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2002 Montero with Rough Idle and Misfires a Case Study by John Rogers
John Rogers uses his high amp probe and igniton trigger pickup to “Work Smarter not Harder” on a 2002 Mitsubishi Montero Sport 3.0 liter towed in for a no start. After the shop owner replaced the timing belt the vehicle starts but idles roughly, runs poorly, and misfires. John is hired to find out why.
John uses his Snap-on scope with the Generic High Amp Probe (AES# 07-600) to check compression.
I don't want to spend all day taking spark plugs out and on this vehicle removing the upper intake to gain access to half the spark plugs. We are going to do a relative compression test. It is one of my favorite tests. Easy to do and packed with information. This test will measure the amount of electrical current needed to turn the motor over as it relative to each cylinder. What we are looking for is uniformity between cylinders. We are also looking for a proper cranking amperage as well.
The compression test shows “a cylinder that has substantially lower compression due to the low amperage load to "turn" that cylinder.” But which cylinder?
To quickly determine which cylinder has low compression, John connects his AES Inductive Trigger Pickup (AES# 01-14) to cylinder #1 and superimposes it over the waveform from the Generic High Amp Probe. Now that the Inductive Trigger Pickup is referencing #1 cylinder on the screen, John quickly identifies the problem cylinder.
“I don't want to spend all day taking spark plugs out and on this vehicle removing the upper intake to gain access to half the spark plugs. “
Instead, he used scope with a high amp probe, ignition synch probe, some reference information, and about 15 minutes.
John conducts a few more tests to verify his diagnosis, to check the timing belt is correctly installed and hands the job back to the shop.
Read the complete story at JWR Automotive Diagnostics.
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