need some help with a 02 Yukon

[BarryB]

It belongs to a friend of mine. He brought it to me hoping I can fix it, but I basically I am old time stuff mechanic. When I first got it, they said they took it by advance for a free code scan they said it could be a bad o2 sensor. They drive it for a little while because it was just the cel on. wasn't running to bad,the story I got it started loosing power so he was forcing it had up 4k and it wasn't moving. I get it at my place on the back of a rollback. it's running really rich, missing and just running like crap. I pulled three of the four o2 sensors out. pass side looked fine. drivers side the one behind cat was wet with gas. well I put a code reader nothing fancy on it. it first came with p0101 p0102 po300 po420.
heres what I did so far, since it was missing I changed the plugs, and wires, and also replaced the MAF sensor. Did all that now the only codes are showing is the po300. and its still running like crap. The way it sounds almost like it has jumped time, but only one side of the motor and all 4 plugs were black. the other side looked normal. So if it had jumped time wouldn't it effect both sides? since there isn't a dist. on this thing how would you check timing? where is the crank sensor cam sensor? would these things going bad not trip a code reading? I'm stumped for now.

[bwood]

I kinda doubt it jumped time. I would check the fuel pressure. Also check the harness that feeds the coils on the side that appears not to be firing. I was thinking the crank sensor was located at the front of the engine near the crank/balancer. I'm not sure on the cam sensor. Does this truck have 1 or two cat converters? If two, pull the O2 sensor out before the cat and see if it runs better (clogged cat). If you are only getting the P0300 misfire, it may be necessary to step up to a full scanner and monitor some live data. A failed cam/cranksensor should set a code also. I also don't believe you have the option to check timing. There are no adjustments that can be made, the computer does it all. If your where to put a light on it, it would constantly be changing as the computer is constantly making changes.

You know the more I think about it, my Mom's Buick has a simialar issue with a P0420 code also (Catalyst efficency below threshold). I blame this on a improperly sized/installed cat. He orig cat rattled and she couldn't stand it, so my Stepdad took it to the muffler shop and they took off the stock 2" (or close to it) and installed a 2.5" high flow cat with a couple "custom" reducers. Well, my theroy is the increased flow with the increased turbulance of the reducers messes with the rear O2 sensor and the engine managment compensates with changes to the fuel managment. The code didn't show up until after the cat was replaced. My point to all the rambling is that a bad cat/O2 sensor maybe messing with the computers mind.

[BarryB]

Bwood I'll try pulling the plug on the front O2 sensor see if it changes.
I don't have a guage for checking fuel pressure, what do i use for that? and also what is the pressure suppost to be?

[bighaas79]

I can see this is already headed to replacing a bunch of stuff that it doesn't need. Save yourself the trouble and headaches and take it to someone that can hook up a scanner and possibly a scope and let them do a proper diagnosis. From what you have said you have and don't have I can tell that you don't have the necessary equipment to properly diagnose. DON'T unhook sensors and try to see if things improve you cause more problems than you can find because the pcm will try to compensate possibly covering up the original problem. That and when and if you take it to someone to diagnose if makes it harder to do so because you have now set off false codes so whoever is working on it has to determine which are relavent to the problem and which aren't. If nothing else let someone who knows what they are doin do the diag. and just replace the parts yourself, unless those new parts need to be relearned or reprogramed then you are better off letting the tech do the whole works. And fwiw the crank sensor is down behind the starter in the block, it is very much not likely the problem and you would need to be able to do a relearn on it once its replaced neway. See how complicated things can get on these!

