Hi guys, I hope yall can help with this. '05 Tahoe 5.3, non flex, runs rough/knocks and stalls after it warms up completely. Runs fine when cold and while in the process of warming up. Usually you can smell gas and funky smell from exhaust while it's running rough. 200k miles. I bought it used, but to my knowledge it's all original. Has run fine for the few years we've had it prior to this. The first thing I checked was fuel pressure. 60 psi running. In spec. Never fluctuates. It's a returnless setup. Originally only had a knock sensor code and downstream o2 heater circuit code. Replaced knock sensors and harness, and cam sensor. Replaced plugs which were covered with burned oil due to valve seals. Replaced crank sensor. Bought 2 new coils and plug wires and moved positions with them until all coils had been checked. Cleared codes and got a 0300 random misfire code once it started acting up again. Unplugged the MAF sensor and there was no change. To me the fact that it only acts up when it's thoroughly hot suggests an electronic component, and I believe I've ruled out everything but the ECU, but wanted to get some outside input and suggestions before messing with that. Any input is appreciated. Thanks.
2005 5.3 Tahoe running rough/stalling only when hot
- Thread starter 1919
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If it hasn't been done, do a CASE relearn with a capable scanner after having changed the crank sensor. Quick relearn to rule it out for your misfire code.
Also, what do all the fuel trims look like when it's running good, and then when it's running rough?
How does the intake air temp compare when it's acting up?
What's the MAF reading in g/s at idle?
Also, what do all the fuel trims look like when it's running good, and then when it's running rough?
How does the intake air temp compare when it's acting up?
What's the MAF reading in g/s at idle?
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You can also check for leaky intake manifold gaskets. They shrink with age, and add unmetered air, causing a lean condition. Use you favorite spray in the area and observe any change in RPM's.
I don't have the capability to do the relearn. The sensor I bought said the relearn wasn't necessary. In any case, changing it made no discernable difference in the way it runs.
I also am unable to see fuel trims. All I have is a cheap code reader.
IAT and MAF appeared normal, but it's been a few weeks since I was at that point. I'll double check tomorrow and post actual values. Unplugging the MAF entirely also made no discernable difference.
All replacement parts were Delco also, no cheap junk.
Intake gaskets were changed when I changed the cam and knock sensors. If a vacuum leak in the intake area was a factor, would it cause problems before the engine warms up also, or only when hot?
rockola1971
Full Access Member
Unmetered air really shows its ugly head when in closed loop. Spray some starting fluid or the like around the intake manifold flats at the heads and see if RPM raises or sputters. Have any current UPSTREAM O2 sensor codes? Active FLASHING check engine light? All plug wires should be new or newer at 200k miles now. Saw that you did say you put in new plugs. The relearn for crank/cam sensor is mandatory.
I'll spray it today and see what happens.
Haven't had any upstream codes at all. O2 sensors were changed preemptively a few years ago when I bought the vehicle. In my head I ruled out O2 sensors because it acts up in both open and closed loop, but I don't know if that's the right way to look at it.
No check engine light, but I suspect it may have been tampered with if it's not coming on during all this.
Thanks to everyone for the input thus far.
strutaeng
Full Access Member
Umm, yeah that's possible. Injectors hardly ever fail, however.
You need a scanner capable of reading misfire counts on each cylinder. Then swap plugs, wires and coils and see if the misfire moves along.
If you have a misfire, your O2 waveform is going to be of of wack. Focus on finding and fixing the misfire first would be my advice.
ivin74
05 NBS Nali
Did you torque the spark plugs? I would remove and inspect the plugs, one might have cracked and might be causing the issue.
Also do as the other member suggested spray the air intake where it meets the block with carburetor cleaner.
Also do as the other member suggested spray the air intake where it meets the block with carburetor cleaner.
I didn't torque them but I didn't over tighten them. The problem existed before the plug change. The plug change was a response to the problem.
Gonna try the spray today.
Thanks.
rockola1971
Full Access Member
The Downstream O2 Sensors only purpose in life is to tell you that your cats have gone bad. They are not used for any kind of lean/rich calibration for the PCM.
