I've had some odd things happening on my current engine.

Unfortunately I installed a new engine,added 720's, and did the rtek 1.7 upgrade at the same time so any of these could be a lead towards the issue.

First, My car hesitates at the secondary injector switch randomly. Some days it's great while other times it's horrible. At night is when it's more likely to happen(turning headlights on I guess?). It sucks but I can atleast drive with it.

Now the odd part.
Some times out of no where the car will not want to go over 3500 rpm or go into boost. Almost like a fuel cut/rev limiter. It bucks and backfires and sounds very nasty.

I've found a solution to this but I can't find the actual problem. When this happens I turn the car off and then back on(just flip the keys off for a split second then back to ignition position). The car is fixed and drives like normal.

Since a system off/on fixes this I am thinking it's ecu related. Has anyone had this happen to them?
I've run the ecu checker and get no codes but I can't test it while this situation happens.

I doubt it's the injectors or harness since it shouldn't fix itself by turning the car off and on if it was a bad wire or injector.

 

I was going to suggest grounding problems, but the switching it off/on thing is really damn weird. It sounds like you might have gotten a bad flash (or whatever it is) on the ecu upgrade.

 

Ok so i just discovered this one. There is a large transistor on the inside of the ECU. It has a little plastic cover over it to keep it from touching the ECU case. If this is missing, it sends voltage to the case and if the case is in contact with the car, like bolted in, it won't work. Mine was like this but i didn't have it bolted in, so when i hit the brakes and the ECU moved just enough to touch metal, it turned off the whole brains.

Try this as it is really, really easy. Pull your ECU. Get/borrow/steal a multimeter and set to continuity. Clip one lead to the case and test each pin. NONE of them should show cont. If one of yours does. Then you have a problem with the unit.

 

Follow up on this.
My AFR gauge wasn't working for the longest time.. hint hint.. about the same time I move the guage and installed the engine....

So I started to trouble shoot the guage. I got ground and 12v. The reference wire was wacky though...

I pull up the poassenger floor to check out the ecu and the O2 wire I spliced in. Turns out my wire was pinched to the floorboard by the metal ecu shield. So it was grounding out.

So me ecu was reading a wire which expected to see voltage but it was grounded.

So far the car runs much better. Hopefully it stays this way.

 

My car did that when it went into limp mode beacause of bad oil metering pump. I drove it a mile like that and screwed my engine. If your check engine light is coming on pull the code. I don't think s4's had the "LImp Mode" but yours may with the turbo conversion.

 

Just one more who agrees with the above statemant. Must be a series five ECU in the car. OMP needs connecting up or repair.

 

It's not a S5 ecu.
It's a S4 with Rtek 1.7 chip. No way it's S5.

I ran this ecu as rtek 1.5 on the previous engine for 2 years.

So far the car runs great. There is still a slight hesitation at the injector switch but that's pretty much normal and doesn't happen often now.