Hello,

I'm at my wit's end troubleshooting an intermittent jerking issue. It happens under certain conditions and only does it once or twice, then it doesn't happen for the rest of the drive. This is NOT the 3k RPM hesitation. This is my second Rx7 and my first one did NOT have this issue. It's a 93 Base with 65k miles on the original engine. It happens in a few ways: light throttle steady state cruising in 1st, 2nd, and mildly in 3rd gear most prominently between 2000-2500rpm I'll feel a power loss, similar to letting off the throttle, followed by violent jerking that continues unless I release or increase throttle. This issue is NOT related to bushings. This usually happens ONCE and then it will not occur again. This happens COLD and HOT. I have ZERO hot start issues and does not smoke one bit. My Rx7 is BONE STOCK minus a Bonez Downpipe.

Things I have replaced or fixed:
-NEW Plugs and wires less than 3k mi ago
-NEW coil pack harness
-Cleaned ALL grounds, added 4G ground between battery and chassis, tried several iterations of grounds added/moved.
-Cleaned alternator and battery terminals.
-Plug wires are correctly installed and fit tightly. No response when shaking them while the engine is operating
-NEW O2 sensor
-NEW fuel filter
-Tried a known good ECU
-Pulled the fuel pump to inspect pick up sock, clean as a whistle
-All vacuum lines replaced with silicone
-Map sensor connection is solid
-TPS is in spec with no dead spots
-Air filter is clean
-Vacuum is solid at 18" at idle [720rpm] No hunting issues.

Other things:
The issue is occurs more likely with the AC on or the headlights on. My knock sensor has leaked ALL of it's goo out. I'm considering changing the sensor as it could be "hearing" excessive knock due to the insulation issue. However, I feel that the sensor might only fix a slight intermittent sluggish issue but not the jerking. My MPG is typically 17mpg with mostly highway driving. I feel if the TPS was at fault, I would ALWAYS experience a jerking/bucking issue at the same typical throttle, however this is not the case. Typically once it has it's episode, its over and done with. I have had the issue through several tanks of gas, so I cannot see this being a fuel issue. I have NOT inspected fuel pressure, however the fuel pump "looked" like a TT supra unit, [no brown block on top? also didn't look 22 years old]. I have searched and found a few threads that were similar, but most lead to a dead end :/

I wish she would drive as good as she looks :/

Any and all help is appreciated.

Matt

 

Do you have a differential brace?

 

No brace. This is an engine based bucking, not suspension based.

 

Must be something in the air. This sounds a lot like what I'm experiencing...intermittently. Generally only once every couple drives. I intended to check both TPS and MAP sensors but afraid that since it's intermittent...and nothing shows up on the sensor check screen of the commander.

 

Did you end up trying the PFC?

 

I had somthing similar when a single injector diffser Snapped but i was also getting lumpy idle
Just an idea

 

Maybe this problem? See post #42, but read the entire thread (if you have not already).

 

@mazderadi: did not try the PFC. Sgtblue mentioned he has the issue (or similar) with a PFC.

@imitek: injector diffusers were replaced when the vacuum lines were done, along with pulsation dampener and repacking injectors.

@wstrohm: I have performed that TSB and still experience the issue. I have upgraded the ground from the UIM to the firewall and have noticed no change as well.

Hearing that Sgtblue has a similar issue with a PFC leads me to believe it won't be my knock sensor as the PFC does not have knock control and merely reads the sensor.

Matt

 

When you cleaned the ground at the A/C bracket, which is aluminum, did you use an abrasive cloth? The oxide that aluminum forms over time is non-conductive and fairly tough. IMHO Mazda should have connected that ground to steel or iron. In my case I used silicon carbide paper to get down to bare aluminum, and the problem went away. However it did return (my problem was the 3K rpm miss) slowly over time (maybe over 3-4 months). An alternate connection point might work better (have not tried that).

 

Wstrohm,

I used a fairly abrasive sand paper and got the terminal itself to a nice shiny new metal look itself, I also cleaned the bolt and used the same sand paper on the mount. It looked shiny and new when I remounted it. I can try relocating it to the iron as I think there is an under bolt location around there. With all the grounds cleaning, addition, and what not, I have a hard time believing it to be a grounding issue.

But just to be sure, I just cleaned it again, and relocated it here

 

My suggestion would be to check your fuel pressure. A friend of mine, and a few here on the forum have had fuel pumps go bad.

 

Check the fuel pump resistor under the brake booster, you can bypass it and see if the problem goes away.

Since it is a light throttle operation issue, sounds like the ECU is not switching over the 0v on the 1K terminal fast enough or the resistance is out of range.

Could also be the fuel pressure solenoid under the UIM.

I would also look at the Circuit Opening Relay, this is what actually triggers the Fuel Pump Resistor.

 

Nice job, and thanks for the tip! Hope you find the source of the problem.

 

@adam c: I have considered the thought of it being a fuel pump issue, but I feel like if it was, it would accur on a more regular note, or at a broader range of RPM. I will need to get s fuel pressure gauge and take a look at the operating system anyhow.

@banzai: I DID check the fuel pump resistor. It was .6-.7 ohms @85*F or so in my garage. If I were to bypass it, would it be as simple as jumping the two wires? Would this effectively keep my pump always at a higher voltage?
I do have a brand new rats nest solenoid sitting on my shelf. Doing a bit of reading on the Pressure Regulator Solenoid, it appears it triggers under a wide range of functions including fuel temperature, coolant temperature, 90 seconds post start, and throttle position greater than 30%. If I'm feeling froggy, I might just swap a brand new one in to easily eliminate it from the equation. As for the Circuit Opening Relat, are you referring to the fuel pump speed relay? I'm not seeing a circuit opening relay inconjunction with the resistor. As for the speed relay, my seat of the pants tells me it has to be working. I believe it triggers past 2750 rpm (and about where the 3k rpm hesitation is. After 3k it feels like a different car. There is a large noticeable difference in power past 3k rpm. Is this another hint at the fuel pump resistor?

