input: Wierd throttle surging at 3000 RPMS.

[Dieselhound]

Ok, cars a 1996 OBD II GTI, motor has 120K, and recently I get throttle surging at 3000 rpm, when the pedal is held in the same position.

Cam is a 276, at -4 degrees.
P-FLO
Autotech chip,
MAF is new,
IAT is new
All coolant temp sensors are new.
Neuspeed wires
OEM plugs,
new coil,
new oil pressure sensors,

Car had same problem before I started "throwing parts at it"

I've swapped the throttle body, no change
I've swapped the knock sensor, no change
I've swapped the main engine harness, no change
I've swapped cap, no change
I've swapped wires, no change
I've swapped the Fuel rail, injectors and FPR, no change
I've run all diagnostics, no lights.

If you hold the pedal at 3000 rpm in nuetral, the car will drop 300 rpm, ever 5-6 seconds.
Causes bucking at lower engine speeds when under load.

I can drive the car, but its annoying, and it feels down on power. It honestly sounds like its killing fuel and spark for a nano second then coming back on???

Could this be the engine speed sensor? I've checked for vaccuum leaks, couldn't find any, and its too repeatable to be random.....

Any help is appiciated.

I basically have the original motor out of the car on the engine stand in the garage, so I pretty much have 2 of everything drivetrain wise. (except O2 sensors, but I don't think thats it)

Car ran fine on this motor and all old parts for over 7 months. I'm starting to get frustrated. Vag-Com shows nothing wierd. TB adaptation is good.

It does need a new ignition swith, and a heater core (bypassed), but it ran fine for months like this.

I did have the fuel pump go out before the motor swap, has an e-bay one in it now. Fuel pressure in the rail seems good, but haven't put a gauge on it.

Could this be a bad hall sender in the dizzy?

WTF....

[1965truckrod]

Well a very common sign of Bad O2 sensors is irratic idle and power loss. Sometimes its just enough to cause the idle problem but not enough to cause a cell or to have the computer log a error. I am not exactly sure on what error code it would be but I'd try a new set of O2 sensors to eliminate them out of the scenario. When I had my cars clutch and flywheel replaced my car was running around without a speed sensor as apparently the shop forgot to put it in and I didn't have a irratic idle or power loss. If you can get a speed sensor I would try replacing it to see if thats the problem as well. I know Hall sensors go bad all the time so that could additionally be a problem but IIRC they will trip a code and or post a Cell light.

My suggestion is to start with the cheapest parts and eliminate them out of the situation to narrow down the suspects. From my personal experience a problem with Idle or irratic throttle speed is usually related to O2 sensors not reading correct inputs and giving the Computer a false reading. Keep us up to date and hopefully its something simple Good luck.

Jesse

[Dieselhound]

Started hearing the muffler bearings rattle.......

J/K, I think the cat may be damaged. I'll look at it this weekend

[mike]

Check fuel pressure. eBay pump may be no good. I have seen bad new ones not flow enough out of the box. I was going to say check the MAF but missed that you replaced it.

[Dieselhound]

Pump is good, thought that may be the case too. I don't have a way to read accurate fuel pressure, but i have a working VR6 pump that I swapped in and still same problem, however, since i started hearing the rattling and 1200 rpm, I am leaning towards clogged Catalytic converter and incorrect O2 readings.

the strange part is all the ross-tech datalogs (graphed) that I have show all signals dropping out at exactly the same time, and the same pattern, which leads me to ECM fault. I tried swapping in my spare ECM, but the car wouldn't fire up at all, so I swapped back to the original Autotech ECU. I cracked open my spare and realized i sold the chip, woops....

I'll do more diagnostics this weekend when I pull the trans.

[Dieselhound]

FYI, it does have a big cam. 276. most local tuners (neuspeed and TT) say anything over a 260 on an OBDII will cause issues. could this be it?

[mike]

yup. More then likely the MAF doesn't like the overlap. I forgot about that. I had a 260 in it from Autotech when I sold it to Justin so it should still be in the other motor. Thats easy to change.

I also have the stock chip for it in my tool box. Clogged cat, it would run like shit and stink. That car has about 3 screws in the heatshield on the downpipe for the buzz saw rattle since I refused to buy another downpipe that would start rattling. Hit the cat with a hammer to see if you hear the brick.

[Dieselhound]

Pulled the heatshield a LONG time ago. Couldn't stand the rattle. Car is way too low to hit with a hammer. Gonna try and get it in the air tomorrow. It does run like shit, and it stinks, but thats the gear oil coming from the output flange on the drivers side and dumping on the exhaust. Gonna swap the trans this weekend too. The 3.94 R&P is killing my milage.