Ok here is the situation. Wife ran car hot, water was leaking out of exhaust manifold. Changed head gasket, and inspected head visually for cracks. (I know I should have had it checked better but didnt have the $$) Anyway now it is all put back together and it has a skip. So I started pulling off wires. Number 3 came off-no difference. Number 5 came off-no difference. Number 7 came off- no difference. Number one is the only one left on the side the head gasket blowed that is producing power? So what gives? Does my head have an internal crack? Maybe my rings stuck? I checked the cylinder walls when I took the head off. They looked fine and all the pistons looked great. Is it possible I may squirt a little oil in the cylinders and fire it up and see if it picks up anything? I know I need to check the compression but like I say I am really stranded at home broke and have no compression tester. Is there anything I can do to try to narrow this down? I have more heads if that turns out to be the problem. All ideas are greatly appreciated!

 

Chev,

Did you install new spark plugs when you finished the assembly? If the old plugs are fouled with a coolant mixture, they will short the spark directly to ground and kill the affected cylinders.

You should also check for spark at those cylinders with a spark tester or the old screwdriver metod - just make sure you're not in the circuit!

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Later,
Vader
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"Click - Click - BOOM!"
Adobe Acrobat Reader

 

I have spark at all the cylinders. I dont have new plugs in at the moment but they are a top priority. But the plugs in it only have about 300 miles on them. I did swap a plug from one of the dead cylinders to a good cylinder and the problem didnt follow the plug it remained on the same cylinder. I have done the same with plug wires. I replaced the cap because I had an extra one laying around and figured it couldn't hurt.

But after it ran hot it ran the exact same as it does now. Unfortunately, I didnt check to see if the cylinders were dead before. I just saw the water coming from the exhaust manifold and tore it down.

I stuck my finger over one of the spark plug holes today and had a friend crank the engine a few times and it didnt "pop" as hard as the good cylinders so I started disassembling it again. I am assuming the problem is in the head so I am going to have it checked in the morning at the machine shop.

One more question, Where are all the most important places to check for cracks? That way I can eliminate this one if there is a visible crack and check my others until I find one with no external crack and take it in for the machine shop to overlook. Thanks for your time. I will let you know how it goes.

 

Chev,

As for weak places on the heads, the area between cylinders and the area between the exhaust valve side of the chamber and the water jacket surrounding it seem to be the thinnest areas for stock castings. The heads can crack just about anywhere, however, and a shop with an ultraviolet lamp and MagnaFlux kit can usually find any imperfections.

It seems really strange that you would have three adjacent cylinders fail, however. Lost compression on three adjacent cylinders is probably more than just a crack or failed head gasket. I'd be more suspect of head warpage, a cracked block, or even a failed valve train or worn cam lobes on the affected cylinders. I could understand two adjacent dead cylinders, but three in the same bank is rather odd.

I wish I had more ideas for you, but lacking any real compression numbers and more detailed information, diagnosis would be just a guess.

You need to be certain that the dead cylinders have some compresion. I've seen V-8s run surprisingly well with over 40% variation on cylinders with a standard compression test, so the lack of some compression can be pretty forgiving.

You also need to be sure all the cylinders are getting fuel. You didn't mention the type of fuel system, so if you have injectors, you may have a failure of either the injectors, a fuel rail plugged, or an electrical failure in the wiring harness. A carbureted system isn't as likely to be a problem, since the two halves of most intakes are split evenly across the firing order.

You also need ot be sure all plugs are getting healthy spark at the correct time, with no cross-firing and no wet plugs.


A REAL LONG-SHOT

It is possible, given the specific cylinders you've mentioned, that you only really have one weak cylinder (#3), and that the spark plug wires have been reversed on #5 and #7. Since they are both physically adjacent and adjacent in the firing order, it's easy to do. I'd hate to try to remember how many times I've done the 5-7 swap. That would explain how the #3 was dead, and #5 and #7 were producing no power either.

Have you tried to probe the exhaust manifolds around the suspected dead cylinders to determine if they are firing at all or not? If there is fuel and ignition, there should be at least some heat even if no power is being produced. It's best if you have a temperature crayon to do this, but a coloring crayon or even a candle can work for this. If the wax melts a lot faster on the good cylinder's exhaust port, you have a better idea of the problem.

