Fuel Cut: What is it? How do I fix it? [merged]

[Slanted J]

All fuel cut threads are combined here.

I've tried doing a search but to no avail.

I was just wondering what Fuel Cut is. I've heard it tossed around alot, but do not actually know the details behind it. And, naturally, I can gather that its when the fuel is cut off, but when and why.


Thanx,
Jon

 

[ddavisaf]

I forgot

 

[ddavisaf]

When I typed that I mis-spelled a word. Posts was supposed to be pages...

 

[Nautica985]

Get a datalogger. Fuel cut occurs after air volume hits 7. In the mean time, I would try changing the fuel filter, plugs and wires. make sure the plugs are gapped right

 

[DSM90AWD]

Your past few questions in your replys were ignored because they would have no effect on my leaks. When I made this post I had no leaks and one thing came into another and another. From looking thru the post you can see that we have ran into alot of problems but were resolved thankfully to Bruce and Steve.

And they have also been asking for logs which you've yet to provide

I would like to think so but not quite, he still has to figure out why the system is only holding 10psi at the turbo inlet. After he is able to pressurize close to 20psi at the turbo inlet and the problem still persists, that log of the fuel cut would come handy

Turbo outlet elbow/gasket, BOV/gasket, SMIC or one of many couplers are the only possibilities.

I asked about his boost gauge tap as I've seen no indication he actually has an aftermarket boost gauge. Also on a stock setup the gauge will read 2-3psi higher at the turbo outlet than at the recommended FPR line tap due to pressure drop from the POS SMIC and piping.

BTW.. is your Type-S BOV vented?

Not to stroke my ego, but with my compressor set at 20psi and my boost tester at the turbo inlet I've been able to diagnose/correct boost leaks in under 10 minutes in > 40 attempts on 3 different cars. You will not be able to diagnose worn valvetrain parts with an intake tester. Only a leakdown tester/compression tester will accurately provide this infomation.

Now how about those logs?

 

[ddavisaf]

Turbo outlet elbow/gasket, BOV/gasket, SMIC or one of many couplers are the only possibilities.

That's not true.
If the turbo oil seal is bad you can leak air past them through the oil pan and all the way up through the motor.

 

[heppm01]

Fuel cut occurs after air volume hits 7.

Not quite. It's not uncommon for me to hit 7 for a few seconds without fuel cut. But agreed: there's not a lot of wiggle room there.

M

 

[DSM90AWD]

That's not true.
If the turbo oil seal is bad you can leak air past them through the oil pan and all the way up through the motor.

The compressor forces air (compresses it) into the scroll area of the turbo causing a low pressure area at the blades (i.e air is flowing towards it). How would air be forced behind the scroll and past the oil seal?

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If anything the compressor o-ring seal is compromised and blowing out the backside of the compressor (though I've yet to hear of that happening before) but would explain the leak he found.

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[ddavisaf]

I've heard of it once.
Excessive crankcase pressure can be caused by air leaking past the bad turbo seal, through the oil return line, up through the oil pan and past the piston rings.
However, it's not relavent to this specific problem since his turbo would be smoking if this were the cause.

Wait a minute....


...

After reading my own statement I just proved myself wrong.

Doh!

 

[ddavisaf]

Sorry John, thanks for correcting me!

 

[DSM90AWD]

Sorry John, thanks for correcting me!

or I might get a complex

 

[oldman]

You two want a room?

 

[ddavisaf]

Depends if you're chaperoning

 

[Boost95]

OK I have the boost leak tester hooked up one last time. Im trying to pin point it better. I can hear the leak more thru a vacuum tube when I put it to the left of where the oil line bolts into the top of the turbo. Right near the back side of the compressor housing
Thanks,
Mike

Logs are not going to help anyone right now. Like I posted a while ago I have a leak at my turbo. I think the o-ring that seals teh turbo from teh center cartridge is shot. This post was much more helpful when Bruce and Steve were just helping me. They were very precise and accurate with what they posted. Now it is alot of replys that are not helping me at all. Like I said in the 1st post, not to post unless you know what you are talking about.

Thanks,
mike

 

[Boost95]

The compressor forces air (compresses it) into the scroll area of the turbo causing a low pressure area at the blades (i.e air is flowing towards it). How would air be forced behind the scroll and past the oil seal?

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If anything the compressor o-ring seal is compromised and blowing out the backside of the compressor (though I've yet to hear of that happening before) but would explain the leak he found.

