2002 Chev 1500 - 5.3L Code P0300B

[DansAuto]

We have a 2002 Chevy 1500 with a 5.3...the vehicle came in with a rough idle and skip and code P0300b and customer stated vehicle just went from running fine to acting the fool....it showed misfire on about 6 cylinders....we did a complete tune-up and now at 1500 rpms it only shows misfire on 1,2,4 but at idle it shows on 6 cylinders again...it bounces around from showing misfire on 3 cyl to 6.... I will list below all we have done so far with absolutely no help....

Complete tune-up
swapped injectors around still no change and all pressures good
replaced cam and crank sensors...no change
replaced intake gaskets
smoke tested
checked exhaust for converter and 02 sensors
replaced pcm as last resort and took for relearn at dealer.....everything is reading fine and dealer says system is in limp mode and they have no explanation and I find this hard to believe that they have not run into this.

Any help would be really appreciated for we have came to a brick wall on this one.

Thanks in advance,
Dawn

[steven kiser]

Unplug the 02's and see what happens. Another thing is this is one of the trucks that ate converters. You may have severe back pressure causing the cylinder misfire. Get under it and hit the converter with a mallet and see if it rattles. The sudden drop out makes me feel the honey comb has spun around and is now causing a restriction.

[DansAuto]

Unplug the 02's and see what happens. Another thing is this is one of the trucks that ate converters. You may have severe back pressure causing the cylinder misfire. Get under it and hit the converter with a mallet and see if it rattles. The sudden drop out makes me feel the honey comb has spun around and is now causing a restriction.

We dropped the whole exhaust when checking cats no change....but did not unplug o2s but they are all reading fine??
Let me add that the dealer relearned pcm that we got and says it in limp mode and we also tried coolant temp but no change.

[brianp87]

What is the compression on the missing cylinders?

[Pauls Automotive]

I hate to sound stupid but I've had from time to time a problem where someone uses Autolite spark plugs and it causes some weird stuff to happen. Iratic misfires etc. What kind of plugs were used? Also I would make sure the egr valve isn't hanging up causing a lean condition. or even an IAC motor.

[brianp87]

All i use is auto light or denso dual electrodes on toyotas. I didnt see any comp readings in the initial testing which is a basic starting point for me on a miss fire.

[steven kiser]

Ok, now on reading the post again i see that the dealer claims it's in limp. Now i've worked on a fair share of gm's and i've never seen a "limp home mode" without a light. With your scanner attached if you're getting varied readings from the sensors it's not in limp. My understanding of limp is it's a predetermined setting that allows basic function to get from point A to the dealer without causing catastrophic issues.

I know this sounds way, way, out in "i've never thought of this land" but for some reason in my shop this time of the year is time for mixed fuel issues. People are getting ready for snow and not wanting to waste any fuel are dumping the old gas from last year into their vehicles. heck i even do it, but i don't dump 2 cycle into my tank like some of my customers do. While this truck is running do you even get a whiff of dsl fuel. i walked into a dealer to see a buddy of mine and asked him who was working on a bus. He was having a fit with a run like crap issue and after i asked about the bus he dropped his head and said "thanks".

Another tip here is (well actually two) while it's running un plug the M.A.F. and see if it throws a check engine light. this is just to make sure the P.C.M. is reading correctly. now, if you're having issues reading the fuel trims you can introduce carb cleaner into the air intake and see if it smooths out. if it does there may be a fuel issue.

While it's running unplug each injector and plug in a noid light to see if it's firing. if you use a test light remember it may trigger on ground so you may get a false idea. the noid light is still the way i go especially since i will do anything to avoid breaking a seal back probing. i've seen way to many green oxidation blobs.

If this were in my shop i would look at if as not seeing the forest through the trees. Where would i start if it were towed in in this condition. I don't see anything about a compression test, although i don't think it's the issue. In the past i've solved issues by discovering a blown or missing fuse. What did the customer do or have done right before all this happened. Was anyone else driving the truck that you haven't spoken with? Remember, at least it's been my experience that when questioning a customer about their vehicle i get the impression that they feel they're in a smoked filled room getting grilled by two cops with their sleeves rolled up. I'm dead serious, i've seen people get the "deer in the head lamp" look. I had a customer start giving me a string of alibis about how it wasn't her fault, she didn't know what happened, and if her husband found out he was going to be real angry at her. I looked at her and said "Ok, lets go with i'm not going to tell him what happened. Can you help me out a bit." She said "sure, no problem. I swerved to avoid a squirrel , clipped the curb, blew out a tire, spilled my coffee down the defrost vent, Went to sears where we purchased the tires, had a new one put on, paid cash, and now the wheel isn't straight, it squeals when i go straight, pulls into the other lane and i can't control the air ducts". Ok, i'm done with my babble and typing practice.

[DansAuto]

Yeah Steve we didnt understand the limp mode either.....they are at it again this morning so I will follow up soon.