Hi,

I have a slight misfire that's been there for 40 km. I've done the following

1 - Fuel pressure test, passed
2 - Injector balance test - same for all 6 cylinder - 62 psi, pulse drops to 40. on all 6...
3 - All new plugs - gapped right
4 - All new wires - ohms within spec much lower
5 - All new cap - wired correctly following diagram
6- All new rotor - correctly installed
7- fuel filter
8 - evap system fully operational no issues no problem - smoke tested as well - all fine.
9 - CKP retard on modus says +2 I believe? So that is fine?
10 - I tried a Crankshaft Sensor re-learn but the damn modus $5000 tool must be to old, it just wouldn't do anything after revving it 2200-3500. Hell i even bounced it off 5k and nothing.
11 - O2 sensors all under 1 Volt
12 - new battery - load tested perfect, static voltage 12.65
13 - charging system perfect
14 - inside spark plug boot perfect
15 - inside cap perfect


Triple checked all work and appear ok.

Short Term and long term Trim show a slight lean condition, showing +1 or +2 to SFT ( nothing major )

Drive with scan tool on with misfire information showing, all 0's for the most part, an odd 1 or 2 show up but gosh nothing major.

170 000 km on truck

No DTC - No stored codes, no stored codes since last clear.

Any ideas?

1 - Find another machine and try and do CKP relearn?
2 - compression check? Seems excessive
3 - leak down ? Seems excessive
4 - pull distributor and check for what exactly?
5- how the heck do i check spider without taking intake off?

It's been so looooooong since smooth running engine. I'm first year auto tech but teacher is just kinda flip flopping on it. If a customer brought this to me and said idling misfire, i'd be like damn, I just wasted an hour check fuel pressure, driving it to check misfire counters, injector balance test, spark plug wire ohm test, re torque all spark plugs.

 

Misfire because you can feel it or because you caught it on live data?

How many cylinders and how many total misfire counts?

What is STFT plus LTFT on each bank? STFT alone does not tell us the whole story.

George

 

Misfire because visual appearance of engine shows it. You can also feel it when putting your hand on top of drivers door. Feel it in pants while sitting on seat in truck.

All exhaust pipes and hangers are tight.

Ok, the first time I looked at this (2 weeks ago )
( going from memory here ) i should of taken picture

LTFT showed 130 misfires on cylinder 1

34 on cylinder 2

12 on cylinder 3

2-6 on cylinder 4,5,6

So after fully removing spark plug # 1 ( which was a little loose ) - measuring resistance of wire, OK, remove cap and rotor and inspect (ok ) , put back together, clear misfire information, so slate is clean

Go for a drive, 10 mins if that, no misfire driving around, maybe the odd 2 count on cylinder 2 and 3 but again i only feel misfire at idle. every mechanic says these engines should be smooth as glass.

I'll have the truck back in the shop next week and can see what LTFT is after 2-3 weeks of driving.

STFT from what I recall was +2 , so again i don't expect perfection its 170k

You need more precise info then what I'm providing right?

My teacher said spider assembly or fuel pump is his guess, i ruled out fuel pump but they often go at 170km it seems. so i dunno it checks out testing wise. How the heck to test spider assembly, I mean pulse width test should test it? isn't one injector unit that fires fuel down the spider, so really you aren't testing 6 individual injectors , just one that fires down the spider lines?

 

STFT by itself doesnt tell us much. STFT + LTFT tells how much fuel is being added or subtracted for a lean or rich condition. The job of LTFT is to keep STFT as close to zero as possible so that the ECM always has the maximum compensation control in either direction

George

 

Also if you have access to training equipment why don't you get a scope on both the primary and secondary waveforms and do snap throttle tests. Many of the potential problems can be found this way such as a lean misfire shown in the secondary waveform.

George

 

thanks for your help. i don't believe that type of scope training is part of curriculum, maybe in theory but i don't recall much of it. Besides i'm thinking that not that I've thrown every part at this type of issue, not intentionally as tune up had to occur but because i but really there has to be what, 3 parts left that could be? darn teacher is so frig'n selfish and should be fired but damn union is protecting him as he's only 5 years away from retirement so he doesn't give a ****. nothing i can do but try and get the most i can out of it as the course winds down.

I can run the modis scanner and get a screen shot of the stft and ltfp and upload in a weeks time. aside of that one other member on her said the temperature ECT wires were broken causing misfires, is that sensor on the drivers side head?

 

Had same issues on my 2000 Jimmy. Mine was my injector spider.

 

Damn... and no test for that i assume lol of course throw a dart. lol