Please read, need help, crappy running LQ4 in 03 Silverado.

[Samckitt]

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I have had this problem going on 6 months or so now & no one can seem to have any idea what is causing it. I have not taken it to the dealer yet fearing it will cost a fortune with them trying multiple things to resolve it.


The symptoms:
Misses like crazy until something gets heat saturated, water temp will be at operating temperature & it still run like crap.

I start it in the morning before work & let it heat up so it runs fine once warm. For about 20 seconds after starting it, it will idle fine & then start missing bad, running like it has a big cam in it & it is loping. I am assuming it idles fine when in open loop mode, once in closed loop it starts running crappy.

I have had a Tech2 on it & the only code is something like 3300 or 3030 (can't remember, but it was for a general missfire). Nothing else.

In the past 6 months I have replaced the intake gaskets, fuel pump, fuel filter, plugs, wires, & air filter. In the past year I replaced the throttle body. Nothing changed. I had to change the fuel pump because original one went out.

I have checked fuel pressure & it is fine. Had a friend use the monitoring feature in HP Tuner & says the O2 sensors look to be functioning properly.

Other strange symptoms are if I try to drive it as soon as I start it, I cannot accelerate at all, any little gas I try to give it, it misses & have no power. I can floor it & it won't get out of its own way. Other thing that is weird is it seems as if the RPM level where is misses is related to the temperature of whatever is affected by the heat. The warmer it gets, the higher the RPM will get before it starts missing until it finally goes away & runs fine. Almost like the rev limiter level changes with the temp. Friends have followed me while I am driving it & it misses & they say smells rich.

Please give me any suggestions you can think of.

Thanks.

[T_Raven]

P0300 is the code for random misfires. If it was p0303, that would be cylinder 3 misfire.

It's hard to help much over the internet, but you could check compression cold and again warm and see if any cylinders are significantly different than the rest cold, but then normal warm.

One common problem is the fuel pressure regulator leaking into the vacuum line but normally that only causes a problem when restarting with the engine warm. But it's easy to check, just pull the vacuum line off and see if there's fuel in it. I've heard there were problems with the intake not sealing when cold. I see you said you replaced the gaskets but it's possible you have a vacuum leak. If you know anyone with a smoke machine you could pump smoke through the brake booster hose and look for it leaking out anywhere it shouldn't.

My 00 Silverado runs pretty rough on cold morning some times. I did smoke the intake once when it was cold but didn't see anything. Haven't really tried to diag it too much because it doesn't last long and doesn't always do it, but it is annoying.

[GBNL]

Map sensor will do that also .

[Slick SS]

i would try the map sensor

[Samckitt]

The code was the P0300, random misfire. We looked at the graph of the cylinders misfiring. It was all over the place, not just one cylinder.

I will try the MAP sensor. Will one from an 08 Trailblazer SS work on an 03 LQ4? Prob not, one is 58x while other is 24. I have one from the TBSS.

Thanks

[im4u2nvss]

A few years ago one of my friends had the same symptoms with his silverado SS(6.0 also). Turned out it was a plugged cat. He was on a budget at the time so he hollowed out his cats and spaced his secondary o2 sensors out to prevent a CEL. Running perfect ever since.

[Samckitt]

A few years ago one of my friends had the same symptoms with his silverado SS(6.0 also). Turned out it was a plugged cat. He was on a budget at the time so he hollowed out his cats and spaced his secondary o2 sensors out to prevent a CEL. Running perfect ever since.

That had crossed my mind, didn't know how to test the cat without taking it off.
thanks

[Samckitt]

So I am bring back an old thread because i still have this missfire that cannot be found. I hadnt driven the truck for about a year, stopped driving it soon after the cats plugged because of the raw fuel being dumped into the exhaust. They are hollow until I can get this missfire fixed. I just got it back from its second visit to a local shop (not dealer), 2 shops have looked at it. Last shop bill would have been close to $2000 if he had resolved it, but only charged me $400 for a fuel pump.

Parts that have been changed on last trip to shop:
fuel pump
pressure regulator
fuel filter
one fuel injector
one O2 sensor
intake gaskets
crank sensor
cam sensor

Owner of the shop said everyone of his mechanics has looked at it & still cannot find the missfire.

He said it is missfiring on cylinders 1 & 6. Will hardly run when it is cold, but once something gets hot it isnt as noticable.

I dont know what else to do, only thing that hasnt been changed is the ECM. Can I swap it with another truck, or will the VATS keep that from working? Do I have to go to the dealer & have an ECM programmed for my truck before it will work?

Any other suggestions? Thanks

[GeoffP]

I remember reading this when you originally posted but figured you had solved it by now. Does the water temp sensor track properly as it warms up? It could easily cause the poor power when cold you're experiencing but I'm not so sure it would cause the random mis-fire unless it's fouling the spark plugs. I assume both the temp sensor reliability and spark plugs have been checked so I would probably lean towards it being a bad ECM since so many people have looked at it and not been able to determine the problem. Over the last 8-10 years I have seen two separate GM vehicles with similar drivability problems as what you describe where technicians were unable to figure out the problem. Both of them ended up being a bad ECM. You are correct thinking VATS will keep it from running if you simply swap the ECM from another truck.

