Real rough idle at start

[ratled]

Just looking for first thoughts on this one.

02 GMC 5.3 gas with about 120k on it. Has been running like a top and the only thing of note is the change oil light came on 2 days ago. Went to start it yesterday and it had a real rough idle. It kicked itself to about 1500 rpms and smoothed right out. As it went to settle back down to a normal idea it went rough again. Did this several times until I shut it off. At the low end it would barley run, kind of reminded me when someone stick gas in a diesel.

Any thoughts before I start looking into it?

Thanks
ratled

 

[Doubeleive]

Just looking for first thoughts on this one.

02 GMC 5.3 gas with about 120k on it. Has been running like a top and the only thing of note is the change oil light came on 2 days ago. Went to start it yesterday and it had a real rough idle. It kicked itself to about 1500 rpms and smoothed right out. As it went to settle back down to a normal idea it went rough again. Did this several times until I shut it off. At the low end it would barley run, kind of reminded me when someone stick gas in a diesel.

Any thoughts before I start looking into it?

Thanks
ratled

First thought is check fuel pressure...
beyond that based on the mileage and no reference of maintenance being done
plugs/wires are rated for 100k
02 sensors are rated for 100k
intake/air filter, maf, throttle should be cleaned and new filter
if the above has been previously addressed and fuel pressure checks out then throw some injector cleaner in it for a few tank fulls and/or have a fuel injection service performed.
only other thought is crappy gas

 

[Fless]

I would look for a vacuum leak. Check the fuel trims first, though; if they're way positive (>10%) on either bank at idle, and improve with higher RPMs, that's an indicator.

EDIT: also check for unmetered air; the intake tube should have a good seal at the MAF and the throttle body, and no cracks or open holes.

 

[jwth]

if I remember right
the fuel pump needs to see a minimum oil pressure to energize
maybe you need to see if there is a pressure problem or a bad sending unit not passing required minimum for
fuel pump ok to run

 

[ratled]

I forgot about the O2 sensors in the 110-120 range. Everything else is squared away

 

[Doubeleive]

I forgot about the O2 sensors in the 110-120 range. Everything else is squared away

I would do a quick check of the fuel pressure then, I don't know about this oil pressure thing mentioned above I seriously doubt it since when the oil pressure sensors go ***** it has absolutely zero affect on the fuel pump, I could see if there was a low oil pressure warning MAYBE, but very doubtful.
you can order or get a fuel pressure kit for like $20-30 at the autoparts store or harbor freight, amazon, etc. some autoparts stores even rent them for the cost and then you just get your money back upon return of the tool

 

[rockola1971]

Check intake manifold gasket for a leak. Spray starting fluid along intake gasket mating surface and see if idle jumps up. Any codes popping? Check throttle body to see if it needs cleaned. Have a scanner to check for misfire counts for each cylinder?

 

[MassHoe04]

if I remember right
the fuel pump needs to see a minimum oil pressure to energize
maybe you need to see if there is a pressure problem or a bad sending unit not passing required minimum for
fuel pump ok to run

Could be something to this as a possible theory... OP said "Change Oil" light came on. Was it "Change Oil" message on the DIC display or was it the oil pressure light (the oil can icon) that indicates oil pressure.

 

[ratled]

Just a quick update - So the O2 sensors are the original ones. Whether they are my issue or not they need to be replaced (though I think this is my current issue). Just wondering if I HAVE To have a O2 sensor wrench or can I get by with a regular wrench to break them free

Thanks
ratled

 

[OR VietVet]

Just a quick update - So the O2 sensors are the original ones. Whether they are my issue or not they need to be replaced (though I think this is my current issue). Just wondering if I HAVE To have a O2 sensor wrench or can I get by with a regular wrench to break them free

Thanks
ratled

Change all at the same time and use a good penetrant, like Aerokroil, and spray all threads and let sit for a day. Spray about three times that day. When you decide to actually take them out, spray again and be careful. You can use an O2 socket but taking out, you can use a deep 6 point socket too but can use a proper sized box end 6 point or 12 point to reinstall. I would also check all the basics as has been suggested.

At RA it shows two different options of Genuine GM sensors, depending on the assembly plant they were built at. Seems odd but it is listed as to how to decide which plant your rig was built at.

 

[rockola1971]

Could be something to this as a possible theory... OP said "Change Oil" light came on. Was it "Change Oil" message on the DIC display or was it the oil pressure light (the oil can icon) that indicates oil pressure.

I've never seen our chassis with an Idiot Light for the oil. Only gauges. I know gm use to tie in electrically an oil pressure switch (not sensor) in series with the fuel pump relay coil (IIRC) but they havent dont that in many years. We dont even have an oil pressure switch on our engines, just sensor for the gauge.

 

[Doubeleive]

I have driven mine with both low oil level and low oil pressure and it didn't do a dam thing with the fuel pump, just gave me warnings
add bad sensor to that as well and all it did was peg the oil pressure all the way up
pretty sure this "theory is moot"

 

[MassHoe04]

I've never seen our chassis with an Idiot Light for the oil. Only gauges. I know gm use to tie in electrically an oil pressure switch (not sensor) in series with the fuel pump relay coil (IIRC) but they havent dont that in many years. We dont even have an oil pressure switch on our engines, just sensor for the gauge.

Thanks for the correction!
I am still learning about our vehicles...

Just because it exists in the owners manual, doesn't mean it is correct.
Sorry! This was in the front of my manual and I took their word that it was there on the cluster somewhere.

 

[rockola1971]

Thanks for the correction!
I am still learning about our vehicles...

Just because it exists in the owners manual, doesn't mean it is correct.
Sorry! This was in the front of my manual and I took their word that it was there on the cluster somewhere.
View attachment 367958

It could be in maybe a base model? I only have LT's and a Denali but ive been in alot of K1500 and C1500 and havent seen a idiot light in a very long time.

 

[GRAYGEORGE]

O2 sensors are usually 7/8 inch. You can also borrow/return O2 removing socket set.

 

[MassHoe04]

O2 sensors are usually 7/8 inch. You can also borrow/return O2 removing socket set.

When I have been removing a sensor that is getting replaced with new, I just cut the connector off the sensor and slip the wire through a regular box wrench to get the old one out. The old one is getting thrown out, so I make it easy and cut the connector off (the connector attached to the sensor, of course... Not the one on the harness).

Then, I'll start threading the new one by hand and use the open end wrench to tighten. So far, I have managed to get away with my basic tools and avoid having to buy or borrow any special O2 socket tool.