Fuel injected 350 MAG MPI backfire. any suggestions?

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

I got word from my mechanic today. They ran a diagnostic on it under load & said they cannot go any further because it has automotive spark plugs in it & need to replace with marine plugs. He says he is fairly sure this is the problem.

My first response was that could not be the problem because I had the same symptoms before I replaced the plugs. He stated that the original plugs were wrong as well & gapped wrong (both of which he said could have caused the symptoms).

Original plugs - NGK BPR6EFS (they were burnt & gapped to .035) pic above
Current plugs - Autolite AP104 gapped to .060

I have searched the forum & most say to use marine plugs due to rusting issues. Can an automotive plug cause the sort of misfiring I am having.

I am replacing plugs either way just wondering what you guys think. Does this sound right?

 

[Yellow Yota]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

According to the Merc manual, the correct plugs are AC 41-932.

IGNITION SYSTEM
SERVICE MANUAL NUMBER 31
Specifications Spark Plugs
Distributor
Coil
Firing Order
Torque Specifications
72008
Description
Specification
Spark Plug
AC Platinum (AC 41-932)
0.060 in. Gap
Description
Part Number
Distributor
864236
Description
Specification
Coil
Part Number 392-863704
Primary Resistance
0.60 - 0.80 Ohms
Secondary Resistance
9,400-11,700 Ohms

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

I know what the correct plugs are, I have the manual, & the plugs are getting replaced (not till Monday though).

My question is..... Can using automotive plugs cause misfiring? Has anyone had this happen before?

 

[JustJason]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

The only difference between marine and auto plugs is the alloy of the hex and thread part of the plug.
Personally I wouldn't run autolite plugs, especially at the gap. You need to understand what plug gap is, what it does, and how the rest of the ignition system is effected by gap. If the shop says it's the plugs causing a misfire. Ask them what the voltage is on the high tension wires in KV, if they can't tell you or say Duhhhh. They probably don't have a clue.





......injector balance test......

 

[full stringer]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

an air leak at the throttle body or intake manifold can cause the symptoms you describe check around the throttle body and base as well check intake manifold bolts for tightness. a friend of mine found two loose intake manifold bolts in the rear of his 350 v-8 cranked them down to tourque specs and problem was solved good luck i posted this because it would only happen on let down and half throttle just like you stated in your post..

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions? (Problem solved)

Re: Fuel injected 350 MAG MPI backfire. any suggestions? (Problem solved)

Problem Solved!!!!!!!!!!

Lifting eye was bent pushing down two wires at the distributor (see pics below). Dist was also off by one tooth. Rotated dist counterclockwise and straightened lifting eye. running great now.

Thanks to everyone for all the input