half throttle hesitation / bog.
 

the car is a 1994 R2 with 158K on the chassis and a fresh rebuild, It had its fair share of hiccups after putting the motor back in but just about everything has been sorted save one. when I try to accelerate at half throttle or more, the car seems to stop making power, feels like an ignition retard. as I back off the throttle, I can feel a slight bump in torque before it starts to decelerate, and if I keep it under half throttle it will accelerate normally.

I did the rebuild my self with used rotors and housings, it has about 700 miles on it now, and compression is starting to come up. while the engine was out,I replaced some of the electrical connectors that were baked and crumbled with fresher ones from a donor harness, I also wrapped some parts with aluminum tape so there is a chance that if something got pinched it may be shorting. grounds have been tested and replaced as needed, but they are original factory grounds.

This problem has persisted with 4 different ECUs.

The only code I've gotten was during a smog check at 400 miles which failed due to high HC and CO.
code 17: Oxygen sensor inversion

Oxygen sensor shows signal while driving consistent with those described in the FSM, but does not throw a code when disconnected.

Knock sensor shows a steady 1.8-1.93v signal with no change when tapping on the engine hanger.

I could use some input and fresh perspectives from the RX7 club diagnosticians.

Does the hesitation occur both when cold and when warm?

If you measure the TPS voltages per the workshop manual, does it meet all the specs?

I've never been able to see a voltage spike on a digital multi-meter when tapping the engine hanger (both new and old knock sensors); think maybe you need a fast oscilloscope to see that.

Does your MAP sensor meet the manual's specs?

Does your warm idle sit at a steady speed at about 700-750 rpm?

Is your double-throttle system working normally?

working my way down the list.

-TPS is at .92-4.98 in narrow range and .39-4.5 in full range (within spec)

-checked codes again and came up with a code 5 (knock sensor) but ive been driving around with a multi meter monitoring that pin on the ECU for the last two days, cleared and I'll check again to see if it comes back.

-Just checked the MAP, its at 1.4v @ idle and 2.6v when the vacuum line is removed (within spec)

-warm Idle sits at about 900 right where I set it for break in. pretty smooth with
the occasional little hiccup but no significant fluctuation.

-Double throttle is the next thing on the list, I'll try to get those components tested tomorrow. so far i only know that the dashpot will operate the secondary butterflies when vacuum is applied.

All your numbers so far look good. Maybe you have the problem described in the Mazda tech note in post #42 of this thread... any possibility that particular ground is oxidized or loose? (Read the text in the pictures.)

Also, problem occurs at both cold and fully warm?

actually, checking and cleaning the grounds was one of the first things i did, i relocated the lager one from the bracket by the ac to the intermediate iron and replaced the on on the UIM with a newer one from the donor harness, (still the old style though) I will replace it once i get supplies.
Yes it does this both cold and hot.

Sounds like the grounds aren't the problem either. Have you attached a vacuum gauge to the UIM tap and measured vacuum at your 900 rpm idle?

Have you measured fuel pressure per the manual?

Plugs are new? Plug wires check out for series resistance?

vacuum is 18-20in/hg at idle

plugs are new

I'll test wires.

I'll test fuel pressure when I get my hands in a FP guage.

also might be worth mentioning, it as a boost spike issue that was present before the rebuild. during hard acceleration the boost goes up to 15 psi for one- 1.5 seconds then settles at 10

I got some more miles on the motor, so I was a bit more comfortable taking the RPMs up, the bog seems to affect it most noticeably from 2-4k after 4000 its still struggling, but not as badly.

tested a few things today, double throttle valve showed 12 volts with ignition on, 1.18v on cold start and 13v after warm up, FSM didn't describe any operation check, and the voltage stayed the same regardless of throttle position.

crank position sensor (NE+ signal) showed .03v on ignition (should be below 1v) ans between .01 and .04 at idle. (refference voltage is listed as .1 to .4.

checked the Four ECU gounds at the pins, they were all .7 to .8 ohms.

tried two things just now, test drive with the Fuel pump pin jumped on the diagnostic port to verify the speed pump relay was working during acceleration. No change, bog remained.

Then i tried a test drive with the charge pipe removed. not only was the bog completely gone, but the NA power band was kinda awesome.
so, it looks like its boost dependent. I'll check the positive pressure values of the MAP next.

I may have been over thinking this, Leading spark plug wires, one was high and one showing open. tested with a set that was about 50k miles newer, and the problem seems to have gone. not going to celebrate yet,but i'll update when I've got some trouble free miles

If its boost related, maybe it's your secondary injectors? I chased a massive hesitation problem for quite a while, it turned out to be poor wiring connections on my secondaries. The hesitation would appear around 4k and any positive boost, right when the secondaries kick in.

Update, it was spark plug wires.

Glad you got the problem fixed!

What were you looking for by measuring the vacuum at the UIM? And what is the UIM tap?