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So I bought my car, a 2003 1.4 si, hatchback, 95000km and it is practically spotless in and out.

It runs perfectly, starts well, engine is so silent I almost cannot hear it at idle.

Yet...

When driving on the freeway in 5th gear and releasing the throttle slightly, going from 100km/h (60mph) to 80-90km/h (50-55mph), there is a slight but noticeable jerking. It feels like I am driving on uneven asfalt, except the motion is more forwards/backwards than up/down. It is really really slight, but it is definitely there.

It is only in 5th gear as far as I can tell, and only with light throttle, at around 3000rpm. Releasing the gas pedal or pressing it down makes the jerking disappear.

How much should I care? What would the most likely problem be? It seems to be a very specific problem that rules out some causes but I am not sure where to focus my attention.

Considering the car really is in such a good shape I would rather find the problem earlier than later.

Any help is much appreciated.

 
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Could it be bogging down at low RPM in a gear without much torque? My ranger does this in every gear, Cant tell you where, dont have a tach

 
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Sounds like the car is misfiring, any check engine light? Check your spark plugs
For buddy with the ranger; try cleaning your MAF

 
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Ranger here
I am not sure anything is wrong, its only in high load low rpm scenarios, and the power to weight is pretty low so its easy to overpower the engine for me. Ford tolerances are also not fantastic, the car runs fine but I have knock on demand applying too much throttle in 3rd and above uphill. What I mean by this is it could be nothing if he's just bogging down the engine. If it does seem like a misfire, colin has good advice. Depending where you live, I might also check the ecu to see if it was chipped, especially as an Si. Very common here.

 
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Your cats may be starting to clog up, near the end my old truck starting acting similarly when I was pulling a trailer in a hilly area

 
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ok, thanks, I will keep that in mind

 
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Ok, getting back to this now...

Changed the sparkplugs and air filter. Still the same.

​​​​​​More info:
It doesn't always happen. When it does happen, it starts at between 2500-3000 rpm and if I keep slowly releasing the throttle then the jerking keeps happening on the way down to 2000 rpm. Haven't gone lower since that is not really normal usage and that WOULD be bogging it down.

If at any time I press the throttle or release it and go back to the same level of throttle, then the problem seems to go away(!?) until I am again releasing the throttle so it dips from above to below 3000 rpm.

It feels like misfiring, gets a bit more pronounced the lower rpms there are, but it is not bogging down. Why? If for example I cross down below 3000 rpm in 4th gear, then I can feel the problem all the way down to 2000rpm, but if I then switch to 5th gear(!) then there is no problem at all and it will accelerate fine. There are also no problems during acceleration. Ever.

Any ideas? Will try to switch from 95 to 98 octane fuel and perhaps try a fuel additive next. Unsure on how to proceed with troubleshooting this. Is there a way to rule out something?

 
Rep Power: 133

Clogged EGR valve and/or EGR ports with carbon?

 
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Others have mentioned this as a possibility but it seems unlikely since it runs buttersmooth, idles almost silently and otherwise reacts fine. Wouldn't egr problems show other symptoms?

 
Rep Power: 222

Not necessarily, be worth checking/cleaning. Not sure for the 1.4L but the 1.7L the passages that get clogged up are in the intake manifold