Rough Engine Idling/Misfire & Engine Lacking Power

Toyota Alphard
1MZ-FE engine

The complaint was that the engine was shaking, especially when transmission put into reverse.

TECH 1:
Scanned engine for error codes, none found.
No function in scan tool to carry out power balance test.
Bank 2 ignition coils were disconnected one-by-one to isolate possible misfire & there is no change when cylinder 4 disconnected.
It's an awkward engine because the manifold & air ducting has to be removed to access bank 1.
All spark plugs were removed & no carbon deposits due to misfire just wider spark plug gaps (0.1mm over spec).
No improvement after spark plugs replaced.
Bank 1 coils were moved to bank 2. Ignition coil disconnect was carried out & again no change when cylinder 4 disconnected.
KOEO, ignition coil 4 connector checked & all okay. For reference, connector on coil 2 checked & same readings.
Sprayed brakekleen in & around intake manifold & vacuum hoses. All okay

TECH 2:
Had a look at fuel trim data, LTFT bank 2 at 15%, LTFT bank 1 at 0.8%
Removed all injectors for testing & cleaning. All okay
When reinstalling injectors identified broken vacuum hose from EVAP canister to engine.
After replacing hose & cleaning injectors engine idle not as rough & fuel trim improved to 12.5%
Cylinder compression test was also carried out & all cylinders within spec

Owner of car was asked to test drive car & confirmed slight improvement in engine shaking but still lacking power.

TECH 3:
Ran O2 sensor live data. Cycling okay & responding when brakekleen sprayed into intake.
The second day ran O2 sensor live data again & B2S1 stuck at 0.00V. Sprayed brakleen into intake & B1S1 responds a lot but B2S1 barely budges up to 0.07V
Replaced B2S1 O2 sensor & sensor signal is back but the engine is shaking more than it did before O2 sensor was replaced
Bank 2 LTFT still stuck at 12.5%
Carried out individual VVT solenoid activation for both bank 1 & 2 & the engine rpm changes throughout test. The LTFT for bank 1 changes but bank 2 stays the same.

Bank 2 LTFT suggests a lean condition but most if not all possible causes have been looked at & tested.

As far as the O2 sensor for bank 2 is concerned, it is genuine Toyota & wire colors are the same but there are less diffusion holes. There is no error code for the new sensor. It is however for a Harrier & not an Alphard but it is all that was available. What is noticed is that the sensor cycles when engine idling but the interval is narrower than bank 1. What I mean is that B1S1 cycles between 0.04 & 0.847 volts while B2S1 cycles between 0.110 & 0.657 volts. During a snap throttle, both sensors register a reading of 0.9 volts.

Attached are photos of the old & new sensor & the O2 sensor live data at idle:




Any idea what may be causing the LTFT to stay stuck at 12.5% & the engine to misfire/idle rough??

I would do a Relative Compression test. If you don't have a scope, disable fuel and crank the engine. Listen to the engine cadence. It should crank continuously and smoothly. If you hear a repetitive cadence, then you may have a single-cylinder compression issue.

Chad wrote:

I would do a Relative Compression test. If you don't have a scope, disable fuel and crank the engine. Listen to the engine cadence. It should crank continuously and smoothly. If you hear a repetitive cadence, then you may have a single-cylinder compression issue.

Battery was disconnected overnight to reset LTFT. Engine still shakes & after running it LTFT eventually went back to 15%, from 0%.

Unfortunately we don't have the connector needed to obtain voltage readings from the AMP clamp.

Disconnected EFI relay & cranked car over...sounds pretty good.

Carried out another cylinder compression test on bank 2 cylinders. This time bank 1 spark plugs weren't removed due to the amount of work required to remove the manifold, throttle & ducting.

Cylinder 4 has lower compression than 2 & 6. I think 10-15 psi less than the others, if I can remember correctly.

Now going to carry out a cylinder leak down test.

Just an update.

Cylinder leak down test was carried out on cylinder 4 - gauge was playing up but when the oil cap was opened, cold air could be felt.

Was about to condemn the piston rings but it was suggested that bank 2 coils be looked at again.

Cylinder 6 coil was disconnected & no change in engine rpm. Re-installed old cylinder 4 coil into cylinder 6 & engine smoothed out.

Interestingly enough the LTFT dropped from a static 15% to -3%...??

My question is: A LTFT of 15% suggests a lean condition because the ECU is adding fuel. If a coil is not firing OR providing a weak spark then there should be an excess of fuel. The LTFT should then be negative ??

Saverauto wrote:

My question is: A LTFT of 15% suggests a lean condition because the ECU is adding fuel. If a coil is not firing OR providing a weak spark then there should be an excess of fuel. The LTFT should then be negative ??

That's one of the most counterintuitive aspects of fuel trim and misfires. :silly: In the case of either a no fuel OR a no spark misfire, the fuel trims on that bank will go positive.

There's a lot of theory that goes into the explanation. Short version, yes there's unburned fuel going into the exhaust during a no spark misfire. But there's also a lot of unburned air. The oxygen sensor doesn't sense fuel, it senses air, therefore showing lean.

What you said makes sense. I’ll have a look at the video you posted too.