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Old 03-15-2000, 11:21 PM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Mind you I'm going to tell this from a techs viewpoint.

#1 What the H is a tune-up. Your car tunes every known adjustment multiple times a second. If the last time plugs were replaced was 35000mi ago well ... DUH.

#2 You will need a valve job when your valves seal so poorly that you have a compression loss. Since quantity of leakage is a time dependent issue, valve leakage hurts effective compression more at an idle speed as there is more time to leak between cycles. Have a compression test and/or cylinder leakage test done when you put them plugs in.

#3 A scope test can mean many things. In this frame of reference it is being used to monitor primary and secondary ignition. An oscilliscope is a devise that plots voltage against time. A proper spark event will have a given signature describing each stage of the ignition cycle. An activation voltage of around 10,000 volts is necessary to ionize the air gap in the plug till the spark can pass through the gap (at idle things doing well). During the actual spark the conditions in the cylinder can be read (if one is good and works on the same systems daily)lean, rich, timing?, compression (not directly here although a relative compression test can be done on good scopes by measuring the starter current keyed to engine firing order). Mainly one can tell deviations from cylinder to cylinder and see a glimse in a rather rapid event. A good spark will take a minimum of one millisecond to finish its event, probably no more than 2 milliseconds at the most. Events less than 1 millisecond are probably in misfire. From here it gets technical (bg).

#4 Forget you herd dat. Replace the fuel filter and pray.

Well after you verify your compression and do a cylinder balance test to identify the weak or misfiring cylinder, the next step involves determining the weakness that causes the misfire. A cylinder balance test monitoring hydrocarbons will identify cylinders recieving more or less fuel. The concept here is that by killing the ignition on a specific cylinder and watching how much unburned gas appears in the exhaust with an exhaust gas analyser you get an idea of the relative amounts that were put in. This test is done by the machine under precisely timed conditions or it would be impossible.
Once you have determined that there is good reason to suspect fuel flow difference (ooh that rust ain't no good here) it can be proven with a differential flow meter which a few dealers and most BSCs (Bosch Service Center required equiptment) have. We test at 8 cc/min for idle 30-40 for midrange and around 60 for full throttle. The idea is to measure all at a flow rate in the range and the variation cylinder to cylinder can't be more than ten percent. The actual flow value isn't the consideration just a position to compare. This test can be done with just the fuel distributor or also with the injectors to precisely identify that $700 bad fuel dist.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 03-15-2000).]

[This message has been edited by stevebfl (edited 03-15-2000).]
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