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Discussion Starter · #1 ·
I recently took my 93 Silverado into a shop because my engine surges at idle. Before bringing the truck in I tested the EGR valve, EGR solenoid, did a fuel pressure test, improved the ground wires, tested for vacuum leaks, I suspected the problem might be with the computer. I used to have a 350 tbi engine (VIN #: K) in the truck, and I swapped in a crate 350 Vortec motor in (VIN#: R). The technician at the shop wasn’t able to get the surging to stop by checking for mechanical problems. The technician suggested having the computer flashed to match a Vortec motor with VIN #: R. My current setup is still using the TBI. I’m a greenhorn when it comes to computer fixes, so I have a few questions:
1) Does a computer flash seem like a reasonable explanation/fix?
2) Does anyone have a reputable company they can recommend for this type of flash?
3) Could I simply buy a refurbished computer intended for a 96-98 truck with a Vortec motor as a fix? Here’s a link to what I’m thinking: computer
Thanks for any advice!
 

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TBI and Vortec are two different animals, will never work. If your "technician" doesn't know that, run away.
 

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In a past life i used to do these everyday (turbo City)- it will run on the TBI computer but needs some system modification.

Use a vacuum referenced fuel pressure regulator and raise the base fuel pressure to around 15-16 psi ( will reguire different fuel pump in the tank (94-95 454 tbi pump)

increase the base timing to 6-8 Deg (0 is stock)

we used to do a computer chip along with these changes but it should run much better if you make those changes.
hope this helps.

Thanks Tom
 

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Discussion Starter · #7 ·
In a past life i used to do these everyday (turbo City)- it will run on the TBI computer but needs some system modification.

Use a vacuum referenced fuel pressure regulator and raise the base fuel pressure to around 15-16 psi ( will reguire different fuel pump in the tank (94-95 454 tbi pump)

increase the base timing to 6-8 Deg (0 is stock)

we used to do a computer chip along with these changes but it should run much better if you make those changes.
hope this helps.

Thanks Tom
Thanks Tom this is really helpful! I discovered through trial and error that 7 degrees is the sweet spot to get this truck to wake up off the line. When you did the computer chip, did this include an EGR delete? I constantly get Code 32 (EGR) on my truck but it seems to be operating correctly. I'm tempted to just block off the EGR valve.
 

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in all the chips i made, always deleted codes ,.32,43. EGR will always set with freeflow exhaust- 43 is most troublesome.
blocking the EGR will not keep the code from setting

raising the FP and then vacuum referencing really will help drive better

Thanks Tom
 

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the vortec heads don't have the exhaust port in the heads- did you add the external tube to the exhaust manifold?

Thanks Tom
 

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Discussion Starter · #10 ·
I wanted to add some more follow up information from Tom about the TBI to Vortec swap, so I included my questions:
  • Do you have any recommendations for a vacuum referenced fuel pressure regulator?
  • When checking/setting the fuel pressure, should this be done prior to the fuel filter or near the throttle body? In the past, when we did a fuel pressure test, we hooked up the test kit prior to the fuel filter.
  • I have summit headers on the truck. I did not add the exhaust tube. Is this something you would recommend?
 

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Discussion Starter · #11 ·
These are the suggestions Tom (turbocityTom) gave in regards to the above questions:

we used to manufacture a VFPR- i have not followed this for a long time so don't know what is out there now.
GM used to make a housing that was used in the marine TBI systems than could be easily adapted if you can find one. I did see on line that there are several DIY instructions

Pressure will be equal thru out the system - measured at the TBI or fuel filter will not make any difference.
NOTEon FUEL PRESSURE: measure before installing the VFPR- the base fuel pressure must not drop more than 1psi below the original fuel pressure when full vacuum is applied to the regulator- could create lean condition. May take several adjustments to get it correct. we had custom springs made for our regulators and used spacer washers to adjust that base pressure.

The EGR is up to you- to be federally legal it must be included- with your set up- probably won't make any difference. the CEL will be on unless you remove the code in the chip

another very big problem we found is using the wrong spark plugs- the TBI heads use a short reach plug- Vortec heads use extended reach. Using the TBI plugs in Vortec heads will run but creates a lot of drivability issues. Use plugs for 96-99 truck, not 88-95.
 

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Discussion Starter · #12 ·
the vortec heads don't have the exhaust port in the heads- did you add the external tube to the exhaust manifold?

Thanks Tom
Hi Tom,
I talked to a helpful representative at Summit and purchased a new fuel pump capable of 50 psi (fuel pump link) and VRFPR from Holley (Holley regulator link).The rep convinced me to buy a VRFPR with a return on it, because he said it would put less stress on the setup. I have a few questions about the regulator.
1) Is a return necessary to setup or recommended?
2) If I don't want to use the return, can I simply block it?
3) Finally, if I did use the return, would I have to set up an independent line back to the gas tank? Or could I tap into a return somewhere?
I included the links and pictures to both the pump and the regulator. Thanks for your time and any advice.
- Mike

Font Gas Auto part Engineering Metal
Product Font Cylinder Engineering Auto part
 

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Hey Mike. Those items will work but there are some possible problems.
1) Is a return necessary to setup or recommended?
2) If I don't want to use the return, can I simply block it?
3) Finally, if I did use the return, would I have to set up an independent line back to the gas tank? Or could I tap into a return somewhere?

