mystery misfire

[c10 custom]

hi everybody i am working on a 1989 gmc truck 4x4 350 5 speed manual trans
i have a misfire at idle and when accelerating it is misfiring slow and high speeds. New spark plugs wires cap and rotor ignition module map sensor
throttle position sensor the fuel pressure is 13 psi whitch is correct
tested coolant temperature sensor and temperature resistance readings are correct i am wondering how to test the ecm and the electronic spark control module on the side of the throttle body to me it feels like a spark related misfire any help would be greatly appreciated thank you

[speedygonzales]

Do a compression test

[stevejohnson48]

I once had a non computer vehicle that had a miss like your describing??/ I Thought it was the intake valves ??/ (Burnt valve ) I took them in for a valve grinding job and the tech machinist told me it was a flat lob on my cam ??/ Sure enough loaded the heads back in the car the next day or so replaced the cam and timing chain and problem solved ??/

[mountie]

Whenever I hear "new spark plugs / wires", I remember a past issue.
( was the miss-fire BEFORE replacing them?? )

New plugs can have defective-hair-line cracks on the ceramic.

I had new MSD wires with poor assembly in the factory pre-installed boots.

I also experienced this at a high-end race shop, when a customer had a miss, and the wires & plugs were newly installed and didn't consider this.

( A couple years ago, I had a new MSD cap fail at the center coil button within 30 days)

[c10 custom]

Thanks everybody for the replys it was a bad plug wire

[mountie]

Originally Posted by c10 custom

Thanks everybody for the replys it was a bad plug wire

Good to hear! ( Not a bad thing to replace other stuff anyway ) ( MSD wires & products are not what they used to be)

And, Spark Plugs..... There are the 'A' stock & 'B' stock....

'B' stock of plugs are made in places like China, made in huge lots, sold to places like large auto parts stores.... pallets of them at a time.

You will find tiny ceramic cracks, etc, not caught at inspection.

'A' stock of plugs are made in the 'original type factories, sold in smaller amounts, mostly to speed shops, etc. These have a better QA control.