Kangarooing at low RPM

[MADMAX]

I recently started experiencing the car bucking/kangarooing at low speed - nothing too dramatic although a quick clutch dump prevents anything too ugly happening.
It happens at low rpm when the engine is pretty much unloaded - you know those moments when you're cuising at 20mph down the high street, everything is sweet and ticking over nicely then you need the tinyist touch more/less throttle which results in the start of bucking/kangarooing, so you have to dip the clutch.
I'm sure I'm not the only person that's experienced this and I'm sure I read about this before but never paid much attention because I've never suffered with the problem.
Now I am suffering, not sure why or what I've done recently to bring it on.

Another symptom I noticed today (doing static emissions tests in the garage) when I tried holding the throttle with engine at 1500rpm, the engine oscillated between 1200 and 1500 rpm every 5 seconds (my foot was absolutely frozen solid, perfectly still) - eventually the engine settling at 1500rpm after say 60seconds or so. The engine was warm and up to operating temp.

Any ideas?
I'm thinking alignment of the two throttle bodies with each other could be contributing to this...

 

[1.8t]

708 cam? If so, it is nothing but cam surge. Mine does the same thing as will all other 708 cars.

Your other symptom sounds like your ecu dancing with the fuel mixture or something.

 

[BlackSnake99]

They all do it.

 

[Mopar Steve]

Easiest simple fix, ( do all the easy stuff first) Clean the throttle bodies.

 

[alwayscode390]

Do you have stock gears?

Mine does it , but I have 3.73 gears.

What type of HP increase do people with the 708 cam get? It is mainly for the lumpy idle? My car has a pretty wicked idle and Im not sure if the previous owner did a cam swap ---

 

[MADMAX]

Not quite stock - okay, not even close to stock - the issue is that kangaroo only just started - she didn't do it before.
She's a 96 GTS so that's a stock 708 cam.
I've changed the rear gear to 3.54 with an OSG diff.
Lots of head work over winter too - you might remember my post.
SCT tune.

I'll take a closer look at the throttle boddies - they were fitted clean after engine work during Winter.

"ECU dancing with the fuel mixture" - yeh, sound plausable - but why and what's the fix?

 

[vancouver-gts]

MAdmax, it could be a fuel trim issue. A scanner wiith data logging could check LTFT B1 and B2 parameters at idle. If the numbers are over +- 25 ,you may have an issue.
The two, B1 B2 numbers should be close. if you have a high reading like -20 or-30, it means the ECU leaning out the mixture too far. Setting throttle body sync is the easy one but doubt that's your problem.
If the MAF sensor acting up, you can unplug it and see how the car behaves. Again, you need a data logger hooked up to read g/s numbers, checking idle, mid and WOT readings to establish its operation.
I had an issue recently with my 99.Turned out that one [#5 cyl] plug boot got cut by the metal shroud [ the one inserted into the cyl head to protect the boot from heat] and was arcing to it at low rpm's . It caused occasional bucking at low revs until higher rpms reached. Dealer wants $50 for just a single wire,so replaced it with an Accel wire temporarily for $15. Should have asked for a single cable [ same grey colour ] for any other Chrysler product, what the cost would have been ,lol.

 

[Dom426h]

Sounds normal to me. She likes to go or slow. Cruising @ part throttle neither accel/deacll leaves evertthing in the drivtrain unloaded/floating exploiting the natural slack(play/slop) in every connection from the trans to the wheels .
Also, the more you screw with the snapthrottleresponse(how quickly she goes to deacel when you let off the throttle) in the tune, the more the slop can come to life.

Steve,
My original TB's have about 60K miles on em. Shine a flahlight in there and they look damn near new.... Same with the I.Manifold
I remember back in my Miata days. It was common to regularly clean the TB & Intake manifold as the EGR system would gunk everything up. Viper has no EGR though...
I have heard of Vipers IAC Valve getting sticky and causing idle issues but i think thats a diff isue from the OP

 

[Will at RSI]

It's a driveability issue in your tune.