@wstrohm: Since I moved that ground as of Sunday, I have not had the bucking/jerking. This is not to say it is fixed, it's just it hasn't happened...yet. If it goes away for longer than one week I will consider it fixed.

Thanks for the wonderful information everyone.

Matt

 

Pages F-108 & F-109 http://www.banzai-racing.com/manuals...ol_systems.pdf

Yes the resistor can be bypassed to eliminate the voltage drop.

Looking at the schematic, the circuit opening relay is the Fuel Pump relay (speed), which I would focus on and rule out. Intermittent/sticking relays are becoming more common the older these cars get. I have seen flaky EGI, Fan, fuel pump relays all recently

 

Figure I would bump this for an update.

I'm still experiencing the issue. It's still a one time thing and after it occurs it's smooth sailing. It aggravated by heat and electrical load. Ive replaced the entire negative battery terminal wire from the battery to the block and chassis with 2AWG wire and I still experience the problem. This is leading me more towards a possible sticking solenoid. The FPR solenoid has been changed with a brand new one to no effect. My next plan of attack is the ACV. The ACV implements a port air solenoid that supplies air to the exhaust ports to alter the o2 sensor reading. I feel this might be at fault because when it happens, it feels like I hit a brief lean out as power drops, followed by the violent jerking. After I release the throttle and either re engage the clutch, it won't do it again for the rest of the drive. I'm wondering if I should just go ahead and order the solenoids and change them, or if i could possibly unplug the o2 sensor and see if the issue goes away. With the o2 sensor unplugged, will the ECU revert to a set map?

Matt

 

I think I found the problem!



I was doing more troubleshooting today. I've been chasing my tail on this issue for almost a year now but I think I nailed it down today! Today I grabbed a fuel pressure test kit from Auto Zone and hooked it up. All fuel pressure read perfectly. 40PSI following a hot start and 32 psi at idle. I duplicated the bucking issue while watching the gauge and pressure was rock solid :/ So I found out it was in fact NOT a fueling issue. I pulled out my timing light and checked timing at idle and it was spot on and advances above 1500rpm. Again, not my issue. I've looked at my TPS 3 or 4 times but I figured, what the heck and I tried it again. Only this time I actuated the throttle with my foot for a smoother control to look over the ranges.


The first pass is normal speed, the second pass includes a "slo-mo" portion.

Looks like it was a dead spot in my TPS the whole time Happens every time RIGHT at about 2V.

I'll be putting one on order through Ray immediately.

Matt

 

Thanks for the update Matt.

 

The problem persists

So I changed my TPS and the issue is still persistent. Since then I have pulled the extension manifold and checked the ACV and all my vacuum routing. All solenoids work as intended and are routed correctly. The ACV had a plunger that wasn't sealing and was not allowing the relief to open. Changed the plunger and the relief works and the issue still recurs. Pulled the engine coolant sensor and it was out of spec at 3.1ohms @68*F. Changed the ECT and the problem still exists. When I was driving around one day it finally threw a code for O2 sensor. I ordered a new OEM o2 sensor, and the issue still exists.

I think I've exhausted almost everything I can think of that would be the issue with this.


The above video is with me holding the throttle steady at 2k RPM in first. This was a light instance of the problem while it's usually a much more violent bucking and last longer. The power usually comes on bumpy around 2k rpm and I have yet to figure out what exactly is supposed to happen at 2k rpm.

I think lighting the car on fire is the next step.

Matt

 

Get a new engine harness or replace the wires and plug to the TPS

If that doesn't fix it then set it on fire LOL

 

Engine harness was on my list of things. The air pump connector obliterated itself on my last tear down. The insulation on the wires is cracking off on the same connector too. If that one is like that, I imagine the rest are pretty jacked up too. So far I've changed:

Grounds (bigger and added)
Knock Sensor
O2 Sensor
Engine coolant sensor
Repaired various ACV issues
TPS
Known good ECU
Plugged EGR (no change)
Ops checked all solenoids and verified routing
Plugs and wires have about 10K on them.
Coil harness

If a new engine harness doesn't fix this, gonna just burn the car to the ground.

Matt

 

I have the same issue also, and it does seem to happen more so with a/c working and right about 2k rpm like OP has described.

Hopefully, this will get resolved eventually.

Something I have learned from owning older turbo cars like these, my other car is a 1G eagle talon, that they are bound to have some strange weird issues sometimes and if chasing after every issue would drive you insane. My rule of thumb on this, as long as, it doesn't significantly affect your drivability or power output I just learn to live with it.

The funny part, is that a lot of times most of the problems end up working themselves out as you keep owning the car.

 

Matt,

I'm betting if you put a PFC in the car it will go away. You will also have better overall throttle response, gas mileage, nice temp gauges etc.... No it's not as safe but if you keep boost at 10 psi you will be fine.

 

Fritz,

I've been on the fence with getting a PFC for this issue. I wanted to rule out a possible failing component because I fear this issue could manifest itself as destructive as I build for more power. I also don't have a tuner local so I'll need to do a bit of reading on how to tweak everything for daily driving. I know arghx has a really sweet how to for it. Maybe it's time I just go ahead and get the PFC. I've been told by countless rx7 peers to get it and I've just been stubborn :P

Get the PFC, if it fixes it, win. If it doesn't, new wiring harness.

Matt

 

The base map is fine for low boost and as mentioned you'll get good mileage (likely 17 plus).