Considering a TBI Swap

[MrGiggles]

I own a 95 Chevy K2500 4x4 that was previously a diesel. After some engine trouble, I swapped the engine with a carbureted 350. The truck is a manual so the conversion was very straight forward. Anyway I am dissatisfied with my fuel economy, and I don't like the hassle of a carburetor. The engine is a very mild build, a little over 9:1 comp. with a Summit Racing K1102 cam. It runs great and I am happy with it's performance. Being a truck engine it doesn't see high RPM very often. I want lots of low RPM power, where is spends most of its time.

All of my gauges are hooked up and working. I just left most of the diesel harness unhooked and plugged in the oil pressure sender, CTS, re-used the old alternator (the tach signal goes through it on a diesel) to run the gauges. For fuel I placed a filter in the frame where the lift pump was, and used a return-style mechanical pump on the block.

Now what I am thinking for the TBI Setup.

1. I will put the electric diesel lift pump back in, and use a regulator that works with a return line. This will give me adjustable fuel pressure.
2. I was just planning on using a TBI distributor for ignition
3. I will use an adapter to mount the TBI on my 4bbl intake
4. I will be pulling everything from a donor at a scrapyard. I'm hoping they will make me a combo deal.

Now for the questions.

My truck is a bit of an odd duck. It is a late 95 OBD2. So I'm not sure if it would have came with a Vortec gasser or a TBI.

1. Can the TBI system work as a stand-alone? Meaning it basically works as a carburetor, without having to mess with the current harness or wiring. Basically I only want to have the TBI, it's few sensors, and the ECM to control it separate from my current system. I don't think I would be able to integrate them. Having multiple Oil pressure senders and CTS's wouldn't be a big deal.
2. Is the VSS signal required for the TBI system to work correctly? My speedo works just fine now, and I'd rather not mess with it. I can't see why the simple TBI would need a speed reference.

I look forward to any input.

[brian617]

The diesel supply pump only puts out 4-6 psi of pressure not enough for TBI. You could have used it for the carb though.

[MrGiggles]

The diesel supply pump only puts out 4-6 psi of pressure not enough for TBI. You could have used it for the carb though.

I thought that little pump made more than that. That's what I get for assuming, but it's not a big deal. Those pumps don't dead-head well, so I would have had to use a regulator for the carb. It was just easier to bolt on and use the mechanical pump.

What I am really wanting is an isolated TBI system on my engine. I am just apprehensive about whether or not it could work that way. Like I said my truck is a manual so I don't have to control a transmission, but I wonder if that will cause problems if I get a TBI harness that controlled an auto.

[brian617]

Minimum you'll want the VSS signal, it with improve the "driveability" of the TBI. Also need to decide which ECM/PCM you are wanting to use as your setup will need some tuning to get fueling and spark correct.

[dave w]

I'm not sure what the diesel lift pump specs are. Personally, I would try the diesel lift pump and see what the fuel pressure measures. The TBI is where pressure regulator is located. As long as the diesel lift pump can withstand 15 PSI constant service it should work for TBI.

I would use the 1227747 TBI ECM and 1227747 Wiring Harness found in many GM Light Truck type vehicles from 1987 ~ 1990. There are aftermarket sources for a 1227747 "Stand Alone" wiring harness. The stand alone harness has the bare essential wiring for a TBI System and uses a very simple 4 wire to hook up: Battery +, Battery -, Ignition Switch "On / Start", and Ignition Switch "Start Only" power source.

A used factory harness can be simplified for "Stand Alone" service also.

The Pictures below are from a Universal Stand Alone TBI Harness that can be found on the Internet for about $275.

dave w

[brian617]

• "Turn engine off. Disconnect fuel line at inlet to fuel filter assembly. Install low pressure fuel gauge to fuel line.
• Crank or run engine 10-15 seconds. If fuel pressure is not 5.8-8.7 psi (.41-.61 kg/cm² ), check fuel lines and fuel tank sending unit for restriction. If fuel lines and sending unit are okay, replace fuel pump." - Shopkey

5.8-8.7psi to be exact. It is only a supply pump and not used to pressurize the fuel, the injector pump handles that chore. We used to use those pumps for carb engines back in the day when using later engine with out mechanical fuel pump provisions. You'd be lucky if you got that pump to dead head 10psi.