That fuel pump has a very high pressure for a TBI system- The factory 94-95 7.4 TBI fuel pump was about 35 psi.
remember, the TBI should be running 12-18 psi with your stock program- just slightly higher than stock with vacuum applied to the regulator- with no vacuum applied, the pressure will be up to 50% higher under load increasing the fuel at the injestors

with 50psi base pressure you will need to bypass a large percentage back to the tank- will make the regulator VERY sensitive to small adjustments.- what you have will work but may have to "fiddle" with it a lot.

There are a couple different ways to connect the VFPR- Simplest, connect it to the return line on the TBI- the outlet (TBI return) to the inlet of the regulator- the outlet (bypass) back to the return line to the tank. The VFPR will "override" the factory regulator in the TBI (the minimum system pressure will be deturmined by the lowest regulator pressure) if the factory regulator is at 12 psi, it will take over control at the lower pressure (which may be helpful in your tuning)

if you remove the spring in the factory TBI regulator, then the VFPR has full control over the entire fuel pressure range.

Hope this helps
Thanks Tom
 

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Discussion Starter · #14 ·
Hey Mike. Those items will work but there are some possible problems.
1) Is a return necessary to setup or recommended?
2) If I don't want to use the return, can I simply block it?
3) Finally, if I did use the return, would I have to set up an independent line back to the gas tank? Or could I tap into a return somewhere?

That fuel pump has a very high pressure for a TBI system- The factory 94-95 7.4 TBI fuel pump was about 35 psi.
remember, the TBI should be running 12-18 psi with your stock program- just slightly higher than stock with vacuum applied to the regulator- with no vacuum applied, the pressure will be up to 50% higher under load increasing the fuel at the injestors

with 50psi base pressure you will need to bypass a large percentage back to the tank- will make the regulator VERY sensitive to small adjustments.- what you have will work but may have to "fiddle" with it a lot.

There are a couple different ways to connect the VFPR- Simplest, connect it to the return line on the TBI- the outlet (TBI return) to the inlet of the regulator- the outlet (bypass) back to the return line to the tank. The VFPR will "override" the factory regulator in the TBI (the minimum system pressure will be deturmined by the lowest regulator pressure) if the factory regulator is at 12 psi, it will take over control at the lower pressure (which may be helpful in your tuning)

if you remove the spring in the factory TBI regulator, then the VFPR has full control over the entire fuel pressure range.

Hope this helps
Thanks Tom
Great! Thanks for the information. I'm going to hit the reset button on my purchases from Summit. I'll use RockAuto to find the correct fuel pump. The regulator I picked is also getting abysmal reviews so I'll get a regulator that's not a return style. Thanks again.
 

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the stock TBI pump is only capable of 15-16 PSI. With the VRFPR minimum pump pressure needs to be 20-22PSI miniumum. The 94-95 7,4 pump is the closest direct fit at 35 PSI The TPI pumps are anywhere from 60-100psi

Mike- look at some of the DIY instructions for modifing the original TBI FPR. no additional regulator required

Thanks Tom
 

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Not true- the whole point of the VRFPR is to provide much more fuel under a load by increasing the pressure. the vacuum control will lower the fuel pressure to just above the stock setting so drivablity improves WITHOUT any computer programming. This method was used by GM in their smog legal 502 engine packages for the TBI trucks

Thanks Tom
 

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Maybe I can give a little better explanation:

There are 3 variables in the fuel delivery curve>

Injector size (flow rate)- fuel pressure- and injector pulse width

On a 350 TBI = 61# hr injector size- 12-13 psi FP- fixed computer injector pulse width= 210HP

On Vortec replacement- we have the same injector flow and no ability to change the injector pulse width with out programming so fuel pressure becomes the only variable.

With the fuel pressure at 20psi (est) the 61# injector will support the 285-300HP at the programmed pulse width.

The problem becomes that the Vortec cam and heads produce a non-linear air flow change to the program- the engine requires more fuel to produce the power but just increasing the fuel pressure will make it over rich under most conditions. This is where the VRFPR comes in.

The engine vacuum has the capability to lower the idle fuel pressure by about 5-6 psi allowing the full throttle pressure to be in the 17-20PSI range (1psi per 2 inches vacuum)

We did several different methods of making this work.

68# injectors at stock fuel pressure- stock pump- modified chip (increase base timing with reduced high RPM timing)- worked OK

Modified chip only- ( increase fuel- modified timing curve)- (this very limited success as mechanical limits of stock components)

61# injectors with higher pressure pump- VRFPR- modified chip with timing curve- best results with this combination

Without being able to program, increasing the base timing will improve the drivablity but will sacrifice a small amount of WOT power.



Hope some of this makes sense



Thanks Tom
 
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