My trial and tribulations with TBI(not tramatic brain injury, I think)

[channingd]

So, 3 years ago I bought my 1987 and it ran okay, not great. But good enough for me to think minor tuneup. I did all the body work all the interior new wheels tires bedliner you name it. Truck looks and smells brand new. I love it.

The trouble is, with the help of you good folks on this board, I have tried to my wits end to get this thing to run 100%. I started with a lopping idle. Vacuum leak, solved. Now it's running pretty good, infact real good when you first start it, but as it warms up, it looses power and is very sluggish off the line. Plus the idle would drop sometimes coming to a stop sign and trip the oil pressure switch, shutting off my fuel pump. You can figure the rest out.

I have change every sensor. When coming to the MAP sensor, I noticed that the vacuum line running to it was removed and capped. Hmmmm. I plugged it back in. Truck would not run. Need a new MAP sensor. Got it. Installed. Still, will not run with MAP vacuum line plugged in. TPS, CTS, O2 sensor, everything. So, I finally throw in the towel and the guy at O'reileys suggests that I call a friend of his who is good with TBI.

Next chapter.

Charles, very nice guy, asks all that's been done, where is the timing set, 0 degrees, and what it's doing and how it's running. He seems pretty confident.

4 hours later.

Charles calls and says come pick this truck up, there's not much I can do and I'm not sure how the hell it's even running. He claims there is no communication to the computer and that the wiring harness has been so butchered that he doesn't even want to mess with it. Says that's probably why nothing has worked. Catalytic converter has been removed and way too many sensors being bypassed. He suggests starting over with either carb or new everything.

Enough crying. I got the notion to put a new wiring harness into it and see what happens. Upon searching ebay and such, I took a look at Craigslist and there it was. Wiring harness and TBI with brand new fuel pump and ECM all for $250.


Here's the question: it's out of a 1991. Will this all work on my 1987? And do you think this is the correct direction to go?

[406 Q-ship]

The 1992 is a completely different truck than the 1987 and the wiring harness will not work as a plug in. Can it be made to work..... possibly.

[channingd]

Mistake--it's out a 1991. I changed the original post to say 1991 not 1992. I don't think there is much difference between 91 and 92 though.

[Your41Plague12]

1991 squarebody, like a suburban, blazer, or crew cab? Or the GMT800 series?

[channingd]

The guy selling it says it's for a SBC, that's all I really know.

[Your41Plague12]

Ask him what it's out of, a 1991 squarebody or a 1991 C/K series. That will be the determining factor. If it's out of a suburban, blacer, or crewcab, it will take minimal modification to work. If it's from the other style, it can be done, but it's not nearly as easy.

[406 Q-ship]

Thats true if it came off a Suburban, Blazer, or Crew Cab would work (might still need some minor work to fit though). If it off a 1988 and up pick-up it will not fit as is.

[makman1297]

Someone is parting out a 1987 Sub from MN in the parts section, he might have a harness for you.

[FritzA]

Before you go spending $250, are you sure the haness has been butchered that bad? I am surprised the truck would have run so well even cold if it was that bad. You changed all those sensors, did you notice the harness is modified?

1 mans opinion is not Gospel. Why not get a 2nd opinion on the harness.

Your problem sounds more like basic issues with a 24 year old truck. Before replacing expensive sensors and harness I would start with some basics. Vacum lines (great, you did those). Fuel Filter (very important on these low pressure EFI systems), fuel pump (a weak spot on these trucks and yours is probably 24 years old). Coil, cap, rotor and plug wires. O2 sensor (cheap on these trucks). The EGR valve at this age could also be sticking. There may also be a broken regulator spring (common on high mileage tbi units and cheap to fix).

The worst (most expensive) issue I think it could be is the tbi unit itself. They are notorious on high mileage vehicles for the butterfly shaft wearing the slots it is in and causing idle issues. On Ebay you can find reworked ones from a couple companies like ExtremeEFI. Much better costwise (qualitywise too) to get a rebuilt one than a replacement one.

Good luck. Keep us informed on your results.

