3.4 swap backfire and stumble issue - help!

Telly

Hard Core 4+
Joined
Jan 4, 2008
Messages
532
Location
Monument
My son and I made the final wiring harness connections today on his 92 pickup 3.4 swap project. Fired it up and it immediately ran rough and starting backfiring within 30 seconds. Only backfires while idling and stumbles at higher RMP's. Good oil pressure and the engine sounds good otherwise.
We did the following trouble shooting things.
1. Checked timing and its within spec.
2. Checked the cam sprocket marks at TDC and they both line up correctly. (we did a new timing belt)
3. Removed the aftermarket coil packs and reinstalled the original coil packs. Issue did not change.
We bought this motor from salvage yard and were not able to hear it run, therefore I can't confirm if this was an issue prior to the swap. I'm leaning towards a bad ECM? My theory is the ECM is sending incorrect firing signals to the plugs. Any other ideas? Thanks!

Please move this thread to the Tech Section. I put this in Chit Chat by accident.
 
Last edited:

SteveH

Hard Core 4+
Joined
Aug 10, 2006
Messages
2,298
Location
Colo Springs
Does it set any codes?
 

J1000

Rising Sun Member
Joined
Oct 11, 2018
Messages
424
Location
Morrison, CO
Don't know much about these motors but I'd check the plug wires/coilpacks are on the right cylinders/in the correct order on the distributor/in the right connector on the harness.
 

HDavis

Locked
Joined
Feb 13, 2019
Messages
253
Is there a coolant temp sensor that sends data to ecu? My 3rzfe has one and when its unplugged it had these types of symptoms.
 

On the RX

Rising Sun Member
Joined
Nov 19, 2016
Messages
914
Location
Superior
Could the timing be off? After you exhaust the simple stuff, check to see if the timing belt has everything aligned.
 
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