Only 2K RPM Misfire - '84 2m4 Iron Duke (Page 1/2)
Sanhino JUL 20, 12:48 PM
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?

Much appreciated!
Nickoman JUL 20, 01:26 PM
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

http://fierocave.shorturl.com/ecmqrg.htm
Sanhino JUL 20, 01:54 PM

quote
Originally posted by Nickoman:

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

http://fierocave.shorturl.com/ecmqrg.htm



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.
Sanhino JUL 20, 07:50 PM
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.
Nickoman JUL 22, 12:18 PM
Hmm, have you checked the Map sensor in the same way?
edfiero JUL 22, 01:53 PM
Have you checked your timing?
That can cause backfiring
pmbrunelle JUL 22, 08:08 PM
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).]

Sanhino JUL 24, 05:01 PM
Update:

- 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
andreww JUL 27, 10:25 AM
A video would be very helpful here.
andreww JUL 27, 10:31 AM

quote
Originally posted by Sanhino:

Fuel pressure holds solid at 10-11psi throughout.




Shouldn't the fuel pressure be over 34PSI?