 

[Frank Parise]

Before reading all of the above comments I would've told you it was your clutch / pressure plate

 

[DrumrBoy]

Not quite stock - okay, not even close to stock - the issue is that kangaroo only just started - she didn't do it before.
She's a 96 GTS so that's a stock 708 cam.
I've changed the rear gear to 3.54 with an OSG diff.
Lots of head work over winter too - you might remember my post.

THIS. If its as bad as mine was before it was addressed it'll slap your u-joints around in a bad way. You can set the RPM above which the ECU completely cuts fuel to the injectors when the throttle position is decreased. Its basically for spirited driving.....if you're cranking at 4000 rpm and back off the throttle, the ECU cuts fuel because it assumes you want to slow down. It also prevents backfiring caused by unburned fuel in the exhaust. (Personally I like the flames that come out when you back off at 4000 RPM ;-)

When you're doing 4000 rpm, tipping in the throttle and restoring fuel to the injectors is pretty smooth.....but when you're going slow, it bucks fiercly. This RPM level should be set ABOVE normal cruising levels.

My bet is that this RPM level is set too low (maybe 1500 or something) so if you're in 30 mph traffic in second gear....or even doing a steady 65 in 5th - you're above the threshold and the car will cut fuel as soon as you back off the throttle just a tad.

I have an AEM so I have no idea how to accomplish it with an SCT tune, but look for that setting and put it at 2500 or 3000. That way, the fuel cut-off won't bite you when just cruising.
SCT tune.

I'll take a closer look at the throttle boddies - they were fitted clean after engine work during Winter.

"ECU dancing with the fuel mixture" - yeh, sound plausable - but why and what's the fix?

 

[DrumrBoy]

MAdmax, it could be a fuel trim issue. A scanner wiith data logging could check LTFT B1 and B2 parameters at idle. If the numbers are over +- 25 ,you may have an issue.
The two, B1 B2 numbers should be close. if you have a high reading like -20 or-30, it means the ECU leaning out the mixture too far. Setting throttle body sync is the easy one but doubt that's your problem.
If the MAF sensor acting up, you can unplug it and see how the car behaves. Again, you need a data logger hooked up to read g/s numbers, checking idle, mid and WOT readings to establish its operation.
I had an issue recently with my 99.Turned out that one [#5 cyl] plug boot got cut by the metal shroud [ the one inserted into the cyl head to protect the boot from heat] and was arcing to it at low rpm's . It caused occasional bucking at low revs until higher rpms reached. Dealer wants $50 for just a single wire,so replaced it with an Accel wire temporarily for $15. Should have asked for a single cable [ same grey colour ] for any other Chrysler product, what the cost would have been ,lol.

Sorry, I meant to grab this quote for my "THIS" comment......

 

[Jack B]

Obviously this could be several issues, but,one common problem is as follows:

1. This problem is very common in a MAP range of 50 to 70 kPA and 1500 to 2000 rpm's.

2. It can happen on a stock car, but, more often on a car with mods - mine did it for a long time.

3. What is happening is the mods have changed the volumetric efficiency (VE) over the rpm/load range described above.

4. Simply put the car's a/f is fixed during closed loop at something close to 14.7. The a/f is kept near that value by the combination of a base injector pulse width table and the front O2 sensors/PCM creating the STFT's - this is over simplification, but, close. In the oem case the table cells flow uniformly because the VE is known and the cells are based upon those values. When you install mods, the VE for certain cells change, therefore, the base injector pulse widths should be changed. That range (light load) of 50 to 70 kPA is typically where the the cell values conflict with the new VE. Unfortunately, the VE for cells in the cruise range (light load) change along with the wot cells

5. I had that same buck and finally logged the STFT's with the LTFT's turned off. Unless you set up a table (pulse width/load/rpm) and log for a long period of time you will not see the full picture with a scan tool. In my case I had several cells in the above load/rpm range that had STFT's fight each other. As an example my car had two adjacent cells that were +10% and -10% respectively. The problem was, those cells were right in the middle of my cruise range, therefore, the buck

6. If this is the problem the only way to fix it is to:
a. Turn off the LTFT with something like SCT.
b. With a OBD software that allows the use of table, creat a table with STFT versus rpm versus load.
c, Start the log and take a ride for about an hour and do closed loop driving over the complete range of rpms and loads.
d. Use the STFT values to modify the cells that are out of whack. I adjusted mine until all the cells were under 5%. That took three logs.

