FC3S - Backfiring, Overboost after Downpipe Install
01-08-18, 02:57 PM
#1
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Thread Starter
FC3S - Backfiring, Overboost after Downpipe Install
Hello everyone –
I’m 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, I’m 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 (hadn’t 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. It’s 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 I’m 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 what’s called “boost creep.” Too much exhaust coming through the turbo so that it can’t 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 doesn’t 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 don’t 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. 5) wait until I have the time to pull the turbo myself, which might be my best option.
I do not 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 don’t really have any recourse there. And what’s 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!
I’m 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, I’m 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 (hadn’t 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. It’s 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 I’m 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 what’s called “boost creep.” Too much exhaust coming through the turbo so that it can’t 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 doesn’t 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 don’t 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. 5) wait until I have the time to pull the turbo myself, which might be my best option.
I do not 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 don’t really have any recourse there. And what’s 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!
01-09-18, 05:50 AM
#2
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quick response is that you have boost creep due to the less restrictive exhaust.
Look up porting the wastgate on the s4 turbo or changing the s4 turbo to an s5 setup.
I'll let someone else get into more detail..I got some special early appts I gotta attend to.
Look up porting the wastgate on the s4 turbo or changing the s4 turbo to an s5 setup.
I'll let someone else get into more detail..I got some special early appts I gotta attend to.
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enb1413 (02-26-18)
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enb1413 (02-26-18)
01-11-18, 12:16 PM
#4
Sucker for Punishment
Have you considered removing the problem which is the downpipe? Sorry that is your #3 option. Are you running a main cat? I had creeping issues when I went 3" turbo back. I was experiencing boost creep with baffle removed and no cat. In the end my low end torque was best with both the cat and baffle installed. Cat in, baffle out -I would gain a lb or so of boost with no creep but my low end would suffer. Low end torque, to me, is important for a nice street ride.
If you aren't running a main cat I would install one. Especially if there are no power upgrades.You will notice increase in the low end immediately. It will most likely fix your creeping issues. That, IMO would be your cheapest fix. This advice is based on the little info given about your setup. I am also assuming that you don't have a cat. Knowing more would help to diagnose the issue.
If you aren't running a main cat I would install one. Especially if there are no power upgrades.You will notice increase in the low end immediately. It will most likely fix your creeping issues. That, IMO would be your cheapest fix. This advice is based on the little info given about your setup. I am also assuming that you don't have a cat. Knowing more would help to diagnose the issue.
Last edited by Rotary Alkymist; 01-11-18 at 12:26 PM.
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enb1413 (02-26-18)
01-13-18, 10:19 AM
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Engine, Not Motor
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One thing worth mentioning is that you are in dangerous territory with the FCD and open exhaust.
If the FCD is functional, I suspect the cutting out is because the engine is running massively lean. DO NOT DRIVE IN THIS CONDITION! Unless you want to replace the engine.
The FCD fools the stock ECU into thinking the boost level is below the built in boost cut, I believe 8.6 PSI. But it DOES NOT increase fuel. Once the AFM is maxed out, and with the boost sensor seeing only 8.6 PSI, the amount of fuel isn't increased to match the higher boost.
The only thing which saves people in this configuration is that the stock ECU runs the car really rich under boost, so there is some leeway for nominal increases (up to about 10 PSI).
But since you don't have a boost gauge installed, and an open exhaust on an S4 turbo, you most certainly are seeing higher than 10 PSI.
As others have mentioned, you need to either decrease boost by restricting the intake or exhaust.
If you want to keep the exhaust unrestricted, then porting the wastegate is the only solution.
Extreme S4 Wastegate Porting
Another issue is the stock turbo becomes a heat pump much past 12 PSI. If you had some way of measuring intake air temps you would find them ridiculously high, above 140F. That not only decreases power but increases the chance of detonation.
So...
Port the wastegate
Install a high flow cat
Remove the FCD
If the FCD is functional, I suspect the cutting out is because the engine is running massively lean. DO NOT DRIVE IN THIS CONDITION! Unless you want to replace the engine.
The FCD fools the stock ECU into thinking the boost level is below the built in boost cut, I believe 8.6 PSI. But it DOES NOT increase fuel. Once the AFM is maxed out, and with the boost sensor seeing only 8.6 PSI, the amount of fuel isn't increased to match the higher boost.
The only thing which saves people in this configuration is that the stock ECU runs the car really rich under boost, so there is some leeway for nominal increases (up to about 10 PSI).
