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Deathdeelr · Posted: Wed Jul 14, 2010 7:47 am Post subject: Overhauling TBI

I'm experiencing an intermittent sputter while cruising. Once or twice every minute or so. No backfire or knock, just a little blurp. Acceleration is perfect, decel no problems. The sputter gets worse with the a/c on. Interval goes from once or twice a minute to once every five seconds or so. The sputter disappears below 2K rpm. I haven't put a fuel pressure gauge on it yet so I don't know what the fuel pump is doing. Definitely sounds healthy and quiet; I know what a dying pump sounds like. I'm running Accel 8.8 wires in custom looms and an Accel cap/rotor. The inside of the cap looks good and the wires don't look like they're arcing. I pulled the #1 plug and it looks great; light tan color with minor brown deposits here and there. No erosion on the electrodes (Bosch +2) so I don't think this is ignition related. A few months ago I caught one injector dead. Car was running fine but lacked power to get on the highway. I found the plug on the right side injector had worked loose. I snapped the plug back on and away we went. Right now both injectors seem to be doing what they're supposed to be at idle. Too bad the ECU doesn't support live injector data; my life would be so much easier.

My main question is has anyone ever overhauled a GM TBI unit and if so is there anything in particular I need to watch for? The car is coming up on tune up time (200K) and I'm thinking some TBI tlc would do me good.
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Me hid big dings wid da hamma.
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Rob S · Posted: Thu Jul 15, 2010 6:32 pm Post subject:

DD,

The most I've done to rebuild a TBI unit (other than replacing the gaskets) is to have the throttle shaft hole drilled out and fitted with bushings. This was done to eliminate the slight sticking of the throttle butterflies just off idle and to stop any air leakage around the throttle shaft holes. Certainly resulted in a smoothly operating throttle.

I found a place in Toledo, OH, that did the throttle bushing. They had me UPS the TBI base to them and it only took a few days and I had it back ('course, I live very close to Toledo). They also cleaned out all of the carbon in the PCV inlet by soaking the TBI in some wild cleaning solution. When the TBI came back you could eat off of it.

Since the injectors spray pattern looked okay to me, I reused them. Probably should have had some professional really check them out since I was doing so much to the TBI base, but everything worked fine when I re-installed it.
_________________
Rob
1982 Olds Delta 88, now equipped with 9C1 sway bars fore and aft, a 9C1 faster ratio steering box, 9C1 rear springs, and a tired 307.

Deathdeelr · Posted: Sat Jul 17, 2010 11:33 pm Post subject:

Thanks for the heads up on the PCV and the throttle shafts Rob. I'm going to play with it next week. I ordered a gasket/regulator kit from NAPA for a couple of bucks. We'll see what happens.
_________________
Me hid big dings wid da hamma.
ASE Certified Master Tech
http://www.cardomain.com/ride/3311846

Rob S · Posted: Sun Jul 18, 2010 7:01 pm Post subject:

DD,
I've collected a couple of TBI units from high mileage vehicles and am no longer surprised when I find a lot of throttle shaft play in them--all of the ones I have are very sloppy. It seems that the metal the throttle plate base is made out of isn't the best at being wear-resistant. I never would have thought to check for the wear except that I happened to read an article about TBIs that stated the throttle shaft hole wear was a very big reason why older TBI vehicles couldn't be made to run right, especially at idle. I guess the ECM gets a real workout trying to adjust the idle air motor with the vacuum leak created by the throttle shaft.

On the two TBI units that I've had the shaft bushing work done on, the PCV channel in the TBI base was completely clogged with carbon. There was no way that either TBI was ventilating the engine block anymore.

There are certainly a lot of small things that can add up to an ill operating engine.
_________________
Rob
1982 Olds Delta 88, now equipped with 9C1 sway bars fore and aft, a 9C1 faster ratio steering box, 9C1 rear springs, and a tired 307.

Deathdeelr · Posted: Wed Jul 21, 2010 8:14 am Post subject:

I put the unit back together but OH YEAH! There's play in the shaft and there was a load of junk under the PCV port but it wasn't completely blocked yet. The car runs better but still has the occaisional stumble; it didn't fix the problem. I put the car on the analyzer at the job. It seems that the stumble is, in fact an ignition problem but I can't seem to pin it down. It runs good for a few seconds then #3 drops for one cycle and recovers. It runs good for a minute or so then #6 drops and recovers. I tested the wires and the coil. I swapped distributor caps and rotors. Everything looks good but the stumble is still there. I'm thinking possibly an ignition module problem. The ECU reads the signal but I don't think it has anything to do with the actual firing of the plugs except for ignition advance. Next week I'll try a new module and see what happens.
_________________
Me hid big dings wid da hamma.
ASE Certified Master Tech
http://www.cardomain.com/ride/3311846

Deathdeelr · Posted: Sun Aug 08, 2010 6:01 pm Post subject:

I finally got around to installing the new ignition module. The plug for the pickup was shorting against the distributor base plate. I repaired the plug and installed the new module, distributor cap and rotor. The stumble is gone. Cruise is smoth as silk.
_________________
Me hid big dings wid da hamma.
ASE Certified Master Tech
http://www.cardomain.com/ride/3311846