Tbi swap build thread

[cjracing15]

Thanks for the info and the pics. If i were to use a frame mounted pump witch one would you recomend?

[DirtyLarry]

Man, I don’t know. The frame mounted pump I had was a Summit Racing house brand. I hear good things about the Walbro pumps. I actually bought a slightly used low pressure Walbro from a board member, Rust Buster, for my ’72 C20 that I just completed the TBI swap on a few weeks ago. The damn truck is still at my buddy’s getting the exhaust hooked up so I haven’t had a chance to find any bugs with it yet. The Walbro pump is what all the TBI conversion kitters like Howell sell with their complete TBI kits.

Here is the Walbro mounted on my ’72 C20 with a TBI 5.7L. Not a very pretty install but it is not a very pretty truck either.

[joe231]

Hey DirtyLarry, what kinda pump do you use for a transfer pump?TIA

[DirtyLarry]

Originally Posted by joe231

Hey DirtyLarry, what kinda pump do you use for a transfer pump?TIA

Mr. Gasket #12D. Summit Racing pn MRG-12D. It is a low pressure/high volume pump. The pump is actually labled as diesel fuel but it will work as a gasoline transfer pump per Mr. Gasket tech line. I am happy with it so far with only testing it in the garage a few times. Moved 1/4 tank of fuel in less than 4 minutes from the secondary to primary tank. Will be making a run to the mountains in a few weeks so I should have real world results soon.


Mounted on a fabricatedcross member between the frame rails. Operated by Painless Wiring electric fuel pump harness kit PRF-50102 (Summit Racing pn) and a toggle switch.

[rfmaster]

Hey Cj

A while back DirtyLarry and I had a bit of discussion going about EFI retrofits and engine swaps. In this thread I posted my setup details - double posting is never a good idea...

http://67-72chevytrucks.com/vboard/s....php?p=2950627


//RF

[skidder111]

I did this swap in my 72 K5 some years back. It is hooked to the original 4 spd. My question: How important is the vss on this setup? It was never hooked up. Is it required for the ECM to go into closed loop?

[Captkaos]

FYI you don't HAVE to get EFI tanks, just don't run the truck less then 1/4T or so if you are offroad/camber often.

Walbro is a good pump, but in the tank is the best location.

[cjracing15]

Originally Posted by rfmaster

Hey Cj

A while back DirtyLarry and I had a bit of discussion going about EFI retrofits and engine swaps. In this thread I posted my setup details - double posting is never a good idea...

http://67-72chevytrucks.com/vboard/s....php?p=2950627


//RF

Thanks for the link. That does look like a good way to go. I do have one efi tank with the pump in it. I am going to use that right now and try to go with a stock setup latter.
Thanks, Jamie

[cjracing15]

Originally Posted by skidder111

I did this swap in my 72 K5 some years back. It is hooked to the original 4 spd. My question: How important is the vss on this setup? It was never hooked up. Is it required for the ECM to go into closed loop?

From what I have read the vss is used by the computer to see how fast the car or truck is going. The vss is also used to control the egr valve the charcoal canister purge valve, idle speed, and air fuel ratio at speed.

I belive in this way, if chevy did'nt need it why would they spend the money to have it installed in the wiring harness.

How does your truck run with out it?

[skidder111]

The truck runs great. The VSS is for the electric speedometers and cruise control, but I'm wondering if it does anything else. If it is supposed to control the issues you mentioned above, but is unable to, would you get a code?

[cjracing15]

Well being that you have a 4 speed trans what did you do with your prn/nurteal wire for the computer?

[skidder111]

Park/Neutral? That's some thing I would have to check into; I don't recall. The truck has the original Muncie 4 spd.

[cjracing15]

B-10 is the signal wire # for the ecm that is you park nutreral switch wire. If it is grounded the computer thinks it is in nuteral all the time and will not throw a ses light. A-10 is the vss signal wire. it runs the the vss sensor it's self.

[skidder111]

So what you're saying is if there is no VSS signal going to the ECM, it will not run as efficiently as it could? And also stay in open loop? Now that you mention it, B-10 must be grounded because I don't get an SES light other than start up.

[cjracing15]

Are you using a stock, or a aftermarket harness?

[skidder111]

Stock harness from an 89 pickup.

[cjracing15]

Well I would re-trace the wires to see what they are doing.

A-10 is a brown wire from the ecm, and B-10 is a org wire with a blk strip on it. let me know what you find, I am just staring my swap as you can see.

[skidder111]

OK Thanks, man. I will check it out. Will check back here tomorrow.

[cjracing15]

Cool.

[D13]

Why not put in a later model speedo with VSS? Pretty cheap on eflay. Looks to be pretty straightforward.

Is the under dash harness involved in feeding the ECM? One would think this is where the VSS and fused ECM power would come from.

[rfmaster]

Originally Posted by cjracing15

Well I would re-trace the wires to see what they are doing.

A-10 is a brown wire from the ecm, and B-10 is a org wire with a blk strip on it. let me know what you find, I am just staring my swap as you can see.

P/N line Orange/black wire is grounded by a P/N switch when in park or neutral position. P/N switch ECM input is used by ECM to compensate idle speed when additional engine loads (AC, PS pressure, etc) are present while in drive and within idle speed range. ECM commands IAC to add additional 50 to 75 RPM over commanded idle speed under such conditions. In your case (manual tranny) leave p/n wire open (i.e always in 'drive')

VSS is extremely important for complete EFI operation. It is unfortunate that many omit VSS during conversion. In your case, having 4 speed manual, it is imperative to have a functioning VSS. Since you have 1227747 ECM you'll need to have 2-pulse square pulse VSS inline unit. These are readily available (JTR, others). VSS provides vehicle speed to ECM which uses this input to accurately set fuel mixture and timing. One common problem with absent VSS and manual tranny is a severe drive train jerking during deceleration. This due to inability of ECM, which uses TPS, MAP and VSS inputs, to enter DE/DFCO mode (Deceleration Enleanment / Deceleration Fuel Cut Off).

When the ECM detects a sudden drop in throttle position and engine load, it reduces the injector pulse width while continuing to monitor engine load and vehicle speed. The purpose of DE is to produce a lean fuel mixture by reducing the amount of fuel gong to the engine. DE mode conditions are determined by ECM on a continuous bases provided that 1) Vehicle speed is above XXMPH (this depends on EPROM calibration value), 2) engine load decrease is below a limit value (this parameter also depends on EPROM calibration value), and 3) Filtered %TPS drop is <1%.

The difference between DE and DFCO parameters is that DFCO parameters are based on fixed values stored in EPROM. The DFCO fuel mode is entered after moderate to heavy acceleration then followed by abruptly releasing the accelerator. Usually ECM will transition into DE prior to entering into DFCO mode as the deceleration continues. Interestingly enough operating conditions may or may not cause ECM to command DE exit by itself. However when ECM enters DFCO mode DE is automatically disabled. Usually DFCO is a 'long' term (self resetting) duration event - coasting down hill in gear for example.

Both DE and DFCO are used for emission control, (keeps CATS from being burned up due to excess fuel during deceleration), but the two will effect drive train and exhaust longevity. There are many other ECM controlled parameters that are derived from VSS input.

//RF

[cjracing15]

What he said.

[DirtyLarry]

Rfmaster,

What you said above about VSS sounds right for 1996 and later OBD II sequential multiport EFI systems but I can’t find anything in any of my older GM STG service manuals for TBI years that state the VSS has anything to do with Deceleration Enleanment or Deceleration Fuel Cut Off. It could be one of the functions that takes place behind the scenes that isn’t really called out in the service manual.

From what I understood, the main sensor to control fuel on OBD I such as TBI is the MAP as it constantly monitors engine load and adjusts fuel pulses accordingly. Once you get into OBD II on sequential multiport things get a lot more complicated and more inline with your description above.

The only function I can find for the VSS in TBI years is to provide a pulse signal to the DRAC for the speedometer, RWAL ABS module, and cruise control module. The VSS isn’t even mentioned anywhere in any of the drivability symptom flowcharts or anything other than inop speedometers, cruise, ABS, etc.

For what it is worth, my 8.1L running multiport EFI on a Delphi MEFI-4 controller (similar to a GMPP RamJet engines and GM Marine engines) doesn’t have any VSS inputs either to run, and run well even with a manual trans. The whole MEFI-4 system is an entirely weird deal anyway.

I know you are an engineer and know this stuff a lot better than any of us but are you sure about the VSS function on TBI?

[cjracing15]

I thoght that on OBD II they did not use a vss sensor, but the ecm got it's signal from the abs computer.?

[GlennBrittain]

Many thanks for taking the time to post your info and progress. This will be a great help to myself and I am sure many others when I get to my drive train. Right now I have an 88 GMC for the donor but may look for the last year of TBI.