Idle Drop Not Surge Just Drop

[pearlnotchback]

With the cam,heads, blower and all that mess I have, my 92 coupe has always idled its best at right at 1000 or a hair under. If I would let the car idle for a long while it would eventually drop to around 600 rpms like I flipped a switch and would stay there until I would drive it around and then it would be good back at around 1000.
Now I can almost anticipate the drop. Like pulling up to a red light it will idle just fine until the motor reaches around 160 degrees then it's like a switch is flipped and it drops down to around 600. Light turns green I go and stop at the next red light idles fine around 1000 and again after a few minutes it's like a switch is flipped and it drops to around 600. Any ideas and input on how to fix this annoying issue would be appreciated!!!

 

[Mustang5L5]

I have a suspicion. Run your check engine codes. Trans in nuetral, don't press in the clutch. Do you get a code 67 when you do koeo?

If not, my suspicion is debunked.

 

[pearlnotchback]

Ok I will definately check the codes tomorrow. What would a code 67 be?

 

[Mustang5L5]

So the ECU gets a signal to revert to idle strategy from two sources, the clutch pedal switch and the ngs on top of the t5. I've noticed that when the t5 switch isn't hooked up, the idle will hang, and suddenly drop on cue.

Just something to investigate and rule out

 

[jrichker]

Ok I will definately check the codes tomorrow. What would a code 67 be?

Code 67
Revised 18-Mar-2017 to include warning about the necessity of having a 5 speed O2 Sensor wiring harness when bypassing the wiring for test purposes

Cause of problem:
Clutch not depressed (5 speed) or car not in neutral (5 speed and auto) or not in park (auto) or A/C in On position when codes where dumped. Possible neutral safety switch or wiring problem. This code will prevent you from running the Key On Engine On tests.

External evidence from other sources claims that a code 67 can cause an idle surge condition. Do try to find and fix any issues with the switch and wiring if you get a code 67.

What the NSS (Neutral Safety Switch) does:
5 speed transmission: It has no connection with the starter, and the engine can be cranked without it being connected.
Auto transmission: It is the safety interlock that prevents the starter from cranking the engine with the transmission in gear.
What it does for both 5 speed and auto transmission cars:
The computer wants to make sure the A/C is off due to the added load on the engine for the engine running computer diagnostic tests. It also checks to see that the transmission is in Neutral (5 speed and auto transmission) and the clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.

The following is for 5 speed cars only. Do not do this unless you are sure that you have a 5 speed O2 Sensor harness!!!! Smoke, sparks and expensive pain in the wallet may ensue if you don’t.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector from the rear. Jumper the safety pin to the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!

 

[pearlnotchback]

So the ECU gets a signal to revert to idle strategy from two sources, the clutch pedal switch and the ngs on top of the t5. I've noticed that when the t5 switch isn't hooked up, the idle will hang, and suddenly drop on cue.

Just something to investigate and rule out

My car has had a Tremec 3550 in it since I've owned the car. Is there a possibility that something isn't connected at the transmission that could cause this? I don't know if this is related or not but I've also never had reverse lights. Could the reverse lights be related to this problem?

 

[Mustang5L5]

Yes.

Try this. Reach up under the dash where the clutch pedal switch is. There are two plugs. One is the starter circuit safety, the other is one leg of the NSS.

You want to unplug the NSS plug and leave the starter plug in. This will be easy because if you unplug starter circuit plug, you can't start the car.

Now, take the NSS plug and stick a 5amp (or whatever you have) fuse in it. Drive around, and see if that eliminates your idle drop.

 

[jrichker]

My car has had a Tremec 3550 in it since I've owned the car. Is there a possibility that something isn't connected at the transmission that could cause this? I don't know if this is related or not but I've also never had reverse lights. Could the reverse lights be related to this problem?

Check the VIN code and post it here - you car may be an auto to manual trans conversion.
Pull the passenger side kick panel and look for the catch code; if it is an A9P it is an auto trans computer.


The Tremec 3550 has the NSS connection at the driver's side of the shifter; make sure that the O2 sensor harness is plugged into it.

 

[pearlnotchback]

Yes.

Try this. Reach up under the dash where the clutch pedal switch is. There are two plugs. One is the starter circuit safety, the other is one leg of the NSS.

You want to unplug the NSS plug and leave the starter plug in. This will be easy because if you unplug starter circuit plug, you can't start the car.

