1G 90 Talon no spark, fuel good

[White90FWDTSi]

Here's a quick rundown of my situation, which ill follow with a simple question:

While warming my engine up after a head swap and BS elim, and replacing my ignition coil, i was adjusting timing via CAS to get a reasonable idle. Once i found a smooth spot i let it continue to idle to reach operating temp and make a final timing adjustment.
well, sometime between finding the smooth spot and it reaching running temp it dropped rpm to about 3-400(was idling perfect at 950) but continued to run on all 4. At first i thought it was thermostat not opening causing the water pump to fight pressure. There's no thermostat in the housing. After i shut it off i went to check the CAS and i left it loose while idling.
Here's the question (2 parts): 1)If the CAS wiggled around enough to skip just a hair could it throw the ECU into a limp mode and cause it to drop idle that much? and 2) Would something like this cause the power transistor to short out/go bad and prevent spark while attempting to start the car on subsequent attempts?

Some other info here, i went to attach the PT to its mounting bracket AFTER the head/intake was already in the car, so I couldnt get to the bottom screw, I hope this isnt the root of the problem.

 

[steve]

HHere's the question (2 parts): 1)If the CAS wiggled around enough to skip just a hair could it throw the ECU into a limp mode and cause it to drop idle that much? and 2) Would something like this cause the power transistor to short out/go bad and prevent spark while attempting to start the car on subsequent attempts?

1. While timing plays a factor in idle speed the primary parts are the FIAV, ISC, BISS and IPS to tell the ECU to manage the idle speed.

If the coolant is cold the FIAV is open and provides most of the air for fast idle (stock is around 1500 rpm). As the coolant heats up the FIAV starts closing and the BISS and ISC take over. While the IPS is closed and the ECU is trying to manage the idle speed it will lower it as the coolant temps rise. Once it hits 186F it will target 750 rpm as programmed by moving the ISC pintle in and out. Trying to make the car idle at a different speed when at operating temps will force the ISC out of range.

When warn and the throttle is opened or the IPS isn't working the BISS and butterfly control the engine speed. The ECU moves the ISC based on the TPS input to act as a dashpot when you close the throttle and help keep the engine from dieing.

So you need to check that the IPS works, grounding the signal from the ECU when the throttle is closed. That includes making sure that the factory throttle body ground strap is connected or has been replaced by a ground wire to the intake manifold or firewall.

You need to check the ISC and make sure all the coils measure between 28 and 40 ohms. Also make sure that it moves in and out when you first turn the ignition on and not just click or buzz while not moving.

The BISS needs to be adjusted about a turn and a half to two turns from the bottom as a starting point. You'll adjust it again once you get the CAS set correctly using the procedure in the factory service manual (FSM). If the BISS is fully closed you'll have problems getting air for starting when the coolant is warm and the FIAV closed

Make sure that the throttle cable isn't holding the butterfly open. There should be a small amount of slack in the cable when the throttle is closed.

Set the CAS to about mid-position to start with. Once the car starts and warms up you need to follow the FSM procedure using a timing light to set the CAS for a 5* BTDC base timing and then you can switch to adjusting the BISS. The idle speed has to be below 1200 rpm to correctly set the CAS.

Make sure that the engine coolant sensor is connected to the ECU and functioning. If it's disconnected the ECU will think is -34F and flood the engine with fuel during starting. This by itself can keep the engine from starting.

2. I don't think the Power Transistor mounting screw is going to make a difference. The mount is a heatsink, the ground for the PTM is part of the wiring harness to it.

Make sure all your connectors have their retaining clips and are plugged in tight.
If you don't have a logger you might want to consider making one.

 

[DSM1G90]

Great post! - thx for the breakdown per step on how the system operates from cold start to fully hot.

 

[White90FWDTSi]

Thank you very much Steve. Today after work I'll get at it adhering to your advice, while keeping my fingers crossed that the problem is something cheaper than a CAS/PT combo.

 

[White90FWDTSi]

Coming back to resolve this problem since I didn't know how to back in Jan.
The cause of the low idle was the fact i had set the base timing via CAS before the engine hit operating temp (my newbie mistake) and once the ECU read the correct temp it dropped idle enough to somehow short the Power Transistor. Replaced the PT, then set timing correctly and it worked fine!