Light throttle missfire.

[The Guinnster]

Hello everyone. My 1984 350i has a fairly bad missfire / judder at constant and light throttle that seems to lessen under heavier acceleration. I was mooching around under the bonnet checking for obvious things, the engine was running and at normal temperature, I have to blip the throttle or it cuts out after a while. My question is, and feel free to mock me here for being daft, but if I disconnect the engine temp sensor with the engine running and at normal temperature, should anything happen? I replaced the sensor recently but it hasn't cured the problem.
Thanks

[pwd95]

Could be timing or the throttle pot. Had the same symptoms in the past with both.

[L33]

Appears to be very common fault and many things can cause it - for me it was timing, for others, throttle pot, potentiometer, temp. sensor and ECU to name but a few.

Try the search tool as loads of threads on this subject over the past few years.

Edited by L33 on Friday 12th September 09:09

[The Guinnster]

Thanks folks. I'll be checking these things. I was just a bit surprised that when I disconnected the temp sensor. there was no difference in the running of the engine. I'm not sure what I expected to happen mind!!!

[jon haines]

i have had the same problem on my 350i and it turned out to be throttle pontentiometer, got a second hand one and problem sorted.

[honestjohntoo]

Assuming a Flapper system, it seems your system is running too rich which explains the rough running symptoms, why it tries to die and why it seems better when accelerating.

The Guinnster said:

I was just a bit surprised that when I disconnected the temp sensor there was no difference in the running of the engine. I'm not sure what i expected to happen

What this says is that the temp sensor may well be faulty or the wiring there-to. When working correctly with the engine at say 90-100 deg C the resistance measurement should be about 150-200 ohms.

With the engine cold it should be in the range 2000-3000 ohms at this time of year in the UK (say 20 deg C). With your multimeter, try measuring the resistance in comparison with the chart at this link:

http://www.vintagemodelairplane.com/pages/Rover_Te...

Regarding other possibilities a faulty throttle pot can cause similar symptoms but for the temp sensor to make no difference when disconnected makes it the initial suspect.

There is a complete description of its purpose, function, expected fault conditions and testing process at the mentioned link.

From the accompanying index a comprehensive likewise description of every other major component in the system. Useful bedtime reading.

[The Guinnster]

Thanks again for the help. Another observation which may give a pointer is that if i take my foot of the throttle and allow the car to slow only by engine braking, the missfire is still present , though not as severe. Would this suggest a fuel problem or an electrical issue?

[matt-man]

I had a very similar problem to this and after replacing and testing all sorts of componants it turned out to be the leads which although not that old were not doing their job corrrectly and therefore creating an intermitent missfire - on mine it was mainly between 3-4.5k....

Easy try!!

Cheers!

[honestjohntoo]

honestjohntoo said:

What this says is that the temp sensor may well be faulty or the wiring there-to. When working correctly with the engine at say 90-100 deg C the resistance measurement should be about 150-200 ohms.

With the engine cold it should be in the range 2000-3000 ohms at this time of year in the UK (say 20 deg C). With your multimeter, try measuring the resistance in comparison with the chart at this link:

http://www.vintagemodelairplane.com/pages/Rover_Te...

So, have you rejected this suggestion? Isn't it somewhat unlikely when the ECU would normally see 150 - 200 ohms from the temp sensor when hot for it to make nil difference when the sensor is removed. Something does not add up.

[The Guinnster]

Thanks again. No I haven't rejected the suggestion, it was just an extra bit of info that I should have put on my first post. I will be following up on your advice and checking the readings of the temperature sensor with the multimeter.

[GRIFFONMARK]

I'd check for any air leaks as these can badly affect tickover without much effect at higher speeds. Regards, Mark