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Very helpful. Thats the o-ring im talking about. Its probably all dryed out and cracking.
-MIke

 

[dizuster]

Hey I stumbled onto this thread late cause Boost95 was selling some parts. . . anyway. . .

I'm not doubting that the turbo seals might be leaking (sounds like a good possiblity) but for it to fuel cut at only 12psi or what ever would have to be a really big leak!!!!

The computer cuts fuel when air flow rate exceeds a certain number right? The MAS really doesn't read mass airflow, it really reads air speed. The computer doesn't just use the MAS readings alone, it also uses Baro readings, and air temp to calculate the acual mass air flow rate. You hear of lots of people who get fuel cut in the winter because the air is so cold and dense. Maybe the air temp sensor has taken a crap and is reading really cold, tricking the computer into thinking it's got more (dense) airflow then it really does? I'm not sure where the air temp sensor is (if it's in the MAS itself I'm gonna sound like an idiot because you already switched it off of your '90 car.) Anyone care to comment?

Also I think a good way to test this and/or the boost leak theory, is to take your MBC off of that car, and bolt it directly on another car without adjustment. If it's got a big leak on your car, and it's only showing 12psi at fuel cut. Then on another (same 14b car) the boost should be through the roof!

Sometimes it's not bad to get somebody in late to get some fresh ideas. Hope this helps.

Good luck!!!

 

[Reese92tsi]

Boost 95, you still need to consider the possibility of fuel starvation. It might be a good idea to start changing out the fuel system components just to eliminate some variables. Also, if you are seeing high injector duty cycles this might be an indication of fuel starvation.

 

[Boost95]

Boost 95, you still need to consider the possibility of fuel starvation. It might be a good idea to start changing out the fuel system components just to eliminate some variables. Also, if you are seeing high injector duty cycles this might be an indication of fuel starvation.

True, but 1st things 1st. I need to think what im going to do about the turbo.

 

[Reese92tsi]

True, but 1st things 1st. I need to think what im going to do about the turbo.

If the compressor housing seal is shot you might be able to run a bead of red RTV in that area. RTV does a great job at sealing compressed air.

 

[Boost95]

If the compressor housing seal is shot you might be able to run a bead of red RTV in that area. RTV does a great job at sealing compressed air.

Really?

 

[Reese92tsi]

Really?

Yep, you might be able to do this with the turbo still attached but I'm not sure because I've never tried to take the compressor housing off with the turbo still attached. The best way to do this would be to remove the turbine housing/O2 housing to give you more space to work. First remove the v-band clamp that connects the turbine housing to the center section, then remove the turbo to manifold bolts, and the downpipe to O2 housing bolts, this only takes about 20 minutes. Removal of the turbine housing may not be necessary but if space is limited it may help. The giant C clip that connects the center section to the compressor housing is difficult to remove, you will need needle nose pliers to compress the C clip. After the compressor housing is removed, remove the old seal and run a small bead of RTV around the lip of the compressor housing, wait about 10 min for the RTV to dry. Install compressor housing then leak test it.

 

[DSM90AWD]

After the compressor housing is removed, remove the old seal and run a small bead of RTV around the lip of the compressor housing, wait about 10 min for the RTV to dry. Install compressor housing then leak test it.

Why would he go to all this trouble disassebling the compressor housing just to ghetto-rig an o-ring out of RTV

This is a $6 part at MachV (or FP/AGP/Buschur).

Do it right and do it once

 

[Boost95]

I misunderstood you. I thought you meant that I can run a bead w/o taking the whole thing apart. Thats why I was shocked to hear that. Thanks tho.
-Mike

 

[Reese92tsi]

Why would he go to all this trouble disassebling the compressor housing just to ghetto-rig an o-ring out of RTV

This is a $6 part at MachV (or FP/AGP/Buschur).

Do it right and do it once

My bad, I thought that this was a part that was hard to get.

 

[DSM90AWD]

My bad, I thought that this was a part that was hard to get.

Believe me when it comes to RTV, I've found out the hard way what does/does not work. And, unfortunately, o-rings are one of those that doesn't

 

[Boost95]

Well Bruce, I had a little extra time 2day so I worked on the car for a few. I ran my wg line different. I ran the leak test again and I was able to built enough pressure to blow the top off my leak tester. From 20 PSI, it went from 20-10 within about 10-15 seconds. It then held 10 PSI for a long time.

Thanks,
Mike