hi everyone...well this isnt exactly on an f-body, but its a TBI question!

Ive searched all around but you guys seem to be by far the best knowledge base of TBI...

so basically, I built a 4 wheel drive vw beetle with a rear mounted 4.3 TBI/th400. Couple pics...

http://i28.photobucket.com/albums/c2..._0085Large.jpg

http://i28.photobucket.com/albums/c2...0124Medium.jpg

I bought the 4.3 from a junkyard, supposedly it was a 1993 "chevy truck" (I really wish they could tell me if it was a c/k, s-10, etc). Im assuming a c/k truck because I think the S-10 got CPI in 93?? The junk yard said the engine ran perfectly when pulled.

The engine didnt come with a harness but they let me grab any 4.3 harness/ECM I wanted, so I dug into the first astro van I saw at the junk yard. I think the astro was either a 1987 or 1988. I saw it was a '7747 ECM (I guess the most desirable??) so I grabbed that.

I got it all wired, but there were a couple things I had to change...the astro TPS plug was different, and the astro had a vacuum actuated EGR. The engine I have has an electric servo actuated solenoid. I thought this was odd because everything Ive looked up says all TBI's had a vacuum EGR. I got rid of it and just made a blocker plate.

The engine started on the first try and idled perfectly. However when I put it under load and try accellerating at anything more than maybe 20% throttle it stumbles and backfires through the intake. Couple pops out the exhaust too but mostly just fireballs out the intake. It seems like a timing issue, either the ECM isnt advancing it properly, or what. Problem is I cant find the timing marks on the balancer or block! Being rear mounted in a non-stock app, its a little difficult to see everything, but still Ive used mirrors and everything, and I cant find them. It seems like the water pump and pulley is directly in the way of seeing them. Or are they (the reference marks on the block) on one side and not the top?? Although if the engine was running fine when pulled, I cant see how the timing would be off. I tried disconnecting the timing bypass to make the ecm relearn but it didnt seem to do anything. I pulled the cap and rotor and there seemed to be some metal filings on the underside of the plug wire connectors, as if little pieces of the contact on the rotor had chipped off? Replaced the cap and rotor and plugs and that seemed to help; now It runs fine up to about 75% throttle until it stumbles and backfires.

Idle is smooth but every 30 seconds or so it kinda stumbles and sounds like its about the stall, then the IAC "catches" it (Im assuming thats whats happening?) and it settles back down to a smooth idle. In park just free revving it it runs perfectly, no stumbling, and revs right up to WOT really fast.

Could the distributer mechanism that advances the timing be stuck or something?

It was originally a 700r4 ECM, do I need to ground the park/neutral wire, or just let it float?

Both injectors are firing
Brand new fuel pump, although I will re-check the fuel pressure tonight.

But the wierdest thing of all is I have no check engine light; as in it has never turned on. It doesnt turn on at key-on engine off, and doesnt flash when I jumper the ALDL wires. Got a new known-good bulb, connected one side to IGN +12v and the other side to the brown/white MIL request wire and still nothing. Maybe my ECM be bad?

However when I plug in my tech 2, it reads data fine, and all the bi-di functions work (IAC control, fuel pump control). But when I scan for codes with the tech 2 it says no current, pending, or history DTC's. Even unplugging a bunch of stuff (MAP, TPS) it still says no codes. TPS and MAP seem to be reading right, the 02 sensor flip-flops between rich and lean, so it seems to be running at stoich, coolant temp seems to be reading fine, and it does go into closed loop (according to the tech 2) when it gets up to 180* or whatever.

the complete lack of check engine light is what makes me think its maybe a bad ECM, but other than that Im stumped.

thanks
ben

 

If you put the timing light on the #4 plug wire, you should be able to see the timing mark from the underside of the car. Not real accurate way to check it, but with the EST disconnected, it should be near the 180 deg position on the bottom of the balancer, about aligned with the CL of the motor. With teh EST connected, it should move about 20-30 degrees toward the right side of teh motor when you look at the engine head on. If its still stuck in the same position after you reconnect the EST and restart the car, then you have no timing advance.

 

awesome thanks thats a huge help...Ill check it out right now.