As a caveat - this may not be the problem.

 

[MADMAX]

Sorry for late come back on the above - very much appreciate the comments.
Looks like I've gotta do a bit of investigation work.
In the meantime I'll just have to drive her harder - only to avoid bucking of course.....

 

[GTSHolgi]

Madmax,

Any updates on your issue? I am just in the process of learning how PCM works and found this thread very interesting. And now I am wondering, what you did to (hopefully) get rid of the bucking?!

Cheers,

Holger

 

[Fatboy 18]

Ade is sorting out a Diff leak at the moment and When I spoke to him during the week the car was in the garage. We have a big track day coming up Sept 14th so the car should be ready by then Not sure the Bucking issue has been fixed yet as the Diff leak has taken Priority.

Im sure Ade will update you soon

 

[GTSHolgi]

Sorry to hear that another issue came up, hope he'll get it fixed on time!

Holger

 

[MADMAX]

Madmax,

Any updates on your issue? I am just in the process of learning how PCM works and found this thread very interesting. And now I am wondering, what you did to (hopefully) get rid of the bucking?!

Cheers,

Holger

Ade is sorting out a Diff leak at the moment and When I spoke to him during the week the car was in the garage. We have a big track day coming up Sept 14th so the car should be ready by then Not sure the Bucking issue has been fixed yet as the Diff leak has taken Priority.

Im sure Ade will update you soon

Cheers Mark.

Hi GTSHolgi - Good to meet a new member.
Not sure the following is gonna help you much though.

Time to bring 'closure' on this issue - my Kangarooing has been cured.
Fatboy18's unwittingly hit the nail on the head when he mentioned my diff oil leak, which is a symptom of the fault.

Turns out my OSG diff output shafts were not securely fixed allowing slap/play in the final drive line – this gave the impression (at idle throttle when the car was moving) that it was an engine/ignition issue due to the kangarooing.
You can see from this bulletin that the output shafts are secured with M8 bolts on an OSG diff.
http://www.osgiken.net/upload/d41.pdf
Basically, these bolts were too long (by about 1.5mm) - whilst they were fixed tight, they were not clamping the output shafts 'tight' into the diff internals so the output shafts had a small amount of float allowing them to move "in and out" of the diff.
The overlength bolts were bottoming-out inside the diff instead of tightening against the output shafts.

The oil leak was a symptom of the fault where oil would track up the bolts and out.

I’ve remade the couplings so they do a proper job from now on.
No more slap/play/float whatever you wanna call it and no more kangarooing - it's a different car.

Hope this short writeup helps anyone with problems in the future, especially if you're going for an OSG diff.

 

[Fatboy 18]

Great stuff Ade, how the hell did you find the problem? Or did the Garage find it?

 

[GTSHolgi]

Ade,

Glad you could fix the bucking!

That's really interesting! A completely different "root cause" than anticipated earlier. I really appreciate this forum as it is the best resource for anything related to Viper!!!

Holger

 

[MADMAX]

Ade,

Glad you could fix the bucking!

That's really interesting! A completely different "root cause" than anticipated earlier. I really appreciate this forum as it is the best resource for anything related to Viper!!!

Holger

You're welcome Holger.

Great stuff Ade, how the hell did you find the problem? Or did the Garage find it?

The garage spotted it during my MOT last month.
When the car was over the maintenance pit the mechanic noticed my new zorst had burnt oil streaks all over it near the diff.
Closer inspection identified the diff oil leak.
As my new zorst passes DIRECTLY below the point the diff was leaking oil, all oil leakage was landing on the hot zorst and burning off - no evidence, until you look at the zorst!
This meant I wasn't getting previous symptoms like 'patches of oil on the garage floor'.

 

[Fatboy 18]

OK Lets go a little deeper, so you found oil on the Zorst, this was coming out of the rear Diff cover? or from the side oil seals where the Half shafts join? Im still trying to work out how you found a thread length too long?