Missfire condtion at first start of the day

[obadran]

I have a 96 Yukon with almost 112000 miles.

I've been experiancing a miss fire problem almost everymorning when I leave around 7:30 for work. After some time, miss fire condition will go away and not return until the next morning no matter how many times I start the car during the day after the first start of the morning. It seems like it might be a temperature thing and the problem occurs only after the truck sits overnight.

So far to date, all ignition related items (plugs, wires, cap, rotor) except for the ignition control module have been replaced. The old spark plugs did not show any thing unusual. I had my fuel pressure regulator and fuel pump replaced about 2 months ago because of lack of power/hesitiation problems. This miss fire problem showed up about a week after the fuel work so I'm not sure if it has anything to do with the fuel work. Also since the fuel work, the mornings have been getting a little cooler. With all this work done in the last two months, the truck runs as if it were new when the missfire condition is not present.

My mechanic is at the point of replacing fuel injectors as the next thing to try. According to him, the miss fires seem to only occur on the #5 and #6 cylinder. He hasn't found any other clues to what is going on and since this problem only occurs in the morning and only last for no more than 20 minutes...it's frustrating to diagnose.

Anyone have any ideas???

Thanks,
Omar

[GMMerlin]

My mechanic is at the point of replacing fuel injectors as the next thing to try. According to him, the miss fires seem to only occur on the #5 and #6 cylinder. He hasn't found any other clues to what is going on and since this problem only occurs in the morning and only last for no more than 20 minutes...it's frustrating to diagnose.

I love it when peoples "mechanics" want to replace parts instead of diagnosing the concern..you have probally spent a good amount of money with him "trying" things.

This concern has been around for years and is a fairly easy fix, just a matter of finding someone who has the equipment to perform the repair.
The problem is that your misfire is caused by the ball sticking in the seat of your poppet (part of the injector). This is what is causing your cold misfire (if you would have done a search, you would have found tons of threads about this).

Well there is a GM TSB (00-06-04-003B) referencing the condition and proper repair procedures.
To fix your truck, you will have to take it to the dealer for this procedure because they are the only ones that have the tools.

If the poppets will not unstick, there is a new conversion kit available from GM that works great and I have not seen or heard of any complaints after the conversion was performed.

Oh yea...cleaning injectors with aftermarket cleaning solutions will not unstick the poppets.

[bigred36]

Okay, truck ran fine on Sunday and then Monday morning I got in and didn't think I was going to make it to work. I changed long awaited plugs cap and rotor, wires and it runs now but the same problem exist. Runs like shit! Hooked up the computer and it is trouble code P1305-cylinder 5 misfire. At this point I do not know what to do. Tear it down further and possibly replace the fuel injector on #5 cylinder (don't really feel like doing anything else on this truck) or take it in and have it looked at? Does anybody have any ideas?? Are the fuel injectors under the black plastic manifold on top? Any ideas would be greatly appreciated.
It is a 96 GMC Yukon with a 5.7L.

UPDATE:
I called today and the garage said to let him clean the injectors with a high pressure cleaning system that hooks into the fuel rail and cleans all 8 injectors before I change any injectors. $100. Guess I will try this first and see what happens.

Bigred36

[bigred36]

UPDATE UPDATE:
Well they just called and said the cleaning did not work and now the trouble codes are cylinder #1 & #5 misfire. I guess the next step is to have them start tearing into it to see what they can find. They said some of the possibilites are fouled injectors, manifold gaskets blown,
etc, etc. Really can't tell much until the top cover is taken off where the fuel rail is located. Probably only a couple dollars worth now.

[bigred36]

UPDATE
They had to replace #5 injector and as of right now it runs great. A little under $600. Not too bad I guess. Hopefully none of the other injectors will be right behind this one.

Sorry for piggybacking to the original post "obadran".

[GMMerlin]

You obviously didn't read my post..
That "high pressure" cleaning system was a waste of your 100 bucks

[bigred36]

I was afraid you were going to say that GMMerlin. I did read that in your post but kinda got swayed by all the shops to do the "high pressure sales cleaning first". It does run great right now but if it does this again I think I might pull the plenum and try to do it myself next time. Would you recommend a remanufactured or new injector? I see there is quite a few sites that sell the injectors but haven't looked locally to check the price. Thanks again. Gotta get back to work to pay for the repair.