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Only 2K RPM Misfire - '84 2m4 Iron Duke by Sanhino
Started on: 07-20-2020 12:48 PM
Replies: 13 (170 views)
Last post by: Nickoman on 07-30-2020 01:12 PM
Sanhino
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Report this Post07-20-2020 12:48 PM Click Here to See the Profile for SanhinoSend a Private Message to SanhinoEdit/Delete MessageReply w/QuoteDirect Link to This Post
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!
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Nickoman
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Report this Post07-20-2020 01:26 PM Click Here to See the Profile for NickomanClick Here to Email NickomanSend a Private Message to NickomanEdit/Delete MessageReply w/QuoteDirect Link to This Post
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
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Sanhino
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Report this Post07-20-2020 01:54 PM Click Here to See the Profile for SanhinoSend a Private Message to SanhinoEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
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.
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Sanhino
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Report this Post07-20-2020 07:50 PM Click Here to See the Profile for SanhinoSend a Private Message to SanhinoEdit/Delete MessageReply w/QuoteDirect Link to This Post
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.
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Nickoman
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Report this Post07-22-2020 12:18 PM Click Here to See the Profile for NickomanClick Here to Email NickomanSend a Private Message to NickomanEdit/Delete MessageReply w/QuoteDirect Link to This Post
Hmm, have you checked the Map sensor in the same way?
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edfiero
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Report this Post07-22-2020 01:53 PM Click Here to See the Profile for edfieroClick Here to Email edfieroSend a Private Message to edfieroEdit/Delete MessageReply w/QuoteDirect Link to This Post
Have you checked your timing?
That can cause backfiring
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pmbrunelle
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Report this Post07-22-2020 08:08 PM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
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).]

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Sanhino
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Report this Post07-24-2020 05:01 PM Click Here to See the Profile for SanhinoSend a Private Message to SanhinoEdit/Delete MessageReply w/QuoteDirect Link to This Post
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
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andreww
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Report this Post07-27-2020 10:25 AM Click Here to See the Profile for andrewwSend a Private Message to andrewwEdit/Delete MessageReply w/QuoteDirect Link to This Post
A video would be very helpful here.
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andreww
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Report this Post07-27-2020 10:31 AM Click Here to See the Profile for andrewwSend a Private Message to andrewwEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Sanhino:

Fuel pressure holds solid at 10-11psi throughout.



Shouldn't the fuel pressure be over 34PSI?
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Sanhino
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Report this Post07-27-2020 11:35 AM Click Here to See the Profile for SanhinoSend a Private Message to SanhinoEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by andreww:


Shouldn't the fuel pressure be over 34PSI?


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).
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Sanhino
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Report this Post07-27-2020 11:47 AM Click Here to See the Profile for SanhinoSend a Private Message to SanhinoEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by andreww:

A video would be very helpful here.


https://imgur.com/gallery/v0o5BHi

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).]

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Nickoman
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Report this Post07-29-2020 03:27 PM Click Here to See the Profile for NickomanClick Here to Email NickomanSend a Private Message to NickomanEdit/Delete MessageReply w/QuoteDirect Link to This Post
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.
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Nickoman
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Report this Post07-30-2020 01:12 PM Click Here to See the Profile for NickomanClick Here to Email NickomanSend a Private Message to NickomanEdit/Delete MessageReply w/QuoteDirect Link to This Post
Never mind I just re read your initial subject line. It drives fine other than the specific RPM range. 2 dead holes would not handle that! Im dumb!!!
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