[BarryB]

Originally Posted by bighaas79

I can see this is already headed to replacing a bunch of stuff that it doesn't need. Save yourself the trouble and headaches and take it to someone that can hook up a scanner and possibly a scope and let them do a proper diagnosis. From what you have said you have and don't have I can tell that you don't have the necessary equipment to properly diagnose. DON'T unhook sensors and try to see if things improve you cause more problems than you can find because the pcm will try to compensate possibly covering up the original problem. That and when and if you take it to someone to diagnose if makes it harder to do so because you have now set off false codes so whoever is working on it has to determine which are relavent to the problem and which aren't. If nothing else let someone who knows what they are doin do the diag. and just replace the parts yourself, unless those new parts need to be relearned or reprogramed then you are better off letting the tech do the whole works. And fwiw the crank sensor is down behind the starter in the block, it is very much not likely the problem and you would need to be able to do a relearn on it once its replaced neway. See how complicated things can get on these!

ALready did that dropped it off. I'll know tomorrow what the "real" culprit is. The main reason for me posting originally is because I want to know and understand hows this stuff works. I completely understand alot of people try to bandage up a problem without actually fixing the problem. If those scanners and other equipment needed didn't cost so much I would have already bought them. but since I'm just the friendly neighbor that works on cars for people I don't see the need to purchase all that equipment.
Who wants to bet it probably just a clogged fuel filter, or bad fuel pump?

[BarryB]

well the diagnosis guy says there an air leak in the plenum somewhere,probably bad intake gaskets. I stopped in napa and bought a set of intake gaskets. So do you have any experience in swapping out these gaskets. Anything to look for, or things to avoid doing.

[bwood]

I've never had to swap the intakes on this engine, but the intake is plastic, torque is critical. Make sure the the intake isn't warped/cracked. My stepdad's 04 had a knock sensor and intake leak during the warranty. The intake was visibly warped so they replaced it. I believe intake issues are fairly common in the earilier years of the Gen III engine (99-04ish).

[BarryB]

Originally Posted by bwood

I've never had to swap the intakes on this engine, but the intake is plastic, torque is critical. Make sure the the intake isn't warped/cracked. My stepdad's 04 had a knock sensor and intake leak during the warranty. The intake was visibly warped so they replaced it. I believe intake issues are fairly common in the earilier years of the Gen III engine (99-04ish).

Intake issues has being bad gaskets, or being warped? I'm hoping it isn't a warped intake. but if it is guess it'll have to have one.

[bwood]

I guess I should have said gaskets, just in the case of my stepdad, it was the intake. I looked like it had been overtorqued.

[bighaas79]

Intake gaskets are really easy on these motors. It shouldn't pose any problems for you. Don't even have to drain the coolant. Just have to pinch off the little coolant hoses that run up to the throttle body. Should only be say a 2 hour job or so. Good luck!

[BarryB]

Originally Posted by bighaas79

Intake gaskets are really easy on these motors. It shouldn't pose any problems for you. Don't even have to drain the coolant. Just have to pinch off the little coolant hoses that run up to the throttle body. Should only be say a 2 hour job or so. Good luck!

2 hours is probably gunna be an all day job for me. I usually try to take my time when I do something new. So have you guys done many of these? Are the intake gaskets a fairly common problem with these motors. Oh and another thing what do you think a shop would normally charge for doing this type of job. Trying to figure out how many 12 packs they need to get me for working on there truck.

[clinebarger]

2 hours is taking your time, takes me 20 minutes. The gen III engines are very simple to work on. 2 hours is book time.

[msgross]

my 05 had four fouled plugs and 4 good one's. ended up being the O2 sensor for that side of the engine, changed it and it ran fine...

good luck

[bighaas79]

Originally Posted by clinebarger

2 hours is taking your time, takes me 20 minutes. The gen III engines are very simple to work on. 2 hours is book time.

Lol. ya it doesn't take me that long either but since he hadn't done one before and not knowing what he has for hand tools or place to work in i was giving him benefit of the doubt. The ls based engines are very easy to work on, and in the truck platforms just about everything is easy to access. I love workin on em.

