The AM Forum

I've noticed a weird operating point in my truck ever since around the first of Nov. when the gas is changed. 2001 Silverado, 5.3, 2wd

When the motor comes up to operating temperature and I'm light on the gas the motor will surge once in a while. It has about 125 k so figured it needed plugs so swapped them out (gap was almost .070 for old plugs). Idles like a new motor. Stomp on the gas it goes fine. Mileage looks normal. Also never see the tach drop out, RPM seems constant. Threw in a new Air filter for good measure.
When the motor comes up to temperature the O2 sensor is active and provides feedback to the fuel control. I asked the  local car parts dealer and they told me a bad 02 sensor should give me a red light. Also there is more than one so don't want to shot gun expensive sensors.
Anybody notice this over the past few weeks? 91 octane gas is a bit better.
I run 89 for best enonomy. I get a similar effect if I run 87 with a bit of a ping.
I hear no ping almost seems the motor timing is retarded like it is compensating. ????

I'm not sure if this is related to your surge problem, but have you flushed out the fuel injection jets in the past few years?

How about the fuel filter - is it located in the tank or accessible?

T

Tom,
The first thing I did was run injector cleaner through it. If the filter was bad I would have problems when I stomp on it. My old '88 had a bad filter once so know how that feels, it would bog on hills of if I got into it.  I do need to change it when it gets warmer though.
The weird thing is everrything is FBOM until it gets up to temperature. A bad filter would show up when it is cold since the mixture is richer. It happens in the same spot on the road for the ride to work.
The injector thing is a consideration if they are sticking. Maybe I should run more cleaner through it or Marvel Mystery oil..

winter gas is better around here than summer gas with all the "clean burning" crap they put in it. Crap that does not actually burn and makes you have to burn more gallons and make more pollution to get the same work done.
summer: 15.5MPG
winter : 17.2 MPG

USA made 80's is probably throttle body. Typical USA made auto or truck wouldn't use expensive injector system.
I have throttle body in both of my old Buicks and 27 mi/gal.............3.8L. A model or chassis that is hard to beat. The Century, Regal, LeSabre must be the same chassis. I see them everywhere here in Pa.
FRED

Frank,
         Have you checked the actual operating temperature?? These systems MUST see the engine reach full operating temperature in order to change from "open loop" to "closed loop" operating conditions. Also there could be an issue with your engine coolant temperature sensor not telling the ECM that the engine is hot enough. You may have a sensor issue.

My Ranger does exactly the same thing at just about the same place every morning during my morning commute. It bucks and surges for about 1 block, then stops and runs fine. It has done this since I have had it. Since it only does it for about a block while rolling, and you only really notice it if you're looking for it, I just ignore it. I also run 89 in it and still get 28MPG or better with 135K on the clock!

If you are getting a spark ping, you might wanna check the knock sensor, and/or the EGR
They aint supposed to do that no matter what kind of gas you put in it. If the EGR is not doing it's thing, the ECM will retard the spark to try to stop it from pinging. I just run the 89 because it is a little peppier and I get a few more MPG.

A quick check with a scan tool might tell you whats happening

I can tell when it switches over. Also it happens in the same spot in my ride to work. This morning it was 10 outside and it still happened in the same place.
Kid next door has a scanner so may ask him to let me borrow it.

Is it colder there in the winter? If so, could the improvement in MPG be as a result of a denser colder air mixture, much like we see when racing in the cold air? 

More power, but I'm not sure if MPG changes too to compensate the mixture ratio...

T

Gas? Did someone say GAS??

    "beans, beans good for the heart, the more you eat the more you f**t, the more you f**t, the better you feel, so eat ur beans..!!"

                    ;D

Just by the title of this post, I thought you were referring to sitting around the fire eating roasted peanuts.  Now that, is WINTER GAS! ::)

I drop about 1 MPG in the winter and about 1/2 when I use the AC in the summer.

Hi Frank,

    You can find forums for your particular vehicle where people have been through the same exact thing a hundred times. I drive a Rube Goldberg and that has worked for me. Beats comparing to what worked for someone else on a design 20 years older.
Good luck!


Put this into google:
2001 Silverado 5.3 surge

Got this after a manual filter.

http://www.performancetrucks.net/forums/showthread.php?t=34230

http://www.batauto.com/Forums/index.php?topic=5770.0

I only know what the built-in computerized dashboard instrument says. It does have a lot more power in winter when it is 30-40 degrees than in the 90-100 degree summer..

Old West Gas

http://www.youtube.com/watch?v=qpB3ME_Xem0


Tom - AB3FL

TNX Dave,
I was thinking it could be a vacuum leak. Plastic manifold could warp. I would lap it on 320 wet / dry paper if that is it. It is not using water and oil looked fine when I changed it about a month ago. Again the idle is very stable, it did need plugs because it was not as stable before I changed them. I'm going to look at air connections after the MAS. Maybe I see if I can connect a vacuum gauge somewhere and monitor it.

A true Classic!! "More beans, Mr Taggart?"  ;D  ;D  ;D 

Awwww, just give them to Mongo!

Candygram for Mongo, Candygram for mongo........................................

 ;D  ;D

Well the next logical thing to do was change the fuel filter with 124,0000 miles on it. Today is the first day I feel good after two weeks of a bad chest cold so figgured I'd pay to have it replaced. Went down to the parts store and bought a filter and asked if they knew a good garage. As luck would have it about 100 yards away. Check them out and busy as heck, good sign. Boss tells me to come back at three. Looked in the book 1/2 hour labor which sounded fair time. With tax $46.50 smackeroos to swap out a $14 dollar filter.
Still better than laying on the cold ground.
I had them throw the old filter in the bed. When I got home it was dry so tried to blow air through it. Almost plugged.
Now I know why I get my hands dirty.

Frank,
         A clogged fuel filter creates an interesting scenario on EFI cars. It drives the ECM nuts. The computer calls for more fuel, so the injectors stay open longer. this causes the fuel pressure in the rail to drop further. The ECM thinks it is richening up the mixture, but it is going leaner and it doesnt know how to compensate for it!!
This can sometimes cause a real nasty bucking and surging problem, especially when you put the boot to it.

Frank,
A few days after my first post I was getting on the highway. Usually the truck would drop down to third but this time it went to second and spun up to almost 4000 not making power. This made me suspect it wasn't getting gas.
The change to winter gas (pisoline) just brought things to a head.
The garage guy told me the motor would be unstable at idle if there was a vacuum leak. Also he said I should change out the O2 sensor even though there is no fault. He said it will protect the cat. He likes to change them at 90K.

I changed all 3 of them in the YL's buggy at 162k, mine are all original at almost 140k
No problemz at all. The biggest thing is to get them hot enough to burn the cheese off of them. A little high speed highway driving does wonders for them and the catalyst. As long as you keep them free of cheese, all is happy.

Well I could really tell when the fuel control switched over to O2 mode and things went lean. I do some highway driving every day. 1/2 my ride is at 70 MPH. The girl at the auto parts store told me I should pull faults if the sensor isn't in range
Took the truck for a ride today and problem is gone.