Hi,

I have a 93 RX7 that I think is suffering from a high rpm fuel cutoff due to overboost? The car is completely stock except for a full exhaust put on by a previous owner, which consists of a downpipe, resonated midpipe and the catback portion.

My problem occurs under WOT at around the 5700-6100rpm range. The car has a hiccup where the power gets cut for a split second and then picks right back up again with no problems. It doesn't happen under partial load. I've hooked up a boost gauge to see if I can see any major boost spikes but it reads the normal 10psi when it happens. I'm thinking the gauge itself is probably not super accurate and doesn't read a fast boost spike. From what I've read the major cause of this could be the exhaust not having a cat and not having enough back pressure. I've also read a cure could be putting a restriction plate between the downpipe and midpipe. I was curious if anyone else has any other theories on what could be causing this problem? Thanks

 

What I think you're referring to is boost CREEP...not enough back-pressure. I'm not sure if a resonated midpipe would provide enough back-pressure to avoid creep, but regardless, I don't think that's the cause of your 'hiccup'.
I would more suspect something in the sequential management system causing it. It's usually still pretty cold in MI this time of year. Do you have an aftermarket temp gauge on the car andif so, what are the engine temps when this occurs. Are you confident in the rpm range that it's occurring?

 

I have the car in storage right now. The MI winters are too rough with all the salt and the cold, but this was an issue during the summer. I don't have an aftermarket gauge hooked up so I don't have any exact numbers. I only drive the car once it has been warmed up after it kicks down to low idle. I forgot to mention that the radiator has been upgraded in my first post, if that makes any difference. I've only had it happen in that rpm range.

 

If you are completely stock (i.e. ECU and emissions), I would install a cat to isolate at least one variable. If you still have problems, it probably is the sequential management system (solenoid/valve/actuator/tubing).

 

Yes, everything is completely stock for the ecu, the only major change is the exhaust. Adding a cat was going to be my next step. I also have to do a full vacuum system hose job. I can usually get around the problem by letting off the throttle slightly during that rpm range and then it'll be fine at wot all the way to redline.

 

From new these cars suffered from what was commonly referred to as a 3K hesitation. Sometimes it was at a little higher rpm. A function of the 8 bit ECU of the day and the system design. Mine was always much worse if the car was not at FULL operating temp. It usually helped to add auxiliary grounds. You could practically eliminate it with a standalone ECU. Are you sure we're not talking about the same thing?

 

I've experienced the 3k rpm hesitation, unfortunately this hiccup is more harsh than that. It will happen at almost any temperature, whether it's 5 mins after warm up or 2 hours. I've looked into the ecu but I would prefer to do some larger scale modifications before a full tune. How sensitive is the stock ecu to boost creep?

 

Put a cat on. The stock ECU is designed to control a stock car.

Or go with an aftermarket ECU and get it tuned.

and if you modify it w/ stock twins, port the wastegate. Controlling the wastegate has no baring on how much exhaust the wastegate can expel, i.e. full open can only allow so much flow

 

OK just wanted to rule that out. But added grounds is still a good idea. Cheap and easy too.
I agree with Tom in recommending a cat instead of the resonated midpipe. A good quality hi-flow cat like Bonez or rx7store with prevent creep, make the car a bit quieter and probably smell a lot better too. And pretty sure you won't notice any power loss.
On porting the wastegate, that's a crap-load of work removing and reinstalling the turbos to do the porting. To clarify Tom ( I think), I'd only consider that if you insist on keeping a midpipe.
As for your hiccup issue, if it remains with a cat I'd suspect a sticky actuator or a solenoid.

 

...or it could be a fuel issue, maybe your secondaries aren't working correctly (injectors or control system) which could be the case if you are experiencing normal boost pattern.

If the car is not only stock, but original, you may want to just go through and replace your rats nest and test all your solenoids and actuators as well as send your injectors out for a clean.

What are your mod plans. You may want to just cut to the chase.

Regardless, if you are running a stock ECU, put a cat back on.

 

Would it be possible to add a restriction plate between the downpipe and the midpipe? I was reading in some threads that some people run a restriction plate to 2.25" to mimic the cat being there. The only reason I ask is because I checked my exhaust the other day and it is one piece from the downpipe back. I still have to do the silicone hose upgrade to the rats nest. In terms of a sticky actuator, are there any tips on keeping it from sticking? Or would I have to replace it?