I have a 2002 Chevrolet Tahoe with the 5.3L Flex fuel motor. Which is a joke since you can’t buy E85 in Oklahoma, but that’s not the point. The gas mileage has been steadily declining, even for a Tahoe, down to around 12 mpg in town for the past 2-3 months. Last week the Service Engine Soon light came on, I had it scanned at Autozone and they said it was both front O2 sensors and that the engine was running lean. I ran some fuel injector cleaner through the motor and the service engine light turned off almost immediately which seemed a bit quick. Now in the mornings on cold start the motor will drop idle for a brief moment then it will pick back up. As the rpms drop it almost sounds like a miss then as it picks back up the whole truck shakes. This continues but the severity lessens until the truck warms up to operating temperature. It does not affect driving, it only does this when cold and in neutral or park. Once the truck has warmed up it idles as normal and has no problems. What could be causing this?
Short Version: Sounds like a miss during cold start, truck shakes, rpms drop, then goes away once it warms up. Why???
I’ve got the same vehicle and engine but fortunately, I don’t have any performance problems with mine.
If you do any trailer towing like I do, the engine bogs down on hills as it will have reduced power thanks to the corn. I refuse to use E85 in mine.
Is there any spark arcing during the cold start time? Are the plugs/wires in good condition?
There are a number of things that can cause this problem, among them are vacuum leaks (check for intake leaks around the manifold and plenum), also the vacuum hoses.
Perhaps a bad IAC valve or maybe just a dirty throttle body.
I initially was going to mention a possible plugged fuel filter, but you said it runs good when warmed up.
PVC good? Easy to check. (any suction or does it rattle when shaken?)
Let me know how this turns out, ok?
Sorry I can’t be a bit more positive as to the cure.
Ok well I’ve had two suggestions. First it could be a dirty throttle body, so I cleaned it out tonight, it was very, very dirty, and we’ll see what that does in the morning. The other, more likely, suggestion is that the intake manifold gaskets need replaced to the new “green” gaskets from GM. So we’ll see what the truck does in the morning with the clean throttle body.
If you have been losing coolant slowly but steadily and the oil on the bottom side of the oil filler cap is a creamy color, there is a good chance the intake manifold gasket is leaking.
Does the coolant slowly disappear from the pressurized reservoir? Any over-heating?
If this is the case and you do not see any exterior leak, the coolant may be leaking to the inside and that can become VERY costly. If you suspect this have a coolant pressure test done to verify.
These engines haven’t had this problem anything like the V6s.
Because you said it runs fine after it warms up leads me to think you do not have an IMG leak.
How many miles on the rig? If it’s over 100k, replace the O2 sensors. That won’t do anything for your cold idle situation, but should have a positive effect on mileage (if you go a CEL for the O2s, there’s a reason for that.)
There is no sign of anything on the oil cap. No over-heating problems, and I haven’t been paying attention to the coolant, it’s a little low but I think it’s been like that for quite some time. The truck has 98,000 miles. I’ll work on gas mileage after I get the cold idle fixed.
You could have a couple different problems. For the fuel mileage issue, the flat O2 sensors could be causing the problem.
On a cold start, the engine is in open loop and is not using input from some of the sensors such as the O2 sensors. It is using pre-programmed settings plus temperature readings it is getting from the air sensor and the coolent sensor. So if the problem is a rich or lean mixture issue when cold, it could be the coolant temp sensor or the air sensor giving bad readings to start with. Another issue would be a leaky injector, sluggish fuel pump, or fuel pressure regulator. Also if you had a sticking idle air control motor it could cause the idle fluctuation. Not much you can do except throw parts at it but the best would be to get a diagnostic computer on it when cold to see what it is doing and to check the fuel pressure on start up.
Well got it figured out, it was the intake manifold gaskets like I thought. Thanks everyone!
Thanks for coming back with the final, SLP.