The title pretty much says it all. The car seems to idle fine and driving seems fine but between 1,900 and 2,100 rpm (and only at that range) it misfires pretty badly. It has a new distributor cap, spark plugs and wires, and ignition coil. Fuel pressure holds solid at 10-11psi throughout.
Anyone have any thoughts on specifically why that range?
May be a bad throttle position sensor. You should be able to back probe it and see what the voltage is while at your troublesome rpms. The voltage should increase as you increase the throttle. see if it jumps around in those ranges. also, see the below link. Ogre shared it with me and its saved me so much time on diagnostics over the years
May be a bad throttle position sensor. You should be able to back probe it and see what the voltage is while at your troublesome rpms. The voltage should increase as you increase the throttle. see if it jumps around in those ranges. also, see the below link. Ogre shared it with me and its saved me so much time on diagnostics over the years
That's probably a good idea. I'll have to double check the TPS. I had replaced it and checked it was right around .5V at idle and increasing with throttle when I got the car a little less than a year ago but maybe it rattled out of position since it has the slotted holes.
I just double checked the TPS and when fully closed reads .50-.52V and when fully open reaches 4.46V which sounds pretty common. It also seems so increase gradually rather than be jumpy so at this point I don't think it's the TPS throwing it off.
The problem arises with a free rev in neutral, or while driving in gear?
My Fiero had a bucking (aka trailer hitching) sensation around 2k RPM, while driving calmly at constant speed (in gear). When the driveline is unloaded, the transmission/diff gear teeth can smack back and forth, as there is not enough torque to take up the lash. Any engine irregularities can be amplified while driving in this condition.
In my case, the air/fuel mixture was too lean, so perhaps you have some fueling issues (sensor, injector, whatever).
If it is the driveline lash phenomenon, then perhaps it's just driveline wear too, in which case you'll have to adapt your driving.
[This message has been edited by pmbrunelle (edited 07-22-2020).]
- I backpinned the MAP sensor - A reads 0V (makes sense, ground), C reads 5V (makes sense, power), and B reads about 0.9-1.0V at idle, and the starts to climb with increased throttle, reads about 1.5-1.6V at around 2k rpm. From what I can tell that seems to make sense.
- I double checked timing: with pinning the ALDL timing is right around 8 degrees and runs fine but without pinning it the timing reads at like 20 degrees. Is that normal?
- It does sputter and stall at a free rev but I think I also just happen to have the bucking issue pmbrunelle mentioned. Originally I was thinking it was due to the ignition issue but now it sounds like it's just what it is.
-Revving it by hand and having it sit around 2k rpm I can hear it start to break up. Just for the hell of it I sprayed a bit of carb cleaner into the throttle body (thinking if it was lacking fuel this could maybe make up for it a bit more) and it stalled out, so in my non-experienced mind that puts a tick in the box against it running lean
If I remember correctly it should be that high if it was a V6 or later model but for the 84 iron dukes 10-12 psi is normal. (Someone more familiar please correct me if I'm wrong).
There are 3 videos there: One of throttle body, one of fuel pressure, one of tach (all not the same run of course). The spray pattern looks a lot worse in the video than it does in actuality, I think it's how my phone captures the video. It only seems to capture some of of the sprays.
[This message has been edited by Sanhino (edited 07-27-2020).]
Absolutely dumb question. But did you get all 4 plug wires back on the right hole? I'm only asking because I've in my ultimate wisdom......... have put wire 2 on plug 3 and 3 on 2... and I checked timing on holes 1 and 4... thinking I had no ignition timing issues. https://www.youtube.com/watch?v=ZT0GOnz-AwM don't mind the whining noise that was a brand new timing gear doing its thing.
I have a V6; never had a 4 cylinder Fiero and am too lazy to look it up if the 2.5 has an EGR as well; I had an issue with my EGR valve where it would stick and nearly stall out on returning to idle like it seems to do in your video...
------------------ "Discord" Red 1988 GT under restoration!
I have a V6; never had a 4 cylinder Fiero and am too lazy to look it up if the 2.5 has an EGR as well; I had an issue with my EGR valve where it would stick and nearly stall out on returning to idle like it seems to do in your video...
I had checked previously if it slid when under vacuum. It does (or at least did when I checked it) but I ordered a new EGR and gasket regardless. Maybe under idle vacuum it runs fine but slight vacuum at 2K rpm isn't enough to move it, and then higher RPMs are able to move it.. just spit balling at this point. Or maybe it's just crunchy and doesn't slide like it's supposed to. I order a new PCV valve too since that's still original and it's cheap enough.
I was also looking up videos of GM single carb throttle body rebuilding. If I was to replace the injector I would already be halfway to replacing the fuel pressure regulator so maybe that's worth trying too? Maybe I'll do that if the new EGR and PCV doesn't make any difference.
I have a V6; never had a 4 cylinder Fiero and am too lazy to look it up if the 2.5 has an EGR as well; I had an issue with my EGR valve where it would stick and nearly stall out on returning to idle like it seems to do in your video...
I just replaced the EGR and the PCV valve and it seems to have fixed it, no specific misfire at 2K rpm and if I hold it and release the throttle it doesn't stall out like it used to. I'm so excited!
