Engine jerking back and forth at 1500 rpm

speed freak · Postby **speed freak** » Sat May 17, 2014 5:53 pm

Hey there

A while a go I had an issue where when you are in gear and off the throttle, say slowing down for a set of lights. When the revs got to 1500 rpm the engine/car would violently jerk back and forth. This usually happens at lower speeds say between 20 km/h and 50 km/h. I only notice it when Im slowing down or coasting slowly in traffic. Only happens at 1500 rpm, if you stay very slightly on throttle but revs dropping still it wont do it or if you clutch in and clutch out to get the revs to 1300 rpm or lower it wont do it, only at 1500 rpm, in gear and off the throttle. Any gear too.

Now I replaced the cam position sensor and the crank position sensor. That didnt help that issue, however the car ran a lot better. Iv also replaced plugs and plug leads. I also had the IAC valve zapped with 12 volts to 'wake it up' a little. I was recommended to change the green EGI relay which I did about a week a go. It hadnt really done it at all up until a few days a go after I replaced the EGI relay, now I dont know if its a coincidence or if its just started doing it again at the same time I replaced the relay.

Replacing the EGI relay has made the car much smoother, when it was in warming up mode it was really sluggish and seemed to run really rich, try to slowly open the throttle a little and it would almost choke on itself but now even once warmed up it runs a hell of alot better.

Does anyone think this issue at 1500 rpm is related to me replacing this relay or a separate issue?

Any other suggestions on finding a solution to this problem? Iv had all sorts of mechanics look at it including mx5 plus but no one can find a solution

Cheers
Rhys

de Bounce · Postby **de Bounce** » Sat May 17, 2014 8:16 pm

Try disconnecting the Idle air controller or swapping out the Throttle Body / Idle Air Controller assembly with a known good one.

pezchops · Postby **pezchops** » Sat May 17, 2014 9:10 pm

hi Rhys,
If anytime you want to swap parts to test/eliminate causes, give us a yell.
so long as we are just trying easy bolt/screw on parts i dont mind..may save you buying parts not needed, and we can both learn some things.
cheers...peter

project.r.racing · Postby **project.r.racing** » Sun May 18, 2014 8:31 am

12 volts is too much for the iac.

i'd investigate the iac more. it's probably not openning up enough when required. pretty common issue.

speed freak · Postby **speed freak** » Sun May 18, 2014 11:23 am

project.r.racing wrote:12 volts is too much for the iac.

i'd investigate the iac more. it's probably not openning up enough when required. pretty common issue.

As far as Im aware the IAC valve got 12 volts put through it to wake it up, apparently it can handle 12 volts but usually only see's 8 volts.

This seem to have helped as it didnt really do it for quit a while (many months) but now Iv changed this relay its doing it again, not sure if could be related or just a coincidence.

Tried finding another IAC but no luck yet.

Thanks Peter Ill keep that in mind

speed freak · Postby **speed freak** » Sun May 18, 2014 11:25 am

de Bounce wrote:Try disconnecting the Idle air controller or swapping out the Throttle Body / Idle Air Controller assembly with a known good one.

Should disconnecting the electrical plug make the car do this all the time or should it stop doing what its doing?

zossy1 · Postby **zossy1** » Sun May 18, 2014 7:06 pm

IAC or TPS I reckon.

speed freak · Postby **speed freak** » Sun May 18, 2014 7:56 pm

Yea Im thinking IAC valve too but havent yet found one or a good one and no shop is willing to try a second hand one 'as its probably no better' according to them.

Iv thought about the TPS though. If the car thinks the throttle is still open but its not maybe the IAC isnt engaging (or whatever the technical term is)

Iv found it normally happens if Im in second or third gear going slowish so revs around the 2000 rpm mark if I get off the throttle to slow down or to do a u turn or something it will jerk back and forth. Its kind of like a violent surging feeling. Happens too if Im just coasting slowly to a stop but in gear and off the throttle.

Only thing is it doesnt happen all of the time, it could do it for a few days straight or a few weeks then not do it at all for ages or could do it once or twice and not again for ages. Its so frustrating.

Sconse · Postby **Sconse** » Tue May 20, 2014 12:53 am

By the sounds of what you are describing, this is something I've experienced, both I'm my mx5 and in my last car, a wrx. Does it only happen under some load? Is it possible for it to happen with the clutch in?

After talking to a few different people about it when I had my wrx I put it down to laxity in the drivetrain at low revs under a little load, where the drivetrain is being loaded/unloaded, which leads to the jerking feeling.

