Fuel cut? Misfire?

[RTtim]

I could use some thoughts from everyone. My car runs great as long as it doesn't make boost. As soon as I get to 5 in vac or to 0 my car starts cutting in and out. I just installed a a/f gauge and when it cuts out my a/f Jumps up to 16 or so. I thought it might be the fuel pump so I swapped it with a known working one from a mk3 and that didn't fix it. The fuel filter is new as well.


Is it possible for the mas is going bad? I did try unplugging the coolant temp sensor and then the o2 sensor to make it run richer and it helped. Like that I can boost 1-2 psi before it cuts. Any thoughts????

 

[RTtim]

I think I figured it out, I will post up when I know for sure.

 

[OfnaRcR4]

You've got some serious issues. 16 is waaay too lean. Not going to be fuel cut unless you have a massive boost leak but that wouldn't explain the lean mixture. You're just not getting enough fuel somehow. Could be electrical or a block/worn out pump etc...

 

[airhead04]

A boost leak would explain a Lean mixture. A boost leak is unmetered air in the intake system which the Ecu is not able to see. Thus causing not enough fuel to be produced in the cylinder. Which would explain his 16+ AFRs.

 

[OfnaRcR4]

Boost leak will be metered air not making it into the engine but a fuel trim for boost when there is none.

 

[airhead04]

How could it not make it into the engine, in order for a car to be in boost you have to be running it (driving). The air WILL make it into the cylinders and not be accounted for which would cause a lean mixture in the cylinders explaining his 16+ afr. And would also explain why his car isnt running the best it could be.

 

[OfnaRcR4]

Google boost leak hah

 

[airhead04]

I understand what your saying. I retract my statements from earlier. Was mixing up boost and vaccum leak.:3d_frown:

Sorry about that. lol

 

[jstricker]

No, the air is metered at the AFM first thing in the intake tract. Any leak after that point when under boost is going to vent pressure to the atmosphere and NOT make it to the cylinders. The ECM will assume the amount going through the AFM is getting to the cylinders and put that much fuel with it when in fact some lesser amount of air is actually being pumped into the cylinders.

John Stricker
Russell, KS

 

[CyFi6]

Boost leak = Rich mixture

Boost leak big enough= Fuel cut

Fuel cut= Injectors turned off

Injectors turned off = O2 in the exhaust = lean reading on gauge

If your check engine light comes on when it cuts out, it is probably fuel cut. Look for intake leaks if that's the case.

 

[RTtim]

Well part of the problem was the o2 sensor. I changed that and it got a lot better. Right now the car will boost to about 3-5 psi and it cuts in and out for a second or two then goes away. I can throttle thru it and the cars like it should, when it does cut in and out the afr's go from 11.5ish to 16ish. Do you guys think it could be possible for fuel pressure reg to be having a hard time? Just a thought, this whole turbo thing is kinda new to me

 

[RTtim]

Something for everyone to think about is my car runs great and has no issue at all after it has been parked for a couple hours. Only when I drive it for 40-50 miles does it start to lean out real bad.


I thought that maybe the tps mightbe having a problem so I swapped it with a different one and there was no change. So I am at a loss right now. Any ideas would be appreciated and I will be messing with this car every night until I figure it out.

 

[OfnaRcR4]

First off we need to figure out if out is indeed fuel cut or misfire. Fuel cut is a sudden cutout for a split second and then the power comes back right after. It can be quite violent and your check engine light will come on for a second. A miss is just when the car kinda putters out but is still trying to accelerate.

 

[RTtim]

It acts just like fuel cut but the check engine light never flashes. I did think about it being a misfire but if it was you would think my afr's would be rich not lean.

 

[CyFi6]

It acts just like fuel cut but the check engine light never flashes. I did think about it being a misfire but if it was you would think my afr's would be rich not lean.

Does your check engine light come on with the key on engine off (need to make sure the bulb works)? If you had a misfire it would show as lean on your gauge, not rich. The wideband only measures oxygen, not HC's

 

[OfnaRcR4]

The wideband only measures oxygen, not HC's

+1

 

[RTtim]

Yes the check engine functions as it should. I will spend some time tomorrow to check for misfires and post up what I find.

 

[RTtim]

Well the problem was one spark plug had a small crack on the side. I checked the plugs once and never saw it but decided to check them all again and there it was. Changed the plug and so far all is good. O well at least it was a cheap fix.

 

[3p141592654]

Cost you an O2 sensor that was perfectly good, and couldn't possibly have been the problem anyway.

 

[RTtim]

The o2 sensor didnt cost me anything, I have a complete car as parts car. I'm just glad it is running great now. Thank you everyone for the words of wisdom