Back Fire Through Intake Need Help


New Member
Hey Everyone,

I picked up a 1987 Buick Grand National roughly a year ago. This car sat for about 15 years due to an accident. The previous owner who was a family friend had the engine apart doing some maintenance. It received new valve springs, water pump, thermostat, ported the dog house and an adjustable fuel pressure reg.

When I got the car I emptied the tank, replaced it with a used Racetronics 255 the car came with and put the rest of the engine back together. I went to start the car with no luck. After beating my head against the wall for awhile I decided to pull the injectors out and found out they were extremely nasty and weren't firing. The car also came with new 60 lb injectors and turbo tweak chips to go with it for 93 octane and E85. I was going to wait to put them in until I got the car running but since I was having issues with the stock stuff I put the bigger ones in and the 93 chip. After getting everything back together the car fired right up and ran pretty decent with all things consider.

I did some driving around the block afterwards for awhile and ran into an overheating condition. The car got to 230 degrees when I finally got it parked back home. I popped the radiator cap and the engine had some pretty bad corrosion which ended up plugging the radiator. i put the original radiator back in which leaked from the wreck but took care of the overheating problem. I ordered a gn1 aluminum radiator which I was extremely pleased with the quality.

While I was waiting on the radiator to show up my dad and I decided to realign the body panels since the new ones were just tossed on after the wreck. I also switch the heater box out with a grand national one since someone had put a stock regal one in by mistake.

After getting the car back together with the new radiator I was finally ready to take it for a real test drive. I took it down the street for fuel and threw about a half tank in it. Almost instantly afterwards the car started back firing and breaking up with any throttle at all. The scan master drops to around 500 milli 02 volts when this happens so it seems to have a lean condition.

I was told this is a sign of the factory maf having problems. I ordered a ls1 maf and a maf translator which didn't change the symptoms at all. After that I threw a decent amount of money in it and replaced the ignition module, coil pack, plug wires and plugs. This also didn't change anything. This was last November and it was getting cold here in Iowa. I decided to throw in the towel for the winter. Now that its spring I'm starting to take a look at it again hoping I can get it fixed.

The only other thing I found is two wires that were disconnected on the back of the passenger head. One wire is while the other is black with a while spring. I intially assumed these were engine groups and grounded them to the intake manifold bolt which doesn't seem to of changed anything. Can someone confirm if this is an engine ground, if the intake manifold bolt to the head is a decent spot and could I of hurt the engine letter it run without these grounds?

Any help would be greatly appreciated!




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some will tell you the #3 lobe on the camshaft is wiped, But I hope it is not. I have a popping problem to I am chasing now. I will watch this thread
It was new when I got the car running again. It's been upgraded to the 4 pin o2 kit I think racetronics offers. Can't remember the brand for sure.
If it's a Bosch it wouldn't surprise me if it's bad mine went bad sitting over the winter while my motor was getting rebuilt. Lucky enough I had a wideband to confirm that but I also switched to sd2 from turbotweak.
Confirm plug wires are routed correctly. Then confirm module is wired to coil pack correctly. Could also be a bad tps sensor. Make sure chip is seated firmly in ecu. Make sure ground wires are firmly attached to back of driver's side head.
I finally got a chance work on the car again and confirmed a few things. I swapped ecu and chips out no changed. Did a compression check all cylinders we're 150psi. Confirmed firing order on wires. Did a smoke test on the vac system no leaks. When I pulled the plugs they looked black the car is running extremely rich. I adjusted the maf pro to pull 10% fuel. It helped but still pops.

Any other ideas or anything I missed would be great appreciated.
With the key on, engine off, put the Scanmaster on TPS.
Slowly apply the gas and watch your TPS numbers. They should be linear. IE: 46, 56, 6x etc etc all the way up to WOT and remain stable. If at any point you see the numbers fluctuate down while you are applying the throttle, the TPS has a dead spot and could be causing this issue.

If it remains linear, then it's something else. But verify it is linear a few times.
You might need to get the car up to operating temps in order to replicate this issue, as heat could be causing the TPS to malfunction. If it were mine and the TPS was the original one, I would replace it no matter what.

