Engine misfire, reduced power, no codes.

[austinfranze]

Hi everyone.

new here. just got my first Tahoe on 7/12/2013. it is a 2003 LS with the 4.8. 201k miles. I'm having a problem/problems with it. I've searched both on these forums and with just a Google search. when i got it, i noticed that it had a rough idle. i figured it was just the fuel pressure regulator, so i didn't think much about it. when i rev it up slightly, it runs perfectly smooth, and drives well. I used to have a 2002 Silverado ecsb with the 5.3. had a cam, headers, no cats, true dual magnaflows, k&n intake, and a custom dyno tune. made about 400 hp at the wheels. Anyway, I was used to the power of the Silverado, then got the Tahoe, and thought that it was just really slow haha. The day i got it, i replaced the fuel pressure regulator. When i pulled the vacuum line off of it, gas sprayed out, which told me it was bad anyway. I put the new one on, and it didn't seem to change much, but now it takes a few seconds for it to start. Still no DTC. A few days after I got it, i drove a customers Tahoe (I work at a car audio and truck accessory store) with the same engine, but a 2002. His was a lot peppier, and i knew for sure something was up with mine. Yesterday, I was trying to find a vacuum leak. I'm pretty sure that i hear a vacuum leak somewhere. I then noticed that the vapor purge valve (Delphi P/N 1997278) was loose. I could move it side to side. My Silverado never had this valve, so I didn't know how it was supposed to be. I took it off, everything looked ok on it, and I put it back on, but it still moved side to side, but i didn't think anymore about it. I then took off the throttle body and cleaned it. It was night and day difference. It started up nearly instantly, had a lot more power, much better throttle response, didn't seem to have a misfire, and idled a lot better. Drove it to work this morning, seemed to be the same. Got in it to drive home, and it started how it should, but throttle response was back down, is misfiring, idling rough, and down on power again. it still sounds like it has a vacuum leak somewhere. Everything on it looks ok upon inspection, but having tried spraying all the connections with carb cleaner or anything. Does anyone else have any idea of what might could be the problem?

 

[madmike]

Austin,

Been having similar issues with my 2003 Yukon XL as well. Seems to be a common issue with them is the misfire.

For your question the purge valve is supposed to be loose as it only has a rubber grommet that goes around the plastic part where it mounts to the manifold. I thought mine was bad so I replaced mine, but then again mine had a low voltage code on the MIL light. There actually is a whole new kit for it you are supposed to get from GM as they redesigned the whole system if the valve is bad. They also redesigned the intake gaskets because they had common problems with vacuum leaks and misfires.

Be sure to get the gaskets with the green seals on them and not the red ones. The red are the original OEM ones that GM redesigned due to vacuum leaks. Unfortunately they didn't recall them. Learning there are many things they redesigned on the 2003 models but didn't recall. I had to replace the steering intermediate shaft due to vibration, but that was after two shops inspected my vehicle when I bought it and said the ball joints and more all had to be replaced in the front end. Sadly I replaced all the suspension parts in the front after that and no luck as they sent me in the wrong direction.

But a misfire is a common problem and GM also put out a bulletin on that, but right now I can't recall the wording, but I had tried most all of those steps and no luck on my misfire problem.

If you want to avoid throwing a lot of parts on it in a trial and error fashion like I have done I would suggest ODBII software and a ODBII connector like I got. It helps a lot. I got a USB cable with free software first for under $35 and then I got Android software and bluetooth ODBII connector for under $20 total a couple weeks ago, and they both work great and can read the misfire history and which cylinders it happens on and won't necessarily make the MIL come on or give a 300 code if it does come on.

So as not to plug any specific company I won't mention the brands of the software or connectors in public but will let anyone know what I got, how much it cost, what it does and how much it cost, and even give screen shots for all interested. Just PM me.

 

[austinfranze]

does it matter what brand of intake gasket that i get? one from the dealership is $110. found an acdelco on rock auto for $68. then one at autozone for $40. http://www.autozone.com/autozone/pa...WD/_/N-jk6p1Z93xym?itemIdentifier=438807_0_0_

figured i would start at the gasket and go from there.

 

[madmike]

Hard to tell from the photo as it is b&w if it is the right ones or not, it has to have the green rubber around the 'cathedrals' for the intake runners. Here is an example: http://www.amazon.com/ACDelco-89060...2&keywords=2003+yukon+intake+manifold+gaskets

I know this is a lot more than you found, but for example. Thers is a victor reinz set that shows up on a search for $55 that is the green ones and includes the valley cover and valve covers.

I just remember I went ahead with the ac/delco ones to be safe but felpro has always been a safe choice also.

I would just make sure it has the green around the cathedrals like in the amazon photo because if it is red in color around them that is the old gasket compound material that GM said was defective.

For mine GM says they now suspect my fuel pump assy. so trying to locate someone with a fuel injector pressure tester.

Oh I forgot, if you find a felpro or other brand on line for cheap, just print the page out and take it to the parts store as most will match that price so you don't have to wait on internet part or pay shipping. I do that at Advance Auto Parts all the time so most things I get I save 50% of what I would pay at the counter if I didn't surf the internet first.

 

[austinfranze]

ok. i took my tahoe into a friends shop and had him diagnose it. turns out it is the fuel pump. fuel pressure drops to 0 when the key is off, which explains the long start. fuel pressure is running at 48 psi. will replace fuel pump soon and post results.

 

[madmike]

Fuel pressure is supposed to be 48psi with the key on and engine off. That is the perfect factory spec. GM thought the same thing for mine but we tested it and they said 48psi was perfect, so there is nothing wrong with my fuel pump pressure.

 

[madmike]

misfire issues finally resolved

My misfire issues were finally resolved. Turned out to be two clogged converters. Went ahead and installed the MagnaFlow ones to get a little more performance out of the system, as I got them for only $299 at Advance Auto Parts thanks to their price matching policy and a cheap seller on Amazon. Hope your issues on the misfire were resolved also.

After I had replaced all the o2 sensors diagnostics still showed downstream sensors on both sides running 14.8 or better so thought I had hard vacuum leak still. Turned out to be the converters. Pulled out the two rear o2 sensors and started the vehicle and the misfire was gone. Tested again when old converters cut off and ran fine, no sputtering or oil smoke, so installed new converters and pipes and running like never has before. Only odd thing is while on the throttle the transmission shifts at 5500 rpm range, but that isn't all bad, no codes on the MIL, so I don't know how to act!