Vader

I've got a 151/4 TBI in a winter driver Fiero that ran fine for years. I "unswirled the heads, enlarged valves, bowl, and ports this past spring. Also installed a mildly improved cam. Engine ran fine all summer until the car was struck while parked.

Fast forward two months (the time it took to find replacement parts and fight the perp's insurance company) and after body assembly it runs extremely lean just off idle position.

MAP is new; TPS new and idle position voltage dead on 0.85VDC; TPS made adjustable and set to various positions to cure problem - no help; fuel and air filters new; fuel pressure adjusted from 11 PSIG to 26 PSIG with little effect; new oxygen sensor installed; distributor replaced; IAC removed, cleaned, and lubed; throttle plates aligned, CTS in perfect range; ECM reinitialized more than once; new injector, fuel return line clear. Ignition timing has been adjusted all over the place with no improvement. Exhaust is not plugged. PCV valve is new; EGR is cleaned and operating (disconnected for testing); and the new intake/TBI/head gaskets aren't leaking as best I can tell. Hoses and plumbing are intact.

If I force backup fuel mode, the engine runs great but is defineitely on the rich side. This proves to me that the ignition is reliable, and fuel is the problem. I suspect that I've altered the intake enough to take the normal fuel requirement at cold temperatures off the PROM's fuel map. The cam grind is not tha much different than stock, but the increased valve sizes may have amplified the effect.

Any insights?

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Later,
Vader
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"No matter how hard you try you can't stop us now"
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Pablo

iTrader: (1)

yes vader, actually this is to be expected although its troublesome to hear that it continues to do it when you played with the fp

can i ask how you know its running lean off idle? Its quite common for modded tbi motors to stumble or even backfire through the intake off idle with a stock chip. I know mine did, the timing requirements are way different, more so than the fuel requirements i found. I could barely drive my car unless i physically advanced the dist a substantial amount which of course made me have too much advance in some spots but made the car driveable. I definately think you need a custom chip, if the symptoms i described are similar to yours try advancing the distributor a substantial amount and take it for a spin and see if its gone away or has been reduced

Brent

iTrader: (1)

This statement makes me think it should not require any custom tuning to return to the previous running condition. I'm NOT saying it wouldn't benefit from some tuning. Just that the tuning doesn't seem to be the issue.

My WAG is that there is a ground strap or such that is corroded. Perhaps some connector that has a weak connection. A wire that got pinched in the accident? Maybe a failing alt diode? Just grasping at straws.

These things can be very damn frustrating. Let us know what you find. G'luck to ya.

gtomike66

Do you have an air cleaner with a thermal switch that actuates the vacuum-controlled air flap in the snorkel? At cold start, vacuum is supposed to pull the gate shut so air can't get into snorkel. Air has to come up through exhaust manifold stove for preheat until engine heats up. Is the gate operating properly?

snflupigus

Just a guess here since I will never claim to be a ace mechanic.

Timing chain is due for replacement. You might be adjusting the timing, but are you adjustments null because your chain is old and stretched out too much??

Vader

Pablo,

I've adjusted the timing and FP to varying extremes with no solution. After the initial mods, I adjusted the timing to about 8° BTC base position (factory), and ended up at about 12° base for best overall performance.

Advancing does help some, but I've advanced it to the point where it will barely idle and detonates at part-throttle.

Thanks for the reply. I'm thinking PROM time, too. I just thought I'd toss it out there in case anyone had a way around the problem.

Oddly, the ECM is telling me that it is very lean, and I installed the old, slow O2 sensor and it actually improved a bit.

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Brent,

Good thought. The first thing I considered was something damaged or loose from the impact, but I've ruled out all the connections and devices I could think of. The engine and exhaust system have good, solid ground connections.

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GTO,

Yes, I have the EFE system and it is working properly. I'm even having trouble running it in a 70°F garage without stumble off idle, although it is much better in the warmer air. This system didn't use a MAT/IAT sensor, and I'm thinking that would help immensely.

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Snuff,

Good possibility in most cases, but the POS four-holer has gears instead of a chain. Probably another reason they run forever.

------------------
Later,
Vader
------------------
"No matter how hard you try you can't stop us now"
Adobe Acrobat Reader 4.0