ISC is disconnected but I still have idle surge?

[N2aquatix]

Okay, long story short, I have a 90 laser RS-T. It has the usual idle surge problems that could be expected for this car so I disconnected the ISC and it still surges and also idles very rich and hits fuel cut at 4000 rpms. But, it only does all of this after it reaches normal operating temperature. Until then it runs fine, in fact it runs great. Until it warms up it will idle perfectly and pull very strong with no fuel cut at all. I'm thinking it could be a bad O2 sensor. Any ideas? Has anyone ever heard of the O2 sensor causing this? I've read all of the idle surge threads and none of the ones that I found mentioned the O2 sensor specifically. Any help would be appreciated.

 

[jrohner]

To put it simple: If your idle surges it is because your idle is too fast.

The ISC would not create or cure idle surge (unless it is stuck in the far open position). The reason the idle goes up and down is because the ECU cuts fuel above a certain rpm when the throttle is closed, this is because it thinks you are decelerating so it is trying to save you fuel. This is called rolling fuel cut.

Why it doesn't do it until the engine is warm:
1. There is a map in the ecu and it says what the minimum rpm is to use rolling fuel cut, based on engine coolant temperature.
2. Idle speed is usually lower when your engine is cold -- if your FIAV (fast idle air valve) and/or ISC are not working correctly. The FIAV is a valve that is open when the coolant is cold to help raise the idle speed, and it is supposed to close when the coolant is hot.

 

[N2aquatix]

That's a relevant idea. I haven't tried adjusting the BISS because one of the previous owners filled that part of the TB with some sort of goop so in order for me to mess with it I'll have to drill through the goop and find the screw! OUCH!!! But that still doesn't explain the fuel-cut problem that occurs only after the engine is at operating temperature. That's what had me thinking that it might be the O2 sensor, maybe cutting the fuel when it goes into closed-loop mode and thinks that it's not getting enough fuel. Thanks for your input. She's totally drivable, just not very raceable until I can sort this out.

 

[DSMopar]

Your ECU like many OEM ECU's does not have closed loop capability at WOT... so no the o2 would not cause fuel cut in any sort of way. If you have 'goop' (probably epoxy) over the BISS I'm going to bet they lost the BISS, and thus the reason your idle is too high. As far as fuel cut... are you sure it's fuel cut? And if it is, time to do a BLT...

 

[N2aquatix]

By BLT I'm assuming you mean Boost Leak Test? You may have a point there. A boost leak would also translate into a vacuum leak at idle, right? If that's the case it could conceivably be the source of both problems. I'll check it out in the next few days. BTW, what's the best way to go about checking all of that. I'm guessing that I'll have to check every component from the turbo all the way to the cylinder head. Thanks for the input.

 

[dylnwit]

Boost leak testing info.
how to run the test
http://www.dsmtuners.com/forums/art...-boost-leak-test-how-boostpro-net-tester.html
build your own tester
IC Pipe Tester

 

[jrohner]

Well how much boost are you running? Do you have a real boost gauge?

 

[killerwhale]

Here are the steps I have taken:

Step 1: Check throttle cable (it's behind your throttle body) Make sure it is not too lose (but don't over tighten it )

Step 2: Clean out your throttle body ($4CAD carb cleaner)

Step 3: Check your battery connectors, make sure they are tight & clean. Replace connectors & wires if necessary.

Step 4: Replace your air filter (cost me $25CAD for the filter @ Lordco)

Step 5: Replace your o2 sensor (cost me $95CAD for the part @ Lordco)

Step 5a: Adjust your base idle screw accordingly.
**note** be careful not to break this screw... it is plastic, and to order a new one is $7.00 + shipping

So, I am currently on Step 7 (below). My ISC arrived today and I'm going to put it in later when my engine cools down. Currently, my car is no longer surging as bad as it was (anywhere inbetween +-500 to +-1500). The idle will sit at 800 (It should be 850 but I'm waiting for my new BIS before I start adjusting things). However, when I take the car out of gear the idle drops to just above 500 and quickly rises to 800.

Step 7: Replace your Idle Speed Control & (cost me $131USD + shipping)

Hopefully this fixes it

Good luck

edit:: BTW, my car was running rich until I replaced the o2 sensor, but I did steps 1-5 all in the same day.

 

[killerwhale]

Ehh, I couldn't wait so I went & opened up the hood for about a half hour until the engine cooled down a bit and replaced the ISC...

Anyway, I started the car and the idle went up to 1500rpms. I adjusted the BIS and drove around for a bit but when I got home the idle was jumping from 850 - ~500.

Next thing on my list is checking for a boost leak.

edit: actually I read somewhere that a missing / broken BIS could cause this problem, and my BIS is missing half of the top, so who knows.

 

[N2aquatix]

Sorry I haven't been on-line for a while. Thanks a lot everybody. It's always nice to have friends who can lend advice from experience. DSMers are the best in my opinion. I've loved these cars since they first came out. This is actually my third DSM. I always wanted a 1990 model and I finally got one but like any 20 year old car it needs to be gone over thoroughly so that's what I'm doing. To answer a few questions, I have no idea how much boost I'm running because I don't have a real boost gauge yet but the car is totally stock. I may have a boost leak because it doesn't pull as strong as it did a few months ago. It used to pull all the way to about 6-7K rpms. Then it started leveling out at about 5k rpms. Now it stalls at about 4500 rpms. It also runs extremely rich when idling. I need to go out there and get my hands dirty but at least now I have some idea where to start. Thanks again.

 

[killerwhale]

Taken from Terry's Talon Troubleshooting:

#8 Ok, so now what?

* In discussing this article with a friend, he suggested that an idle surge might also result if an injector got stuck open. The catch is, this situation would affect driving performance as well as idle. This can be easily checked by inspecting the spark plugs. One of the plugs will show up as fuel fouled or at least very black from a rich mix, while the others would show signs of running very lean (white). If a bottle of injector cleaner doesn't solve the problem, show the dealer the condition of your plugs and get him to replace the injector under the emissions warranty.

Link: Terry's Talon Troubleshooting Tips #3 - Fast Idle (aka. Idle Surge)

 

[olmytsi]

You must manually close the isc all the way in and then set the idle with the biss screw. If it's open when you disconnect it, it will let air in and the idle surge will remain.

You have to take the isc off the car, undo the small screws and manually close the thing that extends (extend it all the way out).

I used to have idle surge issues, did this 2 years ago and my idle has been rock steady with no isc!!!!!

 

[N2aquatix]

Killer Whale, I already checked the plugs and all of them look the same. They all look fairly normal. The burn looks like it's in the stoich region (neither rich nor lean). It only runs rich at idle. When I push it, it runs normal and clean, with no detonation. OLMYTSI may have a point in that I didn't manually close the ISC when I disconnected it. It may still be partially open, causing the surge. I'm thinking about just filling the little ducts in front of the TB butterfly valve full of JB WELD. Then all the air will have to go through the butterfly valve, and no bypassing would be possible. I don't need the ISC at all because I live on the gulf coast and it never really even gets cold here.

 

[olmytsi]

Def try closing the isc off manually and leave it unplugged and set the idle with the biss once the car is warmed up. It takes 5-10 minutes!

No need to use jb weld.