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International Automotive Technicians Network
Posted to Technical Tips Forum on 9/15/2013 22 Replies


When the discussion in the forum was about a misfire at idle and someone came up with the statement that this 3800 was known for restricted exhaust problems, you wonder. But when this individual has 16 years under the belt and is now a shop foreman, you wonder how he got there. It is sometimes painful to reply to a dumb observation or comment because you don't want to alienate the person from asking questions. After all learning from each other's experience is the main purpose of these forums.

The title does not imply that equipment is not required to analyze. However, there is a concept of common sense, when we consider at what condition the problem may occur,. Some typical malfunctions fall under two categories. LEAKAGE, has the greatest effect at low RPM and has practically no effect at high speed. On the contrary, RESTRICTIONS are typically more pronounced at high RPM and has reduced or no effect at low speed.

The illustration shows typical LEAKAGE and RESTRICTION examples: Basic Rules of Analyzing

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! Final advice...

Mac from Michigan

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