940T rough when cold

[asimba2]

I have a 1993 Volvo 940 Turbo and am having a difficult time with it when cold. It starts, the idle bounces around and it stalls. It will stall if it doesn't get a chance to warm up for 30 seconds or so, and that's in a temperate California climate. It runs smooth once warm and is averaging 23-24 MPG highway so I currently suspect some component of the cold start system.

Here is what has been done:
-MAF cleaned
-throttle body cleaned
-idle air controller cleaned.

Any other tips would be appreciated.

[billofdurham]

Watch the tachometer carefully when it stalls. If it drops to 0 even before the engine winds down then that would indicate loss of ignition pulses to the coil negative terminal. Primary suspects would be the power stage or the crank position sensor. If the tachometer tracks the engine speed as it winds down then that would indicate a fuel injection problem. Suspect radio suppression relay or loss of fuel pressure due to relay or pump.

Have you checked for fault codes using the On-Board Diagnostic (OBD) in the engine compartment? Not all fault codes will put on the check engine light.

Bill.

[asimba2]

The fuel pump relay and radio suppression relay are brand new OEM Volvo. I have not checked for codes, I forgot these cars aren't as sensitive as my OBD-II 850T. I will watch the tach next time.

I was sort of thinking along the lines of a cold start injector or cold start injector temperature sensor problem because the problem is strictly limited to cold start. Any thoughts there?

Thanks for the info.

[billofdurham]

A faulty or dirty Idle Air Control (IAC) valve can cause problems at start-up if it is not working or is dirty. This provides for more air, or faster idle on start up. The fact that it starts readily when cold indicates the cold start injector is working properly. The IAC is often referred to as the 'cold start valve'.

There are two checks that can be made on the IAC valve:

1. You need to access the Engine Control Module (ECM) multi-plug to check the resistance of the IAC. Be very careful if you run this check as the ECM does not like probes poked into the wrong places. See attached:

940 checking IAC valve.pdf

(150.14 KiB) Downloaded 216 times

2.Clean the IAC.
It is located under the intake manifold below the second cylinder. It is about 3 inches long and about 1.5 inches in diameter It has two rubber hoses and a snap-on electrical plug connected to it. Remove the hoses and electrical connector, then remove the IAC.
Turn the rotating valve to expose the innards. Spray some carburettor or fuel injector cleaner into the opening and shake it around. Make sure you plug the other end with your palm to trap the liquid inside the IAC.
Open and close the flapper valve with a small implement to loosen any dirt contained inside. Be very careful about this step as a scratch will spoil the valve. Some people use a small screw driver but I use either a small wooden skewer or a very fine plastic knitting needle. (My wife often wonders where they go!!)
Turn the IAC upside down to empty out the dirt and fluid.
Repeat the cleaning process several times to ensure you get all the grunge out.

To see if this has worked you can test the IAC on the car without permanently installing it. You might have to reposition some hosing to do this.

Bill.

[asimba2]

Bill, thank you for the detailed reply. I thoroughly cleaned the IAC when this problem started and it made no difference in how the car ran. I used the same technique I used for cleaning the IAC on my 850. But if the IAC is truly bad, then cleaning it would make no difference. But then again, the car runs great once it's warm. I will try checking the resistance next, as you suggested.