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99 Suburban 'jumpy' or 'stutters' at cruising speeds

D

dka99burb

Member
Messages
650
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4
Location
Reidsville, NC
This is my first 6.5L and my first diesel owned personally. I have only had it for about a month. I drive a 2002 7.3 powerstroke for work right often. Sometimes for days all over the country. Never had to work on it much though.

So, after researching this forum I have tried to test and look at what I could and give you guys the following info so you might be able to help me:
1999 6.5L K2500 Suburban with 257k miles on it.
No EGR.
Heath Diesel PDM with about 3k miles on it.
Vaccum at idle is about 25"
New wastegate solonoid valve from Advance Auto Parts.
I can move the wastegate arm when engine is off with resistance hearing a wistle from the solonoid valve (old one). At idle, I cannot move it.
Turbo doesn't seem to have any endplay to worry about and spins nicely.
Lift pump pushes fuel out of water drain and vent valve very well so I don't think it has an issue. I don't have a pressure gauge on it yet.
Starts and idles just fine.
Oil pressure gauge seems to be 'twitchy'.
Throttle pedal is extremely touchy in my opinion. Feels more like an on or off switch instead of a potentiometer.

So, now on to the current question:
After some driving down the highway at cruising speed (60-70mph) RPMs starts jumping. It gets worse with cruise control as CC tries to keep steady and can't. Actually got pretty harsh the other day.
Can the turbo cause this?
The new solonoid?
Anything I should test?

Oh, and after changing the fuel filter today I drove it for short run. Took off hard on the return trip and got a lot of black smoke from rolling hard throttle all the way to 65mph. In case that makes a difference in thoughts??

I do plan to buy some gauges soon so I can monitor boost, egt, and trans temp. I am now going to add lp pressure to my list. Hope this isn't too much to read but I wanted to make it as clear as possible. I am sure I missed something critical.

Thanks in advance for help and feedback.
 
Ahah!!! Grounds, grounds, grounds its all about them grounds, them grounds gotta be purfect!!!! Them grounds are what my newbie self can do good!!! LOL Welcome DKA99BURB I like Reidsville its nice country. When u get ur Burb fixed can u help me with that woodchuck chuckin wood thing plzzz?????
 
When u get ur Burb fixed can u help me with that woodchuck chuckin wood thing plzzz?????

What?? Dam NC'S:hihi:
 
Check for codes. Get a fuel pressure gauge. Rev the engine, 0-1 PSI is ok revved up. A vacuum indicated a plugged screen or lift pump issues. Check the fuel filter - have one on hand to change it just in case the seals fall apart when you pull it out. Yes you can put the fuel filter back in after inspection if it is good.

Fish-biting can be from lack of fuel to the IP. It is also common for the PMD to cause this. So check the ground on top of the IP as well. As in take it apart and clean it...

The OPS is known to be junk. Oil pressure jumping is usually a bad sender aka OPS. It also controls the lift pump on some years. A bad ground will jump the oil pressure as well.
 
Thanks for the input guys.

I am not sure about the wood chucking thing either...

I replaced the fuel filter yesterday and purged the system of water and air. Ran better this morning on the 20 miles of highway. jerking was not severe like before. Actually starting to think the transmission is a factor. Seemed to be surging about 100-200 rpm like the torque converter kept locking and unlocking?? Any thoughts? It still really confuses the CC.

Thanks again for the suggestions.
 
Sounds like a PMD. A symtom for them is flaking out while on cruise control.
A very long shot could be the brake pedal needs ajusted. If its bumping the light swicth it will unlock the TC.

Also try chuckin some wood on those grounds.
 
Grounds.....I added some more, really I did n it helped my OD problem. I think my ground strap itself was bad cause I cleaned the ground good but the problem only went away when I added a ground from the body to the engine block behind the front wheel.
 
Update on progress...
I don't have much time to work on it so it is going slow. I have ordered and received my a-pillar pod with fuel pressure, boost, and trans temp to help with diagnostics. I have also come to the conclusion that one of my problems is the TC locking and unlocking at almost backward times while cruising is causing some of the issue. It seems to lock-up when under load such as going uphill but unlocks when cruising or going downhill???? I serviced the tranny (fluid and filter) and the fluid was gross. So, I have also purchased the electronics kit suggested in the tranny forum. I plan to install this stuff this weekend along with more fresh fluid and I will let you know how it goes.
My next line of thought if issues persist: 1. TPS needs adjustment. 2.OPS needs replacement. 3. Input and output speed sensors on tranny replacement. 4.grounds, grounds, grounds. 5.Sell it?
If anyone has any suggestions or comments it is all appreciated.
Thanks.
 
It's normal for the TCC to unlock on downhills, the rpm's will drop to idle if your foot is off the APP.

If grounds, air or pmd are ruled out it could be in the IP, could be the OS needs an adjustment.
 
When you lift off the throttle the TCC will unlock. Normal.

Any codes in the computer?
 
I am not talking about it unlocking to the point of idle. It does that just fine. What I am talking about is cruising speeds on flat ground, steady throttle, the rpms actually rise about 300rpm. It only makes that last 200-300rpm drop when I give more throttle on a rise in the highway. Once leveled out again it feels a lot like a downshift from overdrive but based upon reading there should be a larger change in rpm if it was making a real downshift.

There is no check engine light. I have never checked for tranny codes before. I will have to look into how to do this.

The APP makes sense to me. Does anyone have directions or a link to how to do an adjustment?

If it is cruising at lower speeds without issue would it still need an OS adjustment?

I really appreciate all of the feedback. I will be checking grounds while working on it this weekend!
 
dka99burb said:
I am not talking about it unlocking to the point of idle. It does that just fine. What I am talking about is cruising speeds on flat ground, steady throttle, the rpms actually rise about 300rpm. It only makes that last 200-300rpm drop when I give more throttle on a rise in the highway. Once leveled out again it feels a lot like a downshift from overdrive but based upon reading there should be a larger change in rpm if it was making a real downshift.

There is no check engine light. I have never checked for tranny codes before. I will have to look into how to do this.

The APP makes sense to me. Does anyone have directions or a link to how to do an adjustment?

If it is cruising at lower speeds without issue would it still need an OS adjustment?

I really appreciate all of the feedback. I will be checking grounds while working on it this weekend!
Click to expand...

OS is likely OK, I thought you had a sensitive throttle.

If you gain 300 rpm and no change in mph then it is trans related.
 
GM computers drop out of lockup for lots of reasons. This RPM change indicates TCC is unlocking. This could be due to engine miss or any other sensor or reading out of normal range. You will need a scanner to read codes on a 99. Trans codes do not turn on a service 'engine' soon light as it isn't emmisions related.
 
I installed the tranny electronics kit last night. Test drove today. Still doing the wierd TCC lockup thing but otherwise driving did seem a little more robust. Definitely likes more skinny pedal to lockup (accelerating). Unlocks when you back off for cruising speed. Not all the way off, just not accelerating.
I tried to check codes with my cheap codescout reader and all it seems to pull is emissions codes. I have a buddy who is going to bring me a better reader tomorrow.
So, I now plan to replace and add some grounds straps to insure grounding is not the problem as well as replace the OPS if I find one local. Hopefully there is a code that will help.
Does anyone know where I can find instructions on adjusting/calibrating the APP and OS in case it comes to that?
Thanks Again
 
I mentioned the OS because I thought you had a touchy APP. It isn't the OS.

You need a Bi directional scanner to read the mm3 at idle fuel rate. Then you manually adjust the OS.
 
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