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2003 GMC Envoy Misfire

 
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TomR01 TomR01
New User | Posts: 10 | Joined: 12/08
Posted: 12/23/08
09:34 AM

I have an Envoy with 77,000 miles.  It has developed a misfire over time from an occassionaly misfire to a constant misfire on #4.  I have replaced the fuel filter, spark plugs, injectors and the fuel pressure regulator.  I have thoroughly cleaned the TB and swapped the #4 coil with #2, with no change in the DTC code (P0304).  Since this started out as a sporadic problem, I really don't think it is valves or cam, and my vacuum is a very good (fairly steady) 20+ in Hg.  Can anyone out there help me?  

waynep712 waynep712
Enthusiast | Posts: 652 | Joined: 10/08
Posted: 12/23/08
10:59 AM

i really wonder... and i am sure it will get answered... if the po304 really relates to the number 4 cylinder... or to the 4th cylinder in the firing order...  

TomR01 TomR01
New User | Posts: 10 | Joined: 12/08
Posted: 12/23/08
11:58 AM

According to all of the information I have, this is a cylinder specific code.  

Wenis Wenis
User | Posts: 132 | Joined: 10/08
Posted: 12/23/08
02:15 PM

Yes it is cylinder specific!
I've run into this before, if the number 4 injector shorts out it can smoke the driver for the number 4 cylinder in the computer. I don't suppose you have the injectors yet to do a resistance test do you? Oh well.


If you can, check and see if the #4 injector wire (at the injector) has a pulse, The computer pulses the ground circuit not the power, you can check for power and ground at the same time using a noid light or l.e.d. test light at that connector.  

TomR01 TomR01
New User | Posts: 10 | Joined: 12/08
Posted: 12/23/08
04:33 PM

I have already replaced the injectors, and taken it apart again to put a contact solution on the connectors, just in case it was a bad connector.

I checked the resistance at the connector on the top, and they are all within limits (10-12 ohms).

I am suspecting that it COULD be the PCM, but I have no way to check it.  Can I check the pulse for the injector at the connector on top?  

jgate-#002 jgate-#002
Guru | Posts: 953 | Joined: 06/07
Posted: 12/23/08
06:34 PM

Yes you can check for pulse at the injector connector side, what you will need is a  NOID light, The light will flash as it receives a pulse from the PCM. No flash no pulse.  

TomR01 TomR01
New User | Posts: 10 | Joined: 12/08
Posted: 12/23/08
07:02 PM

Thanks.  I will probably not get to it until tomorrow.  Will you be around?  

jgate-#002 jgate-#002
Guru | Posts: 953 | Joined: 06/07
Posted: 12/23/08
08:10 PM

Yea I'll be around off and on,This is what you are looking for, you can buy the complete set or 1 of a specific model. Most parts stores carry these. One for a GM MPI should only be around $10.00 or so .


http://www.handsontools.com/Noid-Light-Idle-Air_c_731.html  

TomR01 TomR01
New User | Posts: 10 | Joined: 12/08
Posted: 12/23/08
09:06 PM

Is this a 12VDC pulse?  If so, couldn't I use a standard 12V point probe at the connector on the top of the rocker cover?  You really can't get to the injector connector without taking the intake off.  

Wenis Wenis
User | Posts: 132 | Joined: 10/08
Posted: 12/23/08
09:49 PM

Test lights now a days can short out the computers, not sure if I'd do that or not.

I said earlier that it toggles the ground not the power.  

TomR01 TomR01
New User | Posts: 10 | Joined: 12/08
Posted: 12/24/08
07:59 AM

The GMC 4.2L is an inline 6.  You can't get to the fuel rail or the injectors without taking the intake off.  This requires taking the PCM off and the alternator.  The back bolts on the manifold are extremely difficult to get to, as they are deep in the firewall.  I see where the use of some noids that are self contained (use internal battery, or attach to battery power) are good for use on computers and sensors.  That is what I would get to use on this.  They require miliamps to activate.  

Wenis Wenis
User | Posts: 132 | Joined: 10/08
Posted: 12/24/08
10:37 AM

Tom, your right, my last post that had the picture was for another guy on a different site with an 89 caravan (3.0)  not sure how I posted that here but it was late. Blush

I've done a little research and found a couple of things that may pertain to this misfire problem, but first! When does this misfire occur, always, only when wet or rainy, under a load and does this light flash or is it on steady? lots of variables here.

(Without tearing things apart) You can check this #4 injector for pulse by testing it at the main connector, it's located at the rear of motor mounted by the #6 coil, it's an eight wire connector. The pink wires are the power feeds for the injectors and the lite blue with the black stripe is the control wire for #4, this wire should flash when the engine is running or being turned over. Should be tested with the proper tools, L.E.D type test lights or like someone said a self powered test light.

One other thing, The fuel injector coil windings (which are internal in the injector) are affected by temperature. The resistance of the fuel injector coil windings will increase as the temperature of the fuel injector increases. Just another thing to confuse you. Good Luck Tongue  

TomR01 TomR01
New User | Posts: 10 | Joined: 12/08
Posted: 12/24/08
12:52 PM

Wenis - The misfire is constant now.  It was occurring occassionally (non-specific), but is not continuous.  It was not due to the problem that Envoys typically have, with water dripping into #4.  All is dry.  I am going to make sure the misfire is not a coil, by using a spark tester on all of the coils.  I understand that sometimes the scanner may say #4, but it may be another.  My checking indicates that the self powere lights are better for electronic components (i.e. PCMs), so I will try to get one and check the connector at the top of the engine.  I checked the resistance there already, and all was fine (10-12 ohms on all).  So far, this is not related to humidity, temperature or loads.  

TomR01 TomR01
New User | Posts: 10 | Joined: 12/08
Posted: 12/25/08
08:46 AM

Well, I found the problem.  I was tinkering with it this morning, trying to check the coils, and I made the misfire diasppear.  As it turns out, the wires going into the connector for the coil on #4 have a broken wire inside the insulation.  Since the resonator sits on top of these, I suspect that over time the wires (being bent) just broke.  That's why my misfire was only occassional, and why it has gone permanent.  The more I worked on it, the worse it got.  I will repair the wires, and I'm pretty sure that the car will run fine.  Keep your fingers crossed.  

Wenis Wenis
User | Posts: 132 | Joined: 10/08
Posted: 12/25/08
12:05 PM

Sweet! This is the type of stuff tech's run into often, you can never assume anything with the vehicles of today. Typically people just keep throwing parts, sometimes it works and sometimes we fix them after there's been a lot of money spent at home by the owner.

Happy for ya and way to stick with it, but now you can see why repair bills can get expensive at a repair shop, and justifiably so sometimes.  

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