Help. Bad MAS or ECU? How Do I test?

[shawntalon]

I have been having a problem with my 91 Talon (Tsi AWD).Which has only 5000mi on a remanufactured engine, in which I run 93 octane only. It Idles awful high(about 1500-2000 rpms) until it warms up, and then It idles up and down constantly, unless the head lights are on and the heater blower motor is on high and I push the a/c switch in one notch so the secondary cooling fan comes on.Then it doesn't surge but still idles about 12-1500 rpms. On top of that, It has been getting fuel cut (I think) for about the past week. The mpi relay keeps resetting and cutting the fuel, sometimes it will run fine( other than the rough idle) and sometimes it won't stay running. Just to get back and forth to work, I jumped a wire from a constant 12V to the fuel pump wire that comes out of the relay and goes directly to the fuel pump. Then it runs okay (outside of the idle surge)If I counted the needle sweeps correctly on my multimeter, while checking the diagnostic codes, it points toward the barometric sensor, and the air temp sensor. When I tested the componets the way the haynes manual says to, it all checks out okay. Here's the weird part, if you unhook the MAS without the jumper wire on the fuel pump, it will stay running, but awful roughly, but you can still hear a clicking from under the dash. Any Ideas? Does it sound like a MAS problem, or time to get another ECU?Know where to get an ECU that is in good shape? I took mine apart and it does look like the capacitors could be leaking a bit, not sure though.If so can the board be repaired if it doesn't have too much damage? I need to find a solution before I fry something by bypassing the relay. I have tried swapping the mpi relay, the Idle speed control motor, tested the o2 sensor, checked for air/turbo leaks,and several other tests.

 

[DaveT]

You have a lot going on in your post so I don't know if I can help you. But we'll see. The easiest way to test the ECU and the MAS is to find someone you can swap with. Stick them in and test them out. I would test the continuity for the MAS wiring to see if there are any short/open wires from the MAS to the ECU.

It is hard to say what is causing the car to idle high and surge without knowing if the MAS or ECU are fine. Check the Fast Idle Air Valve found on the TB. I believe the way it works is when the coolant is cold the wax pellet in the FIAV is solid, causing a valve to stay open allowing bypass air through the TB. As the temp increases, the wax softens forcing the valve to push out, closing the bypass valve and less air gets to through the TB. Yours seems to be working since when you start the car, it idles high, and then drops as the coolant is warm. But you might as well test it out in a pot of boiling water.

Also check the Idle Speed Control Servo in the TB. This also controls bypass air. There should be a way to test it out also.

Check to see if you Throttle Position sensor is also working correctly. VFAQ has a way to test that.

When my engine was put back into my car, I had a very similar problem, but I don't think I had any CEL for my MAS. Both my FIAV and my ISCS were bad, so they were replaced. This lowered my idle but it was still surging. I sent my ECU out to get the capacitors replaced/ECU fixed. Put it back in the car (without changing anything else) fixed the idle surge problem.

See what you can do with some of this info, hope it helps.

DaveT
92GSX