MRIGGS

Thanks for the reply! Timing hadn't crossed my mind yet, but it would explain why fuel and air look ok but it still runs rough. My google-fu is failing me this morning. Is there a standard procedure to check timing on these motors? I'm finding videos of people using aftermarket components or just tearing into it but nothing yet on checking a stock motor. I'm assuming there's no visible timing marks but haven't dug into it yet.

So the problem doesn't seem to be brake specific, I can turn the wheel to the stop and get the same results. The same stumble seems to happen within a couple of minutes of startup to complete operating temperature. Open or closed loop doesn't seem to make a difference either, except on first start open loop I'm about 100 rpms higher so the drop under load isn't as pronounced. I think the added load of the power steering pump in use just showcases the issue. After it was a little warm I had the same rough idle without brakes or steering input, in park and in neutral. Any suggestions?

Hi there, I've been chasing this rough idle gremlin for a while now with not a whole lot of luck. hoping a fellow motorhead can help me narrow this down instead of just changing a bunch of parts. I have an '04 Tahoe with the 5.3L Flex Fuel Vin and about 263,000mi. I've changed the spark plugs & wires and fuel filter as they were due anyway. Recently replaced the crank sensor and engine coolant temp sensor for what I think was an unrelated issue. I've got no codes of any kind. I do have a Bluedriver scan tool I've been trying to use to see if I could spot anything weird going on. My fuel trims seem normal (+/- a few percent). Upstream o2 sensors seem to be switching and average around (.42-.5V) so overall that doesn't look too bad. Downstream o2's ~.63-.69V and pretty steady. I do have one bank that if you hold 2500 rpm it starts more closely trending the upstream but normal highway driving and such everything looks stable. I checked my fuel pressure with an actual gauge. From what I read these are normal for the flex vin. Key on engine off - ~54 psi. Idle - 44-48 psi 1500 rpm - 44 psi 2000 rpm - 46 psi I pulled the vacuum line off the pressure regulator while running and pressure shot up to 50 something psi so I'm thinking that's ok. I tried a couple tanks of fuel injector cleaner and that didn't seem to make any difference. I've cleaned the throttle body, there wasn't much build up. This truck has a fixed PCV orifice but I tried cleaning anyway with a bottle brush and some cleaner. I don't have a smoke machine (yet) to check vacuum leaks but I don't see anything obvious when tracking down the vacuum hoses. MAF at idle runs right around .9-1.1 lb/min and from what I can see increases with throttle opening as expected Manifold Absolute Pressure at idle ~12.1-13 in Hg and from what I can see increases with throttle opening I did finally notice last night when I got home that if I mash the brakes the engine stumbles a lot. Avg idle 560 goes down to 460-470 when I hit the brakes. MAP sensor temporarily spikes to 14-15 in Hg. I could hear a hissing from the floorboard. I thought brake booster leak, easy enough fix but this vehicle uses a hydroboost brake system so no vacuum lines. I've started reading about this a little more and seems the hissing can be normal so I don't know if I have a brake issue, a power steering issue, or if somehow this load just makes the condition more apparent. I have had issues with the vehicle turning right since I got it (like there's no power steering in that direction). From what I've read/heard this is a bad steering box and is common for this year just haven't got it changed out yet. Don't know if that has anything to do with power steering pump wear or not but worth mentioning. I plan to check and see if engine temp cold or hot makes a difference after work with the brakes at idle and also see if turning the wheels causes the same condition.