I have a 1987 v6 GT. Start up brings extreme surging from very low to 2500 rpm and cut out. Was running ok but a little fast. Had my auto tech look and he messed with the diagnostic plug on the ecm. Now if I start and keep the revs up till it warms up it runs smooth but won’t run under 2700. Have replaced IAC valve no help. Dose the ecm need reprogramming. Ecm numbers. Serv no1227170. Fab. 8673. M721121325. Please any ideas welcome. Lombok

quote Originally posted by Lombok: Had my auto tech look and he messed with the diagnostic plug on the ecm.

Might want to elaborate on "messed with". What did he do?

He put an Australian reader on it and couldnt get any result

quote Originally posted by Lombok: Was running ok but a little fast.

That usually indicates a vacuum leak.

quote Originally posted by Lombok: He put an Australian reader on it and couldnt get any result.

Okay, I can see how that would be next to useless.


Pretty difficult to diagnose the issue with no history on engine maintenance.

When's the last time the spark plugs have been looked at/replaced?

Has the ignition timing been done property, with the ALDL jumper in place?

Has the plug/cover been removed from the idle stop screw on the throttle body? (...indicating that the screw has now no doubt been misadjusted.)

[This message has been edited by Patrick (edited 09-26-2022).]

A complete guess:

The EGR tube may have been a little bit broken, but now, messing with it has made it very broken. The original equipment is fragile after all these years.

Hi Patrick. Car was running ok when I got it. Removed engine to fix oil leaks engine mounts new spark plugs and removed inlet manifold and rocker covers to paint Ran ok after Idle revs a little high 1500. Had cut out problem. Stall at traffic lights. Replaced trans lockup solenoid. Still idle a little high. Idle screw definitely been altered. Checked for vacuum leaks Seems ok. Then got tec with reader, problem since then. Tried procedure jumper on Aldl key off unplug IAC start car adjust idle screw. The plug in IAC.several times Engine warm. Got idle down Ran nice About 900 rpm. When cooled down reverted to problem. Timing has not been changed. I think this car once had NOS on it. Regards Terry

You might want to confirm that the spark plug wires are still in the correct firing order (1-2-3-4-5-6).

quote Originally posted by Lombok: Checked for vacuum leaks Seems ok.

Disconnect the rubber snorkel from the throttle body. Cover the throttle body opening completely with the palm of your hand while the engine is running. If the engine doesn't stall, there's a vacuum leak. As already pointed out, the EGR tube is the prime suspect for a vacuum leak.

quote Originally posted by Lombok: Car was running ok when I got it...Timing has not been changed.

Don't assume the ignition timing is correct. Needs to be checked... with the ALDL jumper in place.

[This message has been edited by Patrick (edited 09-29-2022).]

I did your vacuum test. Stalled straight away. Did aldl bridging procedure again , car runs at lower revs eg diagnostic mode But reverts when disconnected. Is there a reader available over there?

quote Originally posted by Lombok: Did aldl bridging procedure again...

And.... did you have a timing light connected?

quote Originally posted by Lombok: I did your vacuum test. Stalled straight away.

Good. Now put your finger just over the IAC port at the front of the TB while the engine is running. The engine should stall (with the throttle closed). If not, the idle stop screw is misadjusted.

quote Originally posted by Lombok: Is there a reader available over there?

Check out THIS.... sold by PFF member phonedawgz

[This message has been edited by Patrick (edited 09-29-2022).]

I covered the IAC and adjusted the idle screw until running evenly. Still reving high. Disconnected vacuum line from map sensor and plugged line revs dropped to around 900. Is map sensor the problem?

I would replace the MAP sensor.

quote Originally posted by Lombok: I covered the IAC and adjusted the idle screw until running evenly.

Why would you do that?

quote Originally posted by Patrick: Now put your finger just over the IAC port at the front of the TB while the engine is running. The engine should stall (with the throttle closed). If not, the idle stop screw is misadjusted.

quote Originally posted by Lombok: Still reving high.

Well yeah, no wonder! There's no way the IAC valve is going to be able to control idle speed if you've adjusted the idle stop screw to allow the engine to be "running evenly" with the IAC port covered.

There are plenty of threads in the archive, including This one, explaining how to reset the idle stop screw if it's been tampered with. Keep in mind that any/all of these methods are based on the presumption that the ignition timing has been properly set (as the ignition timing most definitely affects idle speed).

[This message has been edited by Patrick (edited 09-30-2022).]

Why would I do that. I miss read your instruction