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Old 17th January 2021, 14:00   #1
Yorkshire GOC
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Default Slight misfires but not triggering a Code

In late December i did a 125 mile trip - on a couple of occasions i felt a couple of slight misfires and a couple of mini surges. As i got home the flashing EML triggered and the fault code 0300 (multiple random misfire) and 0141 (Post cat O2 heater circuit malfunction. So swopped out the O2 sensor.

However am still getting misfires but no surges , more frequently, but still not throwing a code up .

The idle appears normal and no hunting and the misfires seem to only happen under load.

Using my OBD and my rudimentary knowledge of data stream readings - the Short and Long trim seems fine, the 02 sensors OK, and the MAP and IAT seem OK .

So thoughts and suggestions welcome as to most likely cause.
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Old 17th January 2021, 14:23   #2
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Do the simple things first Martin, plug gaps, leads and coil packs 👍
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Old 17th January 2021, 15:00   #3
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Will do Andy - however just popped to the shop (2 minutes) and parked up and ran the OBD data stream again ( well twice as i have 2 code readers for comparison purposes) and the IAT was showing 39 C - then with 2nd reader a minute or 2 later 42 C - its a cold day and the ambient temp is 7 C - so that does not sound right does it ?

And looking at the symptoms of a failing IAT sensor misfiring features.

Will investigate further tomorrow.
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Old 17th January 2021, 17:25   #4
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I understand the IAT only features on the 1.8 non turbo engine - so where am i getting these temperature readings -the MAF sensor i assume ?
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Last edited by Yorkshire GOC; 17th January 2021 at 17:37..
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Old 17th January 2021, 22:49   #5
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Had the same thing, misfire under load. It was diagnosed as a the coil on number 4. Replaced and ok. I eventually replaced all six. Sweet as a bird now. They were 100,000 miles over..
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Old 17th January 2021, 23:39   #6
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Quote:
Originally Posted by Yorkshire GOC View Post
I understand the IAT only features on the 1.8 non turbo engine - so where am i getting these temperature readings -the MAF sensor i assume ?

The Manifold Pressure (MAP) and Temperature sensors are combined into one on the 75 (not sure why Rimmers show the earlier separate temp sensor) - it's the one on the left hand end of the manifold.

The temps you've quoted sound perfectly plausible, especially if you have the VVC manifold fitted - it gets a lot of heatsoak from the engine.
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Old 18th January 2021, 00:38   #7
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Quote:
Originally Posted by mbonwick View Post
The Manifold Pressure (MAP) and Temperature sensors are combined into one on the 75 (not sure why Rimmers show the earlier separate temp sensor) - it's the one on the left hand end of the manifold.

The temps you've quoted sound perfectly plausible, especially if you have the VVC manifold fitted - it gets a lot of heatsoak from the engine.
Thanks for this info - i do have a metal manifold fitted but surely 800 yards starting as a cold engine and 2 minutes driving wont generate this degree of heat soak -will have a spin tomorrow with my code reader attached to see what the temp gets up to.
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