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Surging in 3rd on decal at 2500 rpm.

1999gmc

1999gmc

Active Member
Messages
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Location
Carmichael, CA
So a month ago we went camping and my lift pump was acting up and not working sometimes. On the way home we had an uphill climb and then a downhill 7% grade for about 4 miles. On the downhill I put it in 3rd gear and about 1/4 a mile into the downhill it started surging bad, 2500-2800 bucking chugging. I was thinking my lift pump had stopped on the uphill and I finally ran the fuel bowl out of fuel on the downhill coast it started to starve. I finally shifted into OD and just used my brakes, and as soon as I shifted into OD the surge stopped.

I got my new lift pump and changed the fuel filter(was time anyway)

This weekend I towed our trailer and there was a 10% grade. Got to the downhill, put it in 3rd and shortly into it it started the same thing, bucking and chugging, I immediately put it in OD and it stops doing it. The next day, I went up this hill, no problem. Coming back down the hill with no trailer, I put it down in 3rd and it started doing it even without the trailer, I even hit the brake to un lock the converter and it still did it. I put it in OD and it stopped surging.

No codes. Any ideas or suggestions.
 
Pull codes. Check trans oil. IMO the overrun clutches that come on with manual gear selection for compression braking are your issue. Trans codes do not trip a Service ENGINE Soon light. The overrun clutch for 2nd gear manual braking is really weak. (I have burned it out 2x now.) Does all kinds of crazy stuff while slipping and grabbing before finally not grabbing at all. Not sure on what is locked/slipping in manual 3rd.
 
Just pulled trans. codes and no codes are present in current or history. And fluid looks good. Still a nice red color.
 
The decel over run clutches can be burned out and will not affect going forward.

You get fishbite before running out of fuel. When you do run out of fuel the engine quits suddenly. Some give you a surge warning. Not like a carb at all.
You could check fuel pressure and the fuel line before the Fuel Filter Manager for a kink.

Quick and dirty test is to mash the throttle when it is doing this and see if you have power. If you do it's isn't fuel related.

I have had this issue on my 1995 and some tunes make surge worse going downhill.
 
I dont think I'm running out of fuel after the new lift pump. The clutches could totally be the problem. The guy I got this truck from took really good care of it but he towed a lot(frequently). I don't know what size his trailer was, but he had receipts from all over for oil changes and stuff and told me he only used it when they went camping. And the last 60K miles I have owned it have been mostly towing our toy hauler. So trans could definitely be suspect. I just don't want to replace it to find out the truck still does it and it was a PMD or inj pump or something. Thanks for the replies and ideas.
 
To be sure check the power patch through the 4L80E when compression braking. I know it applies to 2nd manual - not sure on 3rd manual (direct).
I went 25K after torching the 2nd gear overrun clutches before the TCC clutch went out. It doesn't have to be fixed. Upgrade and keep the rear adjusted. Shoes should run you about $100 and get a good brand of semi metallic for the front.
 
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