T4 Diagnostics

[andy8688]

No problem, Mykel.

[Rob Bell]

Either that rattle is a corrodes heat shield, or the core has disintegrated! The latter would certainly fail an MoT

[andy8688]

I think you are spot on Rob, I am trying to source a new cat at the moment, mine is the one with the sensor on it, so a little more expensive.

[Mykel]

So here we go re the fault readings:

Quite a few, and many of them relating to a “circuit malfunction”, which imho is a serious hint to a knackered wiring loom.

All of these cleared and did not reappear (except for the P0122 TPS Signal Too Low and P0340 Camshaft Sensor Malfunction). The TPS did not give any angle info and voltage is 0V. All this leads me to the idea to get a replacement engine harness.

What do the experts think?

[andy8688]

Thanks for the info Mykel.

[andy8688]

I have been looking for a wiring harness for the engine bay, are these difficult to replace.

[Rob Bell]

Jusdging from the fault codes (a truly impressive number of them!), this is a MY2000 (post 2001) MGF or TF. The majority of these cars share the same loom, but be aware (as I have found out) that MY2005 cars use a slightly different loom!

Changing the loom is not too challenging - other than the fun of relatively limited access in some areas and the joy of being covered in old, dirty oil! The loom runs are generally pretty self evident from the design, and the vast majority of plugs can only be fitted to the intended sensor or connector block!

Note that there is a sub-loom (injector and coil driver) and it would make some sense to change this with a known good one at the same time.

Good luck!

[Mykel]

What I did was to start from the ECU end of the loom, taking the old one off and step by step replacing it with the new one, first towards the starter motor and coolant sensors/reverse switch, then over to the rest. When working like this, no connector is forgotten accidentally. It does help to remove the charcoal canister for access, the rest is quite okay apart from having to remove the o/s/r wheel for access to the part running below the oil pump to the alternator and oil temp sensor/pressure switch. This part is usually particularly sticky with oil and dirt.

On a MEMS3 car, there is less hassle with the loom as there are no nasty coil packs hiding below the inlet manifold. All in all, it can be done in 2 to 3 hours, I’d say. Remember to disconnect the battery, as there is a live wire attached to the starter motor which has to come off.

The only difference between the various MEMS3 MPi looms is that some came with the extra connector for the green inlet air temperature sensor while the later ones didn’t, as then the combined MAP/AIT sensor was used for both functions. Have a look if there’s a connector on the AIT (in the inlet manifold on cylinder 4); sometimes the sensor had been left there but is not connected.

From the readings we got, I’d say the injector loom seems okay, but you never now obviously. I’d try without changing it first, if problems persist (or others come up), you could always have a go.

[Rob Bell]

The only difference between the various MEMS3 MPi looms is that some came with the extra connector for the green inlet air temperature sensor while the later ones didn’t, as then the combined MAP/AIT sensor was used for both functions. Have a look if there’s a connector on the AIT (in the inlet manifold on cylinder 4); sometimes the sensor had been left there but is not connected.

From the readings we got, I’d say the injector loom seems okay, but you never now obviously. I’d try without changing it first, if problems persist (or others come up), you could always have a go.

The main point of raising the loom incompatability was exactly regarding the last point - trying the replacement loom with the existing injector loom The later MY2005 injector loom is incompatible with the earlier version (you know how I found that one out! )

[andy8688]

Thanks for the reply s guys.

[andy8688]

OK guys here is an update on the problem I was having. I came across a local guy who happens to be an auto electrician and I told him about the problems I was experiencing. He decided to come and take a look; asked when the head gasket was last changed (a few years ago); did the usual test with a snap on diagnostics and the same faults came up as Mykels system. He then went on to some multimeter voltage checks; said he thought the TPS and camshaft sensor connectors were connected the wrong way as the connectors are the same. He swapped them over and would you believe it the engine idled at 950rpm, I test drove the car and it was perfect, what a result.

Thank you all for your replies, especially Mykel.

[Mykel]

Ouch, who’d have thunk!

I should have seen that actually. (Note to self: prepare wiring colour sheet for sensor connections to take along).

[andy8688]

Mykel, he diagnosed what you had done, he had a previous problem with someone connecting the TPS and the Camshaft sensor connectors the wrong way.

[andy8688]

Guys, I took the car out today for a good run and it was like driving a new car. So I can honestly say problem solved.

[Rob Bell]

Wow, I didn't see that one coming either - thanks for the enlightening update Andy!