[Gwagensteve]

Hi all

So, since I completed my car rebuild I've had a misfire.

I initially put this down to plug leads as it seemed like it was on cylinder #3 so I swapped leads and it seemed OK.

Then at Bill's block in early December the car was driving fine and then steadily started to misfire more and more until is was basically undrivable. Won't rev beyond 1500rpm under load and basically runs on 2 cylinders.

It starts and idles under no load with no issue.
It will stumble lightly on tip-in under no load, and then rev cleanly to the limiter.

Occasionally it will clean up randomly and run fine for a few seconds.
Occasionally it will misfire at idle.

In gear (auto) under load it immediately breaks down to the condition above if I'm above about 20% throttle.

It has fuel. I haven't verified pressure but there's plenty. Crimping the return shut has no effect, so it's not fuel pressure. Fuel is fresh 95 octane.
Injectors have been out and cleaned.
Cap and rotor is in good condition.
Plugs have been cleaned and gapped. They look fine (and remember it's not one cylinder only once it breaks down)

I get no trouble codes. If I unplug something obvious like TPS or MAF I get codes immediately so the diagnostics is just fine.

Ignition timing is correct and timing advances under throttle, verified with a timing light.

I have swapped coil - no change
I have swapped igniter - no change
I have swapped signal generator in the dizzy (hall effect transducer) - no change
I have swapped ECU - no change

What am I missing?

[MrRocky]

I would normally associate a misfire with a timing issue.
Although the car might seem ok with the light doesnt mean it wont break down with more heat or under more load.
Forget the diagnostic codes, i would try leads, plugs, sensors and dizzy first.
Fuel pressure reg on the rail, slowly dying fuel pump on the fuel side of things.
Last resort pull timing cover off and check nothing funky is going on with tensioner, pulley ect...

[MrRocky]

Also try some new plugs as a hairline crack in the ceramic can cause a misfire i dont even bother cleaning them anymore just get new ones

[Gwagensteve]

I believe it's an ignition issue.
If it was a dying reg, crimping the return line should up pressure sufficiently to clear it - it made no difference.
Yes, it could be lazy pump, but remember it will bounce off the limiter all day with no load but the moment even light load is applied it will misfire at 12-1500rpm. I can't plug the return line with my finger, there's LOADS of pressure and strong flow. Remember too, it will randomly clean up and pull like a train.

I wasn't expecting the codes to help me, but it was an example before anyone says "its the TPS" for example it behaves the same with the TPS unplugged but the moment I do that I get a code.

No evidence of arcing on the cap/rotor.

I have swapped leads a few times, admittedly to used leads, but they're making no difference. I'll do some more diagnostics, but I believe it's randomly dropping cylinders, it's not consistently one (or two) pots.

Timing is absolutely on spec.

[MrRocky]

Alot of the ignition components will seem fine until they get some heat in them then anything can happen.
Id swap or borrow as much stuff as possible i dont think its an issue that can be logically tracked down

[Gwagensteve]

So I’m at leads ecu coil igniter I’m running out of options without throwing cash away.

[ZUZUKI]

Ok, since you've covered to basics, it's down to random stuff that it probably isn't, but just might be.

Make sure everything has good earth paths - ecu, fuel pump, engine, chassis, body - nice fat cables & wires, nice & tight on clean metal.
You've made your fuel tank - any way of coming up with a temporary mounting for the original tank & plumbing that in? The problem does indicate electrical, but.....
You did some extensive re-wiring - any cross interference from fans, fuel pumps, god knows what, affecting ignition or ecu?
For the price of plugs, leads, cap & rotor, just do it.

If this misfire has only occurred since the car has been back together, even if it was apart for 3 years, there is a good chance it has got something to do with something you've done in the mean time - unfortunately for you, that was a whole lot of stuff, so lots to check over.

Good luck with it.
Rgs, Michael

[fordem]

Is this a 16v engine with the plugs down in tubes in the head? Get a set of fresh plug leads and try it - alternatively run the engine until it's misfiring and then take a look at the leads in a darkened place.

I've had very similar symptoms on a Toyota 5A motor - I found the problem by accident - the heat in the plug tubes would destroy the leads, and then the spark would jump from the lead to the tube rather than across the plug gap. Part of the problem was that I was using the incorrect leads, first time around I put back what the car had when I got it, second time around I went out & got genuine Toyota leads and that resolved it.

[Gwagensteve]

I agree Fordem, and I thought this was the initial problem (a couple of plug tubes were in bad nick) but iI replaced the bad ones and it now doesn't seem associated with one cylinder at all.



I'm going to try and do some more diagnostics today - verify fuel pressure under load, and I'm going to do some messing about with the timing light to see if I can see it dropping spark under load. It's my feeling it's going it randomly on all cylinders (pulling plug leads while it's popping bears this out) but I'd like to know.

Michael, I can totally see that reasoning but I actually have done very little to disturb the wiring since I reworked the front of the car - it was lifted out the way and then refitted. I didn't remove any earths etc. I'm not saying I haven't done something silly but something that's interesting is it's got steadily worse since I first drove the car since completion. It would stumble and then clean up initially, now it's undrivable. To me that points to a component failing.

[shep]

If you are sure it’s random then what about the coil to cap lead or the cap itself

[Gwagensteve]

OK, some more to report. This morning I concentrated on the fuel system to rule it out.

I've checked fuel pressure at the outlet of the pump (at the tank) and at the end of the rail as per the FSM.

FSM spec is 35 to 43 PSI at idle.

I have 40psi at idle, and at least 40 psi throughout the rev range.

When I load the car and make it misfire, it's holding 40psi steadily.

On transients as the car stumbles under low load, fuel pressure momentarily increases about 8psi, I believe this is normal as the regulator is vacuum referenced.

I think this rules out pump or regulator.

In the process of putting things back together I now have replaced 3 out of 4 leads. I have previously changed the cap to coil lead.

I'm going to really dig into the ignition side now with the timing light and try and to observe it dropping spark. My timing light has a tach function and interestingly that seemed to be playing up last time I checked timing. I'm now starting to wonder whether that's indicative of the problem.

[Gwagensteve]

OK, got the car right up to temperature and as per previous experience, it started to break down at idle.

Threw the timing light on it and boyoboy there's some weird stuff going on.

It's all good, and then will start dropping the cylinder 1/2 the time.

Checked cylinder two, and it's firing way too often.

This points to (sorry for the pun) to cap, not leads, or arcing between leads. I think?

timing is still consistent and rock steady, so I think that confirms there's no problem with the ECU.

I don't have a spare cap. I'll have a very careful look at mine and see what I can spot.

[Gwagensteve]

Hmm.

I pulled the cap and had a really good look at it.

I think it might be the problem.



That crack/line is running between 1 and 2, which bears out my findings with the timing light.

As you can see there's also a crack on the centre pin.

And on cylinder 3.... and 4



I'd never suspected the cap as it's basically never been off the car and as a screw down, it doesn't really get wet or misbehave. It's also black, so a quick look doesn't show anything obvious - I had to get it clean and out in the sunlight to see that arc between 1 and 2.

As I'm completely out of other ideas and this is clearly not right, I'll order a seat of leads and cap and see how I go.