The Upstream O2 Sensors only purpose in life is to tell the PCM whether your engine is running rich/lean and the PCM adjusts your fuel injection accordingly. The Upstream Sensors are NOT used for rich/lean calibration until you are CLOSED LOOP.
The instrument cluster does an all bulbs lit up check when you turn your key to the run position (right before you goto engine crank position). So turn your key to run (but without reaching engine crank) and you should see your check engine light lit up. If you do not then your check engine light bulb is bad or the part of the instrument cluster circuit board for that bulb has a defect.
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The intake air temp started at 80 and gradually climbed to 91 as the vehicle warmed up. I didn't see any crazy fluctuations.
Unfortunately my cheapo scan tool don't provide MAF data. I had basically eliminated the MAF as the issue because my understanding is that it should just read the fuel maps with the MAF unplugged and should run basically fine, just a little rich. In my case it runs just as horribly with or without the MAF plugged in. Am I wrong in thinking this?
It does give MAP data, which was 5.9 cold and 5.0 hot. Once again no crazy readings, just a linear drop as it warmed and idle rpm decreased.
I also sprayed around the intake as others mentioned and didn't see any rpm fluctuation.
One other thing I did notice... In the past, once it's started acting up it begins to pull timing like I'd expect, from it's normal 18-20 degrees down to 12 or so. This time it initially pulled timing, but then I saw it go to 28 at one point and it was knocking hard enough that I was worried about it damaging itself and I just killed it. The rpm signal seemed to be reasonable and not erratic during all this.
Also I should have mentioned this at the beginning but it slipped my mind. When swapping the coils I thought at one point it might be fixed. It had warmed up and was running fine. I slammed the hood and it started running rough again. This led me to consider the wiring, but it all looks fine. Some of the plastic sleeve junk is broke up, but there is no damage to any of the wires as far as I can tell. I also ran a temporary ground straight from the head to the negative terminal and this made no difference.
Thanks to all yall for the input thus far.
Yes, it appears to be working correctly.
Thanks
rockola1971
Full Access Member
"I slammed the hood and it started running rough again."
This could be a clue. Take a screwdriver with engine running and warmed up and LIGHTLY smack the MAF body and if the engine starts stumbling and then comes out of it MAF is good. If engine immediately dies then the MAF is likely bad. This is an old school check for an internal short across parts of the grid within the MAF.
The MAF "measures" how much air is coming into your intake. The way it does this is there is a grid heater inside your MAF that the air has to come across to get into your intake. That heater is fed current by the PCM to attempt to keep it at a certain stable temp. The more air that moves across the grid (which happens at higher RPM's) the more cooling is acted upon the heated grid which requires more current from the PCM to get temp back to that stable value. The amount of current at any given engine RPM has been mapped out by GM and is a part of your PCM's calibration files.
Get a short within the grid (not a short to ground) and the resistance of the grid has changed and your GM programmed maps for the MAF are now useless because you no longer have a GM spec MAF sensor and the engine runs like total crap if at all.
This could be a clue. Take a screwdriver with engine running and warmed up and LIGHTLY smack the MAF body and if the engine starts stumbling and then comes out of it MAF is good. If engine immediately dies then the MAF is likely bad. This is an old school check for an internal short across parts of the grid within the MAF.
The MAF "measures" how much air is coming into your intake. The way it does this is there is a grid heater inside your MAF that the air has to come across to get into your intake. That heater is fed current by the PCM to attempt to keep it at a certain stable temp. The more air that moves across the grid (which happens at higher RPM's) the more cooling is acted upon the heated grid which requires more current from the PCM to get temp back to that stable value. The amount of current at any given engine RPM has been mapped out by GM and is a part of your PCM's calibration files.
Get a short within the grid (not a short to ground) and the resistance of the grid has changed and your GM programmed maps for the MAF are now useless because you no longer have a GM spec MAF sensor and the engine runs like total crap if at all.
houstontaylor
Full Access Member
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Thermostat?
Unfortunately not yet as I haven't had much time to mess with it. I will update when a solution is found.
