Bucking and Water Temp. Sensor
 

I've had to tolerate a very annoying "bucking" of my beloved 7 for over a year. I've tried adding two ground straps (which I thought worked for a while, but came back). I've tried replacing the 02 sensor to no avail. I changed the fuel filter, changed the plugs, changed the wires. Checked my TPS (read ok) Nothing cured it.
No error codes either (except for airpump, which was removed)

I've read a couple of posts of people running a potentiometer off their water temp. sensor. I was wondering how many have tried this with success. I've never replaced it, and was wondering if this could be the culprit. Also... does anyone have any idea what a water temp. sensor would cost? It looks god awful easy to replace.

Well, I'm determined to fix this problem... (and I can't afford a PFC anytime in the near future). Your help is much appreciated!

Mods:
HKS Super mega flow
HKS downpipe
APEXi N1 duals
Greddy pulley kit
Efini Y-pipe
Racing Beat Ultra wires
NGK 7's and 9's

oops... I forgot to mention
 

I forgot to mention that the bucking is NOT the notorious 3000 rpm hesitation most people have experienced. It's a bucking that occurs anywhere between right off throttle to about 4,000 rpm. It occurs more often when hot... or right after I've run it pretty hard. It's a bitch to drive in traffic... because it almost wants to stall right off the line at times. UGH!!!!!!

Removing the airpump can cause rough idle and engine smoothness. Maybe that's your bucking...

Also, vac leak at the lower intake manifold causes bad idle and engine roughness. It requires replacement of the gasket which means lots of labor.

Perhaps explaining what you mean by bucking would help.

My idle is fine at a smooth 800 rpm. The bucking I'm experiencing is a light throttle. When I step on the accelerator... the engine would hesitate momentarily and then accelerate. This happens anywere from leaving a stop to 4000 rpm. To alleviate this... I would have to constantly lift off the gas ( or shift), and then step on the accelerator more aggresively. There is no "bucking" when I goose it. I had this problem prior to removing the air pump, so I doubt that is it. And my boost gauge indicates proper vaccum and boost, so I doubt there's a leak... so... I don't know....:(

hhmmm, why did you remove your airpump if you still have the stock cat???? It will clog you cat to not have a air pump. Was the airpump going bad or did you just remove it for the sake of removing it???

I can't really offer much help except I have the same problem. It only happens once and awhile but its under light throtle. Does your car run really rich? My car has been running really rich and i think that this sensor might be the problem. Let me know what happens after you change it?

I have this problem too. I just started havng it recently, in the last month. What is the cause of it? I am getting close to getting a PFC, which may resolve the problem, BUT I don't want to mask and existing problem, I want to get rid of the source of the problem.

I am also getting boost fade at about 6,000 RPM, it started to fade from 10 psi to about 5 psi. I've fixed several hose and piping problems already, its nothing to do with hoses or pipes.

I'm running virtially stock, see sig.

Have you tried cleaning the injectors? Redline S1 and Chevron Techron are decent. BG 44K is pretty strong stuff too.

Also, is the fuel filter old?

No, but that is my next setp. I'll probably put som ein this weekend and drive my car a lot.

BTW, here is my sig, since I left it out last time. Injectors was my first thought too. I am showing running less rich then usual which is why I think injectors.

Bucking
 

My 93 did the exact same thing and its obvious fuel cut related which has to be somewhere along the lines of ecu, injectors, ground causing electric problem (mine was worse with the lights on), running rich "choking" etc... To me it seemed more like fuel cut or falling on my face meaning severe hesitation not a stumble and it would continue to buck on light acceleration but once floored normal response was fine sounds fimiliar doesn't it. Your most probable solution is to replace the ecu,
and if you do that and there's a tuning shop close by you might as well get the pfc. I wish I had a proven solution but I sold my car before replacing the ecu. But all my reasoning was pointing there.
I hope this helps with your next decision for a solution.
Fritz

The reason I took my airpump out was because it "froze" and started smoking really badly one day. So I just unplugged it. I got a quote from a dealership that those stupid pumps run 9 bills. I know I could find one cheaper off the parts trader, but I said wtf and got the Greddy pulley set instead. I really think I'm running rich. So I'm thinking it's got to be a bad sensor or ecu/efi related. And since I just replace the plugs and wires about two months ago, I doubt it has anything to do with the ignition.

So, back to my original question... Has anyone tried replacing the water temperature sensor? Or attached a potentiometer to it to adjust the pulse length of the injectors? I really want to get this fixed without having to fork over a grand for a Power FC. Driving home in traffic today reminded me of how much I hate driving this car at times... it's almost embarassing... jerking around in such a beautifully made car.

bucking
 

The next time it acts up on you immediately remove
the cables from your battery to reset the ecu and see if the hesitation is gone. Good luck but I think your gonna have to spend that grand.

If a sensor is bad, you should be able to pull a code. If your ecu is bad, isn't there a code too?

Why don't you get a $6 bottle of Chevron Techron or Redline SI-1 to see if that helps. If indeed you are running rich, you should see lots of gunk on your plugs. Did you see any when you change them last?

BTW, instead of replacing the air pump you should buy a Mid Pipe, for $100, to $350. You shoudn't run wth no air pump and a Cat though.

Hey guys. I've tried running the Chevron techron stuff in my tank. Last time I went to Vegas... I put a bottle in on the way there and a bottle on the way back. It did help for about a couple of weeks before the hesitation kicked in again. Maybe I should try the stronger stuff (from Napa autoparts, right?). The reason I think it might be the water temp. sensor is because the bucking get's really bad when the engine soaks in heat, like in traffic. In cool mornings, the car runs pretty smoothly.

