Resolved Need help with ISC motor

[RSTurboFWD90]

I'm trying to fix a bucking problem. I've tested just about everything except the ECU. The only thing I have discovered is that the ISC is very loud. It sounds like radio static or maybe an egg frying. You hear it with key on before start and sometimes when running if you put your ear on the intake manifold. Is this normal? There is idle surge when cold but not when hot. Is the ISC noise a concern that it may be broken inside? The ohms test checks OK. No CEL ever.

 

[Drifte]

Never noticed ISC noise from cars that I have replaced the isc's in. vfaq has some right ups of what to look for with idle surge. It's a nice step by step of each thing to check and how to check it. Is your coolant lines blocked off from your isc? in the winter i have noticed this causes idle surge when cold. if you find that site (i dont have time) you will have found everything you needed to know.

 

[RSTurboFWD90]

Not blocked off; this car is all stock. I've done the whole throttle body / ISC cleaning process a few years back to fix classic idle surge. I guess I'm going to do it again to check out this ISC. This bucking problem is something that's new and getting worse. I never heard this ISC noise until I started trying to find out what's broke. The symptoms don't quite say ISC trouble.... but hey I'm not an expert. Any other thoughts.... please post.

 

[dsmfreak20]

theres a quick way to tell if your isc is working properly. take it out of the throttle body and have a friend turn the car to the acc position and see if you can see the isc stepping in and out. if it doesnt move or makes noise and twitches but doesnt move then bang you've atleast found part of the problem. ive found that sometimes they can get jammed. that was the case with mine so i took it apart put a small amount of 10w 40 moved the piston in and out a couple of times and ive had no troubles since. it may or may not work for you but its worth a shot before you drop money on another one. good luck.

p.s. the isc provides no feedback to the ecu. it just assumes the isc is in working order. so it'll never throw a cel for a bad isc.

 

[RSTurboFWD90]

Thanks for the tip. I'll pull the ISC this week and check it out. I did find a problem with a jumpy TPS that I'm going to work on too. I'll post again when I'm done.

 

[RSTurboFWD90]

UPDATE: I spent the weekend putting in the new ISC and TPS. The new ISC also makes alot of noise when you turn on the key and the car is doing the exact same thing. I beginning to wonder if the ECU is messed up. I should probably pull it and look at the capacitors.

Here's what it's doing. When the engine is not completely warm, it bucks like you are cutting off and on the ignition. It's almost undriveable. When it's completely warm you can drive it but you can feel mild stutter at cruise. If you get into it, it has good boost and runs smoothly. If you shut it off for 20 or 30 minutes and restart, it bucks again until the temperature is back to normal value. Any ideas why a cool engine behaves this way?

I don't have access to a scope so I'm running out of ways to check things.

This car has new TPS, ISC, PTM, coilpack, fuel filter, plugs and wires. I've tested the EGR and purge system. I have never had a CEL for this problem. This is a completely stock engine.

 

[RSTurboFWD90]

This problem turned out to be caused by leaking capacitors on the ECU board. The ISC noise was not causing any problems.

 

[steve]

A noisy ISC can be one of the side effects of leakage damage to the ECU.

Did the place tell you all that they repaired? I wonder if your was having both TPS and ISC problems.

Steve

 

[Corwin]

This problem turned out to be caused by leaking capacitors on the ECU board. The ISC noise was not causing any problems.

You can get your board repaired or get a new one here.




Mod Edit:
Don't join up to the forums simply to plug a webpage to our members.
98RedGs

 

[merqury]

p.s. the isc provides no feedback to the ecu. it just assumes the isc is in working order. so it'll never throw a cel for a bad isc.

Are you sure about that - I had ISC problems and I got a code -
P0505 - Idle air control motor circuit malfunction

for reference - http://www.troublecodes.net/mitsu/

 

[Rice Over Wheat]

p.s. the isc provides no feedback to the ecu. it just assumes the isc is in working order. so it'll never throw a cel for a bad isc.

This is one of the most incorrect statements I ever read on this site. Do a search for "isc" and try telling us that again, wading through several dozen posts. Then look up DTC P0505.

Edit: Nm, I see now you meant for 1Gs.

 

[steve]

This is one of the most incorrect statements I ever read on this site.

I wouldn't go that far. I've seen much worse posted here.

What I've said in the past is that the ECU doesn't have any direct feedback from the ISC. It can only deduce indirectly from the reaction to moving the pintle that the ISC is working.

On a 1G you won't get a code if the ISC is bad but on a 2G there is at least one tied to the ISC not working.

Steve

 

[Rice Over Wheat]

That's true, I've seen worse.

 

[steve]

Also keep in mind that the OP has a '90 and it won't throw a code.

Steve

 

[Rice Over Wheat]

Ah makes sense in retrospect. I stand corrected. And edited.

 

[doggo]

I wouldn't go that far. I've seen much worse posted here.

What I've said in the past is that the ECU doesn't have any direct feedback from the ISC. It can only deduce indirectly from the reaction to moving the pintle that the ISC is working.

On a 1G you won't get a code if the ISC is bad but on a 2G there is at least one tied to the ISC not working.

Steve

Steve is exactly (as usual ) right. As for why a P0505 CEL gets thrown, if the last post here is accurate, here is why:

http://www.dsmtuners.com/forums/showthread.php?t=245887&highlight=p0505+will+be+generated+mitsu

"P0505 will be generated (on a 2G) when the "actual idle speed has continued to be higher
than the target idle speed by 300 r/min or more for 10 sec" (this from 99 Mitsu
service manual). So if you force the idle speed higher (by Biss or whatever) you
may get P0505's when this condition is met. "

I'm not sure if the above condition is the ONLY condition that can set P0505, but I bet it is the most COMMONLY OCCURING logic condition triggering P0505, when a bad ISC is actually the problem.