DFI Computer?

[Rob Mears]

I'm purchasing an '89 Corvette that has been fitted with a DFI Computer. I'm trying to learn more about the DFI unit. The seller of the car informed me that the last time he had it tuned, the car was hot (supposed to be tuned cold he said). As a result the car tends to surge at idle range rpm's.

I'm trying to determine first of all whether this is a load of nonsense. If not, is the average GM mechanic capable of reprogramming the DFI Computer to solve this problem, or do I need to locate a specialist?

I'm brand new to the Vette game so bear with the 'novice' questions. I'm a quick learner

Rob

 

[Rob Mears]

I'm purchasing an '89 Corvette that has been fitted with a DFI Computer. I'm trying to learn more about the DFI unit. The seller of the car informed me that the last time he had it tuned, the car was hot (supposed to be tuned cold he said). As a result the car tends to surge at idle range rpm's.

I'm trying to determine first of all whether this is a load of nonsense. If not, is the average GM mechanic capable of reprogramming the DFI Computer to solve this problem, or do I need to locate a specialist?

I'm brand new to the Vette game so bear with the 'novice' questions. I'm a quick learner

 

[Drewser]

Welcome

Welcome to CAC. You will learn a lot from the very knowledgable people here, and chuckle at the rest (like me)

First off, you only have to hit the submit button once. (sorry, I don't know dede about DFI)

 

[69MyWay]

Rob,

Welcome to CACC. Tell us more about the other mods done to this car.

I can tell you from experience that putting a stronger cam alone no matter what the computer programming is will cause the computer to hunt for a steady idle. So, it is not nonsense at all that mods can create this problem and or that there may be no way to eliminate it short of putting the engine back to more stock specs.

Before jumping off that bridge, go back to basics. Check for any vacuum leaks. You need to clean and service the idle air control motor, and make sure the TPS is in an appropriate idle volt level. The timing needs to be brought into specs.

All adjustments should ONLY be done after the engine is warmed up. The only "cold" set that I can think of would be for the valves when putting the rocker arms back on.

Good luck!

 

[Rob Mears]

DFI & Other Mods

The car has these mods to the drivetrain:

Extrude-honed / ceramic coated Lingenfelter JL-1 fuel injection & manifold
58mm throttle body
Ported & polished ZZ3 heads w/2.05 valves
MSD ignition
DFI programmable computer
Art Carr transmission w/3500 stall converter
124-amp alternator


There IS a vacuum leak with the power brakes I believe. He told me that the brakes are working, but feel like manual brakes - no power boost. I figure the booster is not getting any vacuum for whatever reason. This symptom arose suddenly after the car had sat idle for a long time. I'm pretty sure the brake booster itself couldn't go bad from lack of use. I hope to trace the source of the possible leak as soon as I take delivery of the car, and see whether fixing it solves the idle problem.

I'm not sure of the cam grind, but it might well be a bit too radical for the computer to maintain a healthy relationship with. I can see where a 'lopey' cam could throw off the computer, but how about the revalved heads, etc. Do you think that might affect the computer's capacity for keeping things balanced?

I'll surely take your advice regarding the idle air control motor and the TPS volt levels before I do anything radical. I only need to know one thing...what is TPS?

Thanks,
Rob

 

[Drewser]

This one I know

TPS is Throttle Position Sensor. It is mounted on the rear TBI unit(on mine) and it contains a variable resistor used to proportion input voltage, based on the angle of the throttle valve. The ECM uses this signal as a reference for throttle valve position.

Good luck and Happy New Year

 

[69MyWay]

Wow,

That dude spent some bucks on that engine. Even with the brakes fixed, you are going to get a lumpy idle, there is just no way around it. In fact, on extreme hot or cold days it might even stall once in a while.

I am assuming you do not have a Mass Air Flow sensor? Even if you do, it won't make much difference. However, when you don't have one the idle side tends to suffer if it is programmed for total wide open throttle performance. Mass Air Flow cars tend to to a little better on all ends of the spectrum, but that is just one more restriction to suck air through. Most DFI systems I have seen where speed density conversions, and the Mass Air Flow was removed.

Yes, those power brake boosters can go bad for no good reason. I had one blow on me in the middle of some serious use and almost caused a crash. The plastic housing can crack or rupture causing it to leak right out. You can usually hear the whishing sound of air if you get up under the dash while it is running if the rupture is in the rear seal, and or by just listening close to the unit on the outside. The good news is that they are much easier to change than the C3. You can get them at most autopart stores on a special order basis.

When you get the car, just disconnect the booster and plug the line. See if that clears the idle to some degree. Then you know.

 

[Rob Mears]

I'm all good with a little lumpy idle The main concern is dealing with the idle rpm surging up and down intermitently.


Beyond the vacuum problem, I'll determine whether the system is using MAF or not. If not, I might consider reinstalling it even if it does rob a slight bit of power. At that point, I'm thinking of contacting Formato's for a new program that I KNOW will match my setup exactly.

Does anyone have any links or URL's for anything regarding DFI Computers? I seriously need to know what DFI is about. So far they've proven enigmatic enough that I'm tending toward replacing that system with one from a more recognizable/accessable company. Who are these guys anyway?

Rob