Analysis Without Equipment by Mac VandenBrink

LOW SPEED PROBLEMS ARE USUALLY TIME RELATED: 1. Leakage,
like a poorly seating valve has a greater chance to be
diagnosed at low speed, typically cranking RPM. The reason
is simple, because it has more time to leak down. Example: A
burned valve may feel like a misfire at idle, but
performance wise there is very little negative effect at
high RPM, because the leakage is so minute at that speed. 2.
A vacuum leak drives the fuel trim positive at idle, but
equals out at high speed when the leak has very little
effect. That is how you know the difference between lean
injector and vacuum leak. 3. A leaky injector has more time
to drip at idle, while at high speed there is less time and
may be only a few drops. 4. Carbon build-up may keep the EGR
valve open causing a rough idle. At high speed the EGR valve
should be open anyway to control NOX. Therefore carbon
build-up at EGR has no negative effect at high RPM or load.

HIGH SPEED COMPLAINTS ARE USUALLY VOLUME RELATED: 1. Here
are some examples of fuel starvation: An 80% restricted fuel
filter is no problem at idle, but you won’t make it driving
uphill when volume and demand is far greater. 2. The same
holds true for a defective fuel pump. It may have a perfect
fuel pressure, but fails to supply enough volume at high
speed when high demand is critical, causing surging under
load, yet may pass every function at lower demand at idle.
3. Also a restricted exhaust has a greater negative effect
when the volume is high at wide open throttle and the
accumulation of inhaled air is at its peak. 4. A lean
injector may pass at idle, but fails at high speed,
typically at fast acceleration when the computer cannot keep
up with compensating for the lean condition. This can be
easily demonstrated with a snap-test (when the computer does
not respond fast enough to compensate), on the ignition
scope pattern.

FAILURE AT ANY SPEED: A dead hole at any speed can mean
almost anything from absence of fuel, or spark, or
compression. The fact that it is at any speed makes analysis
simpler because it does not fall under those above
categories. Reading the code tells which cylinder.

Final advice! Don’t memorize — analyze!

Mac   VandenBrink
Educator/Instructor/Consultant
Dynamic Auto Test Engineering Corp
Kalamazoo, Michigan, USA

Mac’s website is: www.datec,us

www.Vestest.com

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