I always had a slight 3000 RPM hesitation with my old motor/stock sequential turbos. I recently installed a rebuilt motor/turbos and a lower mileage (~50k mile) rat's nest. Everything is as-stock except the double throttle solenoids, lines, actuators have been removed, I'm using Dale's viton check valves, and silicone vacuum lines. I have an M2 ECU. I have aftermarket grounding on the battery, engine, transmission, alternator and intake manifold.

Since the new motor went in, I've picked up a much more severe 3k RPM hesitation under the following conditions:

1) Motor is still warming up from cold state

2) Partial (less than 1/2-to-3/4) throttle

3) Positive boost

Under these conditions, the car shutters violently about 2 or 3 times before finally taking off. It loses all power and makes it look like I'm some n00b who can't shift or something. As soon as the engine warms up the problem goes away. Under light throttle or vacuum the problem doesn't manifest. I don't know what happens under WOT because I don't do that on a cold engine.

Anyone have any ideas where to start? I'm just hoping to avoid having to troubleshoot everything in the system if someone might know exactly where to zero in.

Thanks!

 

Ahhh.... the double throttle control that you removed will affect cold operation as the ECU thinks that it is closed and adjusts the secondary fuel injection accordingly to prevent an excessively rich mixture. Removing it results in an pronounced lean spot.

 

Ah interesting, I hadn't heard that before. This lean spot occurs around 3k RPM or is it throughout the band? I would have guessed it would be something related to turbo control since it was at a specific point.

The motor I bought came with double throttle removed already so I left it off. I'm not thrilled by the idea of a lean spot, so I guess my options are replacing the double throttle or getting a better ECU. Too bad I already sold off my old UIM.

 

Yep, as you know there's no way to totally kill the 3k hesitation unless you go to a PowerFC.

The problem is with the injector staging, the secondaries are kicking in and the primaries are cutting back. It's really a flaw in the stock ECU's design.

With the level of stuff you've done to the car, a PowerFC is a really good idea at this point.

Dale

 

Do you still have the symptoms once the car is warmed up? If not, it's from removing the DT.

 

Slightly OT: I've heard this 3k hesitation is somehow a flaw of the ECU, but why is it that this was never a problem on new RX-7s?

 

The problem clears up once the engine is warm. Then it's just the normal 3k blip you can barely feel at all.

A new ECU may be in my future

 

You could try the fix in post #42 of this forum thread. Also see post #47 and #53. It worked for me, but the temperature phenomenon wasn't there in the first place... just missing going up through 3000 rpm slowly at any temperature.

 

Sorry, I wrote that incorrectly. If it does clear up when the engine is warm, it's from removing the DT.

 

Thanks for the clarification. I feel better about it now that I know the source of the problem.