300's not running correctly... whilst 90% of the time it drives lovely, if left to idle sometimes (quite randomly) the revs can blip then drop down and try to stall, tiny rev and it stabilises again.

 

Sometims under boost (say 1500-2000rpm) cruising in a higher gear you will feel the occasional mis.

 

When going flat out it behaves perfectly under full boost, and will rev-out fine in 1st/2nd, but in 3rd once it gets to 5500 or so it hits a brick wall.

 

So it's not a boost leak, I think a coilpack or PTU is a possibility, and failing that an injector/connection. Given it's very intermittent how do I troubleshoot to find out or is it a case of suck-it-and-see with replacement units?

Get it on a ConZult and look specifically at both the AFM voltage, and the injector dwell, timing advance when revving with a strobe and or any timing wobble.

 

Then a drive out with the ConZult on save session mode and go reproduce the issue, do it a few times and then when you back you can play it back and see what happen`s to the various live signals at the exact point of the miss / hold back.

 

From experience this fault can manifest itself due to maf fail at a given linear point, and the cas unit can do the same thing, failing to advance or produce a stable ignition signal, the problem with these type of faults is as the units are still producing a signal, however bad or incorrect it is the ecu in its limited diagnostic process cannot log a fault code as it does not see it as a fault, more modern OBD2 with canbus will see these signals as "implausible" which basically means its seeing a signal but it is outside of the set parameters, as much as I dislike many parts of canbus and OBD2 it does have good points.

 

What you really need is a very good ConZult guy, not many if any diyer`s can read ConZult as it needs constant use to understand not only what its is telling you but conversely what it is not and know how to judge that information, trust me on this every working day using this system for +15 years in the workshop , throwing parts at it could be a lucky first find, my advise get it to Jimmer.

I'm game to give it a shot Jeff! I'm very good with IT in general, do programming and set parameter programs up for robots so I reckon with a bit of practice and time it should make sense.. Plus more exciting learning something new.

 

What worries me about 'throwing parts' is that, yes theres a likely set of possibilities (ignition components, fuelling components), but with so many interdependant sensors like you've mentioned there's always a chance it could be something else.. I went through this with a Silvia ZX years ago, replaced virtually every component on it an still had an intermittent missfire.

I'm game to give it a shot Jeff! I'm very good with IT in general, do programming and set parameter programs up for robots so I reckon with a bit of practice and time it should make sense.. Plus more exciting learning something new.

 

What worries me about 'throwing parts' is that, yes theres a likely set of possibilities (ignition components, fuelling components), but with so many interdependant sensors like you've mentioned there's always a chance it could be something else.. I went through this with a Silvia ZX years ago, replaced virtually every component on it an still had an intermittent missfire.

 

If anything my advise would be to keep it simple, it is very easy and a common mistake to go too technical too early in the diagnosis stage, despite it`s appearance the zed engine is not as complicated as many consider.

 

Jeff

sound advice :-) I'll do basic checks, compression, timing, plugs soon as I get a moment spare just to see it's not something really fundamental. I'm going to get rid of the early 90's fuzzylogic boost controller too as that screws with the ECU signals and given there's no acctual increase over stock boost it seems a bit pointless. Would you say the most likely candidates (purely hazarding a guess) would be coilpacks/injectors/ptu/CAS or MAF in that order? The only thing that makes me think coils is the occasional dip miss at idle.. I had exactly the same thing with a micra and skyline, and the missing at high-load.. both turned out to be coilpacks.

sound advice :-) I'll do basic checks, compression, timing, plugs soon as I get a moment spare just to see it's not something really fundamental. I'm going to get rid of the early 90's fuzzylogic boost controller too as that screws with the ECU signals and given there's no acctual increase over stock boost it seems a bit pointless. Would you say the most likely candidates (purely hazarding a guess) would be coilpacks/injectors/ptu/CAS or MAF in that order? The only thing that makes me think coils is the occasional dip miss at idle.. I had exactly the same thing with a micra and skyline, and the missing at high-load.. both turned out to be coilpacks.

 

It is very possible to have a coil pack breaking down, what I have seen is the long leg rubber bit that reaches down to the spark plug have insulation breakdown, sometimes from damage caused when removed, this can effectively give the feel of a misfire.

 

The basic checks first as you mention is the way to go and from there move on up through more tech area`s.

 

Jeff

It is very possible to have a coil pack breaking down, what I have seen is the long leg rubber bit that reaches down to the spark plug have insulation breakdown, sometimes from damage caused when removed, this can effectively give the feel of a misfire.

 

The basic checks first as you mention is the way to go and from there move on up through more tech area`s.

 

Jeff

 

 

Okay so did a few things today as had some time off - Checked there was no missfire at idle by removing each of the coil connectors in turn - they all made it run significantly worse when removed so that's fine.

 

Checked resistance on the injectors

 

Checked for vacuum leaks by squirting easy-start all over the shop lol. Possibly a shonky method but it's never failed me in the past.

 

Checked timing by sticking inductor down back of coilpack and left the guns 'manual advance' setting to 0 - now this is at the moment an issue because I can't see the markings, I can see a 10, and to the LEFT of it either a 0 or 20, it's all rusted so can't make it out. Either way it's just under 2 teeth to the LEFT of the 10. Given it's supposed to be 15 I'm guessing this is out.

 

Monitored the idle. 90% of the time it sits around 900, there are times where the idle drops momentarily then raises back up, or has 20 secs running a little higher. Eventually it settles.. also something I've noticed in the past is if you very gently touch the throttle cable the revs can drop before raising, again it's wildly intermittent.

 

Something else i find unusual as I've run alu rad's in cars before... the radiator never fills higher than say 4/5 way up. There's no leak from the radiator I can see (not to say there isn't one) and if I fill more it just bubbles back down to that - it doesn't lose coolant past this point, however to me this seems like a way to get air bubbles in the coolant system..

A further red herring was after running beautifully for 15 mins, I touched the TPS with my finger (both screws are tightly secured) and the revs dropped then came back up, then I wiggled the wires gently 10 secs later and again the revs got very erratic.. unfortunately I couldn't reproduce this so perhaps it was suspiciously coincidental..