1988 TII Backfiring Issue After Downpipe Install - NoPistons -Mazda Rx7 & Rx8 Rotary Forum

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1988 TII Backfiring Issue After Downpipe Install

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Old 01-08-2018, 02:40 PM
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Question 1988 TII Backfiring Issue After Downpipe Install

Iím reaching out to see if anyone has advice about an issue with my 1988 FC3S Turbo II. After years of poking around at the issue, Iím hoping to come to a logical conclusion so I can determine how to move forward. Apologies in advance if this topic has been covered in other threads or if this post is too lengthy.

A few years back, my mechanic encouraged me to install a Borla downpipe and remove the old catalytic converters, which were completely shot (hadnít passed emissions in years). After the install, the car has never performed the same.

It appears that the increased exhaust flow is causing an issue with the turbo. When I press the throttle in the manner that used to engage the turbo and accelerate the car faster, instead, you can hear the turbo really winding (boost gauge goes max) and then engine appears to backfire. The RPMs will increase until maybe 4000 but then power cuts out, and the exhaust makes a loud backfiring sound. Itís a very unhealthy sound and nerve wrecking feeling when driving the car. With that said, I can and always do operate the car without pressing the gas pedal quickly (without engaging the turbo) and it runs just fine. I can reliably go any distance, without causing backfire, by going easy on the throttle and not pushing the car (which pretty much defeats the purpose of having a Turbo II with many upgrades).

I have received a few opinions on the issue. The one that Iím leaning toward the most, which my mechanic and another person with turbo experience is also leaning towards, is that the increased exhaust flow is too much for the stock turbo, and especially stock turbo waste gate to handle, so I am getting whatís called ďboost creep.Ē Too much exhaust coming through the turbo so that it canít escape through the smaller wastegate, and therefore this excess gas pushes back into the turbo.

I purchased and tried installing a fuel cut controller, only to find that one was already installed! (rookie move.. the existing controller must have been installed by the previous owner some 16-20 years ago!). So it doesnít seem to be a fuel cutoff issue.

As of now, my options appear to be slim and/or costly: 1) pay mechanic to remove stock turbo and port the wastegate. 2) pay mechanic to pay replace stock turbo with a rebuilt / upgraded turbo, which I have sourced, but donít want to spend the money (entire job could easily run over $1500) 3) pay mechanic to remove downpipe and replace with something else that will have less flow. 4) I have also received advice about upgrading the computer system, which I am not exactly sure how that would work, but it sounds expensive, and I am not sure if it will fix my issue.

I don't currently have the time, nor the tools or expertise to take on a turbo install or modification myself. I also do not have a ton of extra cash to pay a mechanic to fix the issue for me. My financial priorities have certainly changed over the years, compared to when I first bought the RX when I was in high school.

I do want to keep the RX and have fun driving it, as I have owned it for over 15 years. I would love to get it on the track for trials, etc. I would like to keep the downpipe and fix the issue without any major modifications, but if I need to remove the downpipe, so be it. But then I will need to find a replacement for the downpipe, as my mechanic sold off my old one and the old cats. I am not happy about how this was handled by the mechanic, but it was a few years ago, so I donít really have any recourse there. And whatís even worse, he appears to be the only RX specialist in my area of New England. I do think he is a nice guy and would normally go back to him to get it fixed, but I feel a bit burned by the situation so I would rather not have to pay him money for a problem that they created.

If anyone has advice about how I should go about fixing/diagnosing this problem, or even a potential mechanic referral in the NE area, I would really appreciate it.

Thanks!
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Old 01-21-2018, 02:26 PM
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Default Re: 1988 TII Backfiring Issue After Downpipe Install

First, I'm sorry to hear of your troubles. I do agree that without supporting modifications to the fuel system, opening up the exhaust and running more than stock levels of boost is a bad idea. The stock fuel system isn't going to be able to deliver enough fuel to keep the engine from running lean at high boost. A good way to kill any engine, but particularly a boosted rotary, is via detonation due to running lean under load.

I believe the most economical solution is to restore the car to stock condition. If you later choose to make upgrades, a good roadmap to follow can be found here:
http://fc3spro.com/TECH/FM2W/power.htm

Note that porting the stock wastegate is included in the very first group of mods.

It may also be worthwhile to confirm the integrity of engine grounds on your RX-7. The so-called "3800 RPM Hesitation" may be a factor here.
http://fc3spro.com/TECH/FAQ/3800.html

Good luck!

Last edited by 1988RedT2; 01-21-2018 at 02:36 PM.
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