altron32

ok, so after porting out my intake and getting my injectors cleaned, blocking off my acv and wiring my 5/6th ports open my car seems to have a fuel cut at 3800rpms, as in it misses and bucks, and makes me just about bite my stearing wheel! it only does this after it warms up. i have no smog pump, a/c or egr valve. full straight back 2.5"exhaust with high flow mufflers and no cat or silencer. and a custom air box. its a 88' n/a rx-7 with 60,000miles on the motor. i just finished a full tune up also on, 10mm plug wires, new plugs, fuel filter and oil change.

tinvestor

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.

tinvestor

so I think you may have left off a ground or not regrounded it properly

altron32

if you would have read the rest of my post i had my injectors cleaned. i have replaced my battery ground, and motor to chassie ground. i had a friend that has worked on mazda's for about 25 years, and he said i need to T in a ground on my the black wire going to my MAP sensor, and that might fix it, if not i need to buy a fuel cut defender from racing beat. but its weird that my car would need that since its not boosted.

HAILERS

There was a service bulletin on early 86-87 cars that added a extra ground wire to the boost/pressure/map sensor. It spliced to the existing ground wire and teminated the other end of the wire at the thermostat area.

This was ACTUALLY to fix a problem with the AFM ground. Since all the grounds for the sensors in the engine bay are all spliced together inside the emissions harness, that makes sense. The MAP sensor was just a convienient place to do it. You can see or read the bulletin at ALLDATA.com for verification of the above.

IF that does not fix the hesitation, make sure the vac line b/t the manifold and the sensor has a small orifice inside the line. It's a piece of plastic with a very small hole in it. Made to prevent spikes in the vac signal>

If that does not work, put a fuel pressure gauge on the fuel feed line and make sure it has over 35psi when your driving the car at full throttle. It should be approx 28-32psi at idle.

IF that does not work, here is something that finally worked on my 87non turbo-turbo. I spliced another wire to the ECU ground wires (pins 3A, 3G, 2R). Where I did this is at the ECU plugs. If the harness cover is stripped back about a foot from the middle and small plugs, you will see where these ground wires all come together. I joined them together with the new ground wire (soldered) and put a ground lug on the other end of the new wire. I connected the lug to the ECU support bracket.

I can make my car studder at will by just removing that lug from the ECU bracket. Others have tried this and it does not work for them at all. I'm clueless why it does work for me and some others.

By the way, those ground wires for the ECU terminate on top of the rear rotor housing in a two wire lug held by a 12mm wrench size bolt. Mine is just fine and dandy and is clean and tight. Is yours?

And by the way, when I take that lug off to make the engine bog/stutter, the wideband shows rich. As in the 10afr range. Enough.

SureShot

Using test lights or voltage at idle readings will get your idle setting perfect, but you could still have a worn out TPS resistor.
-The golden test:
Unplug the TPS connector.
Clip a cheap analog (needle type) VOM on the orange & black pins.
Set it to the 1K Ohms scale.
Hold off the throttle & work the short range TPS plunger in & out.
You should see a smooth sweep from ~0 to ~5K ohms.
Any bad spots or drop outs in the stroke - it's bad & will give fuel cuts at part throttle.
(Before you toss it, try spray contact cleaner between the sleeve & plunger.)
- Then set it:
Find the fast idle (warm-up) cam & push it back so the throttle sits on the hard idle stop.
Adjust the setting to 1K ohms.
While holding off the fast idle cam, open & close the throttle a few times to see that the 1K setting repeats.

I just discovered recently that a bad AFM can also cause fuel cuts, but its symtom is a predictable fuel cut. A worn out TPS gives more random fuel cuts.

rotorforce

tinvestor

The rest of your post? I read it three times and no mention of grounding. If you are talking about a previous thread so sorry I missed it but that is why you are supposed to continue the thread not start a new one.

dat-13b

3800 RPM is where the secondary injectors come on line. If I understand correctly duty cycle goes down 50% on the primaries at the same time. If you have a problem with a secondary injector, either mechanicaly or electrically it will lean surge.
The reason I know this is, it happened to me on my TII. At 3800 it would buck and surge, and after checking, found that one of the secondaries had stuck. I re-cleaned it and has worked fine since.
I know you said that they had been cleaned, but I would verify secondary injector operation.