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GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 29447
Experience:  GM Grand Master Technician 2007. 14 years experience.
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2006 Chevy Uplander 3.5 160k. I am trying to diagnose p0300

Customer Question

2006 Chevy Uplander 3.5 160k. I am trying to diagnose p0300 misfire. I have a Snap-on Solus scanner and is picking up a hard misfire on cylinder 4, but also detects on 5 and 6 upon decel/throttle lift. Van has new plugs, wires, coil pack. I swapped #4& 6 injectors with no change to location of main misfire. #4cyl has 180lbs compression. Vacuum check at intake manifold shows 18.5 steady.
I did an injector balance test and found something puzzling. When injector #2 is tested, followed by #3 (or vise versa), the scan tool shows Error - injector has already been tested, when trying to test the second injector. I am hoping this is somehow causing the misfire, but I am not sure what to make of my ohm testings of the injector wiring harness - all the way back to the ECM. I am getting 28 m ohm from ground to each injector pin at the ecm (with connector disconnected at ecm).
Submitted: 1 year ago.
Category: Chevy
Customer: replied 1 year ago.
Front spark plugs (2,4,6) all looked the same when removed. No oil, coolant. I did a coolant pressure test at the radiator and it held 15lbs for over 15min.
Expert:  GM Tech (Cam) replied 1 year ago.

Hi there, I am Cam. Your injector resistance, do you mean across the injector wiring from the ECM you are getting 28 ohms?

Customer: replied 1 year ago.
With the manifold harness disconnected I am measuring 11.2 to 11.8 ohm on each injector. From the common pink + wire to each - driver wire.
Expert:  GM Tech (Cam) replied 1 year ago.

OK, are you able to see if you get a pulse with a noid light on each:

Customer: replied 1 year ago.
No access to injectors under plenum. I could probably backprobe the injector harness connector with some leads to noid light.
Customer: replied 1 year ago.
I used stethoscope to listen to each, and used solus to disable one injector at a time while listening. They are each making similar sounds and shut off when commanded.
Expert:  GM Tech (Cam) replied 1 year ago.

OK, I see you have many new ignition wires and such, but did you check for spark on that one?

Customer: replied 1 year ago.
After I reconnect the injector harness to the main engine harness, then use meter at the ecm harness connector C2 - disconnected from ecm. I am measuring 14.2 ohms from each injector driver pin back to the #16 fuse
Customer: replied 1 year ago.
Checked for spark using inline spark tester.
Customer: replied 1 year ago.
I really thought that the error that my scanner was having with the combination of testing #2 injectors would lead me somewhere.
Expert:  GM Tech (Cam) replied 1 year ago.

No IO don't think it will be causing this although we cannot rule out the ECM yet. Can you do a fuel injector flow test?

Customer: replied 1 year ago.
I used my scanner to do the injector balance test. Primes fuel pump, then pulses each injector for same time period. Each injector dropped the same. From 57lbs to 42 lbs.
Customer: replied 1 year ago.
Fuel pressure is like 57 running. Hold pressure for 20+ minutes when turned off.
Expert:  GM Tech (Cam) replied 1 year ago.

So that means its not injectors, if you get spark and its the same and in proper time then we are looking at base engine. You did a compression test, but did you verify exhaust valve is opening? If its stuck closed it will still have great compression.

Customer: replied 1 year ago.
I did not remove valve cover but can reach both #4 rockers thru the oil fill hole. There is no rocker slack/noise/clearance, like there might,be with a bent pushrod or flat cam. But I am just speculating.
Customer: replied 1 year ago.
Sticking open/closed wouldnt there be fluctuating needle on vacuum gauge? This was steady.
Customer: replied 1 year ago.
What does it mean when the scanner is intermittently picking up misfiring on 5 and 6, but only when I let off the throttle. Bring up rpm to 18000 or so, then let off the throttle and it momentarily shows 5&6 misfiring. Not constant like #4 though.
Customer: replied 1 year ago.
18000 rpm! I meant 1800.
Expert:  GM Tech (Cam) replied 1 year ago.

That is likely just due to the hard misfire on #4.

Customer: replied 1 year ago.
What direction would you go with on this next? Any chance that it would be a lower intake manifold gasket leak? Or wouldthat show up as lean and fuel trim adding fuel.
Expert:  GM Tech (Cam) replied 1 year ago.

It might be. Can you remove the vacuum lines to the crankcase, plug the ports then spray some carb cleaner or such in the oil filler and see if you get a reaction from the engine. If you do then it does confirm they are leaking internally.

Customer: replied 1 year ago.
I have not checked the carb spray trick yet. A few things on my scanner. First is injector pulse is 4.2, seems high. Downstream o2 is hanging at 840 mv. Seems rich. Charging voltage is 15.0 a little high. Spark advance at idle is 21-22.5
Customer: replied 1 year ago.
Maf 4.1 gm/sec, 2710 Hz
Expert:  GM Tech (Cam) replied 1 year ago.

Is there a plugged catalytic converter?

Customer: replied 1 year ago.
Dont know. Has plenty of powr.
Expert:  GM Tech (Cam) replied 1 year ago.

Ok, lets check that intake and see next.

Customer: replied 1 year ago.
Would I be able to tell by exhaust temp before and after.
Expert:  GM Tech (Cam) replied 1 year ago.

For this it would be back pressure as measured from the pre converter sensor. Anything over 1 psi at any time is the limit.

Customer: replied 1 year ago.
Ok I will check intake. Will I see a difference in o2 sensor numbers or just watch/listen to the engine
Customer: replied 1 year ago.
No difference. Doesnt smooth out, speed up, slow down....
Customer: replied 1 year ago.
Is there a diagnostic "flowchart" that is followed for cylinder misfires? I don't have the capability to scope anything, though. Running out of ideas.
Customer: replied 1 year ago.
Van also has "service abs system" on cluster with no codes stored.
Customer: replied 1 year ago.
Any service bulletins for any of this?
Expert:  GM Tech (Cam) replied 1 year ago.

Let me get one. NO bulletins.

Expert:  GM Tech (Cam) replied 1 year ago.

Which engine, 3.5 or 3.9?

Customer: replied 1 year ago.
3.5
Expert:  GM Tech (Cam) replied 1 year ago.

Here is the diagnose for P0300.

Customer: replied 1 year ago.
thanks. I will check the document. Done for today.
Expert:  GM Tech (Cam) replied 1 year ago.

Sounds good

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