Welcome to the forum. The idle speed going up and down is called idle surging and it's usually caused by too much air bypassing the throttle butterfly. SO things like a bad ISC, air leaks, bad PCV valve, EGR, bad Fast Idle Air Valve (FIAV) or mis-adjusted Basic Idle Speed Screw (BISS) among others can cause it.

I'm not following you on the next part, how is the car only running at 500 RPM if you give it full throttle?

Please fill out a profile for this car so we know more about it.

Poor idle is not an uncommon complaint, and as Steve pointed out, whatever mods the car may have are important to let members know what to suspect. The part that interests me is that it works fine for 3-5 minutes. I'm no pro at understanding the fuel injection system, so I'm kind of throwing this out for others to inform me, but how long does a car run in the Open Loop mode before going to closed loop with O2 sensor feedback? Seemingly something is warming up enough to tell the ECU it's ready to go, but then some sensor might be out of whack (even disconnected) and screwing with the mixture. I would think with throttle open it should supply more fuel and run better, but it's not. So no change in the rpm's/ power when opening the throttle? What range does the idle surge span? More clues....

Mech Addict said:

Poor idle is not an uncommon complaint, and as Steve pointed out, whatever mods the car may have are important to let members know what to suspect. The part that interests me is that it works fine for 3-5 minutes. I'm no pro at understanding the fuel injection system, so I'm kind of throwing this out for others to inform me, but how long does a car run in the Open Loop mode before going to closed loop with O2 sensor feedback? Seemingly something is warming up enough to tell the ECU it's ready to go, but then some sensor might be out of whack (even disconnected) and screwing with the mixture. I would think with throttle open it should supply more fuel and run better, but it's not. So no change in the rpm's/ power when opening the throttle? What range does the idle surge span? More clues....

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I looked around for disconnected sensors and didn't find any. But I found were the o2 sensor plugs in its melted not sure if it's a big deal or not (see image attached) And the rpms go from 1300 to about 1400-1500 rpms. and the engine is still cold at that point. And the rpms being at 500 only happen after it's in gear and been drove for a minute. As far as the open loop to closed loop go's I'm not sure how to time. If you could explain that would be great.

Yeah, that doesn’t look ideal, but might be making fine connection still. Open loop and closed loop refer to programming in the ecu where the fuel is controlled one way during warm-up for good driveability, and then transitions to a better emissions mode that uses the signals from various sensors. I’m no expert on the details and how to troubleshoot it, but it’s odd that your car runs just fine for a short time. It might help to know if it is a “California” or Federal model. It can be decoded using the VIN, but also I think there is an under-hood label that shows all the emissions control hose routing. Post a picture of that.

Closed loop is where the ECU uses the signal from the O2 sensor to provide feedback on the engines air/fuel mixture. The ECU adjusts the fuel amount to cause the O2 sensor to read rich and then to read lean repeatedly to maintain a average AFR of 14.7:1.

On a 1G the ECU doesn't try to enter closed loop until the coolant temp is > 87F and once it is several things also need to be true, the TPS input needs to be in range for the engine's RPMs, the airflow signal needs to be in range, and the O2 sensor needs to switch within a half second of the ECU trying to make it switch. The ECU uses closed loop when idling and when cruising. During times when your accelerating or decelerating the ECU runs in open loop and makes fueling decisions from the maps in it's software.

There are also conditions around when the ECU will begin managing the idle speed and this is what causes the surging. The ECU sees that the car should be idling and that the RPMs are too high so it cuts off the fuel until they drop. Then it starts injecting fuel again to cause the engine not to stall and if the RPMs rise past the limit the cycle continues. On a properly working car this part of the ECUs software only comes into play when you take your foot off the throttle and the ECU cut the fuel off to reduce emissions and improve economy. I haven't looked at the conditions in a long while and don't remember them off hand.

In your profile you mention that the ECU went bad and was replaced. What ECU did you replace it with and from where. You also imply that the idle surging started happening after the ECU replacement. Is that the case?

It will stop idle surging when you apply throttle when I stop applying throttle it will idle surge again. I'll look more into open and closed loop because it definitely sound like that's a problem I'm having. And the idle surging was happening before I got the ECU I was hoping for that to fix it but it only fixed some other problems like the no start. I got my ECU from carparts.com it didn't appear to have any rust or decroded stuff on it.

Check the idle stop switch on the throttle body for proper operation.

1990TSIAWDTALON said:

Check the idle stop switch on the throttle body for proper operation.

