Detonation and backfiring

Plans Ahead

New Member
Tony from Branford, Connecticut here, with a higher mileage '87 GN with a worked motor I bought a few years ago after I sold my last Grand National and regretted it. The car I have now is a T Top, it is a driver but runs like a scalded cat. I have had a few GN's thru the years, but there is something about this motor that impressed me enough to buy it . The car breaks the tires free at any speed and pins you back like it should. Straight dump, no cats,no muffler, big turbo and I am assuming injectors. I am not a mechanic, nor even mechanically inclined when it comes to cars. Lately when punched, the motor roars to life, but about mid throttle, starts to break up and detonate, and on the hind end of accelerating,back fires. A few months ago a backfire blew a vacuum hose off and I limped over to my local mechanic who found the problem and off I went. Ever since ,the car just isn't right. It's still a beast, but nailing it when getting on a highway ramp, it's breaking up and not pulling as strong. I've searched some old threads hoping to find a shop in the area who I could have take a look at my car, but to no avail. Does anyone have any suggestions?
I am not looking for any freebies, just hoping to get my car back up to where it was.
Thanks in advance all!
 

9secWH1

Member
First off, if the car is detonating dont keep beating on it, its not gonna fix it self. Youre gonna end up blowing the head gaskets and beat up the bearings. hopefully you have some gauges in the car.
How old are the plugs and wires? how much boost? are you running alky? Check fuel pressure, should be 43# with the vaccum line off. It should also add one# per psi of boost. If it isnt the pump could be at fault. If it checks out then id tighten the gap on the plugs to see if it clears up, if it does that would point to a weak coil.
good luck
 

Plans Ahead

New Member
Thanks for the reply. I'm unsure how old the plugs and wires are, at least two years old. My local mechanic should be able to handle that, I'll have him start there. I'm not beating on it at all since the initial backfire that blew the vacuum line; I just took it out this week because the weather has been so nice and got on the highway and just got on it enough to test it. I refrained from passing everyone at 100 mph + ( yeah that might have been me). I do have an aftermarket boost gauge I had installed because the stock gauge was buried all the time. Before this (no alky) I could spike the boost to over 35 psi with ease. There really isn't a good place I could get to hold a greater boost for any length of time but generally that was good enough for me. It's t top and I put front braces on,seat braces and the engine strap and the thing was still on the cusp of being out of control. Tracks straight as hell though. It also came with a scan master but my with my admitted lack of knowledge I have no idea what those readings mean. I will pass this on the my mechanic, so again thanks for your input. Would you or anyone else know if this guy Otto from Milford still tuning cars?
 

bison

Moderator
Staff member
Tony from Branford, Connecticut here, with a higher mileage '87 GN with a worked motor I bought a few years ago after I sold my last Grand National and regretted it. The car I have now is a T Top, it is a driver but runs like a scalded cat. I have had a few GN's thru the years, but there is something about this motor that impressed me enough to buy it . The car breaks the tires free at any speed and pins you back like it should. Straight dump, no cats,no muffler, big turbo and I am assuming injectors. I am not a mechanic, nor even mechanically inclined when it comes to cars. Lately when punched, the motor roars to life, but about mid throttle, starts to break up and detonate, and on the hind end of accelerating,back fires. A few months ago a backfire blew a vacuum hose off and I limped over to my local mechanic who found the problem and off I went. Ever since ,the car just isn't right. It's still a beast, but nailing it when getting on a highway ramp, it's breaking up and not pulling as strong. I've searched some old threads hoping to find a shop in the area who I could have take a look at my car, but to no avail. Does anyone have any suggestions?
I am not looking for any freebies, just hoping to get my car back up to where it was.
Thanks in advance all!
I'm in CT. My shop is located in New Britain. I can get your car 100% straightened out or at least give you the options of what to do next. I dyno cars about every other week if you're looking for performance. I don't outsource any work except specific machine work on engine components. Every thing is done right here. Sending a PM


BPE2013@hotmail.com
 

Jerryl

Tall Unvaccinated Chinese Guy
There are only 3 people in the world I would feel comfortable dropping my car off and handing over the keys. Bison is one of them.
 
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berserker6

Assistant to the regional manager
I hope I misread the original post "spike the boost to over 35 psi with ease" ?! @_@
 
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