Intermittent loss of power after 3500rpm...help!
 

car: 1988 Mazda Rx7 GXL N/A

mods: K&N intake, 2.5in High-Flo Bonez Catalytic converter, 3in N1 Cat-back, and an Automatic to TurboII 5-Speed Transmission swap.

Problem: Within the past few months I've exhibited an intermittent surge or loss of power from 3500-5000 rpm. At first it was a minor concern that only happened first start up in the morning, but diminished after warm-up or later on in the day.

The only way i can describe it is when i accelerate to 3500 rpm it jumps back and forth and either continues to do so or struggles onward and then evens out. Then after the car has reached operating temperature, it accelerates as smooth as normal.

Just yesterday however, the conditions changed. I was coming home from work around 3pm, it was around 75-80 degrees out so it was warmed up fine. As i was driving i tried to accelerate from a 50mph roll, but when i hit 5k the car began to act as if i hit a rev limiter and refused to go any higher. Even out of gear i couldn't get it to rev above 5k and this continued all the way home.

Later that night i figured i'd be alright to drive it, i just can't open it up too much, but as soon as i tried to take off i hit 3500 and it again started acting like it was on a rev limiter.

after some research i went out today and began diagnosing it, started it up and couldn't get the revs to go any higher than 3500. As it warmed up though, it slowly regained normal revving. I then took it out on the road and it ran like a charm. The only thing i can think i did was I wiggled the wiring harness the fuel injectors tie into to make sure there wasn't a loose connection, but when i revved the engine to 3500 while wiggling the wiring harness the rpm's didn't drop.

So as far as i'm concerned right now the problem went away for now, but i feel it will probably come back, does anyone have or have had this problem? does anyone have any idea what this is?

Feedback is much appreciated :icon_tup:

Bad ground maybe? Check the FAQ section.

Right from the FAQ section that no one bothers to read?

I have this hesitation whenever I accelerate right around 3500-4000 RPM
The dreaded 3800 RPM hesitation is cause 90% of the time by poor ground connections between the engine, the battery, and the car. Clean or replace all the under hood ground connections. On 86 model year cars , replacing/regrounding the pressure sensor's ground wire has helped (do not re-ground a 87 or later model year). If there is still a problem after that, look at cleaning the fuel injectors and (on a N/A) that your 5th/6th ports are operating correctly.

If you disconnect the hose to the Pressure sensor, plug it and disconnect the plug to the TPS this simulates load. You can then try to rev the engine past the point where you have previously had problems to see if the Pressure sensor is at fault or not. It's a quick and easy check to do. Also, you can take a reading from the ECU to check to see if the ECU is sending a proper signal to the secondary injectors to fire them. This test requires an LED light. You take the negative wire to the LED and place it into the back of the ECU pin corresponding to the front secondary injector, for example, and the positive wire of the LED and place it into the back of an ECU pin that has voltage to it such as pin 3I and then try to activate the secondary injectors and see if the light flashes repeatedly. The pin for the front secondary injector is pin 3H and the rear secondary is pin 3F.

A clogged fuel filter or low fuel pressure could also cause your problem as well, in addition to the points suggested by the previous posters.

[QUOTE=Rx-7Doctor;10189369](do not re-ground a 87 or later model year). /QUOTE]

why would you not reground an 87?


:icon_tup: for copy and paste from faq page

I would suggest check main grounds on engine and engine bay. Then if problem persist go to injectors, especially your secondaries. Lastly, 5/6th ports.

That's a good question and the reason is the grounding system was problematic in the earlier models but was supposed to be corrected in later models. But to answer your question there isn't anything wrong in adding the ground at the pressure sensor because the ground that exists at the pressure sensor is one in the same at the ECU which is one in the same connected/grounded to the UIM.

I see you have a high flow cat and cat back system. Are you still running stock secondary injectors?

Considering the OP is n/a i doubt you need to upgrade the secondaries.

Try unplugging the TPS connector while the car is fully warmed up. Then going for a spin. Have you check to make sure the TPS is set correctly?

Oops, didn't realize he was NA. TPS would be my next guess. Adjust it to spec and see what happens.

awesome, i'll check these out next weekend, running fine so far though lol

Please let me know the solution, curious myself.

Edit: Just realized I've woken up an asleep ass thread, sorry folks.

wow, haven't thought about this forum in years lmao.

So I never figured it out, but I kept in touch with the next owner and he determined it had some damaged engine grounds. Never had an issue after he repaired them.

Happy hunting

Thank you for the reply!! Sorry you 7 ended up sold! Good luck to you !

Mine is falling on it's face at 3800 too, I'll be doing the grounds soon.