That's the first thing I'm suspecting, as I had to diagnose a very similar issue with my current one back in Nov.

I'm going to add the tps sensor to my list of things to replace as far as I know it's the original

Someone earlier suggested unplugging the cam sensor after the car was running to see if it made a difference. It didnt have any effect. I assume the ignition systems on these are waste spark and the cam sensor is just used to sync then it doesn't do anything? Definitely could be possible the intial sync isnt right which could be throwing the timing off.

What's the best way to go about testing the cam sensor? A friend of mine has a few extras. Should I swap it out?
After doing some research I now understand what the cam sensors job is. I'm a little skeptical that this could be the issue only because it sounds like when you unplug the cam sensor it should kick the fuel system into batch fire.

I did go ahead and snag a spare cam sensor from a friend and ordered a Casper cam sensor tool. I watched a video on how to swap them out shouldn't be a big deal.

Tps sensor and o2 sensor are still on my list just haven't gotten around to ordering them yet.
Give it a shot

I had a shotgun backfire thru intake I was chasing for a while

Turned out cam sensor was just out of adjuatment.
Ah gotcha thank you for clarifying! I will give that a try tomorrow. Looks like I should beable to get it set by multimeter for now.

We're headed back from Buffalo NY (I live in Iowa). Dad just sold his concourse 70 Buick stage 1 car and we agreed to deliver it. I've had some time to research on the trip.
Please verify fuel pump is working properly and did you replace fuel filter? Timing chain couple years ago my compression was good all cylinders still had to replace.
I unfortunately stumbled upon the problem yesterday. We decided to pull the valve covers and have a look at the rocker movement while turning the engine over. Number 6 exhaust rocker barely moved maybe enough to crack the valve open.

Popped the intake off removed the rockers and lifters. That lifter has a dome shape in the bottom side and the lifter itself is seized up. It also appears to of wiped the cam lobe out also. Definitely a bummer but at least I know what it is now. I'm probably going to buy a little bigger cam shaft awhile it's apart, new lifters and a new timing chain and gear set. It also needs a lot of engine bay cleaning as well.
Man, I hate to hear this kind of news. Looks like Scarab nailed it, just different cylinder.
If you're not going to pull the engine for the cam swap, I would highly recommend dropping the oil pan, cleaning it out, replacing the oil pickup tube, and putting that back together with a new gasket. Make dang sure, if you still have the factory oil cooler adapter and are using the factory oil cooler in the radiator, that you remove that cooler adapter and stop using the oil cooler in the radiator. If not, and it's the stock style oil cooler, metal particles that are trapped in the cooler, will eventually leach back into your oil system and take out the main and cam bearings. As well as the oil pump. Also, if you are using the stock oil feed line for the turbo, chances are those metal particles from the lifter and cam lobe have found their way into the chra's bearings. The turbo needs to be looked at if you have the stock oil feed line on it. I would also get a new oil drain tube and look into a quality oil filtration kit for the turbo. Like the ones that PTE, FTS, TR Custom Parts, Cruz Performance and GN1 Performance sell. The turbo must have filtered oil at all times in order to survive. One more thing, check your MAF sensor for any signs of oil inside the housing or directly on the circuit board as well. You can run your finger over the inside wall of the MAF and easily tell if there's any oil residue. If so, this means the oil seal in the turbo, as well as the bearings are either on their way out, or already toast. If there's oil on the circuit board or heating element of the MAF sensor, get ready to replace it as well. And don't forget the double roller timing chain and roller bearing cam button. Leave off the factory tensioner with the double roller timing chain.

I tell friends all the time, a wiped cam lobe on a stock car, is not a simple fix. You can't just throw a set of lifters and a new cam in, break it in and ride. You gotta check all of those items above, off the list before you start it. And don't forget to run ZDDPlus oil additive, or your favorite zinc phosphorus additive in the oil during and after cam break in. This is critical if you go back with a hydraulic flat tappet cam as a replacement.
At least you found the exact problem and can address it before it slings a rod or looses all oil pressure from the wiped cam.

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