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Author: Subject: MX5 hesitation, Boost Sensor hose

posted on 28/11/19 at 07:45 PM Reply With Quote
MX5 hesitation, Boost Sensor hose

My MX5 MK 2.5 1.8 VVT is hesitating and sometimes missing a cylinder mostly when cold.
the mileage is low (<50k)
I have done a compression test (all ok)
I have changed the plugs, leads and coil packs (standard advice for this type of issue)
But still the problem persists.
Any ideas?

I had a good look around under the bonnet and found a sensor called a "Boost Sensor", it seems to be a pressure sensor.
The car is not turbocharged
The hose to this sensor has been cut off.
Does anyone know exactly where it should connect?
I suspect that this may have something to do with my hesitation.


[Edited on 28/11/19 by JonBowden]

[Edited on 28/11/19 by JonBowden]


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posted on 29/11/19 at 09:29 AM Reply With Quote
Pretty sure you are talking about the atmospheric pressure sensor - I seem to remember it is labelled boost sensor, it does not have a hose as it is just sensing atmospheric pressure!

When I stripped the wiring loom for my conversion I discovered that it has an OBDII port - think it was the first one. if you can get live data out with a scan tool that might give you a clue as with an intermittent problem you are a bit in the dark.
Any one of the inputs to the ECU could have a bad connection or be putting out defective data - Cam,Crank, Temp, MAF, engine speed... or it could be a faulty injector, an air leak, stuck VVT actuator etc.

For a wild guess, as it is when cold and looking at the time of your post, do you have the right spec, clean, oil in it ? The VVT uses oil pressure for actuation, thick or very old might give you issues.

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posted on 29/11/19 at 10:34 PM Reply With Quote
As its mainly when the engine is Cold, Coolant temp sensor or its wiring/connector?
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posted on 30/11/19 at 01:19 PM Reply With Quote
It's definitely got fresh correct oil in it.
The boost pressure sensor has a hose attached but it seems to have been cut.
The manual diagrams indicate that it should go to the inlet manifold somewhere - but give no indication of where.
I could do with seeing a similar car.
I have seen it suggested that the pipe may be cut as a temporary fix for something being gummed up from the EGR.
I have a cheap diagnostic unit (EOBD MS309 OBD2 OBDII Car Fault Code Reader from eBay), it indicates no faults


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posted on 2/12/19 at 07:42 PM Reply With Quote
Are you sure it's been cut? According to this forum post the hose connects to a plastic Y junction and it's very easy to accidentally disconnect it whilst changing the oil filter.

[Edited on 2/12/19 by MikeRJ]

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