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Old 05-30-2009, 10:34 AM   #174
brontotx
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Join Date: May 2009
Location: Space City! (TX)
Posts: 46
Re: Tbi swap build thread

Great thread... wish I'd found it a year ago when I did a TBI swap into my C20, which is a "mutt" made up of my restored, but wrecked, '82 Camper Special (never lend trucks to girlfriends!) and an '85 with a straight body and frame, but bad engine, shot front suspension, brakes, and rear axle, butchered wiring harness, etc., etc. After getting the mutt running in late '05, I had the usual drive/tune-ability issues with the q-jet and got a full '87 TBI set-up off of e-bay. It took me a few months to sort through it all in the driveway (yup - I'm a bona fide shade-treer, subject to working in daylight when the weather is good). I have it running (and it runs very well if I do say so myself) - I'll mention some of my "insights" (more like problems I dealt with) later.

I ended up using the dash and rebuilt steering column from the '82 due to the '85's butchered wiring harness, shot steering column, and butchered metal dash sub frame (think hacksaws and ball peen hammers). While I installed a new stalk switch for the turn signals and cruise control, I never hooked up the cruise as I mainly use my truck for hauling stuff around town. I am about to go on a longer trip and want to get the cruise operational, so my questions are:

1) What do y'all recommend for the cruise: using the '82 or getting the parts for the later '85 or '87? I think the '82 would be cheapest since I have all the parts (I have some of the '85 parts too), but am concerned about the speedo cable wind-up I see, which is mostly at low speeds, so may not be an issue.

2) If I go the later model route, can I use the VSS signal from the JTR adapter I installed at the transmission end or do I need to get a later instrument panel too? My guess is no as I presume the later speedo-based VSS provided vehicle speed to both the cruise and ECM, but wanted to confirm with y'all.

3) Any other considerations I need to be aware of to help my decision and/or on getting this to work?

Some of my "insights" on swapping a TBI into an older vehicle:

1) Get manuals (or at least wiring diagrams) for both vehicles.

2) A factory TBI manifold requires modification if older heads are used (mine are mid-70's 333882's). I carefully used a disk grinder on the drivers side of the manifold to alter the angle for the bolts and made angled spacers for the passenger side.

3) If you want to use the TBI ECM's electronic lock-up for the 700R4 transmission, get a 700R4 from a later year. My '82 700R4 was not compatible with the ECM and, since my '82 was showing signs of wear (some slippage and lots of silver in the fluid), I got a used '87 700R4 that is compatible and has been working great.

3) Using the original 5/16" return line can result in excessive back pressure, which keeps the fuel pressure too high and makes the vehicle run real rich. FWIW, I have dual tanks and decided to use a Carter in-line fuel pump I found on Summit Racing "designed for TBI swaps into carburetor applications". I installed a fuel pressure gauge at the regulator and the Carter was running at 22 psi, no matter what regulator spring I installed, or how many coils I cut off the springs. I ended up disconnecting the 5/16" return line in sections to see where the back pressure was and found that even short sections had measurable back pressure compared to 3/8" line. After replacing all the return lines with 3/8, except for a short section of 5/16 for the tank selector solenoid, my pressure was down to 17-18 psi... close, but still running rich with a stock ECM calibration. I then found a vacuum adjusted fuel pressure regulator on e-bay and installed that... 12 psi at idle and up to 18 when WOT.

4) Even after sorting out the fuel pressure, I was still running a little rich and attributed it to the stock ECM calibration combined with the older heads and RV (high torque) cam I have in my engine. Researching my options, I ended up buying an EBL ECM, which replaces the PROM with flash memory and comes with "optimization" software. It wasn't the cheapest route, but after data logging and re-flashing my ECM a few times, my truck is running unbelievably well. Fuel mileage is up some 20-25% too (yippee!).

Anyway, sorry for the longish first post, but I wanted to share some of the problems I had and my solutions. And I would appreciate any insight anyone might have on getting my cruise functioning.

Mark

Last edited by brontotx; 05-30-2009 at 10:36 AM.
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