I wish I could get a high-flow cat. or midpipe, but with my mods... I need to get an ecu first... which is way out of my budget.

Thanks for all your help!

Did anyone found a solution to this problem? I'm having it too...

I have new engine, turbos, wires, plugs, injectors cleaned, new fuel pump, new fuel filter... I also tried stock ecu and pettit ecu, and it does it anyway. First I put two grounds and it seemed to solve it, but I realized later it was still there. First I tought it's the pettit ecu, but then realized it wasn't.

My observations are as follows:
It get's worse with city driving, under 2k rpms. If I drive it a little faster, only thing that's obvious is slight pause (accompanied by pop from exhaust - running rich?) when I quickly WOT it under 3k rpms. Driving off the line and low rpms response is fine. But if I do city driving, staying at the lights on idle - the problem seems to become worse. Driving off the line get's me moderate to bad bucking, so I need to lighten or increase the pressure to the gas pedal, or shift gear. It seems that while bucking, if I apply more throttle, the bucking get's worse. If I apply even more, it get's worse and then sets free - the car immediatelly boosts and starts accelerating strong. It feels like something is sticking - could some solenoid be causing this kind of behaviour?

Otherwise, I'll also try to test fuel pressure - is that an easy test to do?

I suspect FPR and FPD, but I don't have any issues with flooding or hot starting the car.


I appreciate all the answers and suggestions...

Take a look inside your fuel tank. Look for sandy deposits or corrosion. It may be difficult to see because of the plastic baffle just below to opening.

You may also siphon out some gas after agitating it by city driving. Look to see if it is cloudy.

I cleaned the fuel tank two months ago. No corrossion or anything was present.

I'm sorry but I don't quite understand what you mean with your last sentence, could explain it again? Thanx.

Everyone with this problem, check Tps sensor and report back.

http://www.rx7.org/Robinette/tps_adj.htm

We'll go from there then, if the problem is still there.

Sounds to me like it's either the Water Thermosensor or the Air Intake Thermosensor. Both of mine have gone bad before and no I didn't get any codes because stock ECU's also go bad....

I don't have this problem, but it occurs to me that either, or both, of the following could be at fault:

A faulty fuel pressure regulator and/or damper might contribute to part-throttle stall. FPR could also be a vacuum line or solenoid issue. This would seem to be more significant at very low RPM/throttle openings. Also, I'm not sure that the normal testing of fuel pressure would show both types of failure. I think you'd need to monitor fuel PSI while actually driving.

One other spot to check is that the TPS doesn't have a 'bad' spot; meaning one or both sets of outputs are not working across all throttle-opening angles. In this case the ECU wouldn’t know that the throttle opening had changed resulting in lots of air but no increased fuel injection. Even a short momentary dropout could create the stall. Also, I could see the affect being noticable over a wider range of operating conditons, not just right off idle.

You might want to check your TPS values and then start the car with the intake inlet pipe leading to the throttle body off. Keep the vehicle running with you manually controlling the throttle linkage, rev the engine and clean the hell out of that throttle body.

I used a can of super tech carb. cleaner from Walmart . 99 cents. Damn this is clean as a whistle... after that set your idle and recheck your TPS.

work for me!

all: I will check the TPS again. I did it a month ago (because of the same issue), and the start and end voltages for both narrow and full range were ok, but I wasn't paying attention to the linearity. Actually, on the second tought, there might have been something on the narrow range... need to re-check. Anyway, if I find a flat spot, can it be fixed, or should I get new (used) TPS?

Anyway, the whole rythm of this bucking and it's irregularity is looking like that's some solenoid problem... When it bucks the hard way, it's like:
buck-buck- BUCK-BUCK-BUCK-whoosh goes the boost and car accelerates
This is when I don't let off the throttle, maybe add it slightly.

Also, today I've noticed it does happen if I lightly accelerate in fourth for example from 1400 to 1800rpms and keep enough gas to keep the speed. So it is happend at constant speed to.

PandazRx-7: Why do you think it's either air or water thermosensor? Did you had the same problem with yours FD? Which of two was it? Did replacing it eliminate bucking?
Where exactly are those two located? I think the air sensor is under the UIM? I can it for giving correct reading over this weekend...

FDJunkie: Yeah, if I don't get any other clue, I will probably just buy this two and new fuel lines and go ahead and rip everything apart to reach these bastards. They have probably never been replaced, so they have good 150k miles behind...
How much would that cost? I think FPD is around $110, what about FPR and lines? Should I consider aftermarket FPR?

andre:
The TB was cleaned when I replaced the engine. I took the intake elbow off the other weekend, and it was all shiny :)

I had the EXACT same problem with the bucking you described and it turned out to be a malfunctioning water thermosensor...located in back of the thermostat housing. Btw, it happened only when it heated up to a certain temp, then starts sending bad signals to the ECU and causes the engine to run rich, which explains the bucking, unresponsive throttle, popping exhaust, etc. Drove me crazy until I figured it out...

If your looking to change it...it's not so bad. I would also change the water thermoswitch (miata) which is right next to it and also the thermostat.

OK, I find the instructions and locations of this sensor in the manual section F-169. I will check it during the weekend.

PandazRx-7: Do I really need to remove UIM to get to the sensor, as the manual says?