Last thing I can think of is to be sure to trace the ground wires and make sure you don't have one frayed or broken. The grounds on that engine are on the driver's side in front of the motor mount (this is the ground for the ignition coil circuit) and there are two grounds on the rear of the engine that are a pain to get to but could easily be a contributor to your problem.

It just so happens I also own an 03 Silverado with an LQ4 and I put an engine in it this year - I learned about the coil ground the hard way having forgotten to reconnect it. I spent hours trying to figure out why it wouldn't start with the new engine.

[Samckitt]

Yep, water temp gauge tracks fine, and I changed the sensor anyway because I read somewhere that it may fix it. Ground straps have been changed. I think only thing left to try is the ECM, if it isnt that, then there is a short in the connector somewhere.

Time to trade it in I guess.

[LXCam]

Well I guess it's about time I made my first post. Considering everything I've learned from this fantastic site, hopefully this might help you. I had almost the same identical issue with a lq4, it drove me nuts as I replaced most everything that could cause a vacuum leak. Finally at my wits end I bought one of those cheap party smokers off eBay for $40.00 and smoked the intake assembly. Turned out I had a very tiny crack where a vacuum hose attached to the manifold, you couldn't even see it unless you forced it open. Once I sealed that, problem solved. Good luck!

Cam

[Samckitt]

Well I guess it's about time I made my first post. Considering everything I've learned from this fantastic site, hopefully this might help you. I had almost the same identical issue with a lq4, it drove me nuts as I replaced most everything that could cause a vacuum leak. Finally at my wits end I bought one of those cheap party smokers off eBay for $40.00 and smoked the intake assembly. Turned out I had a very tiny crack where a vacuum hose attached to the manifold, you couldn't even see it unless you forced it open. Once I sealed that, problem solved. Good luck!

Cam

Did you take intake off & tape holes or leave on intake & plug exhaust?

A little crack like that will keep mine from going over 5mph floored?

I'll try anything.......


Party smoke machine..... a fog machine? Like this http://www.ebay.com/itm/The-Fog-Mach...item35ded81564

[LXCam]

No I left everything together. I was convinced it was a vacuum leak and I had already replaced anything and everything that even remotely looked suspect. I did not have the 5mph limitation, but it was constantly throwing P0300 misfire codes and falling on her face, no power, super rich. In stead of taping or plugging I had planned on doing individual compression stroke tests, but didn't need to after all. And yes, that's what I got was a fog machine.

[GeoffP]

You can also spray ether on the intake to see if it idles up. If it idles up when you spray ether at key locations on the intake, you'll know it has an intake leak or crack. Just be careful or you could start a fire.

[Samckitt]

So recently my truck was at a shop near my house (not dealership), they had it for 3 weeks. & replaced the following:
fuel pump
fuel pressure regulator
one injector
O2 sensors
cam & crank position sensors
intake gaskets

Also cut the exhaust before the mufflers in case they were plugged.

And still could not figure it out. They did the smoke test on the intake, that is why they changed the intake gaskets.

He gave up, charged me $400 for the fuel pump, nothing else, but would have been almost $2000.

So I decided to try another ECM. Same results, still misses.



Any other thoughts?

[GeoffP]

At this point it sounds like you have replaced everything that it could be aside from the intake itself?

[Samckitt]

I have not changed intake. Shop said they did a smoke test on it and found small leak, fixed it by changing intake gaskets.

[LXCam]

Dann bud, that sucks. And not trying to ask too stupid of a question, but did they do another smoke test after replacing the gaskets?. I ask cause I did the very same thing, replaced all the gaskets and hoses then smoked it and found that cracked fitting. It was such a tiny leak , I couldn't believe it caused all my issues.

[79loserbluebu]

Find someone near you with a similar truck and swap MAF sensors. Not sure why everyone was saying a MAP. Odd that you only have a misfire usually when a MAF goes, it will give you a lean or rich dtc.

Has anyone taken either of the valve covers off and inspected for a broken valve spring on the misfiring cylinders? Usually the exhaust will go but either could break seeing as they are the same spring. If you have the covers off run it and see if you see either valves on 1 or 6 not moving like the others. Could be a collapsed lifter or a rounded lobe.

It really could be anything but at least you have a pretty good place so far.

I wish you were in IL I would come look at it just to try and beat the truck lol.

I just had a G8 the other day with a L99 with a collapsed lifter. I know different gen engine but still

[Jetfixr320]

My buddy went through about the same thing a few months back. It ended up being a bad lifter. His had the NON-ACTIVE FUEL MANAGEMENT system. I guess there are some people having problems with these lifters.