I,m getting an intermittent misfire ( or partial misfire??)

 

I' ve been playing with conzult and reading the service manual , but cant work out how to tell which cylinder the problem is with

 

Conzult has just given me lots more questions to ask, once i work out how to post pictures now photobucket doesnt work

Misfire happens at idle and sometimes when trying to power at low speeds, but seems ok on fast

 

 

 

 

 

 

 

So heres a couple of power balance results, i asume one cylender is too high and one too low? Does this help me?

 

Also a graph showing the balance test on the left and my misfires at idle on the right

 

And a graph showing an acelerator blip followed by it trying to stall, is this normal?

 

Finaly , i seem to remember o2 sensor readings should alternate up and down , mine are static at idle and start to move around 1500 revs, is this right?

Edited August 6, 20178 yr by ianl

I am not an expert with Conzult but from previous experience the bigger the rev drop when carryout a power test means the cylinder is working well, thats why when its cut the revs drop so much, so a very little effect, a small drop in revs means it is not working so well. This can be caused by poor compression sol would do a compression test to eliminate that aspect from the problem.

So time to look at this again tommorow. Any one got any opinions on my results and questions?

>So heres a couple of power balance results, i asume one cylender is too high and one too low? Does this help me?

 

if unsure, do the test several times...but let the engine settle between tests. Best is not to blip the throttle between tests as it may cause unreliable results or the engine may stall.

if the same cylinder comes up lower then the others during multiple tests, then that cylinder probably has some sort of issue.

Either low(er) compression or a faulty injector/sparkplug/coil

 

> And a graph showing an acelerator blip followed by it trying to stall, is this normal?

 

Yes, this is normal , but see previous answer about accelerator blipping during balance test or when adjusting base idle

Stalling may occur since the IACV (idle control valve) is disabled during these tests.

 

> Finaly , i seem to remember o2 sensor readings should alternate up and down , mine are static at idle and start to move around 1500 revs, is this right?

 

Most of the older 300's up to 1993 or so, don't run in closed loop at idle.

Meaning the O2-sensor readings are not used to adjust the air-fuel ratio at up to 2000 rpm or so.

 

Regarding troubleshooting your misfire problem, there are few things to look for.

Check the IACV percentage at idle and fully warmed up engine, ideally it should be at or near 15%..I've seen some cars never get below 17%, not sure why, but this is fine also.

If it is (much) higher, First check for air/vacuum leaks, clean throttle bodies, clean injector connectors and check mechanical ignition timing (should be 15 BTDC at idle)

These are the most common suspects for misfires or a lumpy idle.

 

After doing these checks, see what the iacv percentage is and if it's still too high, you will need to adjust base idle.