Cylinder #3 down - Heads coming off!

I have replaced them many times on sbc's with track/road use in high winding motors (read routinely above 6000 rpm.) I would think that they would need to be replaced between 30-40 thousand miles or less if the car is driven hard. I have always been advised to replace all of them as a matched set, never to just replace the broken pair....... Scott

 

that's great that it's not something on the short block (like a broken piston or something). I've had valve spring retainers break on datsuns and produce similar effects (even had a cam break in the middle once! . I almost suggested that maybe you wiped a valve lobe, but didn't think about the possibility of a broken spring. That's great it's something you can fix without tearing apart the whole motor.

Keep the pics coming!

Ray

 

Brian, not to hijack the thread too much, but given all the talk of compression and leak down testing, I was going to install new plugs on my car and perform a compression and leak down test as the same time. However, I'm a little concerned about spinning the motor with the coil detached from the plug. I don't want any stray build of up energy to possibly damage the coil. I also don't want to accidentally ignite any air/fuel mixture escaping from the motor. Is there some "best method" to spinning over the motor for a compression test? I'm not sure if i should pull all the plugs so that every cylinder is open while testing each hole, or if i should leave the plugs in and test each hole one at a time (with the rest of the plugs left in). I was thinking I might pull the fuse (or leads) to the fuel pump, then spin the motor while doing the compression test that way. This would keep fuel from pumping through the motor, while still allowing the coils to spark. On the hole I'm testing, I could just leave the spark plug in the coil, then ground the body of the plug back to the cylinder head. This would allow all 8 plugs to spark with no air/fuel mixture in the chambers while I turn over the motor. I want to avoid washing the cylinders with fuel while I do the compression check and want to make sure I don't do anything which could possibly load up the electrical charge in the coils and/or harm the ignition control unit. Do you have any views or suggestions about this? I looked in the factory shop manuals and couldn't find any procedure.

Any suggestions and/or things to be careful of while doing a compression check on the 360?

Ray

 

All plugs out.

If you do not disable the engine management system you risk very expensive trouble. There are many ways to do it. The best is to just go to the Motronic units themselves and pull both plugs on each.

I prefer going straight to the starter and hooking up a starter button. That way the cars electronics do not need to be disconnected and the learned parameters all erased. With the starter button the cars systems do not even know the motor is turning.

 

This is the 5th 360 that is know of with this failure. It would be best if you replace all the valve springs.
Regards,
Fernando

 

Great call very early on!!!! In a porsche I owned this broken valve spring happened to it, the porsche mechanic spent a few hours figuring the problem. He also did comp/leak test but it only showed normal numbers. Maybe if the valve was bent then it would show, but that would not point to the problem itself. Low numbers could be like worn valve guides, bad cylinder to ring seal, head gasket etc. Myself I was thinking a possible cracked spark plug insulation leaking compression. So leakage would not show with tester (because plug would be outside test equation) but then the plug would show carbon on the white ceramic. Congratulations on the diagnosis!!

 

I know is a lot of work, but DO replace the springs on the other head too, you'll save money on the long run; springs are inexpensive.

 

Okay, thanks. Before I fell asleep last night, I had the thought to just run power directly to the starter on the car - thus (as you point out) bypassing the electronics on the car completely and just spinning the motor...

So then if I understand you correctly: pull all the plugs so the motor will spin more freely, and then run a switched 12V hot lead directly to the starter under the car. If I go that route, I shouldn't even need to pull the harness off the ignition units correct?

Do I need to do anything as far as opening up the throttle bodies in order to allow additional air into the intake during the test. I was thinking I might pop off the throttle bodies where they go into the plenum, unless you feel that's not needed.

Thanks very much for the info.

Ray

 

Yes but lets say you got perfect leakdown and you found a bad spring. Then you are more likely able to take the risk that you can just replace the spring insitu and be OK.. If you have not done the diagnosis you take more risk. It really sucks to take chances spend the hours to put it all back together only to find out a valve kissed a piston and the head has to come off anyway becuase while the new spring works perfectly it don't seal. There is no substitute for good diagnosis just like you don't want your doctor "thinking" you got an appendicitis and start cutting only to find out you got a kidney stone.

 

Gotta love it I may pull the injectors and give them a once over while I'm working in there all right.

When you tested them using the champagne glasses, did you just pull all the injectors, lay them into the glasses and then try to crank the motor (and then just watch to see if the injectors all sprayed the same amount) for a minute? My concern on the injectors would be more that - as the utilization percentage went up - a weak injector might not maintain its pattern/flow characteristics under higher loads. Not sure if a test from just cranking the motor would catch something like that (?), but I don't think it would hurt to run a basic test and inspect them.

Ray

 

I agree with the compression/leak test that my mechanic did and like you say it covers the bases and might save parts replacement. I only said it did not show anything abnormal so it puzzled my mechanic until he removed the valve cover and inspected further. I would like to think doctors and mechanics could find everything first time around too. But doctors have much harder process as they must diagnose many times with little or no visible symtoms only verbal feedback from patient.
But this spring breaking is interesting because sometimes it can be just one defective spring or maybe the whole batch. So precautions should be taken to test and check all for tension or maybe replace all of them. Then there is possibility of a defective spring in the new replacements. But hopefully chances will be very small of that happening.

 

Thats good to hear
130k miles and still running.
I swear these cars run better with more miles on them.....

 

Apples and oranges. That is not what I said no my meaning.

 

Not a good idea Ray.

 

not a good idea to pull the injectors or not a good idea to spray gas all over my garage

Ray

 

Sounds like a fairly good method to check both injectors and compression together.