84 Fiero, 2.5, auto, AC. Just got the car a month or so ago.. Previous owner had thermostat out, fan hotwired. I put thermostat in, filled properly, seems to run great, mileage ~20-23mpg, except after it is warmed up, it doesn't want to idle for very long in drive. The EGR, O2sensor, IAC, plugs and more have been replaced. It idles around 850-900rpm in park or neutral, it may idle fine in drive around 600rpm for several minutes, then dip down and up several times and sometimes dies. The timing was way off and I have adjusted it to spec. I replaced the O2 sensor again tonight (with AC Delco) after I measured the output with a DVMM. It was steady at 57mv before replacing. I unhooked the battery for a few minutes and then took it out for a test drive. All seemed fine until I let it idle in drive for 5 minutes or so. It doesn't seem to matter if the AC is on or not. After the idle problem occurs, it stumbles bad on acceleration. Once it clears out, it is OK again until it idles for awhile again. My wife is tired of it already in traffic. My next part was going to be the TPS. (no codes) Any help would be much appreciated.

How old is the fuel filter? I changed the one on my 2.5 and it helped idle and acceleration.

[This message has been edited by Tigger (edited 10-16-2001).]

Just changed fuel filter, still rough idle after warmed up. I even disconnected battery while I worked and then took it for a road test. As soon as it warmed up, I pulled over and with it still in drive, the idle was around 600rpm and started bouncing up and down, almost stalling. I pulled out into traffic and noticed (it was there before, just seemed worse now) a severe stumble at part throttle, worst around 2500rpm... Any other ideas? I am thinking TPS or coolant sensor. Thanks

Could be your idle speed control on the throttle body. First try to spray clean it out and see if that helps, if not replace it. Auto zone has them, not expensive. If you replace it you'll have to drive a bit to adjudt it to the car.

The idle air control was changed first. I have taken the new one out and made sure it was clean inside the cavity. I ran the field service mode tonight and it indicated that all was working normal. I checked the EGR valve, it checked OK. Each time I do anything to it, I disconnect the battery and let the computer relearn things. Any ideas? Thanks, Scot

bump

You never mentioned the MAP sensor.

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Your fuel mileage at 20 - 23 is not real good. I get about 30 with an '84 four speed in a combination of city/highway driving. Engine has about 40k on it. Low mileage may be another indication that something is still pretty messed up.

Have you tried putting a vacuum guage on it? You should get about 21 inches with a reasonably steady needle.

I have the same type of stumbling when I accelerate in the higher gears, elsewise runs fine. I've went about the same route as you did, (plugs/wires/cap/rotor/TPS/pick up coil/fuel & air filters/vac hoses)and although the engine runs better it still misses badly when I give it lots of gas in 3rd and 4th gears. Tried retarding and slightly advancing the timing and that did not help either.

I'm thinking it may be a dirty injector. Things get marginally better when I put a can or two of cleaner in with the gas. Maybe it's time to get it cleaned at a shop.

I was in a parts store (pricing a TPS) last night and ended up talking to a mechanic (full time day job) who has done alot of work on Fieros over the years and he indicated that the MAP sensor was the #1 suspect with the symptoms I described. I couldn't wait to have a look tonight and see what I found out. The MAP sensor measured perfect with the procedure in the Pontiac service manual. I did not get the results I had hoped for. I will keep looking.. maybe visit the wrecking yard and grab some sensors (TPS,MAP) and maybe an injector. We had an 84 4sp back in 84 (still feel the pain for selling it) and we never had a single problem, ran great, got great mileage, no problems at all.

It seems to stumble when under light throttle (such as idle in drive) or when lightly accelerating. It runs much better when driven aggressively. From a stop and lightly accelerating up to 60MPH, it stumbles when starting out and again about 2250 RPM in second gear (automatic).

So... I changed the TPS and MAP with the same results. Ignition/injector on my list now. I still haven't found my vacuum gauge to check what's going on with the vacuum.

check over the vac lines, all of them but especially the one for the MAP sensor. Also make sure it's vac port on the motor is clear.

Have the charging system thuroughly checked. A battery or alternator problem, even subtle ones, can cause flakey idle. check all the other stuff in the wire service article in my cave.

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11-Sept-01, The day the world as we knew it ended.

replaced the MAP vacuum line when replacing MAP sensor. EGR vacuum port on TBI seems to not have much vacuum?? I'll check the items you mentioned. Thanks

Seems to get better after each time I work on it, just not completely. I have a spark jumping at the spark plug boot. I am replacing the wires and going to a smaller gap plug. Will post results.

I had a similar problem with my 84. Replace the gasket under the throttle body and see if that helps. I also found the passages for the EGR system were blocked by carbon and cleaned them up. Took a few trips but the idle smoothed out. Just started getting flaky so I retightened the T.B. and it smoothed out.

OK, here's what I did...

Replaced many sensors,
*O2 (previous owner had done this one but was running without thermostat so the overrich condition ruined it.
*Thermostat (previous owner had removed)
*spark plugs (gapped at .060", later reduced to .045")
*EGR (and vacuum line)
*MAP (and vacuum line)
*IAC (and cleaned cavity)
*TPS
*vacuum line to air snokel thingy for warmup
*gasket under throttle body (thought this was it until it warmed up with the AC on)
*spark plug wires and coil wire

I also cleared the computer, adjusted timing and checked things at each step.

It idles fine now. The biggest item after the O2 and thermostat was the plug wires! All the other pieces had an impact but the wires were probably the previous owners problem. His solution was to remove the thermostat so it never warmed up. He also hotwired (with fuse inline) the fan. This made sure the car ran in open loop mode. It was only after I put the thermostat in and the car warmed up and ran in closed loop did I have idle and mileage problems. I am VERY pleased with how this baby is running now. I did not buy all the sensors at the dealer BTW, only the O2 and IAC. Many of the other parts were from the salvage yard and very serviceable. The plug wires I wanted were the ceramic boot champions (handle high heat very well, such as find in big block motor home environments) but the budget sent me to Accel 8mm v-8 universal (I can make a second set when I need to).

Many thanks to all those that offered assistance.
Scot