[Mk2] [NA] Intermittent misfire / hesitation issue at low RPM / low throttle position

[SonicSW20]

Hi all,

I have an intermittent issue with my Rev5 NA (UK car so Gen3 3SGE), as it's intermittent it's a bit tricky to track down.

Sometimes while pulling away gently with a small amount of throttle, it will misfire/hesitate at ~1500RPM. It's almost as is the ignition has been turned off for a split second. It doesn't stall though. It has only ever happened when the engine is hot, I cannot see a pattern in the conditions leading up to it though.

Another issue (again very intermittent) when maintaining a constant speed in 1st, 2nd, or 3rd at ~2000-2500RPM it will do a similar thing - misfire/hesitation.

This will only happen at very slight throttle positions - if you apply more throttle in these problem areas it's fine. It runs fine above 2500RPM or so. Fuel economy seems pretty normal, and there's no loss of power at wider throttle openings. O2 sensor seems to be oscillating as it should, fuel trim is at 2.51V which fine.

I can't find a pattern to this at all, which makes it tough to work out.

I've tried two TPS's (both calibrate correctly), though the 2nd one was an unknown to me so it is possible that the 2nd TPS was faulty. Both that one and my original calibrated and tested correctly against the BGB values though. I have one more spare to try so as it doesn't take long to swap / calibrate I'll try that one as well.

ECU Temp sensor has been replaced.

O2 sensor has been replaced.

ISCV has been cleaned out.

Most of the vac lines have been replaced.

HT leads, plugs, rotor, distributor cap are all recent - I would expect these to break down at high load / high RPM if they were a problem, not in the conditions above! Edit - tested HT lead resistance, all well within spec. No damage to any of the leads.

Tested ignition coil against engine manual values - primary and secondary are within spec.

Mechanical timing is correct.

Ignition timing is advanced a few degrees - car has been like that since I bought it 2 years ago. For the purpose of ruling it out I'm going to put it back to 10 degrees. If it is this, I'd like to know why it's become a problem now though.

Compression is good - 210PSI+ across all cylinders, all cylinders within range of each other.

Going to work through the troubleshooting section in the 3SGE workshop manual tomorrow, but in the mean time, if anyone has any ideas or has fixed a similar issue then I'd like to know!

[mr2garageswindon]

Have you inspected the coil? Done a code read? Inspected inside of ecu for caps leaking? Intermittent faults are a pita to diagnose at times. Good luck dude.

[SonicSW20]

No codes. Thought I had TPS open circuit code, but that was my fault - started it without plugging the thing back in

Will check the coil and have a look at the ECU.

[SonicSW20]

Just tested the coil as per the engine manual instructions, resistances are within spec. HT leads are also within spec.

Will remove the coil and inspect for damage etc tomorrow, and clean up bracket ground point.

[mr2garageswindon]

Let us know how you get on.

[SonicSW20]

Will do. This is where the intermittent nature of the problem is problematic... Next time it does it I'll pull over real quick and test it

[abovetherim]

Inspect the throttle cable for tightness, kinks etc

Check the throttle body butterfly for loose screws or deposit build up

[SonicSW20]

TB was cleaned at the same time as the ISCV. Throttle operates smoothly, the vacuum holes are all clear as well.

Will check the cable, wouldn't have thought that would cause a misfire/hesitation though?

I bet it won't do it for weeks now I've made this thread!

[abovetherim]

Another thing to consider is a leaky injector. I have found this on a focus that had at misfire at low rpm. Found that one injector was sticking open and over fueling one cylinder.

[SonicSW20]

What's the best way to check for a leaky / problem injector?

[markstevieandmads]

What's the best way to check for a leaky / problem injector?

Ive never done it but seen it from Wheeler Dealers lol....
Remove injector and put inside a clear bottle. Turnover engine and make sure it squirts out ok and nice and even?

[abovetherim]

What's the best way to check for a leaky / problem injector?

The way i did it on the focus was to pull the rail and have it sat in a tray and turned ignition on. The leaky one was obvious.

Now as yours is intermittant it may not leak at this point. When i pulled the plugs one was obviously from the over fueling cylinder as it was black and greasy.

You could run some injector cleaner through the system or if you know which one is playing up take it out and soak it in some injector cleaner (or even a bit of diesel) Then squirt some carb cleaner in it

[SonicSW20]

The intermittent nature of it is so annoying.

Thanks for the tips.

It hasn't done it for a few days now, ever since removing and reinstalling the ECU and the plugs on the coil / igniter. Too early to say yet (and it's a bit of confirmation bias I guess), but I'm now wondering if one of these wasn't quite seated correctly - they were all unplugged earlier this year to remove the engine to do timing and clutch.

Not going to have a chance to do anything to it til the weekend anyway so lets see how it goes this week.