Now, take the NSS plug and stick a 5amp (or whatever you have) fuse in it. Drive around, and see if that eliminates your idle drop.

I will for sure try that. So it's the plug that is plugged into the actual clutch pedal?

 

[pearlnotchback]

Check the VIN code and post it here - you car may be an auto to manual trans conversion.
Pull the passenger side kick panel and look for the catch code; if it is an A9P it is an auto trans computer.


The Tremec 3550 has the NSS connection at the driver's side of the shifter; make sure that the O2 sensor harness is plugged into it.

I do know for a fact that the only thing that is plugged into my transmission is the speed sensor with the speedometer cable. My o2 harness has nothing running from it to the transmission. So I'm kinda confused about this.

 

[Mustang5L5]

I will for sure try that. So it's the plug that is plugged into the actual clutch pedal?

On the clutch pedal box under the dash, you'll see a small gray switch. Looks like this

Plugged into it, you'll see a black plug with red wires, and a "clear" plug. The black plug is the starter circuit. Leave it plugged in. Take the clear plug off, and stuck a fuse to jump it out. Drive around for a bit and see if makes a difference.

 

[Mustang5L5]

I do know for a fact that the only thing that is plugged into my transmission is the speed sensor with the speedometer cable. My o2 harness has nothing running from it to the transmission. So I'm kinda confused about this.

On the T5. There are three electrical connections. The speed sensor in the cable, the reverse lights, and the NGS on top of the T-5 that tells the computer the trans is in neutral and adjusts idle strategy. This is wired in parallel with the "clear" plug you are jumping out. Basically by jumping it, you are telling the ECu the car is always in neutral, and idle strategy changes slightly. If the car doesn't know it's in neutral, the idle tends to hang a bit.

I don't even know if the 3550 has a NGS. Just leave the plug under the clutch pedal jumped out if this is the issue.

 

[jrichker]

@pearlnotchback
@Mustang5L5

I have a 3550 in my Mustang and it does have a NGS switch - see the drawing below

 

[pearlnotchback]

I haven't had a chance to read the responses yet but definitely will when I get home. I just wanted to mention that even if I dont drive the car I can crank it in the driveway and let it run and the idle will still drop at about 160 degrees. Don't know if it matters figured I'd mention it

 

[pearlnotchback]

On the clutch pedal box under the dash, you'll see a small gray switch. Looks like this

Plugged into it, you'll see a black plug with red wires, and a "clear" plug. The black plug is the starter circuit. Leave it plugged in. Take the clear plug off, and stuck a fuse to jump it out. Drive around for a bit and see if makes a difference.

Ok thanks for the pictures and all. That really helps. I will try that tomorrow afternoon and let you know if there were any changes. Also will the problem with that plug cause these same symptoms even if the car hasn't moved as in just cranked in the driveway and allowed to idle?

 

[Mustang5L5]

Yes.

Even with the wiring plugged in, simply pressing in the clutch will close the switch and jump the plug and signal the ECU the car is in neutral.

I notice on my car, when I push the clutch in, the idle will drop. I don't have the NGS on top of the T-5 hooked up in my car. I've just been living with the idle drop.

 

[pearlnotchback]

Yes.

Even with the wiring plugged in, simply pressing in the clutch will close the switch and jump the plug and signal the ECU the car is in neutral.

I notice on my car, when I push the clutch in, the idle will drop. I don't have the NGS on top of the T-5 hooked up in my car. I've just been living with the idle drop.

The weather here in TN has been crap lately so I haven't been able to get the car out and work on it. I'll post the results as soon as I can and I appreciate everyone's help!!!!

 

[fredfifty]

Check the VIN code and post it here - you car may be an auto to manual trans conversion.
Pull the passenger side kick panel and look for the catch code; if it is an A9P it is an auto trans computer.


The Tremec 3550 has the NSS connection at the driver's side of the shifter; make sure that the O2 sensor harness is plugged into it.

my nss is just hanging, so i would connect it to the 02 harness?? i also have a 3550

 

[jrichker]

my nss is just hanging, so i would connect it to the 02 harness?? i also have a 3550

That's what I said...

 

[Mustang5L5]

NSS doesn't hook up to the o2 harness. There a plug for it on the transmission harness that the reverse harness and VSS harness plug into


Edit. NGS, not NSS