surge confusion.... please help

[MkIII FTW]

Hey guys, I've looked on the forum for answers but can't seem to get a straight answer. My issue began about a week ago. I went outside to crank my Supra and it started fine with no hitches. About 2 hours later I go outside to take it for a spin and it begins this horrible surge. It will rev between 800 to about 1700 rpm and will not stop. If I tap the throttle it will sputter likes its going to die and keep surging. My thoughts were iscv because I blocked the hoses from the accordion pipe and metal pipe going to the iscv and it ran like nothing ever happened. My brother has has a Supra that runs fine so I took his iscv and he took mine. Low and behold my iscv ran no problem on his car and my car continues to run like crap even with a working iscv. I then checked TPS....no codes and it is set properly. I thought vacuum leak. When I sprayed the engine bay to attempt to pinpoint one, none were to be found. It seems to run fine with the hose blocked off but this issue has me totally stomped. Can any of you please please give insight so I can get to the bottom of this fiasco??!

 

[ghostmkiiiturbo]

My first guess would to the ISC valve too. My 87 turbo did the same thing immediately after I purchased it. I ordered a good used one on eBay for $25 cleaned the check valve and installed. 7m idled perfectly after install. I assume you have the stock intake manifold?

 

[super51fan]

Have you unpluged the connector to the ISCV. If it does not surge then you need to figure out why the ecu is wanting to change your idle. I am sure the "smart guy" will make some wise cracks but offer no real help except insults.

 

[supraguy@aol]

LMAO.
You know it's bad when people are 'waiting' to get web-bashed by "him", and everyone knows who you mean.

Im not sure I understand what you're saying aboutyour iscv test; It sounds like you blocked the hose running from the iscv to an air source, which means it's effectively not there- a dead end.
Then you tested your brother's iscv...with the hose to the accordian STILL sealed? If so, then of course you'd see no difference.

 

[MkIII FTW]

No when I tried my brother's iscv the accordion wasnt sealed. I reconnected it as it should have been and it still surged. Basically we swapped iscv's to see what happened.

 

[jetjock]

...I blocked the hoses from the accordion pipe and metal pipe going to the iscv and it ran like nothing ever happened. ...

That's a clue but not in the way you thought. I'd offer more (insults are tempting) to point you in the right direction but the above should be enough. It's better advice than what you've gotten thus far...

 

[MkIII FTW]

Well I don't really care about the insults I'd much rather just get my car running. So I did a sanity check and just changed the ECU (once again to see what happened)...the results are the same. I think I will be checking continuity of the harness next. Just another guess.

 

[3p141592654]

Can you hear your ISCV moving to the home position when you turn off the ignition? Interestingly, a disconnected ISCV will set no codes.

 

[MkIII FTW]

Can you hear your ISCV moving to the home position when you turn off the ignition? Interestingly, a disconnected ISCV will set no codes.

It sounds like it is moving back to the home position. I will listen to it again to verify.

 

[3p141592654]

It is likely your surge is linked to this.
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?Section=FI&P=125

 

[MkIII FTW]

Hmm that's a good guess. Wouldn't the computer throw code 51 for the tps however?

 

[3p141592654]

Depends what's wrong, but assuming your problem is that the ISCV is wide open all the time, then the throttle and TPS will be working properly and set no codes. You need to check if the ISCV is moving under ECU control.

 

[MkIII FTW]

I was able to verify that the iscv is not moving under the control of the ecu. So its never getting the signal to close.

 

[jetjock]

How did you verify that? Because, ya know, that shouldn't cause what you're seeing...

 

[MkIII FTW]

But if the iscv never gets input via the ecu would it not do the same thing?

 

[jetjock]

Well, in theory a frozen in place ISCV results in nothing more than a fixed amount of metered bypass air. Assuming everything else is OK the result is a fixed rpm, albeit perhaps not the correct one. It's no different than if it was unplugged. Many here think the ISC system will chase its tail at the slightest provocation but that'll only happen when something else causes it to do so. And aren't you curious why the engine continues to idle well with the idle bypass path blocked? I know I'd be...

 

[MkIII FTW]

You make a very valid point. My theory as to why it idled once the bypass path was blocked was because the stepper motor controlling the iscv is defaulted open. So the excess air is being allowed to enter via the iscv because its open. By blocking off that path it would allow the engine to run as if the iscv was closed assuming that is how the iscv functions. I understood it as a system that would actuate via ecu input to control air flowing through it to sustain an idle..I could be wrong but that's just my thought.

 

[jetjock]

The main point of the ISCV is not to keep the idle stable as much as it is to provide different target speeds for various conditions. After all carbs and fixed idle systems of old held speed well enough.

If you blocked the path where was the air coming from? On a 7M if the idle path is blocked the only bypass air to be had, assuming the TB is set up properly and everything else is working, would be the PCV system and it doesn't supply much. Something to think about.

That said you should still check the TPS. Go into diag mode and look for a code 51 with the pedal slightly pressed. If you get one that part of the TPS is working. It may still need to be set correctly but it's working and should not cause surging.

 

[3p141592654]

How did you validate the ISCV is not moving? I missed that note about it idling fine with the ISCV hose plugged, so it seems you have a metered air leak. Have you checked that the throttle plate is fully closing. If the ISCV is wide open it will idle fast enough to exceed 1600 rpm with a warm engine, which will then activate fuel cut, which is where I was going originally. Course you could have both problems...

 

[jetjock]

He could indeed. I have to say I've ever seen that happen though. Didn't think there was enough conductance in the path to get that high. Close but no. I know because doing valve unplugged restarts is how I get fast idle for AC and emissions work. Beats trying to jimmy the throttle. Done it many times on probably half dozen cars without ever seeing cut. Makes sense it could happen though.