Hi - When slowing down through the 60-45 kph range from a higher cruising speed there is an irregular sudden irritating misfire jerkiness. This does not occur under load, nor when accelerating through 45-60. All COP's and plugs have been replaced. This eased the problem a little. The scratch test shows excellent spark on all COPs. Could this hesitation be attributed to faulty TPS (Throttle Position Sensor)? Suggestions most welcomed. Cheers. (Jazz 1.3i-DSi CVT, with 365+k on clock)

 

When decelerating, do you release the accelerator pedal completely? If so, it should cut fuel and the engine should not be firing at all. I think you're right to suspect the TPS. Does it idle OK? If you can scan OBD data, the throttle position should read around 10% with the pedal released and about 90% when floored. You can also check for the correct voltages and resistances using the troubleshooting guide here.

Maybe not relevant but there is a method for calibrating the start clutch which involves slowing from about 60 km/h to about 30 km/h with the accelerator released (and with the lights on). I don't know if that could somehow suggest a CVT issue (the start clutches are a bit notorious for going juddery).

 

I also wonder if a leaky injector could cause this problem, if the PCM still commands spark during deceleration fuel cut-off. How's your fuel economy and fuel trims?

 

Another thought: it could also be throttle cable adjustment. I've been tweaking mine over the last couple of days and if it's tight the PCM might not properly register the accelerator being released, especially when the engine is hot.

 

Many thanks for the suggestions. I'll chase those up. One other possibility: the valve gaps. Is that likely a candidate? And a further observation -- the problem seems to be less pronounced when the engine is cold. Cheers.

 

Valve lash adjustment is worth doing if it hasn't been done in a while (I think the schedule is every 40,000 km or 2 years), and the fact that your issue changes with temperature could maybe suggest a valve issue, but it's odd that you only have a problem when decelerating.

Changes with temperature could also suggest an issue with the EGR system (rather common) or the evap purge valve, as these (should) only operate when warmed up, or throttle cable adjustment as mentioned.

I think taking a look at the OBD data would be a good first step (I use one of the Jaycar Bluetooth OBD dongles with the Torque smartphone app). When you take your foot off the accelerator while driving, you should see it go into open loop fuel control, with the throttle position reading around 10% (mine reads 9.4%). Check the readings for engine coolant temperature, intake air temperature, and manifold absolute pressure for sanity too.

 

I have been conducting some more on-road observations. Thanks for the tip. Here's what happens:
1. when slowing and foot is completely removed from gas pedal there is no jerkiness, just smooth slowing.
2. when accelerating there is no jerkiness.
3. when slowing and there is a tiny amount of gas pressure then there is jerkiness.
4. this jerkiness can be removed with either: a. acceleration, or b. complete removal of gas pedal pressure.
The hypothesis about engine temperature was wrong.
My current hunch is that the TPS has a dead spot.
I ordered a TPS but it came with no gasket.
The only gasket I can find is a Tegiwa TPS gasket - B/D/H/F series.
Which costs twice as much as a new entire unit and gasket (from China)

Is T-4077045 or 16402-RAC-A00 or A01 or A02 the correct TPS unit?
From my net hunting it seems there are (almost) generic TPS units.
When I search for Jazz/Fit the result is unclear.

Suggestions and comments welcomed,
Cheers

 

Thanks for the update - helpful and interesting. Does the engine idle OK (750 RPM, not rough)? Does the jerkiness happen when accelerating with the same tiny amount of accelerator pedal pressure as when the deceleration problem occurs? What if you lightly press the accelerator while idling?

My understanding is that the TPS is not readily replaceable (it's factory calibrated for the TB and not easily adjustable), so you'd normally have to replace the entire throttle body assembly, unfortunately. It would be good to confirm that the TPS is faulty before trying to replace.

IME the ignition system can be a bit flaky under very light loads at low speeds, but you've replaced plugs and coils recently so that's less likely (assuming you replaced all eight!). Have you tried injector cleaner? If your engine has a mechanical throttle cable linkage, check/adjust as described here. I think you should also test the EVAP purge valve.