But since you don't have a boost gauge installed, and an open exhaust on an S4 turbo, you most certainly are seeing higher than 10 PSI.
As others have mentioned, you need to either decrease boost by restricting the intake or exhaust.
If you want to keep the exhaust unrestricted, then porting the wastegate is the only solution.
Extreme S4 Wastegate Porting
Another issue is the stock turbo becomes a heat pump much past 12 PSI. If you had some way of measuring intake air temps you would find them ridiculously high, above 140F. That not only decreases power but increases the chance of detonation.
So...
Port the wastegate
Install a high flow cat
Remove the FCD
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enb1413 (02-26-18)
02-26-18, 03:42 PM
#6
Junior Member
Thread Starter
Have you considered removing the problem which is the downpipe? Sorry that is your #3 option. Are you running a main cat? I had creeping issues when I went 3" turbo back. I was experiencing boost creep with baffle removed and no cat. In the end my low end torque was best with both the cat and baffle installed. Cat in, baffle out -I would gain a lb or so of boost with no creep but my low end would suffer. Low end torque, to me, is important for a nice street ride.
If you aren't running a main cat I would install one. Especially if there are no power upgrades.You will notice increase in the low end immediately. It will most likely fix your creeping issues. That, IMO would be your cheapest fix. This advice is based on the little info given about your setup. I am also assuming that you don't have a cat. Knowing more would help to diagnose the issue.
If you aren't running a main cat I would install one. Especially if there are no power upgrades.You will notice increase in the low end immediately. It will most likely fix your creeping issues. That, IMO would be your cheapest fix. This advice is based on the little info given about your setup. I am also assuming that you don't have a cat. Knowing more would help to diagnose the issue.
02-26-18, 04:12 PM
#7
Junior Member
Thread Starter
One thing worth mentioning is that you are in dangerous territory with the FCD and open exhaust.
If the FCD is functional, I suspect the cutting out is because the engine is running massively lean. DO NOT DRIVE IN THIS CONDITION! Unless you want to replace the engine.
The FCD fools the stock ECU into thinking the boost level is below the built in boost cut, I believe 8.6 PSI. But it DOES NOT increase fuel. Once the AFM is maxed out, and with the boost sensor seeing only 8.6 PSI, the amount of fuel isn't increased to match the higher boost.
The only thing which saves people in this configuration is that the stock ECU runs the car really rich under boost, so there is some leeway for nominal increases (up to about 10 PSI).
But since you don't have a boost gauge installed, and an open exhaust on an S4 turbo, you most certainly are seeing higher than 10 PSI.
As others have mentioned, you need to either decrease boost by restricting the intake or exhaust.
If you want to keep the exhaust unrestricted, then porting the wastegate is the only solution.
Extreme S4 Wastegate Porting
Another issue is the stock turbo becomes a heat pump much past 12 PSI. If you had some way of measuring intake air temps you would find them ridiculously high, above 140F. That not only decreases power but increases the chance of detonation.
So...
Port the wastegate
Install a high flow cat
Remove the FCD
If the FCD is functional, I suspect the cutting out is because the engine is running massively lean. DO NOT DRIVE IN THIS CONDITION! Unless you want to replace the engine.
The FCD fools the stock ECU into thinking the boost level is below the built in boost cut, I believe 8.6 PSI. But it DOES NOT increase fuel. Once the AFM is maxed out, and with the boost sensor seeing only 8.6 PSI, the amount of fuel isn't increased to match the higher boost.
The only thing which saves people in this configuration is that the stock ECU runs the car really rich under boost, so there is some leeway for nominal increases (up to about 10 PSI).
But since you don't have a boost gauge installed, and an open exhaust on an S4 turbo, you most certainly are seeing higher than 10 PSI.
As others have mentioned, you need to either decrease boost by restricting the intake or exhaust.
If you want to keep the exhaust unrestricted, then porting the wastegate is the only solution.
Extreme S4 Wastegate Porting
Another issue is the stock turbo becomes a heat pump much past 12 PSI. If you had some way of measuring intake air temps you would find them ridiculously high, above 140F. That not only decreases power but increases the chance of detonation.
So...
Port the wastegate
Install a high flow cat
Remove the FCD
By 'FCD'... do you mean Fuel Cut Controller? I should remove this either way?
