My 89gt is running 8 psi with a paxton supercharger. I have upgraded the fuel pump but the injectors are still stock. I have many upgrades to the car so I figured the injectors were a weak spot, but when I floor it I get black smoke, black water out of my exaust and fouled plugs. This seems backwards to me and I am not sure what I should do next. I heard someone talk about a lean misfire, not exactly sure what that is and if it relates to my problem. Anybody know what is up??
supercharged and still rich??
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ninety15.0
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- Mar 10, 2004
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your definately going to need to tune that motor. Stock injectors are way to small even with 8 psi. The only way to truely know how your motor is running is with a wideband 02 and a tweecer with the laptop sitting right next to u....black smoke could be a more than just raw fuel. it doesnt make any sense that you would be runnin rich....the rest of the motor is stock though?
The rest of the engine is still stock. I am not familiar with some of the tuning software that is available for the fox bodies. I just got done doing a fuel pressure check and at wot I am showing over 80psi due to the fuel control unit that came with the supercharger. Is that pressure normal? seems very high.
vikingpower
New Member
Are you SD or MAF? If SD then you require a one way check valve going to the MAP sensor. The stock computer does not know what to do with boost and will dump fuel long before the engine is ready for it.
80psi at full boost with stock fuel injectors is what I would expect from an FMU and 19s. That's not the problem.
80psi at full boost with stock fuel injectors is what I would expect from an FMU and 19s. That's not the problem.
Damn....I thought that was way high......well I still have the stock intake system which is maf. Is the high pressure because the injectors are too small causing restriction? If that is the case then wouldnt the engine be starving for fuel instead of spitting it back out? I now know it def. is not right, but not sure which way to approach it.
SUPRCHRGD302
New Member
I would do some injectors, new MAF and lose the FMU. Those things are nothing but a headache. Just my opinion. Then get yourself a good tune and enjoy.
I have been calling around local shops today and I was told that if my maf was not calibrated for the extra flow from the boost it would screw things up good. anybody else agree with that? If that is the case what is the cost to recalibrate and is it better to do injector upgrade with calibrated maf at the same time?
5.02GO
Founding Member
FMUs are just fine...
Problem is that the FMU fuel pressure step-up ratio provided by most manufacturers' kits are way tooo conservative... Usually 12:1 ratio which is 12 lbs of fuel pressure for every lb of boost. This usually results in ridiculously high fuel pressures - 80 PSI in your case. The excessive pressure causes your injectors to pump out a lot of extra fuel.. It will cause you to run excessively rich, even with 19 lb injectors..
Don't be quick to dismiss the FMU.. With a proper step-up ratio in the FMU you can get it to run well.. Get the car in a wideband and do some logging (or go do a dyno shop and use their wideband) to find out what sort of air/fuel you are running. Then start changing out for smaller disks.. You will probably find out you can cure the rich condition with an 10:1 or 8:1 disk..
Problem is that the FMU fuel pressure step-up ratio provided by most manufacturers' kits are way tooo conservative... Usually 12:1 ratio which is 12 lbs of fuel pressure for every lb of boost. This usually results in ridiculously high fuel pressures - 80 PSI in your case. The excessive pressure causes your injectors to pump out a lot of extra fuel.. It will cause you to run excessively rich, even with 19 lb injectors..
Don't be quick to dismiss the FMU.. With a proper step-up ratio in the FMU you can get it to run well.. Get the car in a wideband and do some logging (or go do a dyno shop and use their wideband) to find out what sort of air/fuel you are running. Then start changing out for smaller disks.. You will probably find out you can cure the rich condition with an 10:1 or 8:1 disk..
Black Sun 5.0
Founding Member
If I'm correct, the MAF is calibrated for injectors, not boost. With the current MAF, the boost would increase the air flow pressure on the sensor to tell the #19 injectors to put out extra fuel. If you upgraded the injectors and not the MAF (or vice versa), you would not get the correct readings for fuel delivery. The mass air system is directly correlated with fuel injectors, not boost or air flow into the engine. I would take the advice of some of the other posters here and take your car to a qualified shop, get the correct injectors and properly calibrated mass air and get the car tuned. One thing I learned over the course of time with my stang is that going cheap always ends up being really expensive in the long run (repairs). Think about it.
I think that folks are muddying the waters here. Ideally, the correct size injectors and calibrated air sensor are the way to go. It gives the most control over your fuel curve through the usable RPM range and with a custom chip, can be customized so that the motor is happy.
An FMU is required if you do not have what is above. With an FMU, the injectors and meter need to be calibrated for the motor normally aspirated. Using an FMU takes control away from the computer and is a mechanical means to ramp up fuel delivery under boost. If you're going to use mechanical means to adjust the air/fuel ratio then you must tune mechanically. It can be done on a dyno or it can be done with a properly functioning AFR meter. Every pound of fuel pressure you add with a FMU is added in a way that the computer cannot see or adjust for. What you're basicaly doing at this point is blindly adding fuel ontop of what the stock meter and computer thinks are ideal. It also sounds like your FMU is ramping up fuel pressure too quickly and too much for your application. There are a couple of things that you can try but they're not worth mentioning unless you have an AFR meter so that you can see what the air/fuel ratio is across the power range. Attempting to blindly adjust the AFR now (without the means to view your AFR) would be foolish. Leaning it out too much under boost would likely send your motor to the grave yard.
So... Plan on a dyno tune, new meter, and new injectors
--or--
Plan on purchasing an AFR meter so that you can tune it yourself mechanically.
If you select option B then get your AFR meter in hand and installed then drop me a line and I'll tell you how you can go about tuning by mechanical means.
An FMU is required if you do not have what is above. With an FMU, the injectors and meter need to be calibrated for the motor normally aspirated. Using an FMU takes control away from the computer and is a mechanical means to ramp up fuel delivery under boost. If you're going to use mechanical means to adjust the air/fuel ratio then you must tune mechanically. It can be done on a dyno or it can be done with a properly functioning AFR meter. Every pound of fuel pressure you add with a FMU is added in a way that the computer cannot see or adjust for. What you're basicaly doing at this point is blindly adding fuel ontop of what the stock meter and computer thinks are ideal. It also sounds like your FMU is ramping up fuel pressure too quickly and too much for your application. There are a couple of things that you can try but they're not worth mentioning unless you have an AFR meter so that you can see what the air/fuel ratio is across the power range. Attempting to blindly adjust the AFR now (without the means to view your AFR) would be foolish. Leaning it out too much under boost would likely send your motor to the grave yard.
So... Plan on a dyno tune, new meter, and new injectors
--or--
Plan on purchasing an AFR meter so that you can tune it yourself mechanically.
If you select option B then get your AFR meter in hand and installed then drop me a line and I'll tell you how you can go about tuning by mechanical means.
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