20V Blacktop intermittent misfire
Posted: Mon Aug 21, 2017 2:01 pm
Greetings once again C4AG!
So I finished the bulk work of getting my 20VBT swap done in my 85 GTS hatch, just ironing out the little kinks and tying up a few loose ends, good news is... it drives! Surprisingly well at that! Hands down one of the easiest swaps I've done, random issues wise that is. I did notice something strange that concerned me though and was wondering if any of you had an idea what it could be?
The trouble with this issue is that the engine it runs absolutely flawless, the issue only occurs around 4-4.5k rpm in 5th at highway speeds cruising consistently for an extended time (5min +) and when it misses, it is an abrupt few shakes. It scares the hell outta me with how random it is, which as an involuntary reaction, causes me to let off the gas. As soon as I do the shaking stops and it cruises like nothing happend. When it happens it is totally random, not worse from going over bumps in the road, flat ground, incline, nothing seems to trigger it more or less often, and like I said it's about every 5-10 min, but not constantly every time I take it on the freeway, but when it does start happening it usually happens a few times in the same trip....
I'm thinking it's most likely ignition, because it revs and has good power everywhere, just like a freshly rebuilt engine should. Not ruling out fuel but it seems strange that it's a problem only at a given rpm/speed while cruising in 5th.
Just to list potrntially pertinent things that have been replaced...
New OEM distributor cap/rotor
New denso spark plugs
New denso spark plug wires
Confirmed all plug wires are fully seated on the plugs
Timing was set per 20vBT FSM
OEM 20v BT manual ecu
Distributor is in good condition
Idle is fine
Starts up and revs perfectly fine, not laggy at all
New fuel filter
Injectors all functional
I might be missing a few things but that's about where I'm at, any ideas fellas? Many thanks in advance!
So I finished the bulk work of getting my 20VBT swap done in my 85 GTS hatch, just ironing out the little kinks and tying up a few loose ends, good news is... it drives! Surprisingly well at that! Hands down one of the easiest swaps I've done, random issues wise that is. I did notice something strange that concerned me though and was wondering if any of you had an idea what it could be?
The trouble with this issue is that the engine it runs absolutely flawless, the issue only occurs around 4-4.5k rpm in 5th at highway speeds cruising consistently for an extended time (5min +) and when it misses, it is an abrupt few shakes. It scares the hell outta me with how random it is, which as an involuntary reaction, causes me to let off the gas. As soon as I do the shaking stops and it cruises like nothing happend. When it happens it is totally random, not worse from going over bumps in the road, flat ground, incline, nothing seems to trigger it more or less often, and like I said it's about every 5-10 min, but not constantly every time I take it on the freeway, but when it does start happening it usually happens a few times in the same trip....
I'm thinking it's most likely ignition, because it revs and has good power everywhere, just like a freshly rebuilt engine should. Not ruling out fuel but it seems strange that it's a problem only at a given rpm/speed while cruising in 5th.
Just to list potrntially pertinent things that have been replaced...
New OEM distributor cap/rotor
New denso spark plugs
New denso spark plug wires
Confirmed all plug wires are fully seated on the plugs
Timing was set per 20vBT FSM
OEM 20v BT manual ecu
Distributor is in good condition
Idle is fine
Starts up and revs perfectly fine, not laggy at all
New fuel filter
Injectors all functional
I might be missing a few things but that's about where I'm at, any ideas fellas? Many thanks in advance!