1999 Cavalier Z24 Intermittent engine knock under Hard Acceleration

[Kugellager]

I have a 1999 Z24 and have been experiencing intermittent engine knock under harder acceleration between 2500 and 4000 rpm in 1st, 2nd and occasionally 3rd gears (manual trans)

I have noticed this in tha last year. I suspect a bad air flow sensor in the intake or an defective ecu.

Only aftermarket part is a cold air intake. I first noticed the problem a month or so after I cleaned and oiled the filter on the intake. Eventually I checked the sensor in the intake and noticed it caked in oil. I cleaned the sensor and the problem went away after 2-3 weeks. I no longer oil the filter.

The problem now re-appears every so often when I start the car after it has sat in a parking lot when going to the store or at work...but not before the car has completely cooled down. The knocking will occasionally appear after I start it again. It can get it to go away if I slowly accelerate through the gears to about 4500 rpm several times.

Could I have damaged the air flow sensor in the intake so that its function is intermittent...and then the ECU has to re-learn the correct settings or am I experiencing som other more insidious problem?

Other Notes: Plugs have less than 10k miles on them
Oil and other fluids are well maintained and changed regularly.
60k miles on engine.

John

[rallyaddict]

First thing. What plugs did you use? Bosch platinum?? Make sure you only use AC delco platinums. That engine will not run properly on anything else.

Also, if you're referring to the sensor in the s tube after the filter (on the stock intake), it's the intake air temp sensor. You can check it with a multimeter by checking the resistance at different air temps. There's a chart somewhere. I doubt that's your problem though.

What makes you think it's the ecu?

[Kugellager]

Hmmm...the replacement plugs my in fact have been bosch platinum...I'll have to dig them out tomorrow and see.

The reason I suggested ecu problems is because of the way the problem seems to be temperature affected...I just assumed since it seems to start with and engine temp change...of course if the temp sensor IS bad that I suppose that could lead to several different air/fuel mixture ratio problems as well as air density measurement problems...which could very easily lead to engine knock under load...

I will have to find the old plugs and test the temp sensor.

Thanks for some leads...I'll you you know what I find after testing.

John