Hesitant, Rough Idle '89 454 with TBI

[Tim Rif]

I just installed a 454 TBI long block in my Silverado. When I first fired it up it ran great. I adjusted the IAC (which I shouldn't have) to get the engine up to 1500 rpm's for about 15 minutes. I backed it off after 15 mins and it ran pretty rough (didn't want to stay running). I drove it and it was real hesitant, but occasionally it would take off and run great, especially under a load, but had a really rough idle. I took it to a shop and they said one of the injectors was barely dropping any fuel, so I had the injectors rebuilt. I also replaced the fuel filter, EGR valve, O2 sensor, and now the truck idles worse (it speeds up and slows down), but runs better when it is warmed up. Now I am getting a 44 code (lean exhaust). I took it back to the same shop that said one of the injectors was bad, and they say one injector still has an abnormal spray pattern. I called the injector place and they said they have rarely seen the TBI injectors go bad, especially after having them rebuilt. The mechanic said he checked everything (fuel pressure, vacuum leaks, injector voltage, etc) and he is stumped too! Any suggestions would be highly appreciated! Thanks, Tim

[87 Sprint]

I'm not that familar with the Silverado, but this worked on my 86 Toyota, many times. It's so simple, most mechanics won't tell you about it.
Hope you have a garage, because it doesn't work very well outside, in the Winter.

From what you said you did and what you've replaced, I'd say your EEC(Electronic Engine Control) is totally confused.

DON'T PANIC, just yet.

HERE'S WHAT TO DO: Get the engine warmed up to NORMAL operating temp and let it run a few minutes, so the block and everything else gets good and warm. Shut it down. Now disconnect BOTH battery cables and let it set at least 30 minutes, preferably and hour.

The engine must still be close to normal operating temp for this to work.

The idea is for the EEC Unit to lose all it's power, thus it's memory.

Reconnect the battery cables, start the engine and let it just idle for awhile. If all goes well, the EEC unit should RESET itself with accurate data from all the sensors AT NORMAL operating conditions.

I know it sounds too simple, but that's why they charge $75 / hr NOT to tell us these things. I once paid $400.00 to find out which wire was broken.
Then fixed it for a Nickle's worth of bell wire, 10 years ago. GOOD LUCK

[Tim Rif]

Thanks for the info 87 sprint.
Right now the truck is in another shop. Once again, they noticed a terrible spray pattern with the injectors. I had them rebuilt, checked and re-checked, and Doctor Injector swears the injectors are fine (atleast when they leave his shop). The shop where it's at now says when they sprayed propane into the TBI the engine smoothed out. They did a smoke test and noticed a small vacuum leak around the TPS solenoid. Also, they mentioned the fuel lines to the TBI looked a little rusty inside. Then again, maybe this is just info they are telling me to justify there $72 an hour. I replaced the fuel filter after I swapped engines and I could not blow through the old one, so I assume it was plugged. Other than the rod knock, the old engine seemed to idle pretty smooth, but then again, I didn't drive it, or run it very long. What it boils down to, is the engine is starving for fuel and throwing a 44 code (lean exaust). Whether or not it is caused by the vacuum leak or dirty fuel lines or a confused computer remains to be seen until the vacuum leak is fixed. I never tried disconnecting both battery terminals (only the negative) after it was warm.

[Ol'Jim]

So, Tim, What did you find out was wrong with your Big-Block truck?
-Jim.

[Tim Rif]

Jim-

The only thing the "highly recommended" shop could find was a slight vacuum leak around the TBI bushings. When they richen the TBI with propane the engine smoothes out and vacuum inreases to 18 inches from 15 (after replacing bushings), so there appears to be another vacuum leak elsewhere. It still is hesitant and idles rough, but it sure takes off when you put your foot into it. The engine only has 100 miles on it, so I think I will put some more miles on it to let everything "break in". I'm not getting any codes, so I suppose that's a good sign. Let me know if you have any suggestions.

Thanks-
Tim