hey i have a question .. i have a 1990 GSx and i have a 16 g turbo and was wondering how much boost is to much. when i drive it it sometimes bucks and i think its fuel cut but i have the fuel pump upgrade so i think i have enough fuel but i still have all stock internals right now the max boost i run is 15 psi... i tried 18 but the car seems to buck a lot more when i do that. so if anyone could help me that would be appreciated..

 

In my expirience which I dont have much of Bucking seems to be related to having too much fuel. Usually this is caused by your MAS missing counts or not being able to meter the air at all (from an intake leak somewhere after the MAS itself). Are you sure it is a buck or a miss? If it is missing it could be several things. I wanna say not enuff fuel since when you turn up the boost it gets worse but your setup should be fine @ 15psi or even 18 psi. It could be ignition too. I would try testing all your ignition components as per a shop manual and make sure they are in spec and check you fuel system. have you replaced your fuel filter recently?

hope some of this helps.

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Joe
94TSiAWD
http://www.cr-racing.com/talonpage.htm
13.001 @ 106.55

 

well ima almost positive its a buck . but not that you mentioned it iam not sure . how would i tell the differance between a buck and a miss? and yeah i changed the fule filter about 7 months ago so i know thats alright. was yours doing this too? and if so did it goaway with the S-AFC? iam just lost . but i cant do anything to it for the next week or so its at the dealer gettin the the recal done.. but yeah you have helped.

thanks

 

Mine was missing at high RPMs with the stock MAS. With my new one that I acquired from a friend I no longer have that problem.

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Joe
94TSiAWD
http://www.cr-racing.com/talonpage.htm
13.001 @ 106.55

 

If by buck you mean the car pitches forward violently during a WOT run usually in the top of a higher gear (like 3rd or 4th) then you are hitting fuel cut. This isnt just really bad sputtering...the whole ignition system is shutdown when you hit fuel cut and the engine just shuts off (which is very scary the first time it happens when in gear at WOT).

If it just acts like a bad sputtering (or miss) at high RPMs then you are probably getting MAF overrun (but a miss is just an indication of lack of fuel or ignition). How many honeycombs have you removed if any? With that turbo you should be very close to overrunning the MAF (if its unmodified) even on 15 psi.

The AFC wont help either condition...BUT you can install larger injectors and compensate with the AFC. Youll still need to mod your MAF or switch to a 2G MAF to get around MAF overrun. Youll also be able to see your Karhman counts on the AFC so youll be able to tell exactly how close to MAF overrun youre coming.

Id try rewiring the pump (from the VFAQ)...that helped my top end quite a bit. Then pull some honeycombs out of the MAF (if you havent already). Last resort is really injectors and AFC but you will need them anyway to run real boost at the track.


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John
90 Talon TSi AWD
2KW K4U

 

hey joe i wounder what friend that may be... and john i have two hinycones removed i think ... i forgot all about selling the mas to joe.. i think that may be the prob . i did do the rewire to the fule pumb so let me get the mas redone and see whats happends . and also would it be doing that too if i had a bad O2?
thanks again for the help guys

 

You should only have the lower honeycomb removed. Get that thing Blaha-ed... or maybe even go with a 2G MAS. Talk to John about that one.

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Joe
94TSiAWD
http://www.cr-racing.com/talonpage.htm
13.001 @ 106.55

 

The O2 isnt checked at WOT (open loop) so it shouldnt be an issue. Does this bucking occur only at WOT?

Have you added an aftermarket fuel pump yet? It is possible you are starving for fuel. Do you have an A/F gauge or EGT? What O2 voltages are you seeing at WOT? Whats the EGT in the top of 3rd?

Probably time you picked up an aftermarket pump if you havent already. Grab a Walbro 255HP Denso TT Supra or Cosmo pump. Id recommend the Denso unit (high capacity high pressure very quiet) but the Walbro is considerably cheaper...both are drop in. Either unit will get you into the 10s so I dont think youll need to worry about fuel delivery again for a while.

You can safely remove the lower and 2 side honeycombs as well as back out flush the adjustment screw in the bottom. That should stave off MAF overrun for a while. I hit fuel cut before ever seeing overrun (with the bottom and side honeycombs out). My Karhman counts were somewhere around 2250Hz which is (supposedly) at or past the max of the MAS. I had a correction factor of +38% at idle on the AFC to richen things back up...it wasnt near that bad at high RPMs and my car WOULD idle without a correction but my A/F Gauge wouldnt cycle like it was supposed to (the ECU fuel trim was too far off for it to compensate).

Before switching to the 2G MAS I added larger injectors and got rid of the fuel cut problems but my AFC was tuned like -30% at idle. After adding the 2G MAS my values are around -10% across the board for low throttle...not all that bad. I was hitting 1600Hz for Karhman when I initially switched to the 2G MAS and now Im hitting 1900Hz with a boost leak so I dont think the 1G MAS would made it this far without some serious hacking (and I still have ALL the honeycombs in the 2G MAS ). The 2G MAS is good for approx. 400-425HP before requiring modification so Im leaving all the honeycombs in for driveability (Jeff has left all of his in too without problems so far). Depends on what you are looking for...I found the 2G MAS for like $100 with wiring harness and turbo inlet piping. Youd also need a new 2G filter w/ adapter or 3KGT filter. If you plan on large injectors the 2G MAS compliments them very well (the 2G would drop in a 1G with 500-550 injectors WITH OUT any fuel control!).

Anyway...make sure you arent running lean first (might need a new pump). If you arent then probably MAF overrun so yank some honeycombs. Then next would probably be larger injectors.


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John
90 Talon TSi AWD
2KW K4U

 

well as for the fuel pump i have a 95 mustang GT pump witch is good for what i think was 255. not sure tho.. but when it Bucks iam running rich . as for the EGT and the Karhman count i wouldent know i dont have a AFC nor a EGT guage . but it only dose it at WOT top of 3rd and 4th but i do think the problem is the MAS.. iam going to do some major gutting as soon as i get my car back..

also would it matter if my ICU was bad? i just found that out to day and i need to get a new one...

 

You have a stock Mustang GT pump or an upgraded? I hope it was upgraded...stock is small as hell.

Is it a violent buck...like the type that makes you think you broke something or just a slight stutter? If its a very violent buck then its fuel cut...otherwise MAF overrun or a boost leak (which I forgot to mention).

Either way hacking the MAS will drop the Karman counts and consiquently lean out the engine...which *SHOULD* cure the MAF overrun as well as the fuel cut. Just make sure you dont end up running the engine too lean (might want to verify what pump you have).

I ran my 1G MAS with the center honeycomb *ONLY* for quite a while without any fuel control and I didnt run into fuel cut until I added the AFC and bumped my fuel trims up (which in part defeated the purpose of hacking the MAF...but it still wouldnt hit MAF overrun).

You may also have a boost leak. It would cause the symptoms you are describing. Car should go VERY rich if thats the case. Normally I see .84 or so on my A/F (90s are lower O2 voltages than any other year for some reason) and when I got a boost leak at high boost the A/F gauge would practically bury itself at 1.00V.