Randomly runs rough/ tach reads incorrect

[slow4g6392tsi]

So I have had this problem in 2 of my DSMs now and have no idea how I fixed the first one but this time it won't go away. I will be driving and it will randomly start running extremely rough and the tach reads very low. It will smell extremely rich like some cylinders must not be getting spark. The exhaust/engine tone is very difficult to estimate rpm especially without the tach working. I 30mph in 2nd said 1500rpms so I know it is wrong. And I can still hit launch control at 5500 but it reads like 2000rpms. I thought I jumped time when it first happened (idling) but it went away and drove normal for the rest of the night.

 

[Hesh724]

check the wires in the plug and stuff if it is getting giggled?. thats all i got . good luck

 

[slow4g6392tsi]

I have checked all plug wires (pretty new NGKs), and I just replaced the plugs. I was thinking CAS also just because the rpm deal.

When I pulled the plugs two were black and two were white. I thought that was pretty weird. I also pulled plug wire 1 or 4 (whichever is on the pass. side??), put a plug in it, touched it to metal, and it did not spark when I started the car. The car also ran exactly the same even though it was unplugged.

 

[turboglenn]

I'm willing to bet a voltage drop is happening... when voltage is low the injectors have timee added to them to compensate for longer dead times at low voltage, and the tach will always read off, mostly higher than what's really going on (but our tachs read a few hundred RPM high anyway) but you will know what it's doing as you are experienceing it already

See if you can log volts, I bet the Alt is getting hot and shutting down and then the car run like poopy

 

[slow4g6392tsi]

Thanks for the reply. Seems logical. Could it have something to do with me just putting a new alternator in? I replaced it because all of the lights started pulsating after driving for a while or high rpm so I figured the voltage reg. might have been going bad or something. It did fix the pulsating problem. It could be a lot of things though. I just had the motor and trans out a few weeks ago and did a bunch of stuff at once.

Edit: Also forgot to mention, during deceleration it sounds normal.

 

[dsmdeliveryboy]

Its the voltage regulator in your alternator, I just had the same problem. And when you take the alternator to have it tested it won't read that the alternator is bad even though it is. Replace the alternator, have your battery checked for a bad cell and check all your grounds and terminals. Has the MAS been hacked?

 

[slow4g6392tsi]

MAS has not been hacked. That sucks it won't read bad. I just replaced it and this one has a lifetime warranty.

 

[steve]

The ECU knows the RPM from the CAS, the tach from the power transistor.
Sounds like your loosing a coil. That can be wiring, ECU, power transistor, or coil.

 

[turboglenn]

Thanks for the reply. Seems logical. Could it have something to do with me just putting a new alternator in? I replaced it because all of the lights started pulsating after driving for a while or high rpm so I figured the voltage reg. might have been going bad or something. It did fix the pulsating problem. It could be a lot of things though. I just had the motor and trans out a few weeks ago and did a bunch of stuff at once.

Edit: Also forgot to mention, during deceleration it sounds normal.

Start checking and adding some good grounds (there's agood write up on here about 4 gauge ground kit DIY style)

I can't explain enough about how important a good ground is, it's every bit as importnat if not more than a good positive terminal connection... I've had cars that would break up, not rev, all kinds of issues only to find corrosion under the ground terminals, a little sand papaer and things were back to normal.... But don't forget to check the positive leads as well, any one of them could be the culprit but at least it sounds like you're chasing a minor gremlin, and not some major nightmare issue... keep us updated

 

[slow4g6392tsi]

Thanks for all of the answers everyone. I have a weird feeling it is my ECU. It had a sketchy socketing job when I bought it so I guess I will try that for now.

 

[turboglenn]

well, I'm not up on stock ECU's and all that but i have a LOW mileage 97 GS-T ECU that's needing a new home and my pocket could use a pack of smokes and a few greenbacks for insulation

 

[slow4g6392tsi]

Fixed. It was the ignition power transistor. Thanks for the help everyone.

 

[turboglenn]

Fixed. It was the ignition power transistor. Thanks for the help everyone.

Thansk you for updating your thread.. this helps so many people when they actually search and far too many people fail to ever say what their fix ended up being