| Author |
Message |
everyhumanvw
Samba Member
Joined: November 16, 2018
Posts: 51
Location: mICHIGAN
|
 Posted: Sat May 03, 2025 3:42 pm Post subject: 91 Syncro - bypass ICU to test - erratic idle up to 2000rpm |
 |
|
Just got running again and new issue reared its head. Think pior no start was bad ground to ECU; ground repaired and ECU replaced.
Idle is newly erratic and after research want to bypass idle control unit - at least to test. Idle at 1000 rpm and then creeps up fast to about 2,000. Warmed up or cold - will come to stop light and there it goes up and up. Parked after drive and idle stayed around 1000 rpm for 10 minutes; then bumped gas pedal and it started creeping up again.
After research here, found it is fine to bypass ICU and that the two plugs to ICU fit together to bypass. But, may have to give it a little gas on cold starts or possibly when running AC or other load.
Should idle stabilizer valve stay connected as is when bypassing ICU for test? Anything else suggested when bypassing idle control unit? Also read to confirm ICU solenoid not causing short. Cheers for insights on this! |
|
| Back to top |
|
 |
RawUmber
Samba Member
Joined: June 23, 2019
Posts: 297
|
| Posted: Wed May 07, 2025 2:01 pm Post subject: Re: 91 Syncro - bypass ICU to test - erratic idle up to 2000rpm |
|
|
It looks like you're mixing information about different idle systems. Your '91 is likely Digifant and uses an Idle Stabilization Control Unit (ISCU) and Idle Stabilization Valve (ISV) combo. This type stabilizes idle by supplying additional air to the mixture as needed. Earlier Vanagons used Digital Idle Stabilization (DIS), which stabilizes idle by advancing ignition as needed. Unlike yours, this type has the two plugs that can be connected together to bypass the DIS unit.
With an ISCU/ISV, an easy way to disable the idle system is to unplug the harness connector from the ISV. An original equipment ISV will close (no air) when disconnected like this, so you may need to use your pedal to manually maintain idle. If your ISCU is really bad and somehow affecting the input signals it shares with the ECU, it's even more conclusive to eliminate the ISCU by unplugging it instead of the ISV, though a little less convenient. |
|
| Back to top |
|
|
1988M5
Samba Member
Joined: January 23, 2016
Posts: 822
Location: Tucson, AZ
|
| Posted: Wed May 07, 2025 2:06 pm Post subject: Re: 91 Syncro - bypass ICU to test - erratic idle up to 2000rpm |
|
|
Sounds like a worn out or sticky throttle body to me. Easy check, reach in with your hand a slam the throttle closed, then try to slowly open it. If it's kinda stuck bingo.
BK
_________________
1991 tin top GL
2002 Winnebago Vista. VW VR6 24V Eurovan front clip powered class C 21' RV.
Some BMWs. |
|
| Back to top |
|
|
everyhumanvw
Samba Member
Joined: November 16, 2018
Posts: 51
Location: mICHIGAN
|
| Posted: Mon May 12, 2025 4:39 pm Post subject: Re: 91 Syncro - bypass ICU to test - erratic idle up to 2000rpm |
|
|
| 1988M5 wrote: |
Sounds like a worn out or sticky throttle body to me. Easy check, reach in with your hand a slam the throttle closed, then try to slowly open it. If it's kinda stuck bingo.
BK |
Thank you. Just before extended period of no start, replaced throttle cable, and throttle body with Gowesty's custom. After install by shop, it stalled going around block - shop was now closed. I turned the idle screw up. It started. Took it on highway, brought it home, and did not start again for 18 mos. First couple times it was really rough and would die after seconds. Finally found another shop to work on besides me - turned out to be bad ground to ECU. They also said they adjusted something else on throttle body. It clicks at wide open and return to idle. That's about all I know. Bad TPS already? I think GW throttle body was connected correctly to mounted connector directly in front of it. GW does warn about mixing up the throttle body connector with another and frying TPS I guess. |
|
| Back to top |
|
|
everyhumanvw
Samba Member
Joined: November 16, 2018
Posts: 51
Location: mICHIGAN
|
| Posted: Mon May 12, 2025 4:48 pm Post subject: Re: 91 Syncro - bypass ICU to test - erratic idle up to 2000rpm |
|
|
| RawUmber wrote: |
With an ISCU/ISV, an easy way to disable the idle system is to unplug the harness connector from the ISV. An original equipment ISV will close (no air) when disconnected like this, so you may need to use your pedal to manually maintain idle. If your ISCU is really bad and somehow affecting the input signals it shares with the ECU, it's even more conclusive to eliminate the ISCU by unplugging it instead of the ISV, though a little less convenient. |
Thank you. I thought I read the idle control unit behind right tail light could connect the two connectors together. Did not realize this is only on earlier models. But, I can disconnect it regardless to test if idle steadies. If the idle valve is left connected, will it operate at all when the control unit is disconnected? |
|
| Back to top |
|
|
|
