Well heres the deal....

My car cuts out and backfires BAD at half throttle while driving. If I floor it it almost dies. While driving it around town it will start up, but if I leave it overnight it will not start unless I turn the cap counter clockwise about 1 inch even though I timed it with a light too 8* btdc. I put the jumper key in and have gotten two codes today. 1. code 24 Wich is VSS and 2. 42 wich is Electronic timing control. It wont start easily unless I press on the TPS slightly. I have a 1991 Z28 TPI with a 383 I'll post my mods if it will help.
Thanks alot

 

I don't think the VSS has anything to do with it, code 42 pops up when you unplugs the EST bypass to set the timing.

I'm assuming you set the timing with the EST disconnected,to 8* BTDC?

Now CCW would be advancing the timing, and inch rotation would indicate about a tooth on the distributor gear, that's a lot to have to start it.

Now, after you advance it to start, are you setting it back to where it was?..i/e retarding the timing again?

(bear with me, I'm thinking and typing at the same time)

Backfiring out the exhuast ususally means you're reburning unspent fuel, now if the fuel delivery is correct, and you're not running rich, then the only way you can get unburnt fuel to the exhaust to reignite, it if the timing is off, or the firing order (which is actually timing per each cylinder) is off, or if a cylinder is not firing at all. I'd guess that incorrect valve lash could be an issue, but I'd tend to rule that out, being that adjusting the timng allows you to start it keeps bringing me back to the firing/timing.

How long did the car sit waiting for the new engine? It's possible moisture has affected the PU coil or module, both of which can cause missing/backfiring issues as well.

Man, I'd go back and look hard at the plug wires again for routing, and make sure none are wrapped around each other causing spark interference.

 

Mateo,

¿Que es la problema? So you need to check the VSS generator, gear, and wiring. And you need to bypass the EST when you set the timing, and maybe leave it in bypass until you can determine if it has failed or not. If the engine starts well with the EST bypass connector hanging loose, you might have a failing EST module.

These are tough to diagnose when they start to "flake out" and running without it will at least give you a baseline for initial spark timing. If everything seems normal in bypass (except for a bit less power and fuel mileage) you might want to consider a replacement. I normally wouldn't advise just throwing parts at a problem, but I haven't found a reliable method to test the EST control module (yet). Before you go that far, take one last look at the reluctor, pickup coil, and amplifier module connections in the distributor. A minute crack in the reluctor can really throw off the timing signal to the ECM and ESC.

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Later,
Vader
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"I'm'a do Things My Way - It's My way or the Highway."
Adobe Acrobat Reader

 

The 1990-1992's don't have an ESC module. The ESC is part of the MEMCAL.

 

I did disconnect the EST (brown and black wire by the AC canister). Tonight I set the timing about 1/4" above 16* BTDC and it sounded good but it sounded better when I pressed down on the TPS a little bit so I removed the cap and screwed the throttle out til the engine sounded better. I also took the vaccum tube off the throttle body and messed with alot of stuff to make sure nothing was misplaced or clogged and took it for a test drive. It ran GREAT I chirped 2 and 3 and it felt great. So I came back and set the timing to 12* to bring some calm to mind. and it did the same thing as before CUTTING OUT AT FULL THROTTLE. I checked all wires several times and I have two of them zip tied together because I dont want them touching the headers, and they are brand new MSD super conductor's so I dont think they are the problem.

Whenever I get the timing to 8* it misses real bad and I'm just at a loss as to what else can be done. What will happen if I run without the EST hooked up Vader? I'll try some different stuff tommorow morning but I'm just not a tuner. I wish I were. Please continue to help guy's, this sucks.

 

In addition to the above:

Faulty pickup coils will produce results like you describe. When diagnosing backfires/misses/etc that's one of the first places I look. I wrote an article on the subject [ http://www.racer-x.f2s.com/proc/pcoil.htm ] that also gives steps for a very easy quick-test.

The VSS won't do much until the vehicle is actually moving, so I wouldn't bother looking there yet, unless your speedometer doesn't work.

If you have an aftermarket spark box (like an MSD), try bypassing it. I had an MSD go on me once and it produced all kinds of headaches like that.

Don't forget the TPS itself; they can fail without setting a code.

You might also want to have the ignition module tested; they can fail yet still allow the engine to run.

If all else fails you might want to check your valve timing, and check for any slack in the timing chain, especially if you do a lot of racing (on or off the track)

 

You might make sure you didn't shear the pin on the distributor shaft. A worn distributor or high drag oil pump can do that and leave you chasing the timing around.

 

I'm checking all this stuff right now. BRB

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