[MrGiggles]

Minimum you'll want the VSS signal, it with improve the "driveability" of the TBI. Also need to decide which ECM/PCM you are wanting to use as your setup will need some tuning to get fueling and spark correct.

After some research it seems that I would be best off to have a VSS of some sort. Obviously my truck has one on the transfer case but what type and whether it will play nice with the TBI system, while still running my speedo, is the problem. It has an nv4500 transmission with a manual shift transfer case.

I'm not sure what the diesel lift pump specs are. Personally, I would try the diesel lift pump and see what the fuel pressure measures. The TBI is where pressure regulator is located. As long as the diesel lift pump can withstand 15 PSI constant service it should work for TBI.

I would use the 1227747 TBI ECM and 1227747 Wiring Harness found in many GM Light Truck type vehicles from 1987 ~ 1990. There are aftermarket sources for a 1227747 "Stand Alone" wiring harness. The stand alone harness has the bare essential wiring for a TBI System and uses a very simple 4 wire to hook up: Battery +, Battery -, Ignition Switch "On / Start", and Ignition Switch "Start Only" power source.

A used factory harness can be simplified for "Stand Alone" service also.

The Pictures below are from a Universal Stand Alone TBI Harness that can be found on the Internet for about $275.

dave w

That's good to know they can work with as a stand-alone. I will look around the junkyard tomorrow morning and see what I can find. There are tons of TBI trucks out there. What is wrong with using a 91-95 harness? Is it because they started controlling the transmissions electronically?

• "Turn engine off. Disconnect fuel line at inlet to fuel filter assembly. Install low pressure fuel gauge to fuel line.
• Crank or run engine 10-15 seconds. If fuel pressure is not 5.8-8.7 psi (.41-.61 kg/cm² ), check fuel lines and fuel tank sending unit for restriction. If fuel lines and sending unit are okay, replace fuel pump." - Shopkey

5.8-8.7psi to be exact. It is only a supply pump and not used to pressurize the fuel, the injector pump handles that chore. We used to use those pumps for carb engines back in the day when using later engine with out mechanical fuel pump provisions. You'd be lucky if you got that pump to dead head 10psi.

I will have to figure something else out then. I have a couple more questions too. Would I be able to pull all the physical parts (sensors, TB, dist.) from a newer model year, but pull the harness and ECM from an older model to get the better ECM? Last time I was there they had a Suburban that had just arrived and hadn't been touched, but I think it as a little newer than 90. I would pull the parts from that, and find another old truck with an ECM and harness, since they are more likely to not be pulled.

I would also have to address EGR. My current manifold has no provision for it and I don't want to change it. Can this, along with some other issues, be tuned out by somebody? If so, how much would it cost to have that done? Since my engine is very close to stock, would it run efficiently (assuming all the sensors were working, and in closed loop) without tuning?

[dave w]

Basically the TBI Trucks, Suburban's, Astro Vans from 1987 to 1990 used the same ECM / Wiring Harness. To provide the lowes cost TBI conversion, the aftermarket focused on the early versions of TBI to support. Nothing wrong with using 1991 and newer TBI parts. It's can be complicated if all the TBI parts are not from the same donor vehicle after 1990.

The 1995 NV4500 most likely has the 40 Pulses per Driveshaft Revolution VSS, which feeds a converter know as a DRAC to produce 2000 Pulses per Mile input to the computer. 1987 to 1990 computers require also require a VSS with 2000 Pulses per Mile. In most aftermarket stand alone TBI wiring harness (1987 ~ 1990), the "Park" switch input to the computer is grounded so the need for a VSS is eliminated. Using the VSS makes for a much better TBI conversion, but a VSS is not absolutely necessary for a TBI swap.

The software and hardware for the 1987 to 1990 TBI is the simplest to learn and work with. After 1990 the TBI software and hardware, especially (1993 ~ 1995), is not so simple.

dave w

[MrGiggles]

Basically the TBI Trucks, Suburban's, Astro Vans from 1987 to 1990 used the same ECM / Wiring Harness. To provide the lowes cost TBI conversion, the aftermarket focused on the early versions of TBI to support. Nothing wrong with using 1991 and newer TBI parts. It's can be complicated if all the TBI parts are not from the same donor vehicle after 1990.

