Vehicle Jerks while going downhill

[ledzz1]

Hey guys
Anyone had an issue with vehicle bucking while going downhill on cruise control or going at a slower speed in lower gears with it engaged

Thanks

 

[Intuit]

Check for history codes. Could be lean condition. Are sure this only occurs when cruise is engaged? Can you reproduce the issue without CC?

A prior non-turbo EEC-IV car only did this when the tube running between the exhaust manifold and DPFE sensor (operating the EGR system) was torched. If I disabled EGR (by unplugging the sensor in this case) the issue went away. The old EEC-IV system only produced a trouble code when I unplugged the sensor.

Unknown at the time, there was excessive EGR flow due to clogged cat. (had been running cold for a long time) That's why that tube burnt up. Had to replace it with fuel line instead of vacuum hose.

EGR only opened under the conditions of light throttle or cruise on a warm engine.

The excessive EGR flow caused lean condition.

Lean condition is why it was jerking. Jerking will reverberate off the clutch springs, engine mounts, suspension and turn into bucking.

Check for codes. If your exhaust is excessively lean, an OBD-II car will set a code for it.

If you cannot reproduce this outside of cruise control, then maybe something else is going on.

 

[ledzz1]

Check for history codes. Could be lean condition. Are sure this only occurs when cruise is engaged? Can you reproduce the issue without CC?

A prior non-turbo EEC-IV car only did this when the tube running between the exhaust manifold and DPFE sensor (operating the EGR system) was torched. If I disabled EGR (by unplugging the sensor in this case) the issue went away. The old EEC-IV system only produced a trouble code when I unplugged the sensor.

Unknown at the time, there was excessive EGR flow due to clogged cat. (had been running cold for a long time) That's why that tube burnt up. Had to replace it with fuel line instead of vacuum hose.

EGR only opened under the conditions of light throttle or cruise on a warm engine.

The excessive EGR flow caused lean condition.

Lean condition is why it was jerking. Jerking will reverberate off the clutch springs, engine mounts, suspension and turn into bucking.

Check for codes. If your exhaust is excessively lean, an OBD-II car will set a code for it.

If you cannot reproduce this outside of cruise control, then maybe something else is going on.

Thankyou so much for the response

Scanned it many times with different equipment and no codes at all

Even with my professional garage like reader

Its definitely not running lean.. the tune is set up to run more rich and the spark plugs being darker prove this

Its not been possible to reproduce this just using the throttle alone

Its fine on straights unless in a low gear but that sounds normal

Literally just downhill when would assume the fuel injectors turn off

 

[Intuit]

Must be something with the tune then. How long have you used this tune and how long before this condition developed or was encountered/noticed? Is there an option for 'engine brake' in the tuning? With some performance vehicles, even when completely off the throttle there may still be some combustion occurring as a form of reducing the amount of engine brake.

Side note, to my surprise, it's possible to have a lean condition turn plugs dark and even foul them. Happened with a prior motorcycle. Based on every chart I thought the plugs read rich but my Brother took one look at the plugs and immediately identified lean. I enriched a little and all issues (not the issues we're discussing) were instantly diminished. I'm not saying it's lean. Just something to be aware of.


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