Yaris d4d 04. Sudden power loss and erratic idle

September 8, 2016

Yaris 04, erratic idle / rough idle / variable idle / hunting.
Also occasional sudden loss of power when driving, not sure if these are connected.

Idling issue usually happens from when car is first started. The inlet air (throttle?) valve actuator lever, from vacuum actuated diaphram, can be seen moving as revs change.
If the vacuum pipe is either pinched shut or taken off, then the engine idles correctly. When vacuum reconnected, idle usually stays OK, but after revving the problem comes back. Shown here: VIDEO

After power loss, no acceleration and only 5mph on the flat. So pull over, switch off and wait a minute, then restarts normally.

I've found quite a few posts with bits of information, so have included links to them.

The power loss had been gradually getting more frequent, over last two or three years. Sometimes happening up to 3 or 4 times an hour, but usually no problem on short journeys. First thought it was a fuel supply issue, as new fuel filter would "fix" it for a while, but becomming more frequent
and since last change it only lasted 2 months.
Then checked fuel pick up filter in tank. This coubik nylon 'teabag' filter was black, and when assembly was lifted out from top of tank, the residual fuel in pick up pipe only drained out slowly. See d4d cutting out
Removed filter bag and cleaned out with powerwash then dried with compressed air. Now can see light through the filter. - No improvement.
Air getting into fuel line was suggested as possible cause, but in that case I would have expected restarting to take a while while any air was purged.

Various forums suggested problems with carbon build up in EGR valve or the air intake (throttle?) valve, with the diaphram that operates either of these having an air leak, or with either of
the two VSVs (vacuum switching/solenoid valve or vacuum control solenoid).
Both diaphrams checked out OK. Diaphram test
The VSVs both held vacuum, but passed it when 12V applied so looked OK (though not a conclusive test as in use, a variable voltage is used, not just on/off). VSV test

EGR had fair carbon build up, but 80% still clear, and the valve could turn freely. Cleaned - no change. See: EGR Cleaning
Made and fitted an EGR blanking plate Making a blanking plate (as suggested to another owner by a Toyata mechanic) - still no change.

Then checked the air intake valve assembly. Eratic Idle - Clean air valve I found the smaller air channel was completely blocked. There are two airways from this casting into the engine, make sure both are clear. In my
case the main air channel (about 45mm diameter) had 5mm of carbon build up, only when scraping this out did the second chanel (about 12mm) become visible.
Question: What's the second small air channel for? I guess the largeer channel is air to the cylinders.
This must have helped?? - but still an erratic idle.

So then swapped over the two VSVs (as EGR blanking plate was still fitted, the EGR VSV isn't doing much).
Started up, but engine limiting at 3000rpm. Switched off, then wouldn't restart, wait a minute then restarts but revs still limited --much like the power loss problem??
Reseated VSV connectors, and pushed vacuum pipes further on. Retry, now OK, full revs and NO erratic idle!

Now managed a 100 mile round trip, 1hr20 each way with no problem.

If problem stays gone I'll try to get a replacement VSV, Denso part no. 25819-33010, New is around £300, so will get used. Sometimes on eBay around £25-40, but nearby breakers will charge around £10+. Only downside is the age of the replacement.
This post says 25819-33030 was successfully used instead. I think that was used on the later Yaris, so might be available newer.

Having seen the amount of deposits built up, which will probably also be found around the turbo vanes, the occasional use of a fuel system cleaner seems like a good idea. Some claim to also clean post combustion chamber. Turbo Cleaners I'll maybe start using Archoil AR6400-D at every regular service, though the PDF suggest every 5,000 mile.