P38A Engine twitch on idle 4.6 yr 2000

Lots of work done over the last year and now the beloved has a "twitchy engine" on idle. Fuel consumption increased over time and about the same time the idle became a tad rough, nothing audible, just a twitch but enough to feel. Replaced coil packs, plugs, fitted Magnecor leads...compressions all good...all on an engine with 26k on it.

Diagnostics shows Air Flow rate at idle as 6.38 grammes per sec (which I believe is 22.9 kg per hour)
at 2500 rpm as 18.25 grammes per sec (65.7 kg per hour)

I believe the rate should be 17.2kg at idle and 54.72kg at 2500 rpm, with an allowed variance of plus/minus 3kg.

Outside air temp was 12c but Intake Air Temp was showing 25c when the ignition was just switched on (was this a default or failure) and on idle when fully warm (720 rpm) changed to 30c. O2 sensors appear within limits. It does appear to run fast when first started up...1200-1300 rpm.

My questions would be;
1. Is this far enough out to cause the engine twitch?
2. Are the intake temps a fault.

Thanks in anticipation of some clever people pointing me in the right direction.

Check the O2 sensors the live reading (Inputs, Fuelling) and it will show what it is reading. Normally when they die they give 0V which the ECU reads as lean so it cranks the fuelling up to make it richer which will give high emissions and fuel consumption..

It'll run rough too

My vote is o2 sensors also. They can get lazy and sluggish before they die. A drop in fuel consumption is a classic sign

Henry_b said:

Check the O2 sensors the live reading (Inputs, Fuelling) and it will show what it is reading. Normally when they die they give 0V which the ECU reads as lean so it cranks the fuelling up to make it richer which will give high emissions and fuel consumption..

It'll run rough too

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And the IAT on the Thor question?

From further t'internet reading is it correct that the IAT is a spurious reading as the MAF replaced the original set up?

Engine coolant sensor readings also.
These can bugger about with every other sensor reading that should correspond at any given temperature.

Just had a short run and warmed it up...engine coolant readings appear to be rising and falling normally.

The voltage on the O2 sensors doesn't look right, but I am not au fait with this end of the car so please bear with me. (I am only using an Autel MOT Pro, not a Nanocom or similar)

It shows 4 sensors (Bank 1 Sensor 1 Bank 1 Sensor 2 etc) but I only have 2 so I take it the fixed reading of 0.445v on the second sensors are defaults. Sensor 1's on both banks are showing fluctuating figures whether idling or revving to 2500rpm anything from 0.0v-0.780v. No static figures, constantly changing on a fixed rpm, so impossible to give a reading at idle or 2500rpm. Surely these should change with revs?

Coolant temp shouldn't move much once it gets up to temperature. The dial on the dash should stay at 12 o'clock.
It's normal function for the o2 output to 'swing' between 0 and 1. If it had a tendency to stick near any one value for too long then you have a problem

My poor explanation...the in car needle does just run up to 12 o'clock...the readings on the gizmo machine showed a steady rise as driving...no apparent coolant problems. It's a Turner engine with 26k on it so hopefully those problems are in the past and fingers crossed.

Good news on the O2 readings but I was hoping it was the problem as they are on sale at the moment.

The o2 readings should change quickly and react almost instantly when you rev the engine. If you rev it and it takes a second or so to react then it's old.

the first lambda sensors the engines emissions and the second the cats performance.
they fluctuate to show that extra oxygen is released into the cats to help the burn cycle.
if a lambda live data reading shows a flat line, it could be fuel trim issues or a duff lambda sensor.
you can check if a lambda sensor is dead by introducing a separate fuel into the air intake like butane (slowly). the lambdas will react by lifting in performance and the fuel trims will drop to fight the over rich mixture.
they do as said also react to the throttle. typically show a rise and fall.
take a mooch on the tube of you for fuel trims and lambda sensors.
interesting watching. the coolant sensors can fault and tell the engine its cooler or hotter than actual which then confuses the lambdas as they read other info that doesnt correspond to the normal parameters set in the pre programming.
i may be out on details, but these can all be checked before chucking money as a lot of garages can do and some owners.

have a listen to the fuel tank with the cap off for an irregular pump noise.

is the fuel pressure regulator a vacuum or electonic unit on these?

Thor uses 0-1V Zirconia sensors with 0V, or pretty close to 0, being lean and 1V being rich, they should fluctuate and not read a solid reading..

You should get a reading from the pre and post Lamda sensors..

I'm assuming yours is a Thor..

Mark Piercy said:

is the fuel pressure regulator a vacuum or electonic unit on these?

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Electronic being a non return on the thor it is in the pump assembly..
The Gems has one on the rail.

Henry_b said:

Electronic being a non return on the thor it is in the pump assembly..
The Gems has one on the rail.

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Thanks enners .

Henry_b said:

Thor uses 0-1V Zirconia sensors with 0V, or pretty close to 0, being lean and 1V being rich, they should fluctuate and not read a solid reading..

You should get a reading from the pre and post Lamda sensors..

I'm assuming yours is a Thor..

Click to expand...

Yes Thor ....but vehicle only has two sensors...one on each exhaust, both before the cats, the fluctuation is anything between 0 and 0.780 but is rapidly fluctuating with no relevance to the rpm ...ie; at idle it doesn't roam around a nice low figure it shows every number possible from 0 .0 up to 0.78. Revving the engine does nothing to the figures, they still show low and high numbers. I think these may be the original sensors so I think I will try some new ones and then work from there...I will report back.

Mine is also Thor and also only has pre cat sensors.
Sounds like your sensors are working perfectly

kermit_rr said:

Mine is also Thor and also only has pre cat sensors.
Sounds like your sensors are working perfectly

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That's not helped

I will have a chat with the nice man at the garage before I jump in then...many thanks for all the advice.

Revititllclear said:

That's not helped

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Always glad to be of service