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Backfiring 85 T Type


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OK guys and gals... . This one has me stumped. My 85 T Type has been primarily sitting for 12+ years. About a year ago I decided I needed to get her back on the road. Replaced the MAF and was able to get her to start and idle reliably. However, I found when I was cruising and let off the gas and then feathered the throttle it would backfire. But if I gave it more gas the backfire went away.

Did some troubleshooting and indications were I needed a new ignition coil. I had already swapped over to an 86/87 style, so ordered a new AC Delco coil and ignition module. Had to replace the ignition module connector, too (thankfully Casper's Electronics carries the right one). But in replacing the ignition module connector I mixed up two wires and the car wouldn't start. Eventually I figured that out and got everything wired correctly, but it still wouldn't start.

Did more troubleshooting with the Factory Service Manual and figured out I had a bad ECM (probably caused by my wire swapping fisaco). Anyway, I put a different ECM in it today and it fired right up. BUT, the backfire is still there.

I'm thinking I have either a timing or fuel delivery problem. Given that this car's ignition timing is electronically controlled, I'm inclined to think its a fuel delivery problem. But I don't really know.

What ideas do you have?

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It sounds like it's too lean unless you rich it up with some accelleration. How old are your injectors? I overlooked mine for years and once I changed them my lean situation went away. Also after the new injectors it still ran bad untill I reset the ECM after installing the new injectors. I like to always let the ECM figure new fueling values when new hardward is installed. Let's see what some of the others say.

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OK guys and gals... . This one has me stumped. My 85 T Type has been primarily sitting for 12+ years.

I'm thinking I have either a timing or fuel delivery problem. Given that this car's ignition timing is electronically controlled, I'm inclined to think its a fuel delivery problem.

Definitely timing AND/OR fuel problem. Even though the timing is electronically controlled, it needs to get the right info (garbage in/garbage out). I doubt the cam sensor has changed - but, and this sucks, you may want to check the crank sensor. It is not unknown for the bracket to loosen, and then the sensor does not read the crank position correctly. Worse, the damper can knock the magnet off and then you are dead in the water (Don't ask how I know this).

The sitting for 12 years part is the thing that I would look into first. 12 year old gas sitting in that injector would very probably turned into what I call gas gunk. Imagine what you find in the bottom of a carb after sitting 12 years. That stuff does not dissolve easily. I suspect that the pintle is not closing completely. When you feather the throttle and turn the manifold from pressure to vacuum, it is pulling fuel through. That high rich condition in the cylinder is not being completely combusted and goes POP in your hot exhaust. Could be a little tough on your turbo seals. If you have an extended crank when starting hot or have to open the throttle a bit to get it to start hot, I would think that would confirm the leaking injector(s).

How would you fix this condition? Could be as simple as running a high concentration of an aggressive fuel injector cleaner through such as Lucas or Chevron Techron. I have found excellent results by installing Chevy 350 TPI injectors, a 160 degree thermostat, and the big smile on your face when it hits second gear at WOT and your front tires chirp.:D

Good luck, keep us posted on what you find.

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I also had 25yoa injectors that I kept ignoring and running cleaner through. I had the dealership do it once on the machine and another shop did it again. I think those two jobs would of paid for the new set of injectors. lol

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