It may be worth noting that there is a Jacobs Electronics Ignition Kit installed, mounted above the radiator fan. It has been there for 15+ years from prev. owner. I'm not sure if it's still functional, but I suspect it still is. From what I understand, this is supposed to produce more spark, but I don't have any other upgrades to compliment more spark.
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09-12-18, 03:26 AM
#8
Junior Member
Thread Starter
Have you considered removing the problem which is the downpipe? Sorry that is your #3 option. Are you running a main cat? I had creeping issues when I went 3" turbo back. I was experiencing boost creep with baffle removed and no cat. In the end my low end torque was best with both the cat and baffle installed. Cat in, baffle out -I would gain a lb or so of boost with no creep but my low end would suffer. Low end torque, to me, is important for a nice street ride.
If you aren't running a main cat I would install one. Especially if there are no power upgrades.You will notice increase in the low end immediately. It will most likely fix your creeping issues. That, IMO would be your cheapest fix. This advice is based on the little info given about your setup. I am also assuming that you don't have a cat. Knowing more would help to diagnose the issue.
If you aren't running a main cat I would install one. Especially if there are no power upgrades.You will notice increase in the low end immediately. It will most likely fix your creeping issues. That, IMO would be your cheapest fix. This advice is based on the little info given about your setup. I am also assuming that you don't have a cat. Knowing more would help to diagnose the issue.
09-12-18, 03:41 AM
#9
Junior Member
Thread Starter
One thing worth mentioning is that you are in dangerous territory with the FCD and open exhaust.
If the FCD is functional, I suspect the cutting out is because the engine is running massively lean. DO NOT DRIVE IN THIS CONDITION! Unless you want to replace the engine.
The FCD fools the stock ECU into thinking the boost level is below the built in boost cut, I believe 8.6 PSI. But it DOES NOT increase fuel. Once the AFM is maxed out, and with the boost sensor seeing only 8.6 PSI, the amount of fuel isn't increased to match the higher boost.
The only thing which saves people in this configuration is that the stock ECU runs the car really rich under boost, so there is some leeway for nominal increases (up to about 10 PSI).
But since you don't have a boost gauge installed, and an open exhaust on an S4 turbo, you most certainly are seeing higher than 10 PSI.
As others have mentioned, you need to either decrease boost by restricting the intake or exhaust.
If you want to keep the exhaust unrestricted, then porting the wastegate is the only solution.
Extreme S4 Wastegate Porting
Another issue is the stock turbo becomes a heat pump much past 12 PSI. If you had some way of measuring intake air temps you would find them ridiculously high, above 140F. That not only decreases power but increases the chance of detonation.
So...
Port the wastegate
Install a high flow cat
Remove the FCD
If the FCD is functional, I suspect the cutting out is because the engine is running massively lean. DO NOT DRIVE IN THIS CONDITION! Unless you want to replace the engine.
The FCD fools the stock ECU into thinking the boost level is below the built in boost cut, I believe 8.6 PSI. But it DOES NOT increase fuel. Once the AFM is maxed out, and with the boost sensor seeing only 8.6 PSI, the amount of fuel isn't increased to match the higher boost.
The only thing which saves people in this configuration is that the stock ECU runs the car really rich under boost, so there is some leeway for nominal increases (up to about 10 PSI).
But since you don't have a boost gauge installed, and an open exhaust on an S4 turbo, you most certainly are seeing higher than 10 PSI.
As others have mentioned, you need to either decrease boost by restricting the intake or exhaust.
If you want to keep the exhaust unrestricted, then porting the wastegate is the only solution.
Extreme S4 Wastegate Porting
Another issue is the stock turbo becomes a heat pump much past 12 PSI. If you had some way of measuring intake air temps you would find them ridiculously high, above 140F. That not only decreases power but increases the chance of detonation.
So...
Port the wastegate
Install a high flow cat
Remove the FCD
Couple follow up questions:
By FCD - do you mean fuel cut controller? I can easily unplug that and see if it helps. As you suspect, the overboost / boost creep / open exhaust is probably the main issue.
As for boost... boost is much higher since running the open exhaust. I don't have a proper boost gauge (only the stock one) which I will also add to the list. But the stock boost gauge is showing levels that I haven't seen since the new setup. Also I must note that I never let the turbo spool up anymore and basically drive it like a granny, to avoid this issue of the engine cutting out.
Do you recommend any vendors for a high flow cat?
If the wastegate is ported, is it still recommend to install the high flow cat?
Lastly, when installing the high flow cat, will this require cutting / modifying or removing the current downpipe? (stupid beginner question I know)
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