Thread: Motor not Compressing on two cylinders

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So here is the thing,

I have a 70's Chevy truck with the beloved 350small block. However it's been giving me some hard times. To begin with it had some valve bank (not quite sure if that's the official term, but as accelerating you would hear crackling noise) and when heated it got worse. Long story short after over heating it sat on the side f the road for a few hours, it now misfires on two cylinders. I plan a engine overhaul but before I start do you have any suggestions of what might be causing the two problems? The misfiring and Valve crackling?

Thanks!

p.s. I changed the spark plugs and wires not 300miles ago.

Originally Posted by VikingRed

To begin with it had some valve bank (not quite sure if that's the official term, but as accelerating you would hear crackling noise) and when heated it got worse. Long story short after over heating it sat on the side f the road for a few hours, it now misfires on two cylinders. I plan a engine overhaul but before I start do you have any suggestions of what might be causing the two problems? The misfiring and Valve crackling?

I'll take a S.W.A.G.
The motor is detonating and you've either cracked or holed a couple of pistons or smashed the top ring lands down, pinching the rings so that they cannot build compression.
Detonation in a motor has the same effect as swinging a sledgehammer down onto the pistons over and over. It has several causes, including running a high-compression motor on fuel that is insufficient in octane to keep the motor from detonating.

When you get the motor torn down, come back on this forum and tell us what you found. Most of us on here can lay out a plan for the rebuild that will preclude this happening again. We've been there, done that.

Yep, sounds like you were detonating and now have bigger problems as Tech points out. Before you pull it apart see if you can get an accurate measurement of your base ignition timing, just for grins. That might help in the post mortem of your tear down.

what two cylinders????????????

and find out why it overheated .. do you have steam coming out the exhaust ?

If the two cylinders are next to each other, you might have blown the gasket (or worse) between the cylinders.

Like Tech said, let us know what you find out. PICTURES PLEASE!!!!!! We do like pictures.

My first thought was if it had something to do with the gasket, however I have not seen any steam from the exhaust. But I will be sure to post pictures of the tear down, and I guess it is just a waiting game to see what the pistons look like. It is at least misfiring on two, I could feel the exhaust from the middle cylinders were not heating up as much as the ones at the ends. First thing first I need to lay my garage floor with some epoxy coating! Then I will get cracking.

Thank ya´ll for the responses!

If the head gasket blows between the cylinder walls, there might not be any water close to that area. I had a in line 6 in a 67 chevy van that had a blown head gasket between the cylinder walls and the PO had run it like that for who knows how long/far. It ran crappy and when you stepped on the loud pedal you could hear it popping through the carb. Upon teardown there was actual metal missing on the head and on the block. Believe it ot not, liquid steel worked for quite a while (as long as I drove it easy) before I dropped in a 327 SB. You do what you have to when you are young and do not have a lot of $$.

So I got a reading on the engine, and the ratio is wicked to say the least.
B1 (205-45-55-185)
B2 (205-80-100-185)

I thought I'd throw that info in there for kicks, I set up my engine stand today and went shopping for some tools. Hopefully the engine will come out tomorrow.

Do you have an EGR valve on it? Sometimes when that goes bad it will run like crap. It usually sets right next to the carb close to the middle of the passenger side. Never checked the compression on a motor that had a bad EGR valve though.

Originally Posted by Bug

Do you have an EGR valve on it? Sometimes when that goes bad it will run like crap. It usually sets right next to the carb close to the middle of the passenger side. Never checked the compression on a motor that had a bad EGR valve though.

All the EGR valve does is modulate to introduce a measured amount of exhaust gas into the air/fuel stream to slow the burn time, equivalent to increasing the octane rating in the fuel. I don't see how an EGR could affect compression measurements, but if my understanding is off I'd like to learn.

Usually, when two adjacent cylinders have low compression, it is a blown head gasket. On SBC, the center adjacent cylinders are the most common to blow a head gasket. And on a SBC when the head gasket is blown on both sides at the center adjacent cylinders, the cause is usually a motor that has overheated.

Originally Posted by rspears

All the EGR valve does is modulate to introduce a measured amount of exhaust gas into the air/fuel stream to slow the burn time, equivalent to increasing the octane rating in the fuel. I don't see how an EGR could affect compression measurements, but if my understanding is off I'd like to learn.

Like I said, I don't know if it would or not . I never checked it. I believe that all it will do is let too much exhaust gas into the intake which will make it run like crap. I do know that it will do just that though. It has happend to me.

Originally Posted by Bug

Like I said, I don't know if it would or not . I never checked it. I believe that all it will do is let too much exhaust gas into the intake which will make it run like crap. I do know that it will do just that though. It has happend to me.

No denying a bad EGR valve will definitely make the motor run poorly!

Just thinking out loud here.

If the EGR valve is no good and it leaves an opening/vent from the exhaust into the intake, would you get low compression on the two passenger cylinders that it sets between?

Not exactly sure how it even works.