Advance timming or Adjust BISS screw

[maxash0775]

Hey all. When I first bought my gvr4 and had it smoged, the mechanics said the timing was advanced and that he had to adjust it so it would pass smog so of course I said OK. I know I read somewhere that these cars (stock) run better with the timing advanced 5 degrees I believe. When I am at a light or stopped the idle drops to about 800 and the car has more vibration than I like. At 1000 RPM's it is perfect. Should I advance timing or raise the BISS to make RPM 1000 so it is smoother or am I totally off base. Just for the record car runs great this is only due to annoying shaking at 800 rpm.

Thanks as always guys.

 

[fuel]

ideally you want the car idling at 750-800rpms unless you have aftermarket longer duration/bigger lift cams. The vibration is probably another issue such as a broken balance shaft belt or balance shafts out of phase - perhaps the balance shafts have been removed altogether but even then you don't experience that much vibration at idle. Could be one or more of your engine mounts is toast and transferring too much vibration to the chassis - or do you have solid mounts fitted? More info regarding your setup/mods would be great.

 

[beaner]

The ECU assumes ignition timing is 5 degrees BTDC at idle. It has no way to check if it's actually set to that or not, so it's important to properly set it with a timing gun. Having the base timing more or less than 5 degrees will globally skew the actual timing across the power band. The 1g/gvr4 timing map is aggressive as is, so I wouldn't mess with the base value unless you have a good understanding of what you're doing.

The BISS is basically a slave to the ISC. Adjusting the BISS in and out gets the ISC in it's operating range. Using the BISS to adjust idle is wrong. The idle is determined by the ECU.

I don't think it's a good idea to blindly adjust the timing or BISS. These sensors should be set to what the ECU is expecting. Otherwise it's a garbage in garbage out kinda deal.

 

Check the TPS(the black one) sensor on the throttle body, which tells the computer how far open the throttle plate actually is. It can cause pour idle.

The green/black wire will be the sensor ground.

The green/red wire will be a sensor power..

The green/white wire will be the sensor output.

Take a Ohm meter and check it to make sure it's acting proper.

first hook up the sensor ground(green/black), and the sensor power(green/red) it should read from 3.5 - 6.5
if it doesn't then it will need a new one..

Then hook up the Sensor ground(green/black), then hook to the Sensor Output(green/white) for base idle it should be set to 0.48 - 0.52
If it's not loosen up the screws to set it back that way, once it is..Slowly turn the throttle and make sure it going up correctly and not jumping numbers horribly. if it is it's faulting as well..

Hope that helps.. I've learned on my GVR4 not to turn up the Biss screw(if you mess with it check the O-ring) without checking that and having it all set right..And make sure to use a timing light to make sure it's in proper specs.

 

[Specter]

Okay just for my understanding, if i adjust the idle, lets say a 100-200 rpm up or down by adjusting the throttlte cable on the manifold, the throttle plate would be opening/closing a little bit, so what does the tps indicate to the ecu?

 

[fuel]

you DON'T adjust the idle speed by adjusting the throttle cable. You MUST have some SLACK in the throttle cable when the throttle is shut. The ECU is meant to control the base idle at all times with the idle stepper motor and the BISS is there for fine tuning adjustment.

 

[turbowop]

Trying to override the ECU idle control in any way via throttle cable adjustment, BISS screw, etc, is just asking for idle surge issues. It is all controlled by the ECU through movements of the ISC motor. If you want a higher or lower idle RPM, you need to have an Eprom chip programmed specifically for it.

 

[DR1665]

Quoting Specter:
Okay just for my understanding, if i adjust the idle, lets say a 100-200 rpm up or down by adjusting the throttlte cable on the manifold, the throttle plate would be opening/closing a little bit, so what does the tps indicate to the ecu?


If you adjust idle by tensioning the cable, the throttle plate (and therefore, the TPS) will show the ECU the idle is not where it should be. The ECU will probably burn up the ISC trying to compensate or something equally irritating.

The simplest way I've found to deal with the convoluted idle controls on these cars is to delete the ISC and FIAV with a simple blockoff gasket. There's a nice how-to in the How-To section. Once you've disabled the ISC and FIAV, THEN you can adjust idle via BISS without any idle issues. (In fact, my car has never idled better in the 6 weeks since I've done this mod.)

 

[cheekychimp]

Something else you might want to check is if your electrical system is all healthy. I am chasing similar issues at the moment and a few things we have found/done that have helped but not completely resolved the problem are as follows.

(1) Replaced the alternator with a 90 amp one. This is worth checking! Often owners replace alternators with junkyard DSM ones rated at 75 amps
(2) Checked the alignment on the charging belt. Unlikely to move but mine wasn't perfectly in line and spacing it out helped.
(3) Checked and replaced the alternator pulley belt with a new/OEM one. See (2) above as one reason for premature wear.

One other thing I haven't looked at yet but intend to is clutch adjustment. My car idles well in neutral but in parking lots when reversing turning around etc the rpms will also drop with the clutch pushed in. I am wondering whether the clutch could be dragging.