Sorry, kinda long, just don't want to leave out any details.

89 Formula 350, AFPR (set about 50 psi), SLP muffler, ported plenum, Accel coil, stock wires/plugs, 185 thermo, nothing else modded.

At startup, it usually cranks all to heck for about 3-4 seconds, and right when you think the battery will have an anurism, it kicks and runs. Sometimes though (especially if it's still warm, or been sitting several days, nothing in between) it starts in half a crank or so.

It almost ALWAYS idles low to about 500 and wants to die right after starting and shotting up to 1100 or so. When it's cold, if I step on the accelerator lightly, it will bog down and die, without exception. I have to goose it to 50% throttle or more. If I goose it and let off, it jumps up fine, settles down to 900-1100 RPM. If I hold my foot at 5-% or so, it will backfire, bog down and die.

At wide open throttle, it bucks and takes off beautifully. ANYTHING less then 70-90% throttle though, and it bogs, backfires, and wants to die, and doesn't accelerate, period. It actually slows down at 50% throttle when it's cold. When I'm rolling, or especially from a stop, no difference really.

When it's warmed up, it runs better, but will still backfire and bog occasionally at partial throttle, especially from a stop.

I really need to crack this one. As of now, before it's totally wamred up (185), I literally have to floor it and coast to get wherever I'm going, no part throttle at all. Rain is falling recently though, and turning to ice, so flooring easily lifts the back end right off the ground. NOT GOOD, not to mention the embarassment of backfiring off every light for the first 15 minutes of every drive. *Help!*

-Reno
"I like to call it 'Hydra.' Every time I fix a problem, two more spring up."

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"We are the middle children of history. Our Great War is a spiritual war, our Great Depression is our lives" - T.D.

 

That sounds like the TPS.... check it out. Do you know how to check the TPS voltage?

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1988 Pontiac Trans Am GTA L98 5.7L, Black on Black with 114,000kms.

Best ET 14.559 @ 95.25MPH
Custom 3" Stainless Exhaust Y-Pipe back (High flow Cat and Flowmaster Mufler)

 

As nblanchard said, may be the TPS..........but I had much the same symptoms and it was my MAT (manifold air temp) sensor bad.........$6.31 later it is fixed and runs perfectly cold. It did not throw codes so needed a scanner to figure this out........best advice I can give to you or anyone that does NOT have a scanner is to buy one - I went with the new Auto Xray 10110 and am glad I did. Paid $150 new and it is very user friendly.

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'79K5 Blazer with 406TPI

 

I unplugged the TPS, letting it throw the code, and the situation improved, but only marginally. Maybe I'll try the MAT now...umm...where is it?

-Reno
(unrelated private message: thanks for trying Colin, call me anyway)

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"We are the middle children of history. Our Great War is a spiritual war, our Great Depression is our lives" - T.D.


[This message has been edited by Reno (edited November 24, 2001).]

 

if i am not mistaken the MAT sensor is located under the plenum on the plenum's underside.

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86',88',89' IROC-Z, 350 TPI, 700-R4

94' Formula, LT1, 6spd

 

If you unplugged the TPS and the situation improved, even if not completely, you should definitely check the voltage on it and adjust it because that's part of your problem. Instructions can be found at the following link and it's very easy to do.

https://www.thirdgen.org/newdesign/tech/tpimod2.shtml

If you adjust the voltage to the appropriate level it might fix your whole problem but if it doesn't then you can move on to the next step. As mentioned earlier, the MAT is locted underneath the plenum on the back side. You won't be able to get to it to unplug it w/out removing the plenum. If you've never done so, it's quite easy... just remove the top 4 runner bolts on each side and tap on the plenum lightly with a rubber mallet and it'll come right off. Usually too the gaskets don't rip or tear and alot of people reuse them. Of course, you can remove the plenum w/out removing the throttle body, just make sure you unhook everything from the throttle body including the small vaccuum line directly underneath it that you may not be aware is there.




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1986 Trans AM
355 TPI
4 Wheel Disc Brakes
9 bolt Borg Warner Rear (2.77's....oh joy) :P
6" rods, approx. 9.7:1 Compression, Stock TPI (soon to be ported), 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, 1.52 RR (for now), Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech 53,000 volt coil.

Track times to come.