seal properly. It sounds like this is a likely diagnosis based upon the symptoms and the visual condition of the heads. I suppose the machine shop will tell me the same. I am still wondering if I should consider buying a newer set of heads. If so, which years would be a fit for this engine while maintaining original compression?

Is there anything special I should do to protect the cylinder walls and internal engine parts while the heads are off? I have sprayed wd-40 over it, but want to be safe from rust- Thanks

 

[Steve]

Is there anything special I should do to protect the cylinder walls and internal engine parts while the heads are off? I have sprayed wd-40 over it, but want to be safe from rust- Thanks

Keep it clean and dry.

The compression readings don't make sense to me. A brand new engine may not even have 160 psi readings, and you have two that you say have that, while others have as low as 10. That just doesn't make sense. ?:?

 

[76 bronco J]

,,,,as mentioned above leakdown testers are the only way to almost pinpoint the issues versus playing a big guessing game just using a compression tester alone,,,,either build or buy one ,but I prefer the 2 gauge type shown below,,,, on the valve seat recession...I have that problem in my little 1971 honda trail 70's original engines even though I run a leaded additive,,,, the valves start getting tight(mainly the intake valve on these particular engines) causing compression leaks/hard starting & loosening them bring the pressure back up,,,, as far as putting wd-40 on the cylinder walls just remember that it's really a solvent cleaner with a touch of lubricant,, i just wipe regular oil on the cylinder walls

 

[needabronco]

If the cylinder walls aren't worn very much, I would go ahead and at least re-ring it and put new bearings in. For a couple hundred more dollers you could save yourself alot of second guessing and headaches in the future with a fresh rebuilt engine. With the heads off the shortblock shouldn't be too hard to pull out. Take it down to a good machine shop and have the cylinders mic'd to see if a re-ring is feasable. V-8s aren't very hard to rebuild and you will gain alot of understanding on how they run. I'd bet it could be done for as little as $300.00 more.

 

[Bilgamesh]

Thanks to everyone who has replied.

Yes, all around, I could keep saying well for only X$ more I could do this and then for only Y$ more I could do that until I drop $2000 more into a nice new/rebuilt engine for this $3000 vehicle. However, at the end of the day I would still have a $3000 vehicle that I may put 10,000 miles on in the next 10 years.

The plot thickens, as I dug out an old digital caliper and found the cylinders are over 4", which means it has been bored at some point in the past. The battery on the caliper was dead, so I could not tell precisely what the cylinder bore was, but it was definitely over 4. I think my approach will be to see what the machine shop says and assess from there. Then I may do what "oldwinghunter" suggested with a new compression check before I put it all back together, just to be sure. I am still open to other suggestions about this as well as advice about rebuilding these heads, or looking for another set as core etc. - Thanks

 

[Bilgamesh]

I just dropped the heads off at the machine shop. He noticed what I didn't, i.e. that one side had double nuts on the rocker arms. He said I should be looking for a cam as this can mean the cam is flat and that they have used double nuts to maximize the adjustment to the valves.

 

[ugly74]

If I were you, I'd just yank the motor and rebuild the whole damn thing.
the last thing you wanna do is put on a new top end and then find out the bottom's all wore out too.

even if you only drive it 10,000 miles a year, the time and money will feel well spent...when you turn the key and it starts every time, and when you mash the gas and it gets up'n'boogies without spittin and sputterin and stinkin and smoking and being a pain in the ass.

I might only drive mine once a week, but every time it fires up and runs strong, I'm oh so glad that I spent that $2000 building an engine for my $1,800 Bronco.

I bet the double nuts are there, because as the valves recess into the head, the stem above the head becomes longer...necessitating an adjustment.