Diagnose friend's combo issue
Thread Starter
Joined: Nov 2001
Posts: 45,329
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From: Chicago, IL
Diagnose friend's combo issue I've got a friend with swapped 72 Malibu, gen III 5.3, twin GT45s, T400, MSD 6014. He's just hit it with 7 psi, maybe that's 600hp. After a year of driving it, it misses at 4500 and breaks up. He's changed sensors, wires, checked voltage, even does this NA. Summit stage 1 cam + their beehive springs. Any suggestions besides "change sensors, wires" he already did. Thrust looks ok. He was running BR7EF gapped at .028 and then .020. Now with the turbos off he's been running TR5s gapped at .040. 8 lbs fuel pressure.
Last edited by Pro Stock John; Feb 4, 2022 at 02:43 PM.
So it drove fine for a year and started breaking up at 4,500 rpm and higher or just at 4,500 rpm?
Was it N/A for a year then started breaking up once the turbos went on?
8 lbs. of fuel pressure, is it carbureted?
Coil packs?
Was it N/A for a year then started breaking up once the turbos went on?
8 lbs. of fuel pressure, is it carbureted?
Coil packs?
TECH Fanatic
Joined: Feb 2015
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I suspect failing coil(s). Anyway to see misfire data? With a scope and current clamp you could record coil power (sync off crank sensor or #1 fuel injector) and see how coils are responding under load.
Last edited by tblentrprz; Feb 5, 2022 at 11:55 AM.
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Thread Starter
Joined: Nov 2001
Posts: 45,329
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From: Chicago, IL
2nd set of coils.
Blow-thru group on FB suggesting various things, base FP is too high, some concerns about whether the carb is set up right. OP is at 8 lb base and a guy in another FB group Boosted LSX said he ran 5/6 lb base and went 9.0@149.
Blow-thru group on FB suggesting various things, base FP is too high, some concerns about whether the carb is set up right. OP is at 8 lb base and a guy in another FB group Boosted LSX said he ran 5/6 lb base and went 9.0@149.
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Scope and clamp, power and grounds. You want to see the current and voltage of the coils.
Check also the voltage source to the coils, relay etc.
Throwing parts at a possible electrical problem wont fix the source if it is that.
I dont know how to diagnose or isolate the box itself.
Does he have AFR if so whats it read before and during the misfire?
Has he checked the valve train mechanically by chance? Thinking out loud.
Check also the voltage source to the coils, relay etc.
Throwing parts at a possible electrical problem wont fix the source if it is that.
I dont know how to diagnose or isolate the box itself.
Does he have AFR if so whats it read before and during the misfire?
Has he checked the valve train mechanically by chance? Thinking out loud.
Last edited by jester1; Feb 5, 2022 at 10:34 PM.
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Joined: Oct 2016
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From: PDX-OR-USA
Is the MSD mounted on vibration absorbers? if its mounted solid their boxes in general will fail.
I've had all of the CDI boxes fail on road race cars due to vibration, (Broken circuit traces).
Now I use a piece of sheet metal mounted to the car with isolators and then put isolators
under the msd to attach to the piece of sheet metal.
I've had all of the CDI boxes fail on road race cars due to vibration, (Broken circuit traces).
Now I use a piece of sheet metal mounted to the car with isolators and then put isolators
under the msd to attach to the piece of sheet metal.
I ran my blowthrus with a base of 6 lbs pressure, was fine. Does he have any data on AFR's. I have seen a ruptured reg. dia. cause weird problems, but if did it unboosted, prob not the reason. Prob need to see if pump pressure stays constant, no obstructions. Since he has changed a lot of elect. stuff, kind of rules out everything but the box, or fuel supply.
^^^ agreed, the solder in the new electronics do not last, in addition they have a lot of cold solder joints that eventually fail. They pass QC but once mounted in vibration prone applications they fail. Car stereo amps are a good example of that.
Thread Starter
Joined: Nov 2001
Posts: 45,329
Likes: 1,767
From: Chicago, IL
Thanks he reads in here and will see your comments.
I also suggested he join this blow-thru group on FB, lots of guys in there with this combo.
I lean towards wanting to drop base FP, him at 8 is higher than everyone else. I have zero experience with a blow thru setup but he's running 2-3 lbs base more than pretty much everyone else who chimed in.
I also suggested he join this blow-thru group on FB, lots of guys in there with this combo.
I lean towards wanting to drop base FP, him at 8 is higher than everyone else. I have zero experience with a blow thru setup but he's running 2-3 lbs base more than pretty much everyone else who chimed in.
Is the carb he's running setup for up for a FI blow through application?
As I'm sure you know, blow through carbs are significantly modified for those applications when compared to an off the shelf NA unit.
I remember Vortec would put the entire carb in the plenum box to get around this, looked stupid in my opinion lol.
As I'm sure you know, blow through carbs are significantly modified for those applications when compared to an off the shelf NA unit.
I remember Vortec would put the entire carb in the plenum box to get around this, looked stupid in my opinion lol.
I posted above I'd suspect the MSD first lol especially if it's mounted in the engine bay. Mount that stuff inside on rubber isolaters and away from heat. If we know what the AFR was doing or a trend of how it acts before it breaks up that might help also.
Interesting, either the MSD changed what it likes or the thrust on the engine opened up? Do you know if it was a 24x or 58x internal wheel/sensor and what he replaced it with?