Quote:
Originally Posted by Shern
Wouldn't that come up in a compression test? I was within the 10% tolerance.
No skip or miss... it *sounds* like a healthy idle.
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Valves that don't seal consistently due to worn guides or tuliped heads can pass a compression test but still leak enough gas during combustion to cause an inconsistent "shake" at low RPM's. A leakdown test will turn it up, you'll hear the air leaking from the intake or exhaust on the problem cylinder.
My SDL was a good example of this. My compression numbers were all centered around 380PSI. High end was ~395, low end around ~365. The low cylinder was due to leakage past the intake valve (confirmed with a leakdown test). 10% of 380PSI (average) is 38PSI, or a range of ~361-399PSI (+/-5% of 380). My maximum difference was 30PSI from highest to lowest, or ~7.6%. If you ran the engine with the intake off, you could hear the intermittent pops from the intake runner of the weak cylinder when the valve didn't seal fully. I only use my car as an example here because all of the numbers from the engine said it should be fine, and it fell within the accepted range of compression values, but it always suffered from that idle shake, regardless of what I did to the fuel system. I never pulled the head to repair it with the intention to replace it with a #17 or later head in the future. Other than the shake at idle, it ran well, started when cold, made good power, and got good fuel economy.
The bottom line is that if you swapped the DV's and the problem remains, it's more than likely to be a mechanical issue with the engine or injector problems. The latter can be tested by substituting a different injector and checking for a change in behavior. Don't just move the injectors around to different cylinders, you'll easily be fooled by it.
It's certainly possible for the injection pump to have some sort of problem, but it's unlikely. The pumping elements won't have an intermittent problem that comes and goes. It'll be there all the time and will never change. The governor mechanism affects all of the pumping elements at once, so a governor issue will usually result in a very profound rocking/cadence that affects the cylinders at random as the mechanism hunts.
I dealt with this last specific issue in my SDL when I got it. The rear ball bearing on the camshaft in the IP had come apart and was allowing the camshaft to wander around. Impressive that the car ran, but it was very poor, an extreme lope, primarily on cyls 4-6 that would go away with an increase in RPM. The replacement used IP was varnished internally and would refuse to rev or return to idle. I got it freed up enough to try to work, but gave up and sourced a 3rd. The 3rd IP had very worn DV's that caused excessive nailing and a pronounced "soft miss" at idle when hot. That IP is the one that the car lived out its days with and was the basis for my writeup in the DV nailing thread.
When I share information and experience on the IP and how it works, it's from personal first-hand experience with relatively uncommon problems, not someone else's misguided ramblings or armchair thoughts on an Internet forum. Hopefully it helps someone at some point.