Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I was a mechanic back in 1996 after graduating tech school. Unfortunately these scanner were very new and very expensive so I had little experience with them. I'm hoping this is the place to learn how to use scanners and scopes. Working on a 2003 Hummer H2 and I'm looking forward to diagnosing it without throwing parts at it. Currently it has a P0300 code. Misfire data shows a couple misfires on Cyl 1, 20 misfires on Cyl 5, and 50 misfires on Cyl 6. Misfire history shows 500 misfires on Cyl. 5 and 2500 misfires on Cyl 6. I am using a Snap On Modis EES300. I bought an OTC Low Amp Probe for it and have had zero luck setting the probe up to get any readings. The scanner was left to me by my father in law when he died last year.
Make sure it got a good battaery. Connect the the banana jacks accordingly to the scope and select the corresponding channel. The low current clamp probably features 2 ranges, one equals to 100mV/A and the other 1V/A. It also may indicate 20A and 60A.
You may select Volts DC or low amp current clamp in the channel settings. When switcing on the clamp to the desired range you should see now the channel graph hovering around the zero value.
Open and close the clamp in order to "degauss" it and calibrate the clamp or by pushing/turning the zero button. Put the clamp around ONE wire of the consumer you want to measure (if you put it around + and - at the same time both currents will cancael out each other resulting in a zero reading ).
With the consumer switched on you should get a reading now. If you still see nothing you might need to invert the clamp physically or through the software (a negative current flow may fall out of the selected screen range and would show up as a missing signal.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
Can you elaborate on the conditions when the misfires happens? Scandata like fuel trims temperatur, freeze frame etc will be of help.
What have you checked so far?
Specifically the 3.5l, 5 cylinder engines are known for dropping valve seats/valve problems. You may want to put a ut a vacuum gauge on it....
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
I just ordered new clamps as the OTC one I have is only 3/8" and doesn't fit around my positive cable.
As for the Hummer it is a 6.0 Vortec. The random misfire appears to be concentrated on #5 and #6 cylinders with #6 misfiring mostly. Misfires happen at idle and under load. What I did do is pull the air cleaner filter. The filter was completely clogged. Now it misfires a lot less. Was 50 misfires per cycle and now it's down to 8. Fuel trims were STFT 0 on both banks and LTFT 25 on both banks. After pulling the air filter the LTFT is 0 on B1 and 8 on B2.this is at operating temperature. I tried a CKP relearn and that did nothing.
You dont need the current clamp to do relative compression. Connect the test lead of one channel to the engine block and the negative lead to battery negative, then crank the engine over. Adjust DC voltage in the 100 or 200 mV range, total. Or connect the negative test lead to bat+ and the signal lead to battery -. AC couple the signal and crank the engine over. Both ways will give you a relative compression reading, spikes showing the pistons at TDC compression.
What model is the Hummer?
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
JupiterBandit wrote: What I did do is pull the air cleaner filter. The filter was completely clogged. Now it misfires a lot less. Was 50 misfires per cycle and now it's down to 8. Fuel trims were STFT 0 on both banks and LTFT 25 on both banks. After pulling the air filter the LTFT is 0 on B1 and 8 on B2.this is at operating temperature.
Really! :ohmy: That's very interesting. I get why a plugged air filter would impact engine performance, but I don't get why it'd cause a lean condition. Unless the filter was clogged or damaged in a way that disrupted laminar airflow over the MAF.
I'd love to know if the high trims return with the old air filter reinstalled.
Back on topic, has the P0300 returned after replacing the filter?
Topic Author
