Idle Surging

[PJalbanesi]

I have a 1990 GT convertible and the idle was surging after a few minutes of running so I did some research and replaced the IAC valve and when I started the car it ran great for about 10 minutes then it started to surge again so I replaced the TPS sensor as well and adjusted it to about .96 and once again it ran perfect right from the start up and did for about 15 minutes then when the car was warmed up it started surging little by little till the point where it drops so low it stalls idk what else it could be any help??

 

[jrichker]

You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix.

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 200,000 hits, which indicates it does help fix idle problems quickly and inexpensively.

 

[Mustang5L5]

If you replace the IAC, you need to perform the idle reset procedure. A quick search will find results how to do this.

Also, I wish the "adjust tps" myth would die. if you don't have an engine code for it, don't touch it. For the most part the TPS should be considered non adjustable

Step #1 should be to read the engine codes and perform the surging idle checklist above

 

[PJalbanesi]

If you replace the IAC, you need to perform the idle reset procedure. A quick search will find results how to do this.

Also, I wish the "adjust tps" myth would die. if you don't have an engine code for it, don't touch it. For the most part the TPS should be considered non adjustable

Step #1 should be to read the engine codes and perform

If you replace the IAC, you need to perform the idle reset procedure. A quick search will find results how to do this.

Also, I wish the "adjust tps" myth would die. if you don't have an engine code for it, don't touch it. For the most part the TPS should be considered non adjustable

Step #1 should be to read the engine codes and perform the surging idle checklist above

Why does the idle surge only after its been running and warmed up?

 

[PJalbanesi]

If you replace the IAC, you need to perform the idle reset procedure. A quick search will find results how to do this.

Also, I wish the "adjust tps" myth would die. if you don't have an engine code for it, don't touch it. For the most part the TPS should be considered non adjustable

Step #1 should be to read the engine codes and perform

 

[jrichker]

Because before the engine is warmed up, the engine control computer is running in open loop with predefined settings. It only uses a minimal set of sensor data. After it gets warmed up, it switches modes to closed loop and uses all the sensor data to mange the engine and idle. At that point if you have a bad sensor or sensors or faulty sensor wiring, the computer gets bad sensor data and can't manage the engine or idle properly.

 

[PJalbanesi]

Because before the engine is warmed up, the engine control computer is running in open loop with predefined settings. It only uses a minimal set of sensor data. After it gets warmed up, it switches modes to closed loop and uses all the sensor data to mange the engine and idle. At that point if you have a bad sensor or sensors or faulty sensor wiring, the computer gets bad sensor data and can't manage the engine or idle properly.[/QUOTE/]

I'll have to try the idle reset tomorrow after work hopefully that's the issue

 

[PJalbanesi]

I tried the idle reset and my car wouldn't start with the IAC unplugged and when the car was warmed up it surged till it stalled about and then acted like the Battery was dead wouldn't even jump start so I left it for a few hours and when I was gonna push it into the garage I figured I would try one more time and it started up perfectly and ran fine

 

[Mustang5L5]

If car won't start with IAC unplugged, the throttle stop screw is set too low. Unplug the IAC and crank open the screw a few turns and start engine. Idle should be high. Back the screw down until the engine is barely idling consistently. 600-750rpm or so.

Then, unplug battery for 15-20 min to reset computer, plug IAC back in, and startbit up

 

[PJalbanesi]

If car won't start with IAC unplugged, the throttle stop screw is set too low. Unplug the IAC and crank open the screw a few turns and start engine. Idle should be high. Back the screw down until the engine is barely idling consistently. 600-750rpm or so.

Then, unplug battery for 15-20 min to reset computer, plug IAC back in, and startbit up

Car runs great now!

 

[PJalbanesi]

Alright so my problems seem to continue...the car still runs and drives great I started her up perfect this morning and when I went on lunch a few hours after the car started up fine went somewhere like 4 miles away turned the car off sat for like 15 minutes and when I went to start it it stumbled to start but did and was running fine voltage was good on dash drove back to work parked tried to start her up again before going in to see if it would happen again and it really stumbled to start battery almost clicking basically. Put a brand new battery in last week so idk what could be causing this, maybe the electric fan running when the cars off killing the battery???

 

[90sickfox]

Check your ground wires ( if you haven't already ). There should be one that looks like a metal strap going from the back of the intake to the firewall to the left of the speedometer cable hole...and another going from the drivers side of the block to the sway bar mount area below the power steering pump.

Check the battery cable that runs from the battery to the solenoid by the fender. Then check the wire from the solenoid to the starter. You can take a screw driver and bridge across the two bigger posts...this should turn the starter over.( make sure the ignition is off and the car is out of gear or you could run yourself over ) It will spark but as long as you are only touching the handle you'll be fine. If it only clicks and there's at least 11 volts at the battery then the problem is somewhere else ie. starter or cables.

The solenoid on the fender by the strut tower could be bad.....or the starter could be bad...or the wires and ends could be bad.

Seeing that a jump wouldn't work I suggest checking those things. A lot of parts stores will put the relay or starter on a machine and check it for free.... I know Advance Auto does.

 

[PJalbanesi]

Check your ground wires ( if you haven't already ). There should be one that looks like a metal strap going from the back of the intake to the firewall to the left of the speedometer cable hole...and another going from the drivers side of the block to the sway bar mount area below the power steering pump.

Check the battery cable that runs from the battery to the solenoid by the fender. Then check the wire from the solenoid to the starter. You can take a screw driver and bridge across the two bigger posts...this should turn the starter over.( make sure the ignition is off and the car is out of gear or you could run yourself over ) It will spark but as long as you are only touching the handle you'll be fine. If it only clicks and there's at least 11 volts at the battery then the problem is somewhere else ie. starter or cables.

The solenoid on the fender by the strut tower could be bad.....or the starter could be bad...or the wires and ends could be bad.

Seeing that a jump wouldn't work I suggest checking those things. A lot of parts stores will put the relay or starter on a machine and check it for free.... I know Advance Auto does.

I'll have to check the lines I have a msd 6al that is newer so i hope its not that

 

[General karthief]

Do a voltage drop test on your connections, most starting problems are poor connections, bad wires that will eventually result in starter/alternator failures.
There are at least 4 grounds in the engine compartment and one by the computer in the passenger kick panel.

 

[Mustang5L5]

Have you run codes?

 

[90sickfox]

The MSD won't make your starter click and not start. The first thing you should do is check or voltage at the battery. If you have 12v at the battery posts and get a click
when trying to start the connections, wires, solenoid, or starter are bad.

On rare occasions I've seen bad timing cause a slow crank condition when the motor is hot...but that's rare and probably not your case.

Do you own a multimeter ? They're only about 7 bucks or a cheap one.