rear leaf spring "s" wrap and unwind hop from torque variation

[schiker]

Do any manual truck guys have these problems?....

I get some truck hop from rear leaf springs "s" wrapping and recoil that can repeat in 3rd gear towing. Today with gooseneck and 600 lbs loaded in middle. I can also get some hop with just heavy stuff in bed of truck like several hundred pounds up to 1700 lbs +/- of landscape stuff so I don't think its a tongue weight issue. I also get something like clutch chatter launching in 2nd sometimes. I can usually corelate the launch chatter to poor fuel. If I lube it with SAE 30 ND and Stanadyne it launches smoother. It has progressively gotten worst with age. I think its the IP getting tired and not being able to precisely control the fuel metering and corrections to metering from PCM feed back at idle. I launch from idle then roll into fuel.

But this 3rd gear hop I am into the throttle and think its more spring wrap issue I can get it to stop but have to clutch and change fueling or down shift and wind it up pretty high then shift again. I am not lugging it either RPM's are 1600 at lowest more like 1700 and can do it a bit higher at bottom of 3rd gear.

Shocks are Belistiens and maybe a year or 2 old less than 40K miles iirc and the truck seems to drive ok otherwise. When I first put them on they didn't stop the launch chatter it stayed correlated to a fuel issue

What do ya'll think it is tired motor mounts, spring shackles, and or springs general aging mostly that allows the motor to move more and gives the PCM and IP a fit trying to compensate and adjust to stay smooth??? (has not been beaten but tows heavy occasionally more so early in life a 5'er)

Kinda related every once in a while I can get into a buck hop and skid rolling through a parking lot in 2nd gear just off idle but have gotten much better at anticipating for this and staying out of it.

 

[3500GMC]

Clutch slippage or u-joints.

 

[schiker]

Clutch has about 60K miles its a Luk OEM. I replaced flywheel with clutch, also slave/throwout and pilot brg with bushing. I have tried launching in 1st with brake applied, slow 3rd gear launch, plus varied launch style and it doesn't seem to repeat. Well 1st use to be completely smooth but now it very suble and has some chatter depending on load. Its very subtle right around idle control. It also does it in reverse. ???? What makes me think its not exactly a mechanical drivetrain issue is the severity varies considerably with fuel. Also sometimes its somewhat variable due to temperature and throughout the day with shut downs and restarts. I have a fuel pressure gauge and its not a supply pressure variation.

Ujounts are OE and are probably sloppy but I don't get any completely worn out driveshaft "tink, ping, or clunk". There does seem to be appreciable normal accumulation of backlash from age though.

 

[6.2 turbo]

Sounds like an injector pump issue,and the clutch hub springs and the rear leafs amplify it,seems like the engine is simply surging .

 

[schiker]

That's what I think too IP fuel metering related. Have heard other guys complain about the buck, hop, and skid occasionally in parking lot driving very low throttle creeping along in 2nd. But I haven't heard many complain about the "clutch chatter" or buck/hop in 3rd. Maybe just never connected other's description of the issue with mine.

 

[bison]

I have the same thing in a 78 dodge powerwagon, only in reverse and when pulling away in low sec gear.
IMO an issue with the clutch,could be broken or weak center springs on the plate's damper or could uneven pressure plate pressure(uneven diaphram)when letting the clutch out.

 

[6.2 turbo]

A set of homemade traction bars would help the loose springy feeling,but the surge might still be there,I made some for my 97 ,and really like them.

 

[schiker]

Been thinking about Cognito's bar plus rear greasable poly bushings. Not any time soon but thinking that would help.

This last tank of fuel I put in 1 qt of SAE 30 ND as it was getting a bit more pronounced. But I did not have any Stanadyne with me and the truck was still pretty surgy so I double dosed the tank when I got home with Stanadyne and it almost took all of it away after it circulated a little while. Still there but light chatter and the truck was more responsive in general. The truck actually runs much better with the Stanadyne too. Probably that much oil lowers Cetane and affects actual timing. So again and again I can corelate a lot of it to "fueling".

The pressure plate was new as well but I do think the weak clutch hub and general age of the truck contributes (motor mounts, rear springs, and bushings etc). Thing about the clutch hub is its really sensitive to load and idle level as in gearing. 2nd and reverse are the worst. When good fuel 1st and 3rd are smooth as can be I would think weak hub would affect other gears some too.

Do the automatics have one more torque support for drivetrain than the manuals? I think I read a blurb once there was ??? The drivetrain is really only held in 3 places: 2 motor mounts that take all the torque and a transmission mount (basically for tail end height). 3 points are always true and don't bind plane wise but I wonder if better torsional drivetrain support would benefit the weak lower end of the block so its twisted less from drivetrain torque? Thinking dragsters usually get an engine mounting plate to torque against.

When the transmission mount is bad it causes excessive vibration to U-joint (especially if U-joint is tight) I wonder if a poly bushed transmission mount might help the clutch engage smoother too? That would be easy to change and not cost too much.