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.


.

 

[ledzz1]

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.


.

Thanks again
The tunes been run for about 2 years and it was fine until a few months back then started doing this
Monitoring this last night you may be onto something
When viewing my afr.. when using the throttle and letting off it reads 15.5 max generally
When the bucking happens it jumps upto 18 or even 19
Thats the main thing so may be the lean part

There is no engine braking part of the tune however

My rmm was over torqued by some idiot at the garage around the time it started happening
It was at least 100 ft lbs
Didnt think that was relevant though

 

[Intuit]

If an engine mount is broke, that could be it.

Securely setup your cell phone to record the engine while you hop in and slip the clutch with the e-brake engaged. Place wheel chocks on multiple wheels. Don't want the vehicle to move. Look for an unusual amount of engine movement.

The lean must be expected condition otherwise I'd think it would set a code? Are there any corresponding fluctuations in throttle input from the ECU while in Cruise Control or is it steady? (corresponding doesn't necessarily indicate causality) Any fluctuations from the speed sensor? Provided you don't have changes to hardware that require the tune, can you restore it to original programming and then try to reproduce? O2 responding sluggish? (that should still set a code)

 

[ledzz1]

If an engine mount is broke, that could be it.

Securely setup your cell phone to record the engine while you hop in and slip the clutch with the e-brake engaged. Place wheel chocks on multiple wheels. Don't want the vehicle to move. Look for an unusual amount of engine movement.

The lean must be expected condition otherwise I'd think it would set a code? Are there any corresponding fluctuations in throttle input from the ECU while in Cruise Control or is it steady? (corresponding doesn't necessarily indicate causality) Any fluctuations from the speed sensor? Provided you don't have changes to hardware that require the tune, can you restore it to original programming and then try to reproduce? O2 responding sluggish? (that should still set a code)

You are a legend thankyou so much

Tried everything you mentioned and all seemed ok

But in the end it was the rmm

The stupid mechanic who I said specifically to him not to over torque it

Overtorqued it by at least twice the amount as recommended

Wasnt able to undo it.. had to get a big bloke to undo this and even he really struggled

All resolved
Thankyou so much