304 Deaccelaration idle issue - 355 VP auto

[vp_exec]

Boys I have a gremlin with my injected 355 VP.

I get no engine light.
Upon deaccelaration off throttle the tacho drops below 1000rpm, when I slow to say 30kmh the dash lights flicker - she wants to stall. Upon stopping (having used both pedals to keep it alive) idle resumes as normal.

Have swapped tbodies, cleaned IAC and TPS, new plugs, reset ECU by battery disconnect etc..no better.

I admit very infrequently I lose a speedo reading.
Also it infrequently surges the tacho reading up and down, upon 100% deaccelaration, say around 100kmh (whilst lockup conv) so maybe VSS?

I have avoided the tbody idle screw as a last resort but have had a suggestion to do this, and a procedure outlining exactly how do this properly. Should I, or do u have another idea?

Fire away...

 

[vp_exec]

For added info, I got car tuned by HPF couple of years ago in current guise, trans, big conv, exhaust n gears.

 

[Darren_L]

the ECM needs a speed sensor input in order to determine IAC position. When my speed sensor spat the dummy, I had to keep tapping the throttle when I stopped at intersections etc until the idle settled or it would stall. As soon as I replaced the speed sensor, problem was gone.
If your speedo is dropping out intermittently then that's the first thing I'd be sorting out.

 

[GTP 85]

speedo problem 'could be' the little gear in the dash. could also be a speed sensor fault?

The return to idle can be adjusted in the tune up with the IAC steps. I had that issue with my cammed VS. The car had to be fine tuned both hot and cold to ensure it was spot on and didnt hunt or stall

A tad of fine tuning with the throttle screw wont do any harm, i did it on mine. the return to idle wasnt quite perfect and i cbf taking it back to the tuner to just did a quarter turn on the tb screw... fixed

 

[vp_exec]

I do need to keep tapping throttle, sounds like my first port of call then cheers.
Genuine best or is there a suitable aftermarket VSS I should consider?

 

[Darren_L]

I've only ever used genuine speed sensors, but if there are aftermarket sensors available, they might be worth a try because the genuine ones aren't cheap.

 

[vp_exec]

If I remember rightly though, VN-VP ECU gets signal from dash. VR and later ECU signal comes from VSS direct.
May check readings first.

 

[Warranty Void]

There is an issue with speedo that is related to reversing light not working, I had this a while back and did speed sensor etc first... Check your fuse, if it's not blown then discount reversing light issue. (if it is then check trailer plug loom)

 

[Darren_L]

VN-VP still use a gearbox mounted speed sensor. However the speed sensor output goes via the speedo circuitry which includes a divide x5 circuit. It then outputs the modified signal to the ECM. VR speed sensor output goes direct to ECM (and speedo)

So in a nutshell, VN-VR all use a gearbox mounted speed sensor. And I'm fairly sure they all use the same sensor. Just check which sensor applies to your model when you order it. It could be a problem in your speedo circuit, which means the speed sensor signal isn't making to the ECM intermittently, but unlikely. Speed sensors usually are the culprit. Also make sure the plastic drive pin from the speed sensor adapter (gearbox end) isn't twisted or broken.

 

[vp_exec]

Good advice - cheers boys.