[channingd]

The wiring harness has definitely been butchered. I have rewired many things and fixed a lot of "bad" wiring problems. The fuel pump was rewired directly to a switch on the dash. So, when you wanted to start the truck you had to flip the switch on. And when you shut the truck off, you had to remember to shut the fuel pump off as well. I rewired the fuel pump to the fuel pump relay and it worked. Although, it takes about twenty cranks now to start the truck. I still have it hooked to the fuel pump relay.

Like I said, the mechanic cannot get anything from the ECU. Nothing. But, it has to be doing something in my mind, because the spark is advancing( I checked it without pulling the brown wire and it was at least 8 degrees advanced), and the injectors wouldn't work without some kind of computer intervention. Yes?

In doing research, a new fuel pump, wiring harness, ECU, and TBI would cost more than $250, so I think I'll get them and use what I can. The wiring harness is the only thing that scares me but if it's close I can make it work.

I freaking love this truck to death and will make it run, guaranteed. I even painted it the same color code as my 2009 Z51 Corvette. I park it during the winter and want to drive the truck in the winter.

All of your input is most welcome. And thank you, Channing

[Jonboy]

I wouldn't bother to buy the parts you don't know will work, but rather buy an aftermarket harness. If you aren't sure of teh interchange of the used harness, check out www.fuelinjectionconnection.com . I put that harness in my '74, and it was pretty easy. You already have the throttle body, fuel pump, and ECM. The FIC harness is $245, so it is apples to apples and it will fit. It will also do away with most of the emissions stuff as well.

[channingd]

UPDate: I went to look at the TBI setup the guy was selling on craigslist, thinking that having a spare ECM and fuelpump is not a bad thing and hopefully the wiring harness was good. Wrong! It was all just a box of junk. The wiring harness looked worse than mine. PASS!

So, I started to think, it can't be that hard just to clean up the wiring harness that I have. It really wasn't that bad. Yeah, things were cut and left hanging, but really not that bad to where I couldn't put it all back together. I actually had everything hooked back up, I just needed to make it cleaner.

Anyway, so at this point I'm a little pissed because of course the truck still doesn't run right. I got the idea to see if the temp sensor was talking to the computer. Tried to start it early in the morning with it unhooked. Truck wouldn't run. That's obviously working. Decided to pull the ECM and just have a peek. Yep, right there on the board, a 14 prong chip and black resister completely melted. Ouch. Pretty sure this might be my problem.

Going to UPULL today to buy new ECM. Actually, they have 11 of them for $17 a piece so I'm buying 2 just to have a spare.

Let you know what happens. Hopefully whatever blew this thing in the first place has been corrected.

[wyattglock]

Good luck with getting it all straightened out. I have been having a lot of the same issues with my tbi 454 in my 89 R3500. I have a new idle air control valve and a gasket kit for the tbi along with a new diaphragm and regulator spring coming this week. I am going to rebuild it and fix some other minor issues with small vacuum leaks and see how that does. I am definitely interested in following this thread and finding out how things go for you.

[channingd]

Yes! Yes! Yes! FnA double clutch.

Went to UPULL(never again but that's another story), got 4 ECM's(don't ask why I got 4).

Put new ECM in, rehooked vacuum line to MAP, started truck. Starts, that's a good sign. Won't idle for crap but it's still running. Keeps running, idle getting better, ECM is learning(?). 10 minutes later it's running pretty good. Just on a hunch, I decided to unhook the EGR. Took for a drive and runs great. Perfect acceleration. Return home. Reinstall the EGR wire. Retest drive. Horrible acceleration and some of the same old problems. Bad stuck EGR valve. I'll fix that tonight.

Only problem now is when you let off the gas the idle drops a little low. Even enough to make steering tough for a second. I'm sure there is a reason for this and a solution.

I'm happy as hell right now though.

[FritzA]

Glad to hear you have it running and have the EGR problem identified.

There is an idle adjustment screw on the TBI. It is not supposed to need adjstment though....usually the cause is something else.

Has the fuel filter been changed recently? Also, make sure the flap in the air cleaner inlet is opening when hot.