s4 t2 bogs at 3700 rpm help!!
 

So I got the gar running good a little while back and now its starting to bogg at around 3700 rpm if its more than about a quarter throttle if you keep it under quarter throttle it will rev fine all the way to redline almost as if the secondary injectors aren't firing or not enough fuel car was running fine before on 10 psi of boost put it back to stock now because of the issue and dont want to mess anything up here's a list of mods oh and it will sit and idle perfect still at 750 rpms and the tps is right on even tried an extra one I had laying around

MODS

3" dp to straight pipe
3" aftermarket intake
Rats nest delete still left bac and didn't touch the tb
Racing beat fcd
Air pump removed
S5 turbo
MBC
FD fuel pump
Freshly rebuilt injectors
New fuel filter

Could it be the stock fpr that's the next item on my list to upgrade any help is greatly appreciated

Are the secondary injectors firing?

Does it miss, fart or carry on or is it just flat?

Are you getting good voltage with the engine running at the ignition switch?

Are all the earths/grounds good

Oh... Are all the coils firing,, plugs ok?

Coils are both firing I just changed the plugs about 2 months ago and re did all the grounds even added an extra one for the boost sensor to the intake manifold how do you check if the secondaries are firing

Oh and if you give it a little throttle its just flat but if you give it more it pops and sputters

Undo that ^ and see if it goes away.

There was a tsb for that on early s4 t2 cars.

Sounds like your timing could be off or the secondary injectors are not firing

There are 4 wires at the boost sensor. Only one of them is a ground wire. Some people choose the wrong wire as the ground wire. Make sure you did not select the wrong wire to ground.

The plugs to the secondary injectors might not be secure or the two wires in each plug might not be making good contact w/the injector.

There are 4 Green wires grouped together at the ECU (they should register 12 volts w/key to on). Two of them are for the secondary injectors. The voltage should drop to about 6 volts when the secondary injectors are triggered. To recreate load on the engine while idling you can unplug the TPS and remove the vacuum line from the boost sensor. Then you can free rev the engine above 3800 rpm and the secondaries should fire.

my t2 was doing almost exactly that. Under the throttle body, there is the main ground for the harness, I ran a Thick gauge wire direct from that ground to the battery ground. I also adjusted my tps and replaced my spark plugs because they were fouled up. Now the car pulls perfectly to redline, no hesitation or anything.

edit. the wire i used was actually a spare battery ground cable i had, but you get the idea

OK so I threw a fuel pressure tester on the car finally and only got 30psi at start then woul d go down to 0 after I bled the gauge does anybody have a good write up on how to check voltave and rewire the fuel pump I ordered a new fuel pump again and want to do the wiring while I'm at it

Its the 3800 hesitation issue caused by poor grounding.
Make sure the your engine harness grounds are put back correctly. Check Aaron's regrounding thread. As it is obviously a ground issue here.
Start with regrounding the boost sensor, body harness, engine harness ground that attaches to the middle iron, and also check your boost sensor vacuum line to see if it is blocked with something.

I just recently re did all the grounds even sanded down the contact surfaces but the fuel pressure is what's getting me I'm pretty sure is there any write-ups on rewiring the pump

Use the advanced search feature and you find what you're looking for it ( pump rewiring) has been done many times by members over the years.

Are you sure the injector plugs are good. Did you accidentally switch the plugs where the primary plugs are on the secondary injectors?

There is a staple which groups together two of the ground wires near the ECU which needs to be in good order.

Does the car free rev properly?

i had similar symptoms after replacing the injectors one of the bottom mount rings got cracked, it would run fine unless going into boost(making the 2nd injectors to come on) if it was above 3800 rpm. i didnt find it until i pressurized the intake after the afm.

Re did the crimp and added solder that's a little bit up the harness from the ecu the plugs are fine and they are on the right injectors car does not free rev properly same symptoms as under load

The injectors came with same size new rings from Mr injector and even sent them back for a recheck about 3 weeks ago and came with new seals again so I doubt its that it boosts fine before the 3700 rpm then it bogs and popps

Do this because it's easy and quick.

Mine did the same...

Check for good voltage at the key/interior fuse box...

Should be the same or very close to battery voltage, whilst engine is running,

Might want to check the voltage on the Blue/Green wire from the fuel pump relay to the fuel pump and see what you get. You could also temporarily bypass the relay by unplugging it and jumpering one of the Blue/Red wires to one of the Blue/Green wires.

Hey satch.. Do these have two relays with a ballast resistor for slow speed?

Thought I might have read it somewhere..

Basically yes but there's only a single relay.

Rewired the fuel pump by adding an extra 4 pin relay and a wire all the way from the battery to the fuel pump with a 30a fuse in line still had the same symptoms tomorrow I'll have more time to mess around with it going to re check all the grounds again just to be safe

^ this is if I was still running the stock fuel pump wiring right?

Na not necessarily. Low voltage at the coils or ecu could also cause this 'wig-out'.

I realized that when I had the heater and lights on it was worst. At the same time the voltage on the apexi turbo timer was 11.9 whilst idling at traffic lights.

I recreated the same situation the next day and battery voltage was a steady 13.8 (not great but sufficient) Turbo timer still reading around 12v..

So after a bit of time hunting with the multimeter I worked out that the cabin power was fed by a big fat black wire that plugs in near the battery.

This is where the volt drop was.. At that actual connection. After cleaning the connection I managed to get it down to about 0.2v (read across the connector).

Tested the fuel pressure again had about 32 at idle and 40 when reved up does that sound right