1.8 Misfire... any ideas?

[JC]

Is there anyone out there got any sensible suggestions as to why my 1.8 mems 3 engine, may be misfiring .... after slowing down on the approach to a junction you then gently start to accelerate again (1500 - 2000 rpm) it misfires until you reach about 2500 rpm. From a standing start it is barely noticeable. It has 100k on the clock but has the misfire for the last 20k at least. It has had regular, services, I have cleaned and polished the throttle body, replaced the coil packs and leads, looked at the crankshaft sensor,  No DTC codes or probs are logged on the ECU, the head gasket has been done and it's not using any water. Help please !!!

[carbore]

Hello,

I have some ideas based on my experiance of the K Series in my Elise, there are only a few differences (routings etc). First 2 ideas are definites as they have happened to me, laters are more general.

MAP Sensor
This is the manifold air pressure sensor, its actually part of the ECU in the MEMS.  There is a tube that goes form the inlet manifold to teh ECU via a moisture trap.  This tubing may either be blocked or leaking.  Inspect the tube, remove it, blow down it to see if its clear, block the end and blow to see if it leaks.  The moisture trap (on the elise anyway) is a matchbox sized gadget somewhere in the tubing, this could be full of trapped moistre, blow,tap,shake etc to remove it.

Expansion tank cap
This may be leaking pressure which confuses the engine.  It wont necissarily mean your coolant level drops.  If you can hear a hiss/gurle or smell coolant under the engine it may be leaking. On my car the entire tank was past its best.  You may not be able to tell if its leaking so I recomend a replacement cap for around £10

Induction/Air Filter.
Check its not pulling in air after the filter or that there are no leaks around the manifold area.

Bad Earth.
Not especially easy to trace, but some Elises have been poorly due to bad earths.  Ckeck all engine earth points are clean and working.

Injectors.
Check the are "clean" (not that difficult)

Spark Plugs
You have not specifically mentioned that they have been changes, but its worth a try.  Also check that its got exactly the right plugs in as cheaper ones may not be as reliable.

Distributor/Rotor Arm
Again not specifically mentions and rather unlikley, but worth ruling out as its not expensive.

Crank Position Sensor.
You save you have "looked at it" and also that there were not errors on DTC checks. It could still be a suspect, if its intermittent. As its not a hugley expensive bit it may be worth replacing, again to rule it out.

Fule Filter/Line
Possibly the fuel filter is on the way out, but not the first place id look.  Only cheap to relace so worth it when everything else is exhausted.  I had a colapsing fuel line on my elise where the petrol had dissolved the ruber in the line!

[JC]

Thanks for the pointers .. I might get time this weekend to have a look... I'll keep you posted, thanks again.

[Boggert]

have you replaced the throttle set up?
My mate had a freelander and was having a real problem with what seemed like misfiring at low revs after much digging, the garage replaced this all was fine.

[JC]

No, I've not changed the throttle body....  I did strip, clean and polish the black off ..... it didn't make any difference at all.  
As yet I've not had much time to do any more digging into it.
Its new tyres this weekend so I might have to go out to test them.... lol
Thanks for the help ... will continue the saga !!!

[carbore]

More precicley its the Throttle Position Sensor (TPS) or Idle Air Controll Valve.

Both of these chaps can go awary and could casue ths symptons you describe.

The TPS senses how open/closed the throttle is so could loose calibration, although this supposed to auto reset on ignition (I think). Sometimeit gets dirty, but you may have cleaned this when you did the throttle removal.

The IAC adjusts idle and then obviously backedsout when you drive, this could be sticking.

[carbore]

Sorry for not thinking of this before!

More precicley its the Throttle Position Sensor (TPS) or Idle Air Controll Valve.

Both of these chaps can go awary and could casue ths symptons you describe.

The TPS senses how open/closed the throttle is so could loose calibration, although this supposed to auto reset on ignition (I think). Sometimeit gets dirty, but you may have cleaned this when you did the throttle removal.

The IAC adjusts idle and then obviously backedsout when you drive, this could be sticking.

[JC]

Nice one i'll check it out! I can look at the "Throttle Position" via the OBD port, do you know how many degrees it should be on tickover ??.... from memory it reads 3.9...
Cheers for that.

[carbore]

Hi,

I dont know the number for the TPS or IAC, I would suspect though, that if for example its reporting 0 it may actually be a bit open/closed and not reporting the correct value.  Am I making any sense?

I think I am trying to say replace with a known good one.  This is something we did on a mates elise, he borrowed my TPS to rule his out, despite having an Emerald ECU whaich says what the TPS reports (if I remember correctly).