Just had a "Power Balance" Test on my 3.2
 

i went to a new friend from the porsche community in order to get my car smogged for renewal of my California registration. While i was there, the guy suggested that he use the same machine that he was using to do the smog to do a "Power Balance" test. He said that he trusted the accuracy of the machine more so than that of a leakdown test because the "Power Balance" test is done with the engine running while the leakdown test is done while the engine is static. Basically the test consisted of the machine cutting the ignition/spark on each of the 6 cylinders and then measuring the corresponding drop in power at a constant RPM. Results were 100% for 3 cylinders, 94% for 2 cylinders and 89% on the #6 cylinder. This may actually confirm what i have already suspected about my engine's wear and its need to be overhauled this summer. He said that given the results i should just continue to feed it oil (currently drinking a quart every 400 miles or so) and drive it.

any thoughts on the validity of this testing method?

thanks

db

Interesting test. I suspect it may, more to the point, indicate which spark plug leads or injectors are bad, other that necessarily a bad cylinder.

Also I hope your cylinders arn't bad :)

How does it measure the "power drop"?

Is the car on a dyno or something?

db,

It might be useful supplemental information to be used in conjunction with data from an ignition scope, DME diagnostics, injector flow tests, compression, and leakdown tests ... but like many other test methods ... examined independently, the data is essentially useless, because you don't know the status of the ignition or fuel injection system!

warren, i figured as much and didn't really know where to file the results: either complete garbage or somewhat helpful. as far as measuring the power goes, i don't know. my car wasn't on a dyno and only had leads going to the tailpipe, the ignition wire and somewhere else that i couldn't identify.

bill: i don't think i have a bad cylinder but i do think that i will be dealing with top end related wear in the not too distant future. the car has 167K with no top end. the car is running fine but the oil consumption is at about 1 quart every 400 miles or so.

wish i knew more, it was done in passing after my smog. i have a print out that i will post when i get home.

thanks

david

The power drop test is a valid test. Most engine diagnostic/scope machines from way back had this feature - sort of a dynamic compression test. You can accomplish the same thing by disconnecting each spark plug lead one at a time and note the rpm drop (each cylinder should contribute equally). The cylinder(s) with the least amount of drop represents a weaker cylinder. If the engine is actually misfiring, this test will isolate the cylinder not contributing as there will be little change when disconnected.

Caution: To avoid pseudo-electrocution, preloosen the spark plug leads at the distributor cap, then use insulated pliers to remove and test them one at a time. I think K-D and other manufactures make plastic pliers for this purpose.

Repair shops might not do this test as often because the unburned fuel that results from disconnecting the secondary voltage might clog/damage the cat. In addition, disconnecting a high tension lead without grounding it simultaneously may damage modern electronic ignitions. This test is safer on pre-smog and pre-ECI systems.

Sherwood Lee
http://members.rennlist.org/911pcars

The use of insulated pliers is a very good tip. I was watching my neighbor pulling wires on an old chevy truck and noticed a spark hopping from his pants leg to the bumper. Something like a tenth of an amp directly across the heart will make it want to stop.

Also, if there is, say, a bad plug or wire on a cylinder, the voltage gererated in the coil will want to go somewhere and often will internally burn through its windings to get to ground.

The last time a mechanic told me to "just disconnect a spark plug" I asked him, "Isn't that dangerous?" He told me he'd been thrown across the shop a couple of times by it. Hmmm, I thought about it and just disconnected the injector wires one at a time instead. Same test without the shock hazard or unburned fuel.
I must say though, only having 5 cylinder working didn't make much of a difference. ('86 3.2, idle stabilizer disconnected.)
-Chris

Heh,Heh........

At an AX a while back a friend was experiencing a severe miss on his 914 6er and asked me to help. He was on one side and I was on the other. I started pullin' plug wires while it was idling.....bare handed, of course. I found 2 dead holes. He found a leaking wire.......ow,ow,ow. Just fouled plugs, as it turned out. I recommended some new plug wires.......he mumbled something like "no sh**".