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Ground Rat
04-12-2006, 04:48 PM
I have a '90 Voyager w/ a probe IC, .48AR T3, and a MBC. I've been running the van at 12psi for awhile now and decided to turn up the boost a little today. I turned it up to about 14.5psi, but when the boost gets up around 13psi there is some sort of a "cut" where the motor falls on it's face for a second, then keeps building boost. Does anyone know what is going on? Is that what fuel cut is?
cordes
04-12-2006, 04:50 PM
Over boost should have you jerking forward pretty hard when it hits.

If it just falls on its face and then still runs strong again, that is some thing else I would think.
Ground Rat
04-12-2006, 04:56 PM
Over boost should have you jerking forward pretty hard when it hits.

If it just falls on its face and then still runs strong again, that is some thing else I would think.

It does jerk pretty hard, then keeps spooling, then jerks again. It does this rapidly back and forth until I let off the gas.

It did the same thing when I was at only 12psi and no IC. I thought it was the trans so I bought a new one (I wanted a built one anyway). I put the IC in about the same time the trans went in and the problem almost completely went away. I don't know if that means anything.
cordes
04-12-2006, 05:12 PM
Could be cut out I guess. What boost gauge are you running?
Ground Rat
04-12-2006, 05:19 PM
Could be cut out I guess. What boost gauge are you running?

It doesn't say what brand it is on the gauge; it came w/ the van when I bought it. It reads to 20psi. If it is cut, will the zener diode trick take care of the problem?
cordes
04-12-2006, 05:21 PM
Yes, you can use a diode, a grainger, simple bleed etc. to defeat the over boost. However you definatly want to make sure that you have enough fuel so as to not fry your motor.
Ground Rat
04-12-2006, 05:25 PM
Oh, forgot one more thing: After I turned up the boost and made a few WOT pulls, I started getting that same type of cut at only 6psi. I found one of my manifold vaccum hoses had popped off (not one for the MBC). So I just hooked it back up and the 6psi problem went away.
Ground Rat
04-12-2006, 05:26 PM
Yes, you can use a diode, a grainger, simple bleed etc. to defeat the over boost. However you definatly want to make sure that you have enough fuel so as to not fry your motor.

Hmmm, I'm using a g-valve now. :confused:
cordes
04-12-2006, 05:31 PM
Hmmm, I'm using a g-valve now. :confused:


You are using a gvalve to control the boost. However you can tee a gvalve into the map line so that it will vent pressure and not cause cut out.
Ground Rat
04-12-2006, 05:36 PM
You are using a gvalve to control the boost. However you can tee a gvalve into the map line so that it will vent pressure and not cause cut out.

Cool, I didn't even think of that. I'll have to try that once I get a wideband o2 sensor and wideband A/F gauge. :thumb:
cordes
04-12-2006, 05:49 PM
Cool, I didn't even think of that. I'll have to try that once I get a wideband o2 sensor and wideband A/F gauge. :thumb:

Sounds good. :amen:
Phreakish
04-12-2006, 11:37 PM
Drop the gap on your sparkplugs a bit, I had a similar issue where at 9 psi my car would buck like crazy, the coil was shot and dropping the gap to .025 stopped the bucking - then I hit cutout at 11.5 psi (log motor, doh), LOL.

Its something thats an easy check ;)
Ground Rat
04-12-2006, 11:43 PM
Drop the gap on your sparkplugs a bit, I had a similar issue where at 9 psi my car would buck like crazy, the coil was shot and dropping the gap to .025 stopped the bucking - then I hit cutout at 11.5 psi (log motor, doh), LOL.

Its something thats an easy check ;)

I'll try it out man. I dropped the van back to 10psi for now. In the 5 minutes I was gone from my apartment to adjust the boost someone took my parking spot. :mad:
turbovanmanČ
04-13-2006, 12:36 AM
Sounds like classic overboost cut-out. I prefer the Dawes cut-out raisers as if you run a zener and don't get the voltage close enough, it can run leaner up top.
Dave
04-13-2006, 08:42 AM
Sounds like classic overboost cut-out. I prefer the Dawes cut-out raisers as if you run a zener and don't get the voltage close enough, it can run leaner up top.

The problem with electronic cut out eliminators is that it brings the cut out point all the way to 29.4psi. So let's say you pop a vac line off your MBC, it shoots right up to 29psi and you can't cover that much boost. BOOM! I'd rather have an adjustable cut out where I can set my cut out.

About the gap to .025. How the hell was it not missing!? I gapped mine down to .031 and it was missing, cleaned up everything ignition-wise miss was still there. Gapped up to .033 miss went away.
turbovanmanČ
04-13-2006, 11:11 AM
The problem with electronic cut out eliminators is that it brings the cut out point all the way to 29.4psi. So let's say you pop a vac line off your MBC, it shoots right up to 29psi and you can't cover that much boost. BOOM! I'd rather have an adjustable cut out where I can set my cut out.

About the gap to .025. How the hell was it not missing!? I gapped mine down to .031 and it was missing, cleaned up everything ignition-wise miss was still there. Gapped up to .033 miss went away.

Good point. As for your thoughts on gapping, I think you have it backwards. Smaller gap equals less chance of misfire under higher boost and the boost has a tendency to blow out the spark.