2003 Envoy Misfire

TomR01 · 12-23-2008, 11:20 AM

I have an '03 Envoy with over 77,000 miles. It started out with an occassionaly misfire on #4 (P0304). It is now a constant misfire on #4. I have replaced the fuel filter, spark plugs, fuel injectors and the fuel pressure regulator. I have thoroughly cleaned the TB and swapped the coil from #4 to #2 with the code remaining as #4 misfire (P0304). Since the misfire started out as sporadic, I really don't believe it to be valves or cam, and my vacuum is a fairly steady 20+ in. Hg. Does anyone have any suggestions?

maxwedge · 12-23-2008, 03:48 PM

Welcome to AF. Are you getting a code or can you feel the miss? Pretty expensive repair attempt, regulator and injectors, why?

TomR01 · 12-23-2008, 05:42 PM

I am getting a code (P0304) for a misfire on cylinder #4. I swapped the coils, after I did the routine maintenance (fuel filter, spark plugs & clean TB). At this time the misfire was still intermittant. When I swapped the coils, and it started misfiring, I still got the code for #4.

I really started suspecting the injectors after all of this, so I bought rebuilt injectors. When I put the whole engine back together and hooked up the battery, the car ran fine for about 2 days. Then it started again. Upon further checking, I found fuel on the top side of the fuel pressure regulator, so I went ahead and replaced this, as this was an obvious problem. But, it did no good.

By the way, I am not getting any vacuum from the plenum to the pressure regulator. I am not sure if this is correct, and I have not checked it to see if I get a vacuum during accleration.

I need some good advice! Could this be a PCM problem? If so, how do I check it?

maxwedge · 12-24-2008, 10:51 AM

Again can you feel the misfire? Did you clear the code and 04 came back? I would check for injector pulse on #4 using a " noid" lite if ok, then check compression in that cylinder. Or better yet do a leak down test, which is a more definitive method.

TomR01 · 12-24-2008, 01:46 PM

Yeah. You can really feel the miss. I've cleared the code numerous times. It is difficult to get to the injectors on this engine. However, if I can, I will get a tester and check for pulses at the wiring connector on the top of the valve cover. What has me baffled is that the miss went away after I replaced the injectors, and one time after I disconnected the battery for about an hour. My vacuum is good (steady 20+ in Hg).

rhandwor · 12-25-2008, 09:40 AM

If you didn't change the big o-ring on the bottom you coulld have a vacuum leak at the injector. Spray some carb. cleaner around the injector if it goes away buy a new O-ring kit. The dealer sells them.
Try checking the power to the coil a problem in the harness will also cause a miss.

TomR01 · 12-25-2008, 09:47 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.

TomR01 · 12-25-2008, 09:54 AM

Rhandwor - You are the closest so far. I have an "expert" on another blog that has assured me that my problem is a broken spring on my exhaust valve. He was very adamant about this. However, as you stated, the wiring to the coil was the problem. You could not see the break, but it's there. I hate these kinds of problems. They will run you ragged.

maxwedge · 12-25-2008, 01:57 PM

Basic diagnostics, spark , fuel and compression each one has to be verified before the next step, see this all the time in my shop.

TomR01 · 12-25-2008, 08:24 PM

It was found accidentally. I was disconnecting the wires to the coils to verify the cylinder in question. When I started to re-attach the connector, it started firing. When it was fully inserted, it started mis-firing. Connect - disconnect. Fire - Mis-fire. Then I started wiggling the wires, and it started firing. I had suspected the connectors, but not the wires leading to the connectors.

TomR01 · 12-25-2008, 08:29 PM

By the way, the local professionals at the GM dealership said it was a tune up problem and charged me $95.00. Wanted to charge me $350.00 to replace the fuel filter, plugs and clean the TB.

rhandwor · 12-26-2008, 05:56 AM

I looked at what Max Wedge posted which would cover probably 75% of problems of this type. I just picked out items in the other 25% I've had wiring problems before. If you had a broken valve spring doing what he posted you would have a bad compression test.
I would go to a scrap yard and get a new connector and length of wire needed to fix the problem. I've found the connectors at Walmart with screw ends would be your best bet if your not going to solder them. They have a small piece of metal like a pencil sharpened at both ends. You trim a little wire put the screw end on the wire and push over the taper tighten the connector and you have a good connection. Put sicilon on both ends to keep out moisture. Space the connectors a short distance apart so the wire loom fits over the repair.

TomR01 · 12-29-2008, 08:12 AM

Thanks. I'll do that.

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