Diagnosing a missfire/rough idle

[rvander1992]

So I thought my car was running nice, then I got to the dyno and they told me I had a missfire. While on the dyno I got a new coilpack harness just to see if that was the issue(was really close) and it made no difference. I left the shop sad, but car still running relatively nice.

This morning we went and picked it up, I had to keep my foot half way down the gas to keep it running and could barely move it to get it on the trailer. This sucked.


Got it home, I've replaced the plugs, this made it run again though extremely rough. I took off the nissan coilpacks and installed ls2 as a temporary thing. With the ls2's on the car it ran well again though still with the missfire. I pulled each cylinder 1 by 1, all of them made the revs drop except when I pulled 4. No change happened.

Then since the ls2's were just a temp placement I put the nissan coils back on to see if they ran now. Surprisingly all good. To diagnose the missfire with this I took off the plugs to each individual coilpack one at a time. Cyl 1-3 the engine almost died out as soon as the coil was unplugged, cyl 4-6 it made 0 difference. We then swapped the coilpacks(cyl 1-3 on 4-6, cyl 4-6 on 1-3), same thing. If i pull 1-3 it almost dies right away, 4-6 it makes no difference. The car ran about 200rpm less on this setup(1000 instead of 1200) compared to the ls2 coils and nissan coils original position.

My question is:
Since the car runs in either of these orientations(ls2 or either coilpack location) what can I do next to find the missfire issue?

Why when I pull cylinder 4-6 does no change happen to how the engine is running, where with 1-3 the change is instantaneous and almost dies?


How do you diagnose maf's?

[Skym]

As far as I know, stock ECU can run I think was batch at lower rpm, then sequential at higher rpm with injectors. I would assume ignition is the same to suit.

Sometimes it can be the ECU itself, CAS or ignition module. Ignition module failure would happen just after warmup or roughly 10 minutes (especially if earths have gone bad) and engine runs on 5 cylinders. CAS loses sync due to bearing failure, but can make engine hard to start when cold or hot and is due to the bearings failing and dragging, hence loss of sync and engine stalls.

With stock ECU, MAF's, unplug them, then start engine (stock ECU can run without MAF's, but engages a fuel cut rev limiter around 2500-3000rpm). MAF's can make engine stutter (similar to coilpack missfire) if there's a intake airleak before or after MAF's (MAF gets 2x airsources instead of one). If intake airleak before MAF's with clamp onto metal adaptor for MAF type of airfilters, it can make MAF's max out which engages boost cut on ECU.

Sometimes oil from blowby / breathers on engine gets onto MAF sensor hotwire due to reversion and the hotwire can't burn off the oil deposits when you turn the engine off. CRC MAF cleaner can help to solve this problem. Sometimes it's the common solder problem from circuit board inside square part of MAF to MAF plug itself.

[rvander1992]

Skym any tips for diagnosing my missfire? I assume it's in cylinder 4 from the ls2 coil test. I really wanna figure out what is wrong :/

[Skym]

Check the spark plug tip for damage on cylinder no 4 (also check heatrange, gap). Check the continuity of the wiring for coilpacks with a multimeter (do the same test with ECU to MAF wiring). If no problem, replace ECU with a known good ECU to see if the ignition part of ECU has failed or not.

Pull CAS off engine, turn ignition key to just before turning engine on and rotate shaft to see if the injectors click in sequence (you'll need to clear CAS error code on ECU after doing this test).

[rvander1992]

I eliminated coilpacks by switching to ls2 coils(or so I thought) and put in brand new plugs before starting the diagnosing. The tips on the old ones were dirty but none broken. The new ones aren't broken either.
I pulled the cas off and spun(with fuel pump fuse out) to listen to injectors but I could not tell if they were going in sequence, or if they were even all going. I may have to just swap them around a bit(which I don't wanna do since I don't have new insulators or orings kicking around).

After injectors if that doesn't solve my issue where should I be looking next? What else could it be?

[Skym]

My bad, I mean with the LS coilpacks. Might have a faulty coilpack or wiring. Also check dwell settings for coilpacks (might not be set correctly, as factory settings are for factory RB coilpacks).

Dirty in terms of black or brown tip on spark plugs???

The MAF, o2 sensor, TPS can make engine run too rich or too lean which can make engine missfire (check via wideband).

Also check headgasket, as when the seal fails around cylinder the engine can start to sound like it's missfiring (usually after a long drive).

Injectors can leak and make the engine sound like it's missfiring. Usually when injectors have done high km.

Faulty coilpack loom, injector loom are the other known causes of missfiring on RB engines.

[rvander1992]

Engine was running lean(17-18 idle 15-16 wot). Was looking at injectors this morning(pulled cas , spun listened) then moved on to looking at how to swap em around. Figured I would try the first thing from computer problems, "is it plugged in". Gave it a push and *click*. Engine runs fine now... What a shitty thing considering I towed it 2 hours and took a day off to go to dyno then had to walk away after doing nothing just cause an injector wasn't all the way plugged . Will have to go again soon.

[Skym]

Good to hear that you found the problem.

Must have been a lean missfire. Before taking to dyno, put a bore scope on that cylinder and check the top of piston for marks from knock (might not be able to if top of piston is covered in soot). Also do a compression, leak down test to make sure that cylinder hasn't dropped in compression (usually the result of running lean).

[rvander1992]

Guess I didn't put compression numbers in here, 1-6 155 165 165 170 170 165, pretty solid numbers imo. 4 is sitting at 170 so it should be fine. The car has been around the block once or twice and idled in the garage for test purposes, still not driven.

Its got maybe 15 km on it and 2 hours run time since the engine was installed.

[Bruizer]

Have you brought it to a shop to get diagnosed? Best bet=the pros are pros.

[rvander1992]

Bruiser problem solved, it was at the shop and they were going to charge 100$ an hour to diagnose by replacing parts id be paying
For 1 by 1. I can do that too . Ended up being 1: my ignitor is ******, 2: injector not fully plugged.

[Skym]

Ignition module failure is common.