I hope someone can help here. My work truck is an 02 Chevy 6500 cab over with a Caterpillar engine 3126 series and auto trans. With or without a load the truck will start bucking (surging) like their is a lack of air or fuel between 1900 and 2300 rpm. The bucking is actually worse without a load in the bed and real noticable going up hills. We have changed the fuel filter today and the air filter last week. After the air filter change we reset the filter gauge and after a short trip it is pinned like it can't breath again. We have checked the intake system up to the turbo intake tube and can't see any restrictions. Another thing that just started is while driving (doesn't mater if it is 20 mph or 70 mph) the service engine soon light will come on and start flashing. When that happens the truck looses almost all of it's power. It's like it falls into a reduced power state or limp mode as some might say. I think when this happens the turbo doesn't even work. At this point all you have to do is pull over shut the truck off and restart. The SES light goes out and the truck runs fine again with the exception of the bucking.
Can someone shed some light on this for me? dealer can't get it in for a few weeks.

thanks for any and all help
PoolRebel

If the Filter minder is pegged it sounds like there is a restriction before the filter. Not between the filter and the turbo but before the filter altogether.

Thanks Max Power, Unfortunatly as I stated we checked the intake system from the air intake all the way up to the turbo and thier are no visible restrictions. The intake is almost big enough to fit my head into so if thier was a restriction I'm sure it would have been easy to see. Come to think of it I did notice some moisture build up in the filter minder yesterday as well. Not sure if that will help or not.

PoolRebel

One thing to check. Does your truck have a line running from the Muffler/Exhaust to the air intake? If it does, this is a vacuum line that is used to suck out most of the dirt before the filter. If the exhuast has gone bad, you will pull soot into the filter and plug it in a short time. If you have that line, feel if it is hot after the truck has been running for a while. If it is, you are pulling exhaust into the air intake and need to replace the exhaust/muffler.


We had this problem on our 700 Versatile tractor. Air filter would plug in about 2 hours. Clean the filter and it would plug again. The filter was loaded with soot. Replaced the exhaust, and problem went away. The heat destroyed the line running to the air intake.

Check the top of the air intake on the cab. Sometimes they will suck in under load and block off the air tube.

Darin Billing- Thanks but I don't recall anything running off the intake before the air filter. The bucking was still there 30 seconds after replacing the filter so there is definatly something going on.


GMC SID- Thanks I'll take the top of the intake off for a little while in the morning and see if it makes a difference. If this is the source of the bucking would that cause the second problem as well. ( SES light comes on and flashes, truck goes into reduced power mode?)


PoolRebel

Update: Took the top off the intake today and it didn't make a differance in the bucking or the SES light and repuced power occuring. Took the filter minder off and shook the water out of it then reinstalled it. Made deliveries all day and it didn't pop up once but the problems still exist. I did notice that when the bucking would start if I took my foot off the go pedel and then put my foot down again the truck would run like a champ untill I took my foot off the pedel again. I think it has to be a sensor going bad but not sure which one. Any ideas on which one and how to test it?

Thanks again,
PoolRebel

I'd take it in and have them hook it up, w/ the SES lite coming u should have some codes set.


Our Pete w/ cummins had to have a new go pedal switch, had loss of power but cruise worked OK.

Yea, definately get the codes read. They should point you in the right direction.

Thanks for the help guys. I'm in agreement on getting the codes read too but don't have access to a reader. We are just going to have to wait till the dealer can get it in. Unless I can talk the boss into buying a reader for me. http://www.dieselplace.com/forum/smileys/Evil Smile.gif Opps I mean for the trucks at work.

Update- After begging for the dealer to find time to look at the truck, they finaly got it in. Found a code (don't know what it was) and claimed to of put a galon of oil in it. (I checked it before it left for the shop and it was right on the full mark) They said they drove it around for 3 days and couldn't duplicate the proble. Hence it was just our imagination! My boss picked the truck up last night and the ses light came on and went into limp mode 3 times in under 10 miles. This morning we changed the oil being that it was over due for it and now the truck runs like a dream again. No surging or ses light all day. The truck was running from 7:30 am till 5:00 pm and ran over 150 miles without a problem. Could this have been an oil sensor getting gummed up?

That could have been it. The injector are hydraulicaly actuated with engine oil.

What GMCSID said. I recently took a Kenworth/Cat in for service with the same problem. The tech that I spoke to mentioned that he sees it all the time, especially in the local school buses here in Anchorage. --SS