Short version: Can a broken Knock Sensor cause bad idle?



Long Version:

I somehow managed to change spark plugs during the weekend (not much of a mechanic!).

While crawling on the floor under the car, I noticed a black plastic plug between two sparkplugs near the starter, which I assume is the Knock Sensor.

Well. it was broken off and the metal "pin" is exposed to the air. I have been (and still am) experiencing a very erratic idle, especially when the engine is hot. Yesterday, it even died on me. The idle seems to have a cycle. First it will hang around 700 for a few seconds and then drop to 500- them back up to 700 again. The cycle will usally (but not always) repeat.

I have read a lot of postings about this sensor and it was touched upon somewhere that if it is broken- the car could give a bad idle. So before I go unscrewing the IAC and looking for vacuum leaks I just want to keep this off the check list. Is there any truth in this?

Besides the idle, the car runs fine- no loss in performance and no error codes. It always runs on 95 octane fuel (can´t get leaner gas in Sweden even if I wanted to!) The old spark plugs looked very good when I took them out.

The car is a 1987 T/A 305TPi (L98)

Best Regards

/Joe
Sweden

 

This is just my opinion and I could certainly be wrong, but my scanner always shows 0 degrees knock retard at idle. If the sensor was broken or disconnected the retard would also be zero. Therefore it should not cause a rough idle unless you have a trouble code or are in limp home mode. I once had the coolant sensor for the ECM go bad. It did not set a light, but the car would not idle for more than a few seconds. I only caught it because I watched my scanner and saw the coolant temp jumping from 98 degrees F(as the car was warming up) to -38.6! Unless you have a trouble code, I would not suspect the knock sensor.

 

No, I don't see a disconnected/broken knock sensor causing a bad idle. You should definitely fix it, though.

I'd be looking towards a bad oxygen sensor and maybe vacuum leaks or something with the IAC or MAF.

I'm not 100% sure, but I think Sweden has a different method of rating fuel octane, which makes it appear higher than it would be by US standards. A lot of the world uses RON ratings (Research Octane Number), while the US uses the antiknock index(AKI)=(R+M)/2. M is another octane number(Motor).

 

Thanks guys. I´ll be taking a look at the IAC and TPS first. I suppose that I could have a vacuum leak, but the hoses look pretty good... I just hope the injectors aren´t acting up! They´re NOT cheap in Sweden!

I don´t know how we rate octane compared to the States, but our "95" octane is the one has has always been "unleaded". Nowadays, its all unleaded of course but a few years ago 95 was the only lead free gas here.

 

We rate octane diffrent here in sweden our 98 is like 92 in US....

I always drive on 98 octane.......

/N.