If you are experiencing what I had, it's not anything to worry about.

I hope I explained that alright, I'm pretty tired, I'll re-read it tomorrow and see if it makes sense lol

slimx · Postby **slimx** » Tue May 20, 2014 5:43 am

Hey there,

Definitely in the transmission I can second that. Check CLUTCH / GEARBOX / FLYWHEEL etc. Try getting it to 1500, launching at 1500 - dropping to 1500 etc.

----
Put it on a DYNO or maybe even have it in MID AIR on a hoist and try to drive. Jack the rear end and try driving .. it shouldn't jerk. Because there is no weight on the wheels it should just stall out - This'll show it isn't the engine.

My bet is on the clutch. It's the only thing that gets played with - I've never heard of a gearbox wrecking like that. - But still worth a check if it isn't te clutch.

project.r.racing · Postby **project.r.racing** » Tue May 20, 2014 2:21 pm

So is the engine jerking, or the car jerking? 2 very different things!

Mr Morlock · Postby **Mr Morlock** » Tue May 20, 2014 3:00 pm

you have already replaced parts that may have all been ok. If you can get the car to do this on test then a diagnostics guy can probably find the cause. If you check out the Mazda service and repair data it will go thru the problem solving- no one can tell you what it is just a number of things it could be.

speed freak · Postby **speed freak** » Tue May 20, 2014 5:56 pm

I havent replaced heaps of parts with no outcome as I dont know whats causing it, I was replacing those sensors either way and they helped in other areas.

I can only assume the engine/drivetrain is jerking back and forth but its that violent that the car itself also jerks back and forth. It happens only under some load, only happens if you are in gear and off the throttle.It is something to worry about if it was only minior I could understand but this isnt normal.

When it does it I can hear the transmission/clutch making noises due to the jerking, hard to explain doesnt sound like its about to explode, no whining or anything but because it happens in gear it makes the metal transmission shaking noise thing :lol:

Iv had the clutch and flywheel replaced but it used to do it with the old clutch too.

Hard to just go out and make it do it as its so intermittent as Iv mentioned but when it occurs its bad.

Only way I can describe a similar feeling is say in a 4x4 in low range first gear stabbing the throttle pedal hard and constantly or some really bad bunny hopping constantly.

Again only happens in gear off the throttle and at 1500 rpm. Usually in third or second as it usually happens when Im slowing for a red light or making a u turn or slowing for a speed bump etc and when it does occur once the car/engine has shaken itself enough to get the speed/revs below 1500 rpm it stops.

project.r.racing · Postby **project.r.racing** » Tue May 20, 2014 6:32 pm

My Astina does the same thing. Sometimes near impossible to drive at walking speeds, and doesn't happen all the time.

You got a big exhaust and cloth air filter? Is the clutch heavy duty? Got big heavy wheels fitted?

I live in a rural area nowadays to rarely does it happen for me. But riding the clutch normally is the cure. Also my IAC has been faulty for the past 10 or so years. Same IAC as a NB8.

speed freak · Postby **speed freak** » Tue May 20, 2014 7:51 pm

project.r.racing wrote:My Astina does the same thing. Sometimes near impossible to drive at walking speeds, and doesn't happen all the time.

You got a big exhaust and cloth air filter? Is the clutch heavy duty? Got big heavy wheels fitted?

I live in a rural area nowadays to rarely does it happen for me. But riding the clutch normally is the cure. Also my IAC has been faulty for the past 10 or so years. Same IAC as a NB8.

Walking speeds is ok whilst slightly on the throttle its just after doing say 60 or any speed in fact then coasting down to a stop when the revs get down to 1500 rpm it jerks back and forth.

No big exhaust or CAI just normal air filter and a xforce muffler. The clutch is a HD B6 setup but this problem occurred before I had the clutch changed. Wheels are light weight Enkei's.

Im really thinking its the IAC valve but I havent found one to buy yet and Ill need a TB too as the screws that attach the IAC to the TB are stuffed. Only thing is if I do get one I dont know its any better then the one I have.

Could it be a faulty TPS or would issues with this occur under acceleration?

Its that precise rpm too, 1500 rpm, if you stay slightly on the throttle but still loosing speed/revs and get it to 1400 rpm or even bang on 1500 it wont do it or if you clutch in and out to drop the revs below that point its fine.

Its only when the revs are coming down on their own and your in gear off the throttle when the revs get to 1500 rpm it throws you about. Its near impossible to drive in stop start traffic when it really plays up. Since the IAC had some electrical current put through it, its been great but since replacing the EGI relay its started doing it again.