A little more information about the car and engine would be helpful, too.

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Later,
Vader
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"Click - Click - BOOM!"
Adobe Acrobat Reader

 

Valves too tight??

As Vader stated, we need comp. specs.

I would do a comp. check before I tore it down.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R
and 2 POS commuters...

R.I.P. #3
"He can see air"

 

DOH!

Good call, EB.

Looks like I just missed the obvious, again...

Since it was apparently disassembled and reassembled, there are a lot of variables, like the tight valves, bent pushrods, incorrectly placed head gasket(upside down/front to back), incorrect head gasket entirely, improperly torqued heads, bent valves, yada, yada...

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Later,
Vader
------------------
"Click - Click - BOOM!"
Adobe Acrobat Reader

 

The motor is a carbed 305 so I dont think the fuel would be a problem. I am fairly certain that the gasket is on right. It is a felpro and I put the side with the felpro name on it up.
I did have a problem with number 1's valves being to tight and so I loosened them all up this was after turning it over a few times but the engine never started with them that tight is it possible that could have caused a problem? I am certain that they are not to tight now.
I too have been thinking about why the 3 adjacent cylinders are the only ones affected. It seems like if it were a ring problem caused by the heat it would have affected the 2-5-6-8 side of the engine as well.
I have already pulled the intake should I pull the head in the morning or will it be ok to check the head with the intake off? It wouldn't affected cylinder pressure would it?
I must be honest as wrong as I know it is to do this I torqued the heads without a torque wrench by tightning them really tight but not to the point of risking a bolt breakage. I have installed them with torque wrenches numerous times before and while I know I didnt hit it exact I figured I was close enough it would suffice given my situation.
I did have the 5-7 wires crossed in the beginning and was relieved to have the engine smooth out consiberably once I changed them back around. uh oh as I said that I wondered something how would that affect anything if those were dead cylinders? Hmm.. now that really throws me. But anyway the engine smoothed out after changing them around but still had a noticable skip. But the engine seems to idle the same with them removed but even though I haven't tried it I dont think it will rev as well. Or at least it didnt when the 5-7 was crossed. Maybe it has just enought power in the cylinder to have an adverse affect if fired at the wrong time. But not enought to have a noticable affect when firing at the right time?
Thanks for taking the time to respond.

 

Sorry I just wanted to say that it should read "will it be ok to check the compression with the intake off. Not neccesarily the head.
Thanks again for your time.

 

Yeeehhhhhaaaaaaaawwwww, the ol' 5 7 cross, I did that the other day as well ;-)

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j.wolfe
_ ____ _
91 GTA L98 MSD K&N 194,000
Best time: 14.389@95.49 (VMP) Done with 193,000 miles and no engine work what so ever.
"...Deception..." Brandon Sloan * ...wounds heal and chics dig scars...

 

I've been doin that for 15 years....

Yes, you can check compression with the intake off, just run the fuel line to a can so you dont fill the engine with fuel or spray it everywhere.



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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R
and 2 POS commuters...

R.I.P. #3
"He can see air"

 

If you remove the schraeder (sp?) valve from your compression guage, then connect compressed air to the hose with it screwed into the cylinder @ TDC, you can do a what I call a "poor-mans" leak down test. With air applied to the cylinder listen for air coming from;

• Radiator cap - blown gasket, cracked head or block
• Carb/TB(intake ports w/your intake removed) - intake valve hung or burnt
• Exhaust - exhust valve hung or burnt
• Dip stick/oil cap - Rings/cylinders

This test should pretty much nail down if and where your leakage is occuring. Remember to make sure each cylinder is @ TDC, or put it @ BDC and loosen the rockers so you know it won't turn, and you expose the entire swept area of the cylinder to the test. Also, when I do this test I only use about 30 PSI. That is plenty to indicate a problem, and it is easier to keep the engine from turning off of TDC w/that lower presure. Good luck.

[This message has been edited by Tom 400 CFI (edited September 07, 2001).]