[BarryB]

Well I'm pretty well equiped for tools, I used to be a maintenance mechanic for a factory here. When I Quit that and started my home remodel, handyman business. I brought home my big tool boxes, and all there contents. The thing I'll have to find is the inch pound torque wrench.

[BarryB]

I swapped out the gaskets today. Your right they are fairly simple to work on. My fuel line tool wouldn't fit the clips so I just took fuel line loose from the rails. It was pretty dirty under there so I did take some time to clean up the mating surfaces. razor blade and a scuffy pad with carb cleaner. out it all back together , didn't fix it. still running like crap. I'm going to have to do a compression test on it later.

Are these intakes bad for cracking??? I didn't see any visible cracks. When I put the manifold back I torqued it to spec. 44 inlbs first pass, then 89in lbs second pass. When I put my code reader back on it to see if anything changed, the codes did except one. po106 manifold air pressure sensor, and po>>> barometric pressure sensor. po300(same) multi cylinder misfiring. I won't be able to call or get it back to the guy that scanned it until weds.

[bighaas79]

ya sure ya got the map sensor hooked up? Both of those codes (map and baro) would be from map sensor readings. its the little square sensor on top, at the back of the intake manifold. And if you have him scan it again, have him look at the misfire data to find out for sure which cylinders are misfiring. The misfire data will break it down to which cylinders the misfire readings are on, versus the random 300 code you have. the 300 code is basically to let you know something is up. Specific cylinders will bring up for example cyl.1 = p0301, cyl.2 = p0302... etc. However with out those codes the misfire data will help pinpoint for you. When you were changing gaskets did you notice any chewed wires from critters bein in there? Don't know how many chewed wires i've had to fix causing similar problems. And I have never run into a cracked manifold, not that it isn't possible but from what i have seen and heard not very common. I wish I could see data stream to see what all your readings are at. Would love to see the map and maf readings, as well as o2 signatures and responses, and fuel trim numbers.

[BarryB]

When I talked to the diagnos guy, he said 1357 were all misfiring.
Yes I made sire to hook up the map sensor, , No i didn't notice any chewed wires. What is the fuel pressure supposed to be, He said at idle it was 39 lbs, but he was able to get it up to 50lbs, told me all the injectors were firing.
Henestly I wish I could show you all the data.
would a clogged cat cause these problems?
what about a wiped cam, or burnt valve. reason I thinking about doing a compression test. Oh what do I unhook to kill the ignition to do the comp test?
When it's running and try to rev it up it pops back thru the induction. kinda like it's out of time.

[msgross]

is the O2 sensor on that bank new? that's what caused my left side to misfire, replaced the left o2 sensor (had to pull the driveshaft) and it ran fine.

[BarryB]

Originally Posted by msgross

is the O2 sensor on that bank new? that's what caused my left side to misfire, replaced the left o2 sensor (had to pull the driveshaft) and it ran fine.

the owners had one replaced prior to bringing it to me. Which one I'm not sure. Trust me, I'm listening to everybody, I just don't want to throw stuff at it, until I have a better understanding where the problem is. It's not coming back with a 02 reading so I leaning towards other problems.

Was yours popping back thru the induction, and not wanting to rev up??

[99-2door]

What did you ever come up with for fuel pressure readings? 9 out of 10 running issues I have had have been fuel pressure problems.
I wish I had seen this before you pulled the intake. Would have given you a heads up to replace the knock sensors and seal the boots while you had it off. An 02, and you said it was dirty under there it probably needs at least the rear one. May not be setting a code yet, but it is probably a rusty pile of garbage and the hole is probably full of water. Won't be related to your problem, but would have been a good time to do it.

[BarryB]

FOUND THE CULPRIT!!!! Clogged cat. I pulled the top o2 sensor out, completely out. I had it in my hand, took it down the road. other than the exhaust leak noise it runs out great. sound like a freight train but it runs great now. Gotta get it to the muffler shop for a new cat.

[msgross]

sounds good...

edit:hey that was #10,000!