GS-T Jerks Under Boost

[DuvalGs-t]

I have a '95 GS-T that i bought from a guy that lives down the road from me. He only had it for about three weeks before he sold it to me. The guy i bought from the car, bought it from another guy who had it in his garage as a project car. To get to where im going, i dont know exactly what has been done to my car. It has a Turbonetics turbo with a blue backplate, fmi with Greddy piping, aftermarket rims, aftermarket front bumper, TurboXS RFL BOV, NGK plug wires with NGK Iridium plugs, Apexi 3" N1 exhaust, aftermarket boost gauge and narrow band. Thats all that i know of that has been done to the car, so i didnt think it was a bad deal for $1700. So this is what the car is doing and i dont know why, ive been looking for about three weeks and i still cant find anything. The car runs fine out of boost, but when i hit about 6psi, it starts bucking real bad and blows black smoke. After i do that a few times, i guess the car goes into limp mode or something but it starts sounding like a Subaru. The car only does this with the MAF plugged in. When i unplug the MAF, the car stumbles up to speed, but after the car builds boost, its fine. I thought that maybe a dirty maf would cause it to make the ECU think its running lower boost and hit a fuel cut, but even with the MAF being cleaned, it still does it. Im lost to what to do to fix this car. Help would be much appreciated, thank you.

 

[WES_393]

I see 3 possible issues here. First, you need to do a real boost leak test. Pressurize the intake and spray soapy water on everything intake related. Second, you have a larger than stock turbo while still using the stock fuel system. This would explain the lean reading on the narrowband. 450s cant even handle 20psi on a 14b let alone the average sized Turbonetics. Hopefully you havent melted a ringland. Lastly, the jumpy tach is probably from a poorly connected ICM (power transistor).

But honestly, if a boost leak test is too much to handle then this isn't the right car for you.

 

[Mischif]

Not sure 450's can handle 20psi without raising fuel pressure. 20psi is also way too much for a stock fuel pump.

^ looks like i got beat to it.

 

[DuvalGs-t]

I know i need to do the blt i just never made it around to it. And the fuel system held it fine before the head gasket was replaced, but now its narrowing out at around 6 psi. And i thought that too but the connecting was good. Should i try putting the old power transistor on and see if it continues? I only replaced it for good measurement. I found out after i replaced it that there was a bad coil pack, so the old power transistor is fine.

I dont know if the fuel pump is stock or not. Its not very noisy but it isnt quiet either. On the fuel line going into the fuel rail, there is a brass four way block thats capped off. I dont know what thats all about either. Maybe its someones way of rigging a fpr if the fuel pump isnt stock?

 

[FLASH1970]

Ill try doing a blt out of curiousity even though im selling it. But if its a boost leak then wouldnt the narrowband peg rich instead of lean?

If you're unwilling to figure out the issue and just beat the snot out of it and call it junk, the car is probably better off in someone else's hands. A narrowband is useless. Get a wideband. It'll tell you what your AFR's are. All a narrowband gauge is, is pretty lights... Don't boost a car to 20psi on a stock fuel system. Leave it at 12psi. Do not pass GO, do not collect 200 dollars... Do a boost leak test.

To answer your question...

Your car either runs in closed loop or open loop. When you are cruising down the highway, just cruising at a steady speed, not flooring it, your car's ECU is monitoring whether the AFR is above or below 14.7:1. This is called closed loop. The O2 sensor tells the ECU if it's below or above 14.7:1, the ECU adjusts fuel, the ECU asks the O2 sensor "how's that?", the O2 sensor gives it feed back, the ECU adjusts fuel, the ECU asks the O2 sensor "how about now?", the O2 sensor gives the ECU feedback... it's an ongoing loop as long as you're cruising. It's a "closed loop" of information that keeps going around and around.

The other form is called open loop. Open loop occurs when you're flooring the car. The boost is coming in, the car is accelerating... The ECU ignores the O2 sensor, and just goes off of values in tables. Think of it as a Microsoft Excel table. Across the top of the table in each column is a load value. To keep it simple, Load refers to air pressure. 0-100 is out of boost. Over 100 is in boost because you're forcing more than 100% of atmosphere pressure into the engine. Each row going down the side contains an RPM number. The ECU reads how much air is coming through the MAS, what the engine RPM is at, and picks a cell from that table and tells the injectors to squirt in "this much" fuel based on what value was in the cell of the table. The ECU doesn't bother to ask the O2 sensor for feedback because the AFR is obviously going to be richer than 14.7:1. The ECU in open loop relies on the knock sensor. If you start getting knock when you floor it, the ECU will pull out timing which robs power. If you're boosting 20psi on a stock fuel system, your car is running lean and detonating and knocking and not happy.

That is why YOU NEED A WIDEBAND O2 SENSOR. A wideband can read the air fuel ratios when you floor the car. You also need to datalog your car. You need to log knock, rpm, fuel injector duty cycle, AFR, engine load, throttle position...

 

[PieEyedPiper]

Enjoy your VW Golf!


Could you lay it out for us, the things that you've done to try and get your car fixed? I've read about your hg and such, I was speaking more towards any of the suggestions you've been given in this thread.

 

[kenuwine4g63]

You really need to do a blt. If you have to take it to a gas station and use one of there air hoses. On top of that check your spark plugs, make sure you have a heat range of 6 or higher. Anything below a 6 will cause your car to buck and go crazy in boost it may even develop a misfire at idle or regular crusing speeds. Turn the boost down to about 14psi, or all the way down to stock which is 10psi(to be safe) and figure this thing out. Its the simple things with these cars but that Audi will run you a fortune in performance

 

[xkspikex]

What engine management system are you using?

 

[FLASH1970]

What engine management system are you using?

I don't think he has one. He has an aftermarket turbo, front mount, boost controller, narrowband, and a boost gauge from what I gather.

 

[xkspikex]

Possibly a tune or a boost leak could result in this

 

[slayer450]

Maybe you should try sedsm.com. You may be able to find someone local to you that can help you out. I'm not trying to steal the thunder of this forum, this place is great so dont yell at me please. I'm just thinking he may be able to find somebody right down the road. If you were closer I'd do the blt for ya..