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If the car is surging, the IPS is working. If you unplug the IPS it will stop surging and idle high. The IPS telling the ECU that the throttle is closed and is one of the gating conditions.

This is typically not about open vs closed loop. It's about either a bad ISC/ECU or too much air.

Steve to the rescue!
Thanks for the clarification Steve!

After another cup of coffee it occurred to me that a short to ground of the IPS wire would also provide the signal the ECU is looking for but I think the symptoms would be different than those described.

Coffee is magic! Thanks Steve for the details on the open/closed loop angle. It seems that the 3-5 minutes of normal should be indicating something.
Dequavis; how does it drive during that first few minutes? Any better than later, or same? Any other differences noticed before/after surging? I mean, it sounds like the car is unusable right now.

Mech Addict said:

Yeah, that doesn’t look ideal, but might be making fine connection still. Open loop and closed loop refer to programming in the ecu where the fuel is controlled one way during warm-up for good driveability, and then transitions to a better emissions mode that uses the signals from various sensors. I’m no expert on the details and how to troubleshoot it, but it’s odd that your car runs just fine for a short time. It might help to know if it is a “California” or Federal model. It can be decoded using the VIN, but also I think there is an under-hood label that shows all the emissions control hose routing. Post a picture of that.

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It's very scratch up. And it is a federal emissions

I can just make out that it's color code B14, which I think on Plymouth is Daytona Blue, but same code on Mits is Maui Blue.
Anyway, that's good info about the Federal model. Not sure if it relates to your issues or not, but one never knows these things....
Well, maybe Steve knows.

Mech Addict said:

Dequavis; how does it drive during that first few minutes? Any better than later, or same? Any other differences noticed before/after surging? I mean, it sounds like the car is unusable right now.

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Sorry for the late response. Yeah the car is unusable. I found the ISP isn't working correctly but think that will only fix my idle surge. Other thing I looked at are PCV valve. ERG they seem to be ok

Just a thought: is the hose from the charcoal canister intact? I don’t think that could let in enough air to account for the condition you described, but maybe worth checking. Might be related to a bad MAF? No codes/ check engine light, right?

Charcoal canister hoses look good. I did some testing on the MAF it seems good. Your correct there's no codes on it. One more thing I just realized when the car broke down it was after driving on a washboard gravel road so maybe something just got rattled lose.

Dequavis said:

I found the ISP isn't working correctly but think that will only fix my idle surge.

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Please explain this?

Something to do with internet, Steve. Wiseman indeed!

What's this? Looks like a broken wire but I have no idea what it goes to. Could be part of your problem.


You might also have an issue where an electrical component, like your ISC, works fine when cold but as it warms up the metals expand and causes it to stop working.

This sounds a lot like 'the ECU doesn't know how much air is coming in.' Any chance the MAF is non functioning? (You said you did 'some checks' but did those show output so many Hz indicating air flow actually being measured.) A useful check would be what happens when you disconnect the air intake hose so the air to the engine doesn't go through the MAF.

Did you look at the MAF and intake area? Maybe a blockage there -- mouse nest, etc. and yes they DO go through the filter ...

This is too big a problem for the usual air leak, 02 sensor, etc. issue.

dwb said:

What's this? Looks like a broken wire but I have no idea what it goes to. Could be part of your problem.

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You might also have an issue where an electrical component, like your ISC, works fine when cold but as it warms up the metals expand and causes it to stop working.

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Found where that wire goes I have no idea what it does though (see image). ISC isn't moving in and out its only shaking side to side

waltah said:

This sounds a lot like 'the ECU doesn't know how much air is coming in.' Any chance the MAF is non functioning? (You said you did 'some checks' but did those show output so many Hz indicating air flow actually being measured.) A useful check would be what happens when you disconnect the air intake hose so the air to the engine doesn't go through the MAF.

Did you look at the MAF and intake area? Maybe a blockage there -- mouse nest, etc. and yes they DO go through the filter ...

This is too big a problem for the usual air leak, 02 sensor, etc. issue.

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I disconnected the air intake and it just stalled. I took off the intake hose and looked at the air filter it all looked nice and clean

Dequavis said:

Found where that wire goes I have no idea what it does though (see image). ISC isn't moving in and out its only shaking side to side

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It's for the gauge cluster temp.

Dequavis said:

Found where that wire goes I have no idea what it does though

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Looks like the Water Temp Gauge Unit. Does your temp gauge ever read anything?

That area looks like it’s been toasted. I would inspect every wire and connector in that part of the harness. Also check for exhaust leak between head & manifold which I think could be the cause.