RX-7 FD3S starts, but backfires at WOT
 

Useful mods list:

Single turbo T04B
Power FC

I just had a problem with my car last week. I was running with it pretty strong and suddenly the car backfired like hell, then stop in the middle of the road. Couldn't start it anymore, I discovered later on that 3 of the 4 coil wires were cut.

I repaired them today.

Now it starts fine. But as soon as I give WOT, it just backfires like crazy. If I only give 50% throttle, car runs fine.

You guys got any idea?

-Spark is all fine
-Plugs are brand new
-Compression is fine

I don't have any commander, and no Datalogit neither.

Thanks!

I managed to hold the throttle for a while on the road, boost goes at a steady 13 psi, which is perfect. Can't find the backfires issue though...

Oh well, it looks like it's gonna take a WHILE to solve... Nobody wants to try something, no?

Who cut your wires???

Without any doubt myself while changing the spark plugs. At first, maybe only one of them was cut (when it stopped in the middle of the road). Then, maybe I cut two more when I tried to unflood the car by removing the plugs again. The 4th one was just barely holding, they were very worn.

But thank you, I appreciate that you took the time to read.

Hum, is this forum dying or what? I thought some people would try to help me?

Well, you seem to be checking in every few hours, and the original post is from roughly 24 hours ago :). There's nothing wrong with the forum, you're not going to get answers instantaneously.

I'll offer two generic pieces of advice: have you checked the TPS calibration, and have you checked for boost leaks? Specifically the line to the MAP? Common to blow off.

Might be worth putting a new set of plugs in to rule out fouled plugs. Do you have stock ignition or aftermarket? Ignition amp, like an HKS Twinpower?

Dale

-Line to MAP is OK
-TPS calibration seems to be a good idea.

-New set of plugs already installed (video was taken with brand new 0 miles plugs)
-Stock ignition

Thanks guys!

Could be a bad ignition harness or a bad coil. Had a car I worked on recently with a bad leading coil, the back of the coil had swelled out and broken the plastic. Tested the resistance using the method in the shop manual and it failed big time. New one made a huge difference.

May also want to get the updated ignition harness, you can tell the new one since it doesn't have a ground wire on it. If you don't have the updated one it's worth doing, it's like $80 for the harness.

Dale

Is that A/F gauge accurate? Who tuned the car?

-Fuel filter brand new

I've got a very hard time starting the car now. Definitely doesn't start when cold (I need to get pulled by someone else to start the car).

Bad coil is a good idea. Last time I put my finger on the tip of one of the leading spark plug wires I didn't even feel a spark (but when I plugged it, I noticed a (very) small difference in idle). Felt the spark on both trailing wires though.

If I test it with the FSM method, will it tell me if there's a problem in the coil OR in the harness or I won't know for sure?

It's not, at all. My friend tuned it. :lol: 5 years without any issue on that engine.

What did you use to check compression?
Check the coils per the FSM and you will know if there is an issue with them. How did you repair your cut wires? Were they spark plug wires or actual coil wires?

I used my ears to check the compression. :) I didn't really check the compression with a gauge (I know it's a 5 minutes job). I removed the plugs and I listened. To be honest, I didn't really investigate too far on the problem. I know the sound of a blown engine (I blew 3 engines). I removed the spark plugs, cranked the engine and listened for a "steam train" and it seemed brand new. Also, when I just give 20% throttle, the engine sounds good (you can hear it on the video).

But yeah I know, it can't hurt to test compression, you're right.

I repaired the cut harness wires the simpliest way ever. I just connected an extra 6 inches of wire to each wire (dirty I know, can't wait to change the harnesses).

To do list (in order):

1. Test coils
2. Test compression
3. Test MAP sensor
4. Test TPS

At the moment I'm installing headers on my ZR-1, so that's why I'm a bit slow on the RX-7.

Thanks for your help guys!

Don't know if there's a good test for the coil pack harness, whenever I see one that has the ground wire and the car is having issues I replace it pronto. The FSM method is pretty good for determining a bad coil pack, just checking the resistance between certain pins and where the plug wire goes into the coil.

Dale

Thanks Dale, I know you've been on this forum for a while, I believe in you. I will test coils as soon as possible.

-Coils tested, they are all fine
-Spark plugs wires tested, they are all fine

Impossible to test the MAP sensor without a vacuum pump, impossible to test the TPS without the car running.

For any engine to fire, you need spark in the combustion chamber at the right time, fuel in the combustion chamber at the right time, and compression. Start there and work your way back.

It's all fixed now. When I fixed the cut wires from the harness, I switched 2 wires. Simple as that.

Thanks all!

That would cause it :). Glad you figured it out!

Dale

Which wires? Im having the same issue. Are you talking about the plug wires?

V/R

Daniel

He's talking about the harness that goes to each ignition coil. His case was pretty out of the ordinary - the harness was brittle and he moved it around and broke the wires inside, then tried to repair those wires and got the order of the wires in the harness wrong.

All the troubleshooting here is solid advice, though.

Dale

Sorry for late reply - but YES, exactly what DaleClark said. That's also why my car is out of service since it happened... :blush: