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Old 12-25-2005, 03:18 PM
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Mild TBI 350 won't run consistently

I just finished building my truck, '59 Chev cab on '89 K2500 4WD chassis. We can't get the engine to run quite right. Anybody have any experience with OBD1 systems? I'm not a pro mechanic, I'm a carpenter. I have a good friend, George, helping on this who is a pro, but he rarely works on anything but stock vehicles, so he's not sure about the effect the mods have on the computer system.

Here's the deal: Engine is rebuilt from '89 350 that had about 70,000 miles on it run on propane. It was extremely clean inside. I re-did everything but the valves, they seemed too clean to mess with. Non-stock items are (1) Holley #502-6 670 cfm throttle body, stock intake holes bored out to 2" to match the TBI. (2) Crane cam P/N 114112 (very mild), with new lifters. (3) JBA Tri-y headers with 3" exhaust. The transmission is manual Muncie HD 4 spd.

The O2 sensor fitting was located about 6" from the head on the #7 tube, but we moved it to just aft of the final collector, just ahead of the cat. It's not heated. The reason for the move was that the system was having trouble staying in closed loop, plus we thought it would get a better reading on all 8 cylinders. It stays in closed loop better.

On cold start, it runs, but not smoothly, I usually have to feather the throttle to keep it running. When it goes into closed loop, it will idle okay. When driving, it runs fairly strong, but has some hesitation on acceleration. Steady running seems pretty good, but there's a noticeable sag when you step on it. I'm not looking for neck snapping power, just a little passing power. It takes awhile to recover power.

Snap-On scanner says it is usually running lean. We tried adding resistance to the coolant temp circuit and got the apparent temp down to about 140 degrees. Some improvement, but not even what you would expect from a bone stock motor.

Valves are set at zero lash. George fiddled with that for a while and thought it sounded better that way. Crane says 1/2 to full turn beyond zero. Any thoughts?

We haven't been able to check fuel pressure yet, but the injectors appear to be putting out plenty of fuel.

Timing is set at 4 degrees advanced (unplugged), just because it seemed happier that way.

One other thing we weren't sure about is how the speed sensor interacts with the computer. I just have it connected direct since I can't use the original speedo to buffer the signal.

Any suggestions would be appreciated. I'm sure some of you guys have some good ideas. Merry Christmas and have a Happy New Year.

Don

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Old 12-25-2005, 03:25 PM
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You could crutch it with an adjustable fuel pressure regulator and go up until the O2 sensor reads slightly rich. You've increased the engines ability to breathe and the F.I. can't keep up.

Does this the ECU in your truck have flash memory or a replacable prom chip?

Larry
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Old 12-25-2005, 04:04 PM
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Larry,
It has a PROM. I believe the Holley TBI has an adjustable pressure regulator. I guess I need to plumb in a fuel pressure gauge fitting. Thanks for the reply.
Don
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Old 12-25-2005, 07:07 PM
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You may want to consider one or both of these as well. To compliment the changes you've already made and to help ease the tuning process a bit.

Motorvation Stage 1 chip

Caspers TPS module

Larry
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Old 12-25-2005, 07:46 PM
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Something else to consider is DIY PROM. It is not as difficult as you may think at first.

Your 89' TBI ECM can be converted to accept a 29C256 flash chip. See it here: http://www.moates.net/documentation....entation_id=10

Using flash chips on OBD1 ECMs is pretty straightforward and the total cost of the equipment and software is often less than a "custom" chip.

I converted my 94' Suburban from TBI to TPI and programmed the ECM myself by using equipment bought from Craig Moates and software from Mark Mansur "TunerPro RT". Here is my page: http://www.edgesz28.com/edgesZ28/sub...ban-tuning.htm

You would use a different mask and ECU file, but that is easily found at thirdgen.org. There is tons of DIY PROM information at thirdgen.org: http://thirdgen.org/techbb2/forumdis...?s=&forumid=16

With the mods you have done, a little work on your VE table and maybe spark advance will solve your problems. Moates also has equipment and software to perform emulation (real-time tuning). You can monitor the engine and make changes and instantly see the result.

If you decide to try DIY PROM, do not hesitate to contact me with any questions. While I do have not done TBI tuning, the process is very similar to the TPI speed-density tuning I have done. And there are experts at TGO who have great knowledge when it comes to tuning OBD1 TBI systems.

Good luck, Ed www.edgesz28.com
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