The 1995 NV4500 most likely has the 40 Pulses per Driveshaft Revolution VSS, which feeds a converter know as a DRAC to produce 2000 Pulses per Mile input to the computer. 1987 to 1990 computers require also require a VSS with 2000 Pulses per Mile. In most aftermarket stand alone TBI wiring harness (1987 ~ 1990), the "Park" switch input to the computer is grounded so the need for a VSS is eliminated. Using the VSS makes for a much better TBI conversion, but a VSS is not absolutely necessary for a TBI swap.

The software and hardware for the 1987 to 1990 TBI is the simplest to learn and work with. After 1990 the TBI software and hardware, especially (1993 ~ 1995), is not so simple.

dave w

So would it be possible to tap into the VSS signal after the DRAC on my truck without affecting the Speedo? If it really benefits the driveability, I'm all for a little extra work.

[brian617]

So would it be possible to tap into the VSS signal after the DRAC on my truck without affecting the Speedo? If it really benefits the driveability, I'm all for a little extra work.

Yes sir, thats what I was going to suggest.

[MrGiggles]

My scouting mission went pretty well. There are tons of TBI trucks and parts out there. And they will sell me everything I need for around 60.00. Dist, throttle body, everything.

Throttle bodies are kinda hard to come by. Is a 95 TBI unit the same as an older model? What about the 95 distributor? These are late 95, with the rounded dash and OBD2. There is a suburban out there in good shape that still has all of the motor parts. That was one of the only complete TB's I could find.

The one thing that I couldn't find was a 1227747 ECM. Every 87-90 truck I could find (and there were several) had the ECM's pulled. I might be able to find one if I look harder. There was one that was possibly the right one, it was laying on the seat of an 89, but the sticker was faded. Is there any other way to identify these?

[dave w]

There are two styles of 5.7 liter TBI throttle bodies, early and late. The early style use a 3 pin flat Throttle Position Sensor, and late style use a 3 pin round Throttle Position Sensor. See attached picture.

There are TPS adapter plugs flat to round available.

Distributors are the same from 1987 ~ 1995.



dave w

[MrGiggles]

Since that is the case I am gonna go Monday morning and pull the distributor and TBI for sure.

I am wondering if the CTS and OPS already on my engine would work for the TBI setup. I know for sure that the OPS is the same, but I'm not exactly sure about the CTS.

And lastly, are there any major differences in the sensors from a 95 TBI system?

Here is a list of what I plan to grab from the 95.

1. Throttle Body
2. MAP sensor
3. ESC Module
4. Coil
5. Distributor
6. 02 Sensor (if it's not too difficult to remove)
7. Knock sensors
8. CTS and OPS (if they don't cost a whole lot)

What do I do about the EGR? I don't plan on running one but I'm sure the ECM will be programmed for it.

Then I will find the a good 88-90 wiring harness, and hopefully an ECM.

[dave w]

Since that is the case I am gonna go Monday morning and pull the distributor and TBI for sure.

I am wondering if the CTS and OPS already on my engine would work for the TBI setup. I know for sure that the OPS is the same, but I'm not exactly sure about the CTS.

And lastly, are there any major differences in the sensors from a 95 TBI system?


Here is a list of what I plan to grab from the 95.

1. Throttle Body
2. MAP sensor
3. ESC Module
4. Coil
5. Distributor
6. 02 Sensor (if it's not too difficult to remove)
7. Knock sensors
8. CTS and OPS (if they don't cost a whole lot)

What do I do about the EGR? I don't plan on running one but I'm sure the ECM will be programmed for it.

Then I will find the a good 88-90 wiring harness, and hopefully an ECM.

The OPS that your currently using might work, depends on which switch type (gauge, or light) and the OPS connector (Male vs. Female plug).

Most all 1995 sensors are the same as the 1987 ~ 1990 sensors. Two sensors that I can think of that are different. One would be the Knock Sensor (100K vs. 4K resistance). Knock sensors for Manual and Automatic transmissions show different part numbers starting in 1991. Starting as early as 1991 the ESC module was moved inside the computer and incorporated with the Memcal. The 1995 TPS is round not flat.

dave w

[EagleMark]

Alright, I give? What's an OPS?

O2 sensors wear out and are to cheap to pull used, same for CTS.

Don't expect miricles when you get the conversion done on